JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. B7, PAGES 16,541–16,561, JULY 10, 2000
Near-ridge seamount chains in the northeastern Pacific Ocean
David A. Clague, Jennifer R. Reynolds, and Alicé S. Davis
Monterey Bay Aquarium Research Institute, Moss Landing, California
Abstract. High-resolution bathymetry and side-scan data of the Vance, President
Jackson, and Taney near-ridge seamount chains in the northeast Pacific were collected
with a hull-mounted 30-kHz sonar. The central volcanoes in each chain consist of
truncated cone-shaped volcanoes with steep sides and nearly flat tops. Several areas are
characterized by frequent small eruptions that result in disorganized volcanic regions with
numerous small cones and volcanic ridges but no organized truncated conical structure.
Several volcanoes are crosscut by ridge-parallel faults, showing that they formed within
30 – 40 km of the ridge axis where ridge-parallel faulting is still active. Magmas that built
the volcanoes were probably transported through the crust along active ridge-parallel
faults. The volcanoes range in volume from 11 to 187 km3, and most have one or more
multiple craters and calderas that modify their summits and flanks. The craters (⬍1 km
diameter) and calderas (⬎1 km diameter) range from small pit craters to calderas as large
as 6.5 ⫻ 8.5 km, although most are 2– 4 km across. Crosscutting relationships commonly
show a sequence of calderas stepping toward the ridge axis. The calderas overlie crustal
magma chambers at least as large as those that underlie Kilauea and Mauna Loa
Volcanoes in Hawaii, perhaps 4 –5 km in diameter and ⬃1–3 km below the surface. The
nearly flat tops of many of the volcanoes have remnants of centrally located summit
shields, suggesting that their flat tops did not form from eruptions along circumferential
ring faults but instead form by filling and overflowing of earlier large calderas. The lavas
retain their primitive character by residing in such chambers for only short time periods
prior to eruption. Stored magmas are withdrawn, probably as dikes intruded into the
adjacent ocean crust along active ridge-parallel faults, triggering caldera collapse, or
solidified before the next batch of magma is intruded into the volcano, probably 1000 –
10,000 years later. The chains are oriented parallel to subaxial asthenospheric flow rather
than absolute or relative plate motion vectors. The subaxial asthenospheric flow model
yields rates of volcanic migration of 3.4, 3.3 and 5.9 cm yr⫺1 for the Vance, President
Jackson, and Taney Seamounts, respectively. The modeled lifespans of the individual
volcanoes in the three chains vary from 75 to 95 kyr. These lifespans, coupled with the
geologic observations based on the bathymetry, allow us to construct models of magma
supply through time for the volcanoes in the three chains.
1.
Introduction
Short chains of seamounts are common near mid-ocean
ridges and have been studied extensively, particularly along the
East Pacific Rise, because they provide insight into the dynamics of mantle upwelling beneath ridges [e.g., Davis and Karsten,
1986; Schouten et al., 1987; Shen et al., 1993, 1995] and into
magma genesis at and near ridges [e.g., Barr, 1974; Batiza and
Vanko, 1984; Allan et al., 1987, 1989; Batiza et al., 1990; A. S.
Davis and D. A. Clague, Seamount structure and petrology of
basalts from the President Jackson Seamounts, northern
Gorda Ridge: Magmatic relationship between on- and off-axis
volcanism, submitted to Journal of Geophysical Research, 2000,
hereinafter referred to as Davis and Clague, submitted manuscript, 1999]. These chains have been proposed to parallel
absolute or relative plate motion vectors [Davis and Karsten,
1986; Batiza et al., 1990; Mukhopadhyay and Batiza, 1994],
migrating nontransform offsets [Lonsdale, 1985], melting
anomalies that migrate along the spreading ridge [Lee and
Hammond, 1985], or plate motion relative to the subaxial asCopyright 2000 by the American Geophysical Union.
Paper number 2000JB900082.
0148-0227/00/2000JB900082$09.00
thenosphere [Schouten et al., 1987; Lonsdale, 1991; J. R. Reynolds and D. A. Clague, Pacific Plate absolute motion: New
constraints from near-ridge linear volcanic chains, submitted to
Earth and Planetary Science Letters, 2000, hereinafter referred
to as Reynolds and Clague, submitted manuscript, 2000]. They
appear to be especially common near fast spreading ridge
segments but are also found at moderate spreading ridge segments. Near-ridge seamounts have also been proposed to be
preferentially located near kinks or transverse structures in the
ridge crest [Menard, 1969; Barr, 1974], ridge offsets [Batiza and
Vanko, 1983; Lonsdale, 1985], and topographically high areas
of the ridge crest [Fornari et al., 1987]. The distribution of
seamounts near intermediate to fast spreading ridges is commonly asymmetric, with far more seamounts located on one
side of the ridge axis than on the other [Davis and Karsten,
1986; Scheirer and Macdonald, 1995]. Many, if not most, nearridge seamounts have the form of truncated cones [Batiza,
1982, 1989; Searle, 1983; Lonsdale, 1983; Smith, 1988; Rappaport et al., 1997]. In addition, many near-ridge seamounts have
complex nested calderas [Lonsdale and Spiess, 1979; Batiza and
Vanko, 1983; Batiza et al., 1984, 1989; Fornari et al., 1984,
1988], commonly offset toward the ridge axis [Hammond,
1997]. The volcanoes located nearest the ridge are generally
16,541
16,542
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
de Fuca Ridge. The high-resolution data provide new insight
into the tectonomagmatic processes that construct and modify
near-ridge seamounts.
2.
Figure 1. Location map showing the three near-ridge chains
studied. The Vance Seamounts are on Pacific Plate crust between 2.55 and 0.78 Ma. The President Jackson Seamounts are
on Pacific Plate crust between 3.97 and 2.18 Ma. The Taney
Seamounts are on Pacific Plate crust between about 30.3 and
29.3 Ma and formed near the Pacific-Farallon spreading center.
thought to postdate those located farther from the ridge [e.g.,
Barr, 1974; Barone and Ryan, 1990]. Other small volcanoes on
the seafloor far from ridges have similar shapes [Batiza, 1982;
Searle, 1983; Smith, 1988; Smith and Jordan, 1988; Mukhopadhyay and Batiza, 1994; Bridges, 1995, 1997] and have been
inferred to have formed close to ridges.
Lavas that comprise near-ridge seamounts are chemically
diverse. They consist primarily of normal mid-ocean ridge basalt (NMORB) but may include enriched MORB (EMORB)
and even some alkalic basalt [e.g., Batiza and Vanko, 1984;
Allan et al., 1987, 1989; Batiza et al., 1989; Leybourne and Van
Wagoner, 1991; Davis et al., 1998; Davis and Clague, submitted
manuscript, 2000]. The lavas are usually more primitive (higher MgO) than those from the adjacent ridge, and are usually
aphyric or sparsely phyric. The primitive nature and uncomplicated mineralogy indicate that most near-ridge seamount
lavas have undergone little or no modification in crustal
magma reservoirs. These lava characteristics have been used to
argue against the existence of shallow crustal magma reservoirs
within or beneath the volcanoes [e.g., Fornari et al., 1984,
1988].
In this paper, we describe new 30-kHz high-resolution swath
bathymetric and backscatter data for three near-ridge seamount chains located in the northeast Pacific Ocean (Figure
1). These chains are the Taney Seamounts offshore central
California, the President Jackson Seamounts on the Pacific
Plate adjacent to the central Gorda Ridge, and the Vance
Seamounts on the Pacific Plate adjacent to the southern Juan
Methods
High-resolution swath bathymetry and side-scan images
were collected using a hull-mounted 30-kHz Simrad EM300
system on the M/V Ocean Alert by C&C Technologies, under
contract to the Monterey Bay Aquarium Research Institute
(MBARI). The system consists of 137 beams that can be divided into three or nine sectors for independent active steering
according to ship roll, pitch, heave, and yaw. For surveying
deeper than 250 m, beam width is set to 1⬚. The pulse length is
variable and is operator-controlled. Depth is determined by a
combination of phase and amplitude detection, resulting in
vertical and horizontal resolution of 0.2% and 2% of water
depth, respectively. All navigation was done using shore-based
differential Global Positioning System (GPS).
The bathymetric data were gridded at 30 m for the Vance
and President Jackson Seamounts and 50 m for the deeper
Taney Seamounts. The backscatter data were mosaicked at
10 m for the Vance and President Jackson Seamounts and at
20 m for the Taney Seamounts and were utilized to determine
where the volcanic aprons surrounding the seamounts end and
sediment-covered seafloor begins. The location of the sediment/lava contact, in turn, was used to estimate the average
depth of the seafloor underlying each volcano.
The seamount volumes were calculated by summing the
volume of columns extending from the estimated depth of the
underlying basement to the seamount surface for each pixel,
using an ArcView script written by G. Hatcher at MBARI. The
volume estimates have large errors (probably ⫾10%) due to
uncertainty in determining the depth of the basement beneath
each volcano. Several of the volcanoes, particularly several of
the larger Taney Seamounts, were not completely imaged, and
we adjusted the volumes to account for the unimaged flanks
assuming that the volcanoes were symmetrical about the axis of
the chain. The volumes were calculated using the present-day
bathymetry.
3.
Descriptions of the Volcanic Chains
General parameters that describe each chain are listed in
Table 1. We have tabulated the age of the crust beneath the
southeastern and northwestern seamounts in each chain, the
ridge spreading rate and azimuth during the time the chain was
forming, the azimuth of the seamount chain, the rate and
azimuth of absolute Pacific Plate motion estimated from several plate motion models, the number and spacing of volcanoes
in each chain, and the total volume of each chain calculated
from the volumes of individual volcanoes listed in Table 2. The
crustal ages are estimated from identified magnetic anomalies
[Wilson, 1988, 1989; Fernandez and Hey, 1991] and the ages are
calibrated using the Cande and Kent [1995] geomagnetic timescale. The spreading rate is calculated from these calibrated
anomaly ages for the crust immediately adjacent to each chain.
We have identified the seamounts alphabetically starting
with A for the oldest, or most northwesterly volcano in each
chain. Coalesced volcanoes have been identified as two separate volcanoes based on the locations of calderas and the
presence of saddles between the adjacent volcanoes. The three
near-ridge chains are identified as V for Vance, PJ for Presi-
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
Table 1. Measured Characteristics of Three Near-Ridge
Seamount Chains
Seamount Chain
Vance
Azimuth of chain (measured)
324–331⬚
Length of chain, km
57
Number of volcanoes
7
Volcano spacing, km
9.5
235
Volume of chain, km3
Age of crust at east end, Ma
0.78
Age of crust at west end, Ma
2.55
⫺1
Half-spreading rate, cm yr
2.75
Azimuth of spreading direction
292–294⬚
Azimuth of absolute Pacific Plate
313⬚
Absolute Pacific Plate rate, cm yr⫺1
5.1
President
Jackson
Taney
318–320⬚
64
8
9.1
376
2.18
3.97
3.0
290⬚
309⬚
5.3
294–298⬚
53
5
13.3
430
⬃29.3
30.3
4.9
⬃268⬚
296⬚
7.0
Crustal ages are determined using magnetic anomalies from Wilson
[1988, 1989] and Fernandez and Hey [1991] calibrated to the Cande and
Kent [1995] timescale. Spreading rates are calculated from crustal ages
at each end of the chain and the azimuth of spreading. Azimuth of
chain (measured) indicates the widest possible range of azimuths that
could fit the locations of the volcanoes in each chain. See text for
discussion of spreading vectors and absolute motion vectors.
dent Jackson, and T for Taney. For each volcano, Table 2 lists
the depth to the adjacent seafloor, the volcano height, the
depth of the shallowest point on the seamount, the calculated
volume of the seamount, the number of shields on the summit,
the number of calderas on the summit and flanks, the depth of
the saddle between adjacent volcanoes, and whether that saddle consists of volcanic (high backscatter) material or sediment-covered (low backscatter) seafloor. The lower slopes of
adjacent volcanoes coalesce and cover the original seafloor
except between PJ-C and PJ-D and between T-C and T-D.
Table 3 summarizes the size and depth of each caldera (⬎1 km
across) or crater (⬍1 km across) and indicates its location and
16,543
shape. The calderas are numbered in stratigraphic order determined by crosscutting relationships, where caldera 1 is the oldest.
None of these chains appears to be surrounded by flexural
moats, although our surveys rarely extended far beyond the
base of the volcanoes. The Lamont Seamounts near the East
Pacific Rise are surrounded by a moat up to 150 m deep
[Barone and Ryan, 1990]. The absence of moats around the
three northeastern Pacific chains may simply reflect the much
smaller sizes of the volcanoes in these chains compared to
those in the Lamont Seamounts or that the northeast Pacific
volcanoes formed on older, thicker lithosphere than did the
Lamont Seamounts.
3.1.
Vance Seamounts
The Vance Seamounts comprise the southernmost of several
near-ridge chains located on the Pacific Plate near the Juan de
Fuca Ridge (Figure 1). No symmetrical chain occurs on the
Juan de Fuca Plate. The eastern portion of the chain (five of
seven volcanoes) was previously surveyed using SeaBeam
[Hammond, 1997] that also showed an area between the southeastern seamount and the ridge axis characterized by numerous small volcanic cones. The chain projects roughly to the
offset between the Cleft and Vance segments of the Juan de
Fuca Ridge at 45⬚20⬘N. As presently mapped, it begins ⬃19 km
west of the axis. However, data from a 1971 geophysical mapping cruise suggest that at least one more seamount may exist
farther to the northwest along the strike of the chain at
⬃131⬚20⬘W, 46⬚0⬘N [Wilson, 1993, also personal communication, 1999]. The few chemical analyses that have been published for lavas from the Vance Seamounts [Smith et al., 1994;
M. Perfit, written communication, 1999] indicate that V-E and
V-F are constructed of depleted NMORB that are relatively
primitive.
The volcanoes (Figure 2) form a roughly linear array. Just
west of V-C, the underlying sediment-covered seafloor crops
out in a ridge-parallel graben. Between V-A and V-C lies a
Table 2. Characteristics of Individual Volcanoes
Seamount
V-A
V-B
V-C
V-D
V-E
V-F
V-G
PJ-A
PJ-B
PJ-C
PJ-D
PJ-E
PJ-F
PJ-G
PJ-H
PJ-I
T-A
T-B
T-C
T-D
T-E
Base,
m
Height,
m
Summit,
m
Volume,
km3
Shields
Calderas/
Craters
Saddle,
m*
2700
2700
2650
2600
2600
2725
2725
3080
3025
3070
3100
3100
2950
2950
2925
W2930-E2730
4150
4150
4150
4250
4250
1050
770
830
1140
1040
850
440
1310
1200
1700
1430
1280
1340
1470
1200
⬎330
1990
1300
1310
830
490
1652
1932
1820
1460
1560
1876
2281
1772
1826
1366
1669
1819
1615
1483
1723
2471
2157
2847
2837
3420
3761
35
67
36
25
23
34
15
38
34
53
68
45
58
56
24
?
187
91
116
25
11
0
0
0
1
1
0
0
0
1
1
1
2
2
1
1
0
1
0
0
0
0
1
0
2
3
3
2
0
5
3
2
4
4
4
6
1
0
3
3
3
5
0
2430v
2620v
2135v
1650v
2280v
2360v
2230v
2597v
3343s
1974v
2665v
1765v
2476v
3116s
3750v
3750v
4110s
3880v
The depth of the saddle between A and B is listed under volcano A, etc. The volcanoes are designated
alphabetically from northwest to southeast for each chain with V, Vance; PJ, President Jackson; and T,
Taney. PJ-I is only partly imaged, and the base slopes from 2930 m to the west to 2730 m to the east.
*v, volcanic; s, sediment.
16,544
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
Table 3. Calderas and Craters
Volcano
V-A
V-C
V-C
V-D
V-D
V-D
V-E
V-E
V-E
V-F
V-F
PJ-A
PJ-A
PJ-A
PJ-A
PJ-A
PJ-B
PJ-B
PJ-B
PJ-C
PJ-C
PJ-D
PJ-D
PJ-D
PJ-D
PJ-E
PJ-E
PJ-E
PJ-E
PJ-F
PJ-F
PJ-F
PJ-F
PJ-G
PJ-G
PJ-G
PJ-G
PJ-G
PJ-G
PJ-H
T-A
T-A
T-A
T-B
T-B
T-B
T-C
T-C
T-C
T-D
T-D
T-D
T-D
T-D
Caldera
1
1
2
1
2
3
1
2
3
1
2
1
2
3
4
5
1
2
3
1
2
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
5
6
1
1
2
3
1
2
3
1
2
3
1
2
3
4
5
crater
crater
crater
crater
crater
crater
crater
crater
crater
crater
crater
crater
crater
Size,
km
Depth,
m
Type
2.5
2
1.5
2
1.5
2
1.5
1.5
3.5 ⫻ 2
3 ⫻ 2.5
4
1.2
1
0.75
0.75
0.75
2 ⫻ 1.5
1
0.5
0.5
0.5
2
1
0.5
0.5
3 ⫻ 2.5
1.3 ⫻ 0.8
0.5
0.4
2 ⫻ 1.5
1.5 ⫻ 2
1
0.2
2 ⫻ 1.5
0.7
1
2 to 2.5
2?
1.5
1.2
4.5
3.5
5
8.5 ⫻ 6.5 (?)
6.5 ⫻ 4.5
2 ⫻ 1.5
4.5 ⫻ 4
2.5 ⫻ 2
2.5
2
1.5
2
2.2
0.1
shallow
shallow
150
100
140
40
125
230
450
300–400
150
100
90
150
40–110
120–230
150
150
100
115
210
50
60
130
170
shallow
shallow
150
100
shallow
50–120
90
60
100–150
180
300
200
170
400
150
50–400
150
⬃400
unknown
⬃100
80
75
370
575
shallow
190
290
420
60
centered
off-center
flank
off-center
offset
offset
centered?
offset
offset
centered
flank
off-center
offset
offset
offset
flank
centered
nested
summit edge
centered
summit edge
off-center
off-center
summit edge
summit edge
centered
nested
nested
summit edge
centered
trapdoor
nested
nested
off-center
nested
offset
flank
flank
flank
centered
centered?
offset
flank
somma
nested
nested
centered
nested
flank
off-center
offset
offset
flank
off-center
For size, along-chain dimension is listed first, across-chain dimension is listed second. Calderas are ⬎1 km across, whereas craters are
⬍1 km across.
region of rough, elevated seafloor consisting of lava cones and
flows (V-B). A small conical seamount [Smith et al., 1994] occurs
on the axis of the Juan de Fuca Ridge on strike with the chain, at
the eastern end of a second region characterized by many small
cones [Hammond, 1997]. The volcanoes have an average volume of 34 ⫾ 15 km3 with a range of 15 to 67 km3 (Table 2).
Volcano V-A has a circular base and a gently domed top. A
subtle depression is centered in the summit platform and represents either the remnant of a nearly filled deep caldera or an
unusually shallow caldera (Table 3, caldera 1). The nearly flat
summit slopes down to the northwest and is modified by numerous small cones. Other cones, the largest ⬃1.3 km across,
and shields, the largest ⬃2 km across, occur on the seafloor
adjacent to the central volcano.
Volcano V-B is shown in Figure 3. It is a volcanic constructional feature that covers an area at least 25 by 10 km. The
region consists of high-backscatter material, presumably lavas,
that forms a broad 700-m-high rise with superposed roughness.
Much of the roughness can be resolved into individual cones,
some with summit craters, and small volcanic ridges aligned
parallel to the ridge axis. The largest of the cones is nearly 2 km
across, and the smallest appear to be only a few hundred
meters across. The volume of the dome and cones (67 km3,
Table 2) is roughly equivalent to any two of the circular volcanoes in the Vance chain. The volume erupted per kilometer
of length of the chain is similar to that of the central volcanoes,
but the volcanic activity was less centralized.
Volcano V-C has a roughly circular base and a flat top that
is modified by two calderas (Table 3). The first is very shallow,
having been flooded and filled almost completely by subsequent flows (Table 3, caldera 1). Only a low northern rim of
this caldera still exists. The younger caldera is located at the
southeastern edge of the flat summit platform (Table 3,
caldera 2). Three ridge-parallel faults cut the east central part
of the summit platform. These faults have only low relief, of
the order of a few tens of meters. They do not step systematically down to the southeast but instead have both down-tothe-southeast and up-to-the-southeast throws. The summit
platform is domed upward, and the shallowest several points
on the summit platform are the tops of small cones. None of
the cones appears to be offset by the faults.
Volcanoes V-D and V-E are shown in Figure 4. The northwestern volcano V-D is flat-topped with a broad central shield
(S1) whose remnant is surrounded by the 1500-m contour on
the northern part of the platform. This shield was truncated by
three separate caldera collapses that progress towards the
south-southeast. The first caldera (C1) left only the northern
rim of the shield. The second caldera (C2) truncates the first,
leaving only the northern portion of its flat floor. The flat floor
in the second caldera slopes gently down to the southeast. The
third caldera (C3) has a scalloped northern rim, and a flat floor
that is only 20 m deeper than the floor of the second caldera.
The summit of volcano V-E has the northern rim of a shield
(S1) that partly filled in the three calderas from volcano V-D
and was subsequently modified by three more caldera collapses. The remaining rim of the shield is the shallowest point
on the volcano. This is the only place where the data confirm
the general hypothesis that the southeastern volcanoes are
younger than the northwestern volcanoes. The shield is truncated by the first caldera (C1) that was then filled in by a
second shield volcano (S2), centered a few kilometers farther
southeast than the first. The second shield was subsequently
truncated by formation of a second caldera; only a small remnant of the flat floor still exists, and the size of the caldera is
not well constrained. A third caldera (C3) removed most of the
evidence for the second caldera and is elongate parallel to the
trend of the chain. Most of the calderas have smooth, flat
floors, probably representing fluid lava flows, that formed prior
to caldera collapse. The floor of the final caldera has a hummocky area, probably a landslide deposit, on the flat floor
below the northeast rim and several volcanic cones. The flat
summit plateau of V-E is tipped down toward the southeast,
and the last caldera removed much of the southeastern part of
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
16,545
Figure 2. Illuminated bathymetry of the Vance Seamounts. The seamounts are labeled from northwest
(V-A) to southeast (V-G). The grid cell size is 30 m, and the contour interval is 250 m. Inset boxes indicate
the areas shown in Figures 3, 4, and 5. Two ridge-parallel faults are shown west of volcano V-C.
the summit platform and rim. A lava shield (Figure 2), 1 km
across, occurs near the base of the volcano several kilometers
to the northeast.
Volcanoes V-F and V-G are shown in Figure 5. The northwestern volcano, V-F, is roughly circular in shape, although the
southeastern portion of the edifice is missing, leaving the impression of a doughnut with a bite removed. It is a flat-topped
volcano that was modified by two caldera-forming collapses.
The first caldera (C1) is roughly centered in the summit. Its flat
floor has a large landslide/talus deposit at the base of the west
wall. The rim of the flat summit platform dips down toward the
east-southeast, as does the floor of the first caldera. The flat
summit platform has been dropped down to the southeast
along three ridge-parallel faults in the east part of the summit,
although the offsets are only several tens of meters and cannot
account for the tilt of this summit platform. The second caldera
(C2) truncates the first on its southeast side and also removed
the entire flank of the volcano. We have identified C2 as a
caldera because there is no hummocky region of debris to its
southeast, as expected if it were a landslide scarp. Its floor is
only a few tens of meters deeper than the floor of the first
caldera. The second caldera was flooded with subsequent lava
flows, probably erupted from volcano V-G. V-G is a low shield
located south of the second caldera on V-F. The shield has
been modified by three ridge-parallel faults that each step
down 30 – 60 m to the southeast. A semicircular depression
offset to the eastern edge of the low shield may be a caldera.
A number of flat-topped and conical volcanic cones up to 1 km
across, located toward the northeast, are included in the volume
calculation. These cones could be part of seamount V-G or V-F.
3.2.
President Jackson Seamounts
The only near-ridge seamount chain off the Gorda Ridge is
the President Jackson Seamounts, located on the Pacific Plate
16,546
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
Figure 3. Shaded contour map of volcano V-B, with a contour interval of 100 m and shading changes every
50 m.
(Figure 1). The most easterly volcano is located ⬃53 km west
of the ridge axis. A previous survey with the GLORIA system
[EEZ-Scan 84 Scientific Staff, 1986; Masson et al., 1988] established the rough sizes and locations of the volcanoes. A SeaBeam bathymetric survey conducted by the National Oceanic
and Atmospheric Administration (NOAA) imaged most of the
chain (R. Embley, written communication, 1985), but poor
navigation and incomplete coverage of the volcanoes limit the
usefulness of the survey. The SeaBeam data guided a sampling
cruise by the U.S. Geological Survey [Clague and Holmes,
1989] that recovered pillow fragments and hyaloclastite from
four volcanoes. These samples consist entirely of NMORB that
is generally more primitive (most have Mg ⬎ 65; Mg ⫽ 100
Mg/(Mg ⫹ Fe2⫹) than that erupted along the nearby Gorda
Ridge (Davis and Clague, submitted manuscript, 2000).
Our new survey imaged eight volcanoes (Figure 6), of which
four are isolated volcanoes ⬍10 km in diameter and nearly
circular in plan view. The other four form two morphologically
complex, coalesced flat-topped structures. In addition, a transit
line to our survey crossed an elevated hummocky area of volcanic cones and flows ⬃13 km southeast of PJ-H, which we
have designated PJ-I. The volcanoes form a linear chain. The
volcanoes are all relatively small, with an average volume of
47 ⫾ 14 km3 and a range of 24 to 68 km3 (Table 2).
Volcano PJ-A is the westernmost in the chain, with a base
roughly 10 km in diameter and a flat top 3.5– 4 km across. The
summit is modified by five coalesced calderas and craters (Table 3). Caldera 2 and crater 3 are identified by only small
portions of their walls. Craters 4 and 5 are on the southeast
edge of the flat summit platform. The northeastern portion of
Figure 4. Illuminated bathymetry of coalesced seamounts V-D and V-E. The contour interval is 100 m. Caldera (C) sizes and depths are listed in Table 3, and shields
are indicated with S; both are numbered sequentially starting with 1 for the oldest
feature.
Figure 5. Illuminated bathymetry of seamounts V-F and V-G. Explanation is
as in Figure 4.
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
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CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
Figure 6. Illuminated bathymetry of the President Jackson Seamounts. The volcanoes are labeled from
northwest (PJ-A) to southeast (PJ-I). The grid cell size is 30 m, and the contour interval is 250 m. Inset boxes
show the areas depicted in Figures 7 and 8. Several large ridge-parallel faults are evident between PJ-C and
PJ-D and between PJ-H and PJ-I.
the summit platform also has numerous small cones. A large
cone, nearly 0.5 km across, lies at the intersection of three of
the coalesced calderas and craters.
Volcano PJ-B is the only one in the chain that does not have
a broad flat top. The base of the volcano is roughly 10 km in
diameter, and the summit is modified by three calderas and
craters (Table 3), with calderas 1 and 2 nested in the center of
the summit and crater 3 on the southern rim of the summit.
The southeastern flank of the cone has numerous small cones.
Volcano PJ-C is located next to a sediment-covered, ridgeparallel graben that separates it from PJ-D. The seafloor to the
southeast of PJ-C drops as deep as 3343 m in this graben. The
volcano is circular with a base ⬃9 km in diameter and a flat top
⬃2.5 km in diameter. There are two craters in this summit
(Table 3). The first is centrally located, whereas the second
cuts the southern rim of the flat summit. The east-northeast
and southeast flanks are fluted with erosional gullies that may
be small landslide chutes. A large hummocky apron, probably
of debris, extends to the southeast. A small step at 2100 m
depth in the northeast slope of the volcano may be the remnant
of an earlier large caldera. Such structures are known from
subaerial volcanoes as sommas.
Volcano PJ-D, shown in Figure 7, is notably elongate in a
southeast-northwest orientation, parallel to the chain. The
base of the volcano is ⬃9 km across, perpendicular to the
chain. The summit is a strikingly flat plateau marked by numerous cones, including one with a crater that forms the sum-
mit peak. Four calderas and craters modify the summit platform; all are located toward the southeastern end (Table 3).
The northeastern caldera 1 is circular and evident from a rim
that stands above the lava-flooded floor. Caldera 2 is less
obvious but is apparently oriented with its long axis parallel to
the seafloor grabens. Craters 3 and 4 cut the southeastern rim
of the platform. The southwestern flank is fluted by erosional
gullies or landslide chutes.
The base of volcano PJ-E, also shown in Figure 7, is ⬃8 km
in diameter. The summit is, again, a strikingly flat plateau that
has a number of small cones and the remnants of four calderas
or craters still exposed. Caldera 1 is filled so that only a string
of small peaks define its margins. Nested within this large
caldera is circular crater 2, also partially filled, whose floor is at
⬃1885 m depth. Nested within caldera 2 is a small, deep crater
(Table 3, crater 3). Crater 4 occurs on the southeastern rim of
the summit platform.
Volcanoes PJ-F and PJ-G are shown in Figure 8. The base of
seamount PJ-F is ⬃10 km in diameter, and four calderas and
craters modified its summit. The oldest caldera (C1) is surrounded by the remnants of a shield (S1) that rises to 1627 m.
The caldera has since been filled with lavas that overflowed it,
leaving only pieces of the rim; the southern rim is completely
buried. A second shield formed (S2), located almost due south
of the first. The rim of this shield includes highs of 1653 and
1615 m depth. A trapdoor caldera (C2), whose floor slopes
down toward the southeast, destroyed most of this second
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
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Figure 7. Illuminated bathymetric map of coalesced volcanoes PJ-D (northwest half) and PJ-E (southeast
half). Explanation is as in Figure 4.
shield. A circular caldera (C3) is nested inside the eastern end
of the trapdoor caldera, and a crater (C4) is nested within the
circular caldera. The volcano’s southwestern flank has three
embayments, probably formed by significant landslides, ranging from 100 to 600 m wide. The base of seamount PJ-G is ⬃9
km in diameter. A shield grew toward the west edge of the
summit platform (S1) but was followed by at least six collapse
events that modified the summit and southeast flank. Several
of the younger calderas (C4 –C6) crosscut the flanks of the
volcano. These six calderas are offset so that the younger ones
are toward the southeast and south with the exception that the
second crater (C2) is nested inside the first caldera (C1). Since
C2 is located within C1 but is unmodified by any of the other
calderas, it could have formed at any time after C1. A 1-kmdiameter cone is constructed in the southern part of the fourth
caldera, and several additional cones are located on the southeast flank of the volcano. The south-southwest flank may have
been modified by a broad landslide that created a large scallopshaped embayment in the summit platform.
The base of volcano PJ-H is ⬃7.5 km in diameter, and the
flat top is ⬃3.5 km across. The sole caldera is centrally located.
The eastern half has been faulted downward several hundred
meters. Southeast of PJ-H, the seafloor is downdropped ⬃75 m
by a ridge-parallel graben.
PJ-I is an elevated region of hummocky volcanic terrain that
shows up as an acoustically bright region in GLORIA images
[Masson et al., 1989] and is interpreted as seamount-type volcanics surrounded by acoustically darker, sediment-covered
abyssal hills. PJ-I was incompletely imaged, so Table 2 lists
parameters for only a part of the feature. Two other patches of
rough, acoustically bright seafloor 10 km south and 17 km
south-southeast of the chain were also imaged by the GLORIA
surveys and are probably similar features.
3.3.
Taney Seamounts
The Taney Seamounts are located on the Pacific Plate west
of San Francisco. It is unknown whether a symmetrical chain of
volcanoes occurred on the subducted Farallon Plate. GLORIA
imagery [EEZ-Scan 84 Scientific Staff, 1986, 1988] shows that
the chain does not extend beyond the region that we have
imaged, that several of the seamounts have summit calderas,
and that several additional small seamounts occur in the same
region. These small seamounts may be aligned in two discontinuous chains that are roughly parallel to the Taney chain.
Dredges on the northwestern volcano in the chain have recovered EMORB and mildly alkalic pillow basalts [Davis et al.,
1998].
The Taney Seamounts consist of five aligned volcanoes (Figure 9). The volcanoes have an average volume of 86 ⫾ 64 km3
and a range of 11–187 km3 and include both the three largest
and the smallest volcanoes surveyed (Table 2). EEZ-Scan 1984
Scientific Staff [1988] incorrectly inferred that the Taney Sea-
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CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
Figure 8. Illuminated bathymetric map of coalesced volcanoes PJ-F (northwest half) and PJ-G (southeast
half). Explanation is as in Figure 4.
mounts were young volcanoes based on their morphology;
however, an Ar-Ar age of ⬃26 Ma for volcano T-A (G.B.
Dalrymple, as cited by Davis et al. [1998]) demonstrates that
their age is close to that of the underlying seafloor. Uncertainties in both the Ar-Ar age and the geomagnetic timescale
during this time period (D. Wilson, personal communication,
1999) indicate the differences in age between the seamounts
and the underlying crust are poorly constrained. The summits
and flanks of the volcanoes have high backscatter in our new
Simrad EM300 data and in GLORIA data [EEZ-Scan 84 Scientific Staff, 1986, 1988]. While the 6.5-kHz GLORIA can
image volcanic basement through up to 20 m of pelagic sediment cover [Mitchell, 1993], a 30-kHz sonar like the EM300
cannot penetrate through more than a meter of pelagic sediment [e.g., Barone and Ryan, 1990]. Thus our EM300 data
indicate that little sediment has accumulated since the volcanoes formed.
Volcano T-A is shown in Figure 10. It is by far the largest
volcano in any of the three chains, with a calculated volume of
187 km3. The volcano has been so severely modified by three
successive caldera collapses that most of the summit platform
has been destroyed. The volcano is ⬃15 km in diameter at the
base. We did not image either the southwestern or northeastern flanks, but they can be identified on the GLORIA image
[Masson et al., 1988]. The first caldera (C1) is the most northwesterly. Its flat floor is tipped down to the south and the rim
is taller to the southwest than to the north. The second (C2) is
nested inside the first, and its floor has been modified by
eruptions that formed numerous small cones. This caldera may
have truncated the southeastern edge of the first caldera, or it
may have been completely nested within it. The reason for this
uncertainty is that the third caldera (C3), an even larger
caldera, has truncated the southeastern portions of the first
two calderas. The flat floor of caldera C3 is roughly 200 m
below the floor of the caldera C2. The southeastern portion of
the floor of the last caldera has also been modified by eruptions
that formed many 0.5–1.0 km diameter cones and several
roughly 1-km-diameter, low, flat-topped shields. A large region
of such cones and shields extends southeasterly between seamounts T-A and T-B. The large embayment north of the
southeastern caldera may be caused by a landslide or by faulting along a ridge-parallel fault.
Volcano T-B is shown in Figure 11. It is the most unusual
volcano imaged in this study owing to its large caldera. The
volcano is oval, with the long-axis parallel to the chain. The
base is about 12 by 10 km, and the summit is about 6.5 by 4.5
km. The entire summit platform is a large caldera (C2) that has
a second, either incipient or mostly buried oval caldera (C3)
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
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Figure 9. Illuminated bathymetry of the Taney Seamounts. The volcanoes are labeled from northwest (T-A)
to southeast (T-E). The grid cell size is 50 m, and the contour interval is 250 m. Inset boxes show the areas
depicted in Figures 10 and 11. The lines of roughness on the seafloor adjacent to the chain and parallel to it
are an artifact caused by poor data in the outer beams of the EM300 system. Such poor data quality results
from surveying near the depth limit of the EM300 system.
nested in its southeastern end. The nested caldera (C3) has a
cone constructed near its center. The rim of the large caldera
varies in height up to ⬃150 m tall. The east-southeastern rim
has been overtopped by lava flows erupted within the caldera
that flowed from the floor down the flank and then ponded at
the base of the slope to form a lava delta. A small step on the
southern flank of the volcano, seen as an illuminated band
about halfway up the flank, may be a somma, the rim of an
even earlier, and larger, caldera (C1) that was almost entirely
filled by a subsequent shield.
Volcano T-C has a circular base ⬃12 km across and a flat
summit platform ⬃8 km in diameter. The summit has numerous small, low lava cones. There are three calderas, with the
first centered in the summit platform, the second nested within
the southeastern end of the first, and the third truncating the
southeastern flank of the volcano. These calderas are oval with
their long dimension parallel to the chain. The third caldera
downdropped the southeastern rim of the summit by ⬃575 m.
A cone, 0.5 km across, occupies the northeastern part of
caldera 3 and another 1-km-diameter shield is located at the
base of the slope ⬃3 km south of the center of caldera 3. The
eastern rim of the summit platform is scalloped, apparently by
several landslides.
Volcano T-D has a circular base ⬃8 km across and a flat top
⬃5.5 km in diameter. A pit crater (Table 3, crater 5) in the
northwestern part of the flat summit is ⬍100 m across and ⬃60
m deep. This is the only collapse feature in the Taney Seamounts that is ⬍1.5 km in diameter. A cratered cone, roughly
0.5 km across and breached to the north, occupies the lower
west flank of the volcano. Several other, much smaller, cones
occur on the flat summit platform. The first of four calderas to
form is offset from the center of the summit toward the northeast. It is now seen mainly as a semicircular low ridge, all that
remains after infilling by younger lava flows. The second
caldera is located near the center of the flat-topped summit
and truncates the western half of the first caldera. The first two
calderas are in turn truncated by the third, located to the
southeast. The fourth caldera truncates the southeast side of
the third one and cuts through the flank of the volcano. The
eastern edge of the fourth has been buried beneath lavas
erupted from T-E.
Volcano T-E is a broad low dome centered ⬃6 km east of
T-D. It is ⬃6 –7 km in diameter and has no other distinguishing
features. It is unique in being located off the main trend of the
chain. Its northern margin was not imaged.
3.4.
Comparisons Among the Three Chains
The morphology of the volcanoes in the three chains are
broadly similar in that all consist mainly of more-or-less circular, steep-sided volcanoes with flat or slightly domed tops that
are modified by collapsed craters and calderas. There are,
however, important differences between the chains.
For example, craters ⬍1 km in diameter are common on the
President Jackson Seamounts where they comprise 12 of 29
collapses, but there is only one of that size on the Taney
Seamounts and none occur on the Vance Seamounts. The
calderas on the President Jackson Seamounts, except those on
PJ-G, were either shallow or have been largely filled in by
Figure 10. Illuminated bathymetry of volcano T-A. Explanation is as in Figure 4.
Figure 11. Illuminated bathymetry of volcano T-B. Explanation is as in Figure 4.
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
subsequent lava flows, whereas on both the Taney and Vance
Seamount many calderas have flat, lava-covered floors, but
they did not fill to overflowing.
Summit shields occur on several of the Vance Seamounts
(V-D and V-E) and on most of the President Jackson Seamounts (PJ-A through PJ-F), but a remnant of only one occurs
in the Taney Seamounts (on T-A). Late cones are common
near the older, northwestern ends of each chain where they
occur on both the summits and flanks of the larger volcanoes
(T-A, V-A, V-C, and all the President Jackson Seamounts).
Regions of disorganized, noncentralized volcanism occur at
the young end of both the Vance and President Jackson chains,
as well as within the Vance Seamounts (V-B) but are absent
from the Taney chain. These differences mainly reflect differences in the magma supply rate during the growth of the
volcanoes, as discussed below.
4.
Discussion
4.1. Volcanic Propagation Rates and Orientation
of Near-Ridge Seamount Chains
The orientations of near-ridge seamount chains are customarily described either as parallel to the spreading direction of
the ridge axis or parallel to absolute plate motion, i.e., fixed in
the hotspot reference frame [e.g., Davis and Karsten, 1986;
Batiza et al., 1990; Stoddard, 1989]. In cases where discrepancies between these directions and the trends of seamount
chains are recognized, the chains have been labeled “oblique”
[e.g., Vogt, 1981; Allan et al., 1987; Macdonald et al., 1992].
Thus the chains are still characterized with reference to absolute and relative plate motion. Stoddard [1989] proposed that
the President Jackson Seamounts formed as a hotspot trace on
the Pacific Plate, with the northwestern seamount formed
about 1 Ma and the southeastern seamount formed, or forming, at present. As we discuss below, the President Jackson
Seamounts are not aligned parallel to Pacific Plate absolute
motion vectors, nor are the lavas recovered from PJ-H (Davis
and Clague, submitted manuscript, 2000) young enough, based
on their observed alteration, to support Stoddard’s model.
Schouten et al. [1987] have suggested, as an alternative, that
the migration of volcanic centers along fast and slow spreading
ridges is related to motion over melting anomalies upwelling in
the shallow subaxial asthenosphere (SAA). This suggestion is
primarily based on a correlation between the migration rate of
the volcanic center along axis and the along-axis component of
absolute migration of the ridge. Their model assumes that
mantle upwells vertically with respect to the spreading center,
so that there is no component of motion parallel to the spreading direction. A melting anomaly embedded in the upwelling
mantle would have only along-axis motion relative to the ridge
axis. At the slow spreading Mid-Atlantic Ridge, Schouten et al.
[1987] attributed migration of groups of volcanic segments
bounded by nontransform offsets to this phenomenon. At the
fast spreading northern East Pacific Rise they attributed chains
of near-ridge seamounts at 10⬚N and 5.5⬚N to the same sort of
melting anomaly.
At 5⬚–10⬚N latitude on the Pacific-Cocos plate boundary, the
region to which Schouten et al. [1987] applied their model, the
predicted orientation of seamount chains relative to the SAA
is unfortunately similar to the absolute plate motion direction.
The region around the Pacific-Rivera plate boundary provides
a better test of the model. Lonsdale [1991] applied this model
to 11 near-ridge seamount chains near the East Pacific Rise on
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both the Pacific and Rivera Plates, between 20.5⬚N and 23⬚N.
The predicted orientations are distinctive and the superiority
of the Schouten et al. model is clear. Reynolds and Clague
(submitted manuscript, 2000) have evaluated the Schouten et
al. model for the three chains studied here and find the orientations of the chains to be consistent with Pacific-SAA vectors
and inconsistent with absolute plate motion vectors. The Pacific-SAA motion vectors are shown in Figure 12, listed in
Table 4, and used in the subsequent discussion.
Although it is generally thought that chains of near-ridge
seamounts form in sequence with the young end near the ridge
[e.g., Barr, 1974; Barone and Ryan, 1990; Lonsdale, 1991; Hammond, 1997], there is only scant evidence for a volcano-tovolcano sequence in our surveys. The best evidence is that the
shield on volcano V-E partially filled the calderas on the more
northwesterly volcano V-D (Figure 4) and several flank
calderas appear to be filled by flows erupted from the volcano
immediately to their southeast. However, there is overwhelming structural evidence that within individual volcanoes, the
northwestern calderas formed prior to those to the southeast.
This pattern supports sequential formation of the chain as a
whole from northwest to southeast. We use this concept to
derive propagation rates of volcanism along the seamount
chains.
The relative motion rates for the Pacific-SAA vectors at the
locations of the three chains are the same as volcanic propagation rates along the chains, just as the rate of absolute plate
motion is the rate of volcanic propagation over a hotspot. The
Pacific-SAA vector tracks motion over an anomaly embedded
in the mantle upwelling around the mid-ocean ridge. The volcanic propagation rates can be used to estimate formation time
for each chain by simply multiplying the propagation rate by
the length of the chain. The time to form the chains varies from
0.9 to 1.9 Ma (Table 4), with the Taney chain forming the most
quickly and the President Jackson chain forming over the longest time period. The time period listed for the Vance chain is
regarded as a minimum because the chain is still active and
may not have reached its full length, and its old end may extend
beyond our map. Likewise, the total volume and time to form
the chain can be used to calculate the average eruption rate
during the formation of each chain. The average eruption rates
vary from 0.14 – 0.20 km3/kyr for the Vance and President
Jackson chains, respectively, to 0.48 km3/kyr for the Taney
chain (Table 4). The more rapid the rate of spreading at the
nearby ridge, the shorter the time to construct the chain and
the greater the eruption rate.
The near-ridge chains in the northeast Pacific have similar
short lengths (⬃60 km long) and seem to invariably occur
off-axis, in contrast to hotspot-related seamount chains which
can extend onto the ridge axis [e.g., Karsten and Delaney, 1989;
Macdonald et al., 1992].
At the intermediate spreading rate of the Vance chain the
initial volcano formed on ocean crust ⬃0.9 Myr old, or at a
distance of ⬃26 km off axis. At the fast spreading East Pacific
Rise the crust under the first of the Lamont Seamounts was
only ⬃0.55 Myr; however, its distance from the ridge axis was
52 km, using the age estimates of Barone and Ryan [1990] for
successive volcanoes in the Lamont chain. The first seamount
therefore formed twice as far from the ridge axis at twice the
spreading rate on the fast spreading East Pacific Rise compared to the intermediate spreading Juan de Fuca Ridge.
Other near-ridge seamount chains have not been adequately
dated for this purpose. Additional data will be necessary to
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CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
Figure 12. Vector diagrams (Reynolds and Clague (submitted manuscript, 2000) after Schouten et al. [1987])
for each chain showing rates and directions of relative plate motions and absolute plate rates and directions
for relevant poles of rotation (NUVEL-1 from Gripp and Gordon [1990] and Duncan and Clague [1985]). The
vector diagrams show the predicted versus the actual orientations of the three chains and demonstrate that the
chains align parallel to subaxial asthenospheric flow (SAA) rather than either relative or absolute plate motion
directions.
determine whether the initial appearance of a near-ridge seamount chain is dominated by the age of the lithosphere above
the SAA anomaly or by the distance from the ridge axis. The
limited data available suggest that the distance from the ridge
is important, perhaps as a consequence of the geometry of
mantle flow and melt focusing in the upwelling asthenosphere.
For each of the seamount chains discussed here, the geometry of absolute and relative plate motion predicts that the
ridge axis will eventually override the SAA anomaly producing
Table 4. Derived Parameters for Three Near-Ridge
Seamount Chains
Seamount Chain
Vance
President
Jackson
Taney
Azimuth of chain (measured)
Azimuth of Pacific-SAA
Rate of Pacific-SAA, cm yr⫺1
Time to form chain, Myr
Average eruption rate, km3/kyr
324–331⬚
324⬚
3.42
1.67
0.14
318–320⬚
319⬚
3.32
1.93
0.20
294–298⬚
302⬚
5.88
0.90
0.48
Azimuth of chain (measured) is from Table 1. Pacific subaxial asthenosphere (SAA) vector is from Reynolds and Clague (submitted
manuscript, 2000). See text for details about calculation of duration of
volcanism and average eruption rate.
the chain. At some relatively small distance from the ridge,
presumably ⬍19 km (the distance between the Juan de Fuca
Ridge axis and the youngest volcano in the Vance chain), the
magma generated by the melting anomaly can be entrained
into the subaxial magma chamber beneath the ridge and distributed along axis. We speculate that the region of disorganized volcanism and small cones east of V-F and V-G in the
Vance Seamount chain (Figure 5) may result from the seamount magmas being focused toward the ridge axis. This region does not lie along the trend of the seamount chain but
instead forms a spreading-parallel zone between the young end
of the seamount chain and the Juan de Fuca Ridge. The region
of disorganized volcanism at the young end of the President
Jackson Seamount chain, PJ-I (Figure 6), is likewise on a
spreading-parallel trend from PJ-H toward the Gorda Ridge.
4.2.
Ridge-Parallel Faults
Along the Gorda Ridge, on the Pacific Plate, our transit line
between the ridge axis and the President Jackson Seamounts
imaged 20 faults with an average spacing of 1.4 km (range
0.6 – 6 km). These faults have average vertical offsets of 47 m
(range 3–157 m) and an average azimuth of 202⬚ (range 193⬚–
209⬚). To the northwest of the President Jackson Seamounts,
another transit line imaged 43 faults with an average spacing of
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
1.0 km (range 0.15– 6.6 km) having an average vertical offset of
31 m (range 4 –169 m). The average azimuth of these faults is
205⬚ (range 191–226⬚). GLORIA imagery shows that largethrow faults, spaced more than 6 km apart, also occur both
north and south of, and presumably beneath, the President
Jackson chain [Masson et al., 1988]. Ridge-parallel faults along
the Juan de Fuca Ridge south of the Vance Seamounts imaged
with the Simrad EM300 sonar have a spacing of roughly 1 km.
The ridge-parallel faults adjacent to the President Jackson
Seamounts and south of the Vance Seamounts are clearly close
enough together to provide the pathways for magma rising to
supply the volcanoes. The fault spacing is similar in scale to the
distance between offset calderas on individual volcanoes.
Ridge-parallel faults are also visible on the tops of several of
the Vance (V-G, V-H, V-C) and President Jackson (PJ-H)
Seamounts (e.g., Figure 5). We interpret their presence to
indicate that the volcanoes formed close to the spreading ridge
axis where horst and graben structures were still forming. The
volcanoes probably cannot form too far from the ridge axis
because the ocean lithosphere is too thick and abyssal hill
faulting is no longer creating pathways along which the magmas can penetrate.
Several of the calderas, such as C1 in Figure 5, are bounded
by ridge-parallel, northeast-southwest, oriented faults. In addition, the volcanoes probably form centered on ridge-parallel
sea floor faults, and the distance that calderas are offset on a
single volcano may be directly related to the spacing of such
faults. This is because the progression of calderas represents
migration of shallow magma reservoirs, and the locations of
these reservoirs may be controlled by underlying abyssal hill
faults. However, the volcanoes cover most of the faults close to
the chains and make this difficult to demonstrate. Surveys
beyond the flanks of the volcanoes will be required to test this
idea. The volcanic chains may form only as far from the ridge
as active faulting is taking place. Such active faulting could
reopen pathways for magma ascent into the crust and into the
seamounts.
4.3.
Flat Summits
The flat tops of most of the volcanoes are comparable to
those observed on similar seamounts along the East Pacific
Rise. Several previous workers [Batiza and Vanko, 1983; Fornari et al., 1988] have argued that the flat tops are formed by
eruptions from circumferential feeders (cone sheets) in a manner similar to that observed in the Galapagos Islands [Simkin,
1982]. We, however, see no evidence for such structures in our
high-resolution bathymetry. On the contrary, several of the
caldera collapse structures clearly cut large shield volcanoes
erupted from central vents near the centers of the summit
plateaus (Figures 4 and 8), suggesting that central volcanism
characterizes these volcanoes. Batiza and Vanko [1983] show
similar structures on near-ridge seamounts near the East Pacific Rise but describe them as “flattened crescent-shaped
ridges” that they attribute to eruption from circumferential
vents. Our new high-resolution bathymetry shows that these
crescent-shaped structures are the remnants of centrally located shield volcanoes largely destroyed during caldera formation. We propose that the flat tops formed by filling and overflow of early, large calderas that defined the shape of the
summit and the depth of the slope break. Clearly, flat summit
regions that have subdued, mainly filled, calderas (Figure 2,
volcano V-A; Figure 7, volcano PJ-D) have had significant
postcaldera volcanism. Volcano T-B, whose entire summit is a
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large caldera, demonstrates the idea that the flat tops may be
caused by infilling of early large calderas. Eruption of only a
few additional cubic kilometers of lava within this caldera
would have erased all evidence that it existed. This may be the
dominant mechanism to form the ubiquitous flat tops on nearridge seamounts.
4.4.
Tipping of Flat Summits and Caldera Floors
The flat or nearly flat tops of several of the volcanoes are
strongly tipped down towards the spreading axis (V-F, V-E,
PJ-H, PJ-B). Others appear to be tipped in other orientations,
such as PJ-A, which tips slightly down to the north; or V-A,
which tips down to the northeast; or T-D, which is tipped
slightly down toward the south. In each case, on the volcanoes
where ridge-parallel faults can be seen, the summit plateaus
are smooth and without volcanic cones. These volcanoes also
have flat summit plateaus tipped down toward the ridge axis.
Other volcanoes where such faults are not observed have more
evidence for late volcanism (cones) on their summits and
flanks. The volcanoes whose flat tops tip down toward the ridge
axis were probably entirely constructed close to the axis. As the
crust underlying these volcanoes aged and moved away from
the axis, the side of the volcano closest to the axis has subsided
more than the side away from the axis, producing a tilt to the
originally horizontal surfaces. With continued volcanic activity
away from the ridge axis, the flat surface would be rebuilt as a
horizontal surface and no tipping would be evident.
The lava floors inside calderas are also usually flat but are
rarely horizontal. Unlike the flat tops of the volcanoes, they
appear to tip in any direction. In some cases, these tipped flat
surfaces probably result from uneven subsidence of the rigid
caldera floor as magma was withdrawn and the caldera deepened. In other cases, the floor of the calderas have been built
up by later lava flows that formed gently sloping surfaces away
from the vents within the caldera (e.g., Figure 11).
4.5.
Smooth Flanks and Landslides
The smoothness of the outer flanks of the volcanoes suggests
that these slopes are comprised almost entirely of fragmental
materials. These slopes are probably blanketed by pillow breccias. Studies on similar seamounts near the East Pacific Rise
(EPR) have described abundant hyaloclastite on the summit or
in caldera walls but not on the flanks [Batiza et al., 1984, 1989;
Smith and Batiza, 1989]. The recovery of hyaloclastite from the
flanks or summits of each of the President Jackson Seamounts
(Davis and Clague, submitted manuscript, 2000) indicates that
such materials occur on these seamounts as well, but we cannot
tell where the hyaloclastite is located. Landslides have modified these smooth slopes (Figure 8), although they are surprisingly rare considering the steep slopes (up to 30⬚) on most of
these volcanoes.
4.6.
Cones Around and on the Seamounts
Many of the volcanoes have small cones on their lower
slopes (e.g., southwest corner of Figure 4, southeast of V-F on
Figure 5, and on the northeast side of PJ-F and PJ-G in Figure
8), on their flat summits (northwest part of PJ-F in Figure 8) or
within their collapsed calderas (in caldera C3 on Figures 4 and
10). These cones clearly represent some of the most recent
volcanism on these volcanoes. For example, the cones shown in
Figure 10 postdate the youngest of three caldera collapses on
seamount T-A. The presence of such cones indicates that small
volumes of magma continued to be supplied to the volcanoes
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CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
episodically after the main eruptive phase was completed and
calderas and craters had been formed, filled or partially filled,
and formed again. As far as is known, all alkalic samples from
near-ridge seamounts have come from late stage alkalic capping lavas at the summits [Batiza et al., 1989]. On the Taney
Seamounts, where samples from T-A include some alkalic basalts [Davis et al., 1998], these late cone-forming eruptions
probably took place as the volcano drifted away from the ridge.
However, on the President Jackson Seamounts, all samples
recovered are depleted NMORB (Davis and Clague, submitted manuscript, 2000), presumably including the lavas that
comprise the late cones, although we have no way to determine
if such cones were sampled.
Fornari et al. [1984] and Lonsdale [1991] describe small
cones around the base of large seamounts as “satellite vents,”
presumed to be late stage eruptions, but observations from
Alvin dives, camera surveys, and SeaMARC I sonar suggest
that the cones around the Lamont Seamounts [Fornari et al.,
1988; Barone and Ryan, 1990] and around Seamounts 6 and 7
[Batiza et al., 1989] are older than the large seamounts. In the
three chains we surveyed, the cones appear to be mainly late,
rather than early, eruptive constructs. There may well have
been early isolated cones, but these have been largely buried by
subsequent voluminous volcanism that formed the large circular volcanoes.
4.7.
Organized and Disorganized Volcanism
There are several examples of volcanism in near-ridge chains
(V-B, area southeast of V-G, PJ-I) that did not result in large
circular or coalesced volcanoes. These poorly defined volcanic
areas are characterized by numerous small cones and hummocky terrain that can rise 770 m above the surrounding seafloor (Table 2 and Figure 3). The “Cone Cluster” between Red
and Green Seamounts and the EPR at 21⬚N [Lonsdale, 1991]
may be another example. There are several possible origins for
these volcanic regions.
The first possibility is that such volcanism is a precursor to
the more organized volcanism that constructs large circular
volcanoes. In this case, the disorganized volcanism would be
still present only because the growth of these structures was
arrested before the main effusive stage took place. In the case
of the Vance chain the area southeast of V-G may still be
active. In the case of seamount V-B, however, the volume of
erupted lava is similar to that of several of the circular volcanoes. We propose that this region formed during a period with
more frequent, small eruptions instead of the relatively few
voluminous long-lived eruptions that built the large circular
volcanoes. We suspect that such disorganized volcanic activity
is related to increased permeability of the crust that allows
magma to migrate upward more continuously than through
less fractured crust.
4.8.
Calderas and Craters
Nearly all of the volcanoes that compose these three nearridge seamount chains (17 of 21, Table 2), and all of the larger
circular, well-developed ones, have one or more calderas and
craters. Only the four smallest volcanoes (height ⬍ 800 m) lack
calderas or craters. These collapse structures commonly form
nested or offset sequences [Hammond, 1997] whose relative
age can be determined by crosscutting relations. In almost all
cases the youngest calderas are offset toward the ridge axis to
the southeast. They also range in size considerably, with the
largest caldera (on T-D) about 6.5 by 4.5 km across and the
craters as small as 0.5 km across. One even smaller pit crater,
only 0.1 km across, was imaged on T-B. The collapse volumes
are as large as (or larger than) 3 km3 (seamount T-B) and
many have volumes of the order of 1–2 km3. These are minimum volumes because most of these collapses have been partially filled by subsequent lava flows. Most of the calderas form
during single voluminous magma withdrawal events that result
in simple faulted boundaries, but some form during repetitive
withdrawal of smaller magma volumes from the same chamber
that result in nested collapse structures [Marti et al., 1994].
Similar calderas have been described from many other submarine volcanoes [e.g., Hollister et al., 1978; Lonsdale and
Spiess, 1979; Batiza and Vanko, 1983; Batiza et al., 1984, 1989;
Fornari et al., 1984, 1987, 1988; Searle, 1983; Scheirer and Macdonald, 1995] as well as the summits of Hawaiian volcanoes
like Kilauea (Figure 13) and Mauna Loa. Kilauea caldera is
shallower than most of the calderas on the near-ridge volcanoes and successive collapses tend to be nested rather than
offset. Experimental and field studies have established that
calderas form in response to subsurface withdrawal of magma
[Marti et al., 1994; Lipman, 2000; Roche et al., 2000]. There is
no evidence for explosive eruptions that could excavate the
calderas on the near-ridge seamounts, nor any likelihood that
the volume changes could be attributed to degassing of stored
magmas. However, there is also only scant evidence that the
withdrawn magmas erupted on the flanks of the volcanoes or
on the seafloor adjacent to them (see Fornari et al. [1984] for
an example). Batiza et al. [1984] noted this lack of eruptions on
the flanks or adjacent to some of the near-ridge seamounts
along the East Pacific Rise and proposed that the withdrawn
magma fed summit eruptions. This seems highly unlikely as
summit collapse occurs when magma pressure in the magma
chamber is too low to support the roof, whereas summit eruptions occur when magma pressure is greater than the roof can
contain. We suggest that the withdrawn magma intruded along
ridge-parallel faults, presumably during episodes of ridgerelated extension on those faults. At Kilauea, magma withdrawn from the summit reservoir commonly migrates away
from the summit to supply rift eruptions [e.g., Tilling and
Dvorak, 1993]. However, for ⬃7 years following the 1975 M7.5
earthquake, magma withdrawn from beneath the summit did
not erupt but instead intruded the rift zone and filled fractures
opened during the earthquake and slip of the south flank of the
volcano [Klein et al., 1987]. The highly tectonized axis and
flanks of the ridge crest adjacent to these near-ridge seamounts
would form similar fractures that could accommodate large
volumes of withdrawn magma. A 2 km3 caldera would form
during magma withdrawal into such a dike 2 m wide, 5 km long,
and 200 m tall. Withdrawal events that lead to caldera formation may be directly linked to reactivated movement of the
underlying ridge-parallel faults. Such faults are thought to remain active along middle to fast spreading ridges at least as far
as 30 – 40 km away [Lee and Solomon, 1995; Wilcock et al.,
1992]. Injection of withdrawn magma in these seafloor faults
may explain the absence of large volume flank lava flows
around seamounts with such large calderas.
4.9.
What Underlies the Calderas?
Fornari et al. [1984] and Batiza et al. [1984] recognized the
similarities between the calderas on near-ridge seamounts and
those on subaerial volcanoes in Hawaii, the Galapagos Islands,
or Mount Etna. Rather than embracing the well-established
concept [e.g., Ryan, 1988] of caldera collapse caused by with-
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
16,557
Figure 13. Illuminated topography of the summit calderas on Kilauea Volcano, Hawaii, based on a 30-m
digital elevation model developed by the U.S. Geological Survey. The image was produced to be comparable
to the images in Figures 2–11, with the exception that the illumination is from the northwest instead of from
the east. Contour interval is 50 m. Kilauea caldera is a complex collapse structure with a subdued, older,
broader Powers caldera [Holcomb, 1987] surrounding the modern caldera and the Halemaùmaù Crater is
nested within the modern caldera.
drawal of roof support above relatively shallow magma chambers [Marti et al., 1994, Roche et al., 2000], Fornari et al. [1988,
p. 78] proposed that the withdrawn magma occupied a “welldeveloped plexus of interconnected dikes and magmatic conduits that extend deep into the crust and upper mantle 䡠 䡠 䡠 a
cylindrical network of throughgoing dikes.” Their primary argument against the presence of a crustal magma chamber was
the lack of extensive shallow level fractionation of the seamount lavas. We agree that the seamount lavas are mostly, but
not universally, quite primitive. We also agree that near-ridge
seamount lavas are petrographically simple, suggesting that
mixing between evolved and primitive magmas occurs only
rarely, if at all (Davis and Clague, submitted manuscript,
2000). However, we disagree that these lava characteristics
constitute a priori evidence that crustal magma chambers do
not underlie these calderas.
What is established is that most, but not all, near-ridge
seamount magmas do not reside, cool, and fractionate in such
chambers for extended time periods prior to their eruption,
nor is there extensive mixing between successive batches of
magma. One way to maintain the primitive character of the
seamount lavas, yet have them pass through crustal storage
chambers, is simply to have them pass through such chambers
quickly. This could be accomplished by high magma supply
rates and rapid transit from the mantle, through crustal chambers, and to eruption. We propose that substantial volumes of
magma (probably between 1 and 5 km3) were delivered from
the mantle to the volcano episodically. These pulses of magma
must have large volumes and be of long duration, in order that,
as the magma moves through the volcano to erupt at the
summit, it also creates a subcaldera chamber. The time periods
between magmatic pulses (discussed in section 4.12) must be
long enough that any magma stored in such chambers after the
eruption ceases either solidifies or is withdrawn and intruded
into ridge-parallel fractures (triggering caldera collapse), and
then, following a long period of volcanic quiescence, the process repeats itself. The numerous nested and coalesced
calderas and craters indicate that most of the volcanoes had a
succession of crustal magma storage chambers (as shown in
experiments by Marti et al. [1994]) that stepped toward the
ridge axis as the seamounts grew.
4.10.
Magma Chamber Sizes and Depths
The experimental studies by Marti et al. [1994] and Roche et
al. [2000] demonstrate that calderas are bounded by steeply
dipping reverse or vertical faults that encircle a flat central
depression formed by collapse of a coherent central block. In
their models the caldera diameter is comparable to or smaller
than the diameter of the underlying magma chamber. In addition, the deeper the deflating chamber is located, the smaller
the observed surface effects. For the near-ridge seamounts,
magma chamber sizes, based on the sizes of the overlying
calderas, are of the same order as the shallow crustal magma
chambers at Kilauea and Mauna Loa Volcanoes. The Hawai-
16,558
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
ian magma chambers, overlain by calderas about 2.5 ⫻ 5 km
across (Figure 13), have been modeled using deformation data.
Such models suggest magma chambers about 4 –5 km in diameter whose tops are ⬃3 km below the surface [e.g., Decker,
1987]. Thus the Hawaiian calderas are only slightly smaller in
diameter than the underlying magma chambers. The simple
ring fault boundaries of most of the calderas on the near-ridge
seamounts are most similar to results of experiments done with
low (0.2– 0.5) aspect ratios (thickness/width of roof), suggesting
that their magma chambers may be shallower (perhaps as
shallow as 1 km) than those at Kilauea or Mauna Loa.
Many of the calderas on the near-ridge volcanoes are unusual in that they truncate the flanks of the volcanoes, rather
than being centered beneath the summit platforms. In order to
collapse the seafloor beneath the flank of the volcanoes the
magma reservoirs must be located within the underlying ocean
crust rather than within the volcanic edifice. Likewise, if the
withdrawn magma that leads to caldera formation is intruded
laterally into ridge-parallel fractures in the ocean crust, then
the magma is probably stored within the ocean crust. If it were
stored inside the volcano above the ocean crust, then more
withdrawn magma would likely erupt on the flanks of the
volcanoes. On the basis of the heights of the volcanoes (Table
2) and the conclusion that the tops of the magma reservoirs are
within the ocean crust underlying the volcanoes, we conclude
that the tops of the magma chambers are at least 0.8 km below
the surface, consistent with the results of the experimental
studies outlined above.
4.11.
Volcano Lifespans
Hammond [1997] used the offset calderas on northeast Pacific near-ridge volcanoes to calculate the lifespan of individual
volcanoes by dividing the typical distance between the center
of the volcano and the last formed caldera (he used an average
of 1.5 km and a maximum of 3.4 km) by the rate of seafloor
spreading. As we showed in section 4.1, the seafloor spreading
rate is not the same as the volcanic propagation rate, so Hammond’s estimated volcanic longevity for northeast Pacific seamounts are ⬃20% too low. For the volcanoes imaged in this
study we calculate average lifespans of 83 kyr for the Taney
Seamounts, 95 kyr for the President Jackson Seamounts, and
75 kyr for the Vance Seamounts. These durations are estimated using a maximum caldera offset of 5 km for the Taney
Seamounts (T-C), 3.2 km for the President Jackson Seamounts
(PJ-D), and 3.2 km for the Vance Seamounts (V-C and V-F)
and volcanic propagation rates from Table 4. These estimates
are greater than the 40 –50 kyr lifespans for other near-ridge
seamounts from Barone and Ryan [1990] and Hammond [1997]
and shorter than the 260 kyr lifespan from Alt et al. [1987].
These lifespans are used to calculate average magma supply
rates for the three chains.
4.12.
Magma Supply Rates
A magma supply rate of slightly ⬍0.1 km3 yr⫺1 [Denlinger,
1997] is ample to sustain the subcaldera magma chamber at
Kilauea. If a 0.1 km3 yr⫺1 eruption rate were sustained during
the growth of these volcanoes, then each would form in only
110 to 1870 years, depending on the volume of the seamount.
It seems highly unlikely that the volcano could migrate off the
conduit system, resulting in the offset calderas observed, in
such short times. The long-term magma supply rate must be
much less than that of Kilauea, and we suggest that it is insufficient to sustain a long-lived subcaldera magma chamber.
We have already estimated that individual magma batches
are probably large, varying between perhaps 1 and 5 km3,
based on the size of the calderas. Each seamount would therefore consist of the accumulated eruptive and intrusive rocks
from perhaps 10 to 100 such events. Considering the long time
periods over which the individual volcanoes apparently form
(75–95 kyr), such intrusive/eruptive events probably occur on
average only once every 1000 to 10,000 years. Considering this
eruption frequency, it is hardly surprising that any stored magmas either solidify or are drained to form calderas prior to the
next intrusive event. The episodicity of magma intrusion into
the volcanoes may be controlled by the episodic nature of
abyssal hill fault movement in the underlying ocean crust.
The differences between volcanoes of the three chains provide information on the variation of eruption rates as a function of time as the volcanoes grew. In Figure 14 we have
plotted hypothetical eruption rate versus time curves for average volcanoes from the three chains. The curves cover the
period of time we have estimated it took to form the average
volcano in each chain, and the area under each curve is scaled
to the average volume of the volcanoes in the chain. The
different shapes of the curves reflect the different characteristics of the volcanoes in each chain. The President Jackson
Seamounts are depicted as having slowly declining activity following a period of peak productivity. This slow decline is seen
in the declining volumes of individual magma batches recorded
in the sequence of collapsed craters and that the larger
calderas are largely filled by subsequent flows. The Taney Seamounts have a much higher peak productivity than either the
Vance or President Jackson Seamounts, reflecting the large
volume of lava delivered (to form the large calderas) in a
relatively short time period. In addition, we have shown a
low-volume tail at the end of activity. It is during this period of
low eruption rate that the scattered cones formed on several of
the Taney volcanoes. The Vance Seamounts are depicted as
having rapidly, almost abruptly, declining activity soon after
peak productivity. The large, unfilled calderas and general
scarcity of late cones support this interpretation.
5.
Conclusions
The volcanoes that comprise near-ridge seamount chains
typically have the form of truncated cones with steep sides and
nearly flat tops. Their summits have almost universally been
modified by repeated collapses to form craters and calderas,
which commonly step toward the ridge axis. The structures
observed on the seamounts best fit models in which the seamounts form from a sequence of intrusive/eruptive events that
are infrequent and of large volume. The magma supply events
may be linked to episodes of movement on abyssal hill faults in
the underlying crust. Between intrusive/eruptive events, any
magma stored from the prior event either solidifies or is withdrawn, triggering caldera and crater collapses.
The calderas collapse during magma withdrawal from
crustal magma chambers. These magma chambers have volumes as large as 3 km3 are probably 3–5 km in diameter, and
their tops are between 0.8 km and, at the deepest, 3 km below
the surface, within the uppermost ocean crust. The withdrawn
magma probably intrudes into active ridge-parallel faults in the
ocean crust beneath the volcanoes. The flat tops of the volcanoes form by infilling and overflow of early, large calderas.
The volcanoes within the three chains have average lifespans
between about 75 and 95 kyr and eruptive frequencies of the
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
16,559
Figure 14. Plot showing eruption rate versus time for average volcanoes in each chain. The curves have been
constructed to have the duration of activity calculated in the text and have been scaled so the area under the
curve is the average volume of volcanoes in each chain. The differences in the curves reflect the much larger
volume of the Taney Seamounts and the different eruption histories recorded by empty versus filled calderas,
presence or absence of late cones, and caldera sizes as discussed in the text.
order of one every 1000 –10,000 years. The volcanoes range in
volume from 11 to 187 km3. Magma supply rate to volcanoes
differs from chain to chain, as expressed by the size of calderas,
infilling of calderas, and formation of late cones on the volcano
summit and flanks. The paradox that near-ridge seamount
lavas are usually primitive, indicating lack of crustal storage,
yet the seamounts have abundant calderas and craters, indicating shallow magma chambers, is resolved if the melts simply
form and then pass through these magma chambers relatively
rapidly, probably in months to years.
Occasionally, volcanic activity is much less organized, resulting in formation of a broad volcanic mound consisting of the
eruptive products of many small eruptions. These regions of
disorganized volcanic activity may begin the cycle that forms
the central volcanoes that generally characterize the near-ridge
chains but may also form in lieu of a central volcano.
The orientations of the near-ridge chains are not parallel to
either spreading directions or absolute plate motion vectors
but instead map subaxial asthenospheric flow, as proposed by
Schouten et al. [1987] and previously demonstrated by Lonsdale
[1991].
Acknowledgments. We thank Gerry Hatcher and Norm Maher for
their assistance at sea collecting and processing the Simrad EM300
data. Doug Wilson generously provided digital maps of the magnetic
anomalies beneath the three chains and assistance with some of the
plate rotation calculations. Kensaku Tamaki has made a user-friendly
plate motion calculator available on the Web (http://www.manbow.
ori.u-tokyo.ac.jp/tamaki-html/plate_motion.html), which was also
helpful. Mike Matthews assisted with the scaling of the eruption rate
diagrams so that the area under each curve represented the average
volume of volcanoes in the chain. We thank Tracy Gregg and especially Yves Lagabrielle for their careful and helpful reviews of the
manuscript. Funding from the David and Lucile Packard Foundation
supported the collection and interpretation of the Simrad EM300 data.
References
Allan, J. F., R. Batiza, and P. Lonsdale, Petrology of lavas from seamounts flanking the East Pacific Rise, 21⬚N: Implications concerning
the mantle source composition for both seamount and adjacent EPR
lavas, in Seamounts, Islands and Atolls, Geophys. Monogr. Ser., vol.
43, edited by B. Keating et al., pp. 255–282, AGU, Washington,
D. C., 1987.
Allan, J. F., R. Batiza, M. R. Perfit, D. J. Fornari, and R. O. Sack,
Petrology of lava from the Lamont Seamount chain and adjacent
East Pacific Rise, 10⬚N, J. Petrol., 30, 1245–1298, 1989.
Alt, J. C., P. Lonsdale, R. Hayman, and K. Muhlenbachs, Hydrothermal sulfide and oxide deposits on seamounts near 21⬚N, East Pacific
Rise, Geol. Soc. Am. Bull., 98, 157–168, 1987.
Barone, A. M., and W. B. F. Ryan, Single plume model for asynchronous formation of the Lamont Seamounts and adjacent East Pacific
Rise terrains, J. Geophys. Res., 95, 10,801–10,827, 1990.
Barr, S. M., Seamount chains formed near the crest of the Juan de
Fuca Ridge, northeast Pacific Ocean, Mar. Geol., 17, 1–19, 1974.
Batiza, R., Abundances, distribution and sizes of volcanoes in the
Pacific Ocean and implications for the origin of non-hotspot volcanoes, Earth Planet. Sci. Lett., 60, 195–206, 1982.
Batiza, R., Seamounts and seamount chains of the eastern Pacific, in
The Geology of North America, vol. N, The Eastern Pacific Ocean and
Hawaii, edited by E. L. Winterer, D. M. Hussong, and R. W. Decker,
pp. 289 –306, Geol. Soc. of Am., Boulder, Colo, 1989.
Batiza, R., and D. Vanko, Volcanic development of small oceanic
central volcanoes on the flanks of the East Pacific Rise inferred from
narrow beam echo-sounder surveys, Mar. Geol., 54, 53–90, 1984.
Batiza, R., D. J. Fornari, D. A. Vanko, and P. Lonsdale, Craters,
calderas, and hyaloclastites on young Pacific seamounts, J. Geophys.
Res., 89, 8371– 8390, 1984.
Batiza, R., T. L. Smith, and Y. Niu, Geological and petrologic evolution of seamounts near the EPR based on submersible and camera
study, Mar. Geophys. Res., 11, 169 –236, 1989.
16,560
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
Batiza, R., Y. Niu, and W. C. Zayac, Chemistry of seamounts near the
East Pacific Rise: Implications for the geometry of the subaxial
mantle flow, Geology, 18, 1122–1125, 1990.
Bridges, N. T., Submarine analogs to Venusian pancake domes, Geophys. Res. Lett., 22, 2781–2784, 1995.
Bridges, N. T., Characteristics of seamounts near Hawaii as viewed by
GLORIA, Mar. Geol., 138, 273–301, 1997.
Cande, S. C., and D. V. Kent, Revised calibration of the geomagnetic
polarity timescale for the Late Cretaceous and Cenozoic, J. Geophys.
Res., 100, 6093– 6095, 1995.
Clague, D. A., and M. L. Holmes, Geology, petrology, and mineral
potential of the Gorda Ridge, in Geology and Resource Potential of
the Continental Margin of Western North America and Adjacent Ocean
Basins, Earth Sci. Ser., vol. 6, edited by D. W. Scholl et al., pp.
563–580, Circum-Pac. Counc. for Energy and Miner. Resour., Houston, Tex., 1989.
Davis, A. S., L. B. Gray, and D. A. Clague, Non-hotspot ocean island
volcanoes: examples from the central and NE Pacific Ocean, in
Evolution of Ocean Island Volcanoes, Penrose Conference Series,
Galapagos, Ecuador, p. 27, Geol. Soc. of Am., Boulder, Colo., 1998.
Davis, E. E., and J. L. Karsten, On the cause of asymmetric distribution of seamounts on the Juan de Fuca Ridge: Ridge-crest migration
over a heterogeneous asthenosphere, Earth Planet. Sci. Lett., 79,
385–396, 1986.
Decker, R. W., Dynamics of Hawaiian volcanoes: An overview, in
Volcanism in Hawaii, edited by R. W. Decker, T. L. Wright, and
P. H. Stauffer, U. S. Geol. Surv. Prof. Pap., 1350, 997–1018, 1987.
Denlinger, R. L., A dynamic balance between magma supply and
eruption rate at Kilauea Volcano, Hawaii, J. Geophys. Res., 102,
18,091–18,100, 1997.
Duncan, R. A., and D. A. Clague, Pacific Plate motion recorded by
linear volcanic chains, in The Ocean Basins and Margins, vol. 7A,
edited by A. E. M. Nairn, F. G. Stehli, and S. Uyeda, pp. 89 –121,
Plenum, New York, 1985.
EEZ-Scan 84 Scientific Staff, Atlas of the Exclusive Economic Zone,
Western Conterminous United States, U. S. Geol. Surv. Misc. Invest.
Ser., I-1792, 152 pp., 1986.
EEZ-Scan 84 Scientific Staff, Physiography of the western United
States Exclusive Economic Zone, Geology, 16, 131–134, 1988.
Fernandez, L. S., and R. N. Hey, Late Tertiary tectonic evolution of
the seafloor spreading system off the coast of California between the
Mendocino and Murray Fracture Zones, J. Geophys. Res., 96,
17,955–17,979, 1991.
Fornari, D. J., W. B. F. Ryan, and P. J. Fox, The evolution of craters
and calderas on young seamounts: insights from Sea Mark I and Sea
Beam sonar surveys of a small seamount group near the axis of the
East Pacific Rise at ⬃10⬚N, J. Geophys. Res., 89, 11,069 –11,083,
1984.
Fornari, D. J., R. Batiza, and J. F. Allan, Irregularly shaped seamounts
near the East Pacific Rise: implications for seamount origin and
rise-crest processes, in Seamounts, Islands, and Atolls, Geophys.
Monogr. Ser., vol. 43, edited by B. H. Keating et al., pp. 35– 47, AGU,
Washington, D. C., 1987.
Fornari, D. J., M. R. Perfit, J. F. Allan, R. Batiza, R. Haymon,
A. Barone, W. B. F. Ryan, T. Smith, T. Simkin, and M. A. Luckman,
Geochemical and structural studies of the Lamont Seamounts: Seamounts as indicators of mantle processes, Earth Planet. Sci. Lett., 89,
63– 83, 1988.
Gripp, A. E., and R. G. Gordon, Current plate velocities relative to the
hotspots incorporating the NUVEL-1 global plate motion model,
Geophys. Res. Lett., 17, 1109 –1112, 1990.
Hammond, S. R., Offset caldera and crater collapse on Juan de Fuca
ridge-flank volcanoes, Bull. Volcanol., 58, 617– 627, 1997.
Holcomb, R. T., Eruptive history and long-term behavior of Kilauea
Volcano, in Volcanism in Hawaii, edited by R. W. Decker, T. L.
Wright, and P. H. Stauffer, U. S. Geol. Surv. Prof. Pap., 1350, 261–
350, 1987.
Hollister, C. D., M. F. Glenn, and P. F. Lonsdale, Morphology of
seamounts in the western Pacific and Philippine basin from multibeam sonar data, Earth Planet. Sci. Lett., 41, 405– 418, 1978.
Karsten, J. L., and J. R. Delaney, Hot spot-ridge crest convergence in
the northeast Pacific, J. Geophys. Res., 94, 700 –712, 1989.
Klein, F. W., R. Koyanagi, J. S. Nakata, and W. R. Tanigawa, The
seismicity of Kilauea’s magma system, in Volcanism in Hawaii, edited by R. W. Decker, T. L. Wright, and P. H. Stauffer, U.S. Geol.
Surv. Prof. Pap., 1350, 1019 –1185, 1987.
Lee, J. S., and S. R. Hammond, Origin of Heck and Heckle seamount
chains, Juan de Fuca Ridge, Eos Trans. AGU, 66, 403, 1985.
Lee, S.-M., and S. C. Solomon, Constraints from Sea Beam bathymetry
on the development of normal faults on the East Pacific Rise, Geophys. Res. Lett., 22, 3135–3138, 1995.
Leybourne, M. I., and N. A. Van Wagoner, Heck and Heckle seamounts, northeast Pacific Ocean: High extrusion rates of primitive
and highly depleted mid-ocean ridge basalt on off-axis seamounts, J.
Geophys. Res., 96, 16,275–15,293, 1991.
Lipman, P. W., Calderas, in Encyclopedia of Volcanoes, edited by
H. Sigurdsson, pp. 643– 662, Academic, San Diego, Calif., 2000.
Lonsdale, P., Laccoliths[?] and small volcanoes on the flank of the East
Pacific Rise, Geology, 11, 706 –709, 1983.
Lonsdale, P., Non-transform offsets of the Pacific-Cocos plate boundary and their traces on the rise flank, Geol. Soc. Am. Bull., 89,
981–999, 1985.
Lonsdale, P., Structural patterns of the Pacific floor offshore of peninsular California, in The Gulf and Peninsular Province of the Californias, edited by J. P. Dauphin and B. R. T. Simoneit, AAPG Mem.
47, 87–125, 1991.
Lonsdale, P., and F. N. Spiess, A pair of young cratered volcanoes on
the East Pacific Rise, J. Geol., 87, 157–173, 1979.
Macdonald, K. C., et al., The East Pacific Rise and its flanks 8 –18⬚N:
History of segmentation, propagation and spreading direction based
on SeaMARC II and Sea Beam studies, Mar. Geophys. Res., 14,
299 –344, 1992.
Marti, J., G. J. Ablay, T. L. Redshaw, and R. S. J. Sparks, Experimental
studies of collapse calderas, J. Geol. Soc. London, 151, 919 –929,
1994.
Masson, D. G., D. A. Cacchione, and D. E. Drake, Tectonic evolution
of the Gorda Ridge inferred from sidescan sonar images, Mar. Geophys. Res., 10, 191–204, 1988.
Menard, H. W., Growth of drifting volcanoes, J. Geophys. Res., 74,
4827– 4837, 1969.
Mitchell, N. C., A model for attenuation of backscatter due to sediment accumulations and its application to determine sediment thicknesses with GLORIA sidescan sonar, J. Geophys. Res., 98, 22,477–
22,493, 1993.
Mukhodopadhyay, R., and R. Batiza, Basinal seamounts and seamount
chains of the central Indian Ocean: Probable near-axis origin from a
fast-spreading ridge, Mar. Geophys. Res., 16, 303–314, 1994.
Rappaport, Y., D. F. Naar, C. C. Barton, Z. J. Liu, and R. N. Hey,
Morphology and distribution of seamounts surrounding Easter Island, J. Geophys. Res., 102, 24,713–24,728, 1997.
Roche, O., T. H. Druitt, and O. Merle, Experimental study of caldera
formation, J. Geophys. Res., 105, 395– 416, 2000.
Ryan, M. P., The mechanics and three-dimensional internal structure
of active magmatic systems: Kilauea Volcano, Hawaii, J. Geophys.
Res., 93, 4213– 4248, 1988.
Scheirer, D. S., and K. C. Macdonald, Near-axis seamounts on the
flanks of the East Pacific Rise, 8⬚N to 17⬚N, J. Geophys. Res., 100,
2239 –2259, 1995.
Schouten, H., H. J. B. Dick, and K. D. Klitgord, Migration of midocean-ridge volcanic segments, Nature, 326, 835– 839, 1987.
Searle, R. C., Submarine central volcanoes on the Nazca Plate—Highresolution sonar observations, Mar. Geol., 53, 77–102, 1983.
Shen, Y., D. W. Forsyth, D. S. Scheirer, and K. C. Macdonald, Two
forms of volcanism: Implications for mantle flow and off-axis crustal
production on the west flank of the southern East Pacific Rise, J.
Geophys. Res., 98, 17,875–17,889, 1993.
Shen, Y., D. S. Scheirer, D. W. Forsyth, and K. C. Macdonald, Tradeoff in production between adjacent seamount chains near the East
Pacific Rise, Nature, 373, 140 –143, 1995.
Simkin, T., Origin of some flat-topped volcanoes and guyots, Mem.
Geol. Soc. Am., 132, 183–193, 1982.
Smith, D. K., Shape analysis of Pacific seamounts, Earth Planet. Sci.
Lett., 90, 457– 466, 1988.
Smith, D. K., and T. H. Jordan, Seamount statistics in the Pacific
Ocean, J. Geophys. Res., 93, 2899 –2918, 1988.
Smith, M. C., M. R. Perfit, and I. R. Jonasson, Petrology and geochemistry of basalts from the southern Juan de Fuca Ridge: Controls
on the spatial and temporal evolution of mid-ocean ridge basalt, J.
Geophys. Res., 99, 4787– 4812, 1994.
Smith, T. L., and R. Batiza, New field and laboratory evidence for the
origin of hyaloclastite flows on seamount summits, Bull. Volcanol.,
51, 96 –114, 1989.
CLAGUE ET AL.: NEAR-RIDGE SEAMOUNTS
Stoddard, P. R., Kinematic models of the evolution of the Gorda Rise
and President Jackson Seamount chain, in Evolution of Mid-Ocean
Ridges, Geophys. Monogr. Ser., vol. 57, edited by J. M. Sinton, pp.
31–37, AGU, Washington, D. C., 1989.
Tilling, R. I., and J. J. Dvorak, Anatomy of a basaltic volcano, Nature,
363, 125–133, 1993.
Vogt, P., On the applicability of thermal conduction models to midplate volcanism: Comments on a paper by Gass et al., J. Geophys.
Res., 86, 950 –960, 1981.
Wilcock, W. S. D., G. M. Purdy, S. C. Solomon, D. L. DuBois, and
D. R., Toomey, Microearthquakes on and near the East Pacific Rise,
9⬚–10⬚N, Geophys. Res. Lett., 19, 2131–2134, 1992.
Wilson, D. S., Tectonic history of the Juan de Fuca Ridge over the last
40 million years, J. Geophys. Res., 93, 11,863–11,876, 1988.
16,561
Wilson, D. S., Deformation of the so-called Gorda Plate, J. Geophys.
Res., 94, 3065–3075, 1989.
Wilson, D. S., Confidence intervals for motion and deformation of the
Juan de Fuca plate, J. Geophys. Res., 98, 16,053–16,071, 1993.
D. A. Clague, A. S. Davis, and J. R. Reynolds, Monterey Bay
Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing,
CA 95039-0628. ([email protected])
(Received June 11, 1999; revised January 13, 2000;
accepted March 8, 2000.)
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