JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS, VOL. 118, 1–19, doi:10.1002/jgre.20059, 2013
Large shield volcanoes on the Moon
Paul D. Spudis,1 Patrick J. McGovern,1 and Walter S. Kiefer1
Received 29 August 2012; revised 5 December 2012; accepted 4 February 2013.
[1] The volcanic style of the Moon has long been understood to consist almost exclusively
of flood basalts erupted from fissures along with minor pyroclastic activity; large central
vent shield volcanoes that characterize basaltic volcanism on the other terrestrial planets
appeared to be absent. Small (few kilometers diameter) central vent constructs have long
been recognized in the lunar maria and often are found clustered in fields throughout the
lunar maria. New global topographic data from the LOLA and LROC instruments on LRO
reveal that almost all of these volcanic complexes on the Moon occur on large, regional
topographic rises in the lunar maria, tens to hundreds of kilometers in extent and between
several hundred to several thousand meters high. We propose that these topographic swells
are shield volcanoes and are the lunar equivalents of the large basaltic shields found on the
Earth, Venus, and Mars. The newly recognized lunar shields are found peripheral to the
large, deeply flooded impact basins Imbrium and Serenitatis, suggesting a genetic relation
to those features. Loading of the lithosphere by these basalt-filled basins may be
responsible for inducing a combination of flexural and membrane stress, inducing a
pressure distribution on vertically oriented dikes favorable to magma ascent. This condition
would occur in a zone annular to the large circular loads produced by the basins, where the
shield volcanoes occur.
Citation: P. D., Spudis, P. J. McGovern and W. S. Kiefer (2013), Large shield volcanoes on the Moon, J. Geophys. Res.
Planets., 118 doi:10.1002/jgre.20059.
1.
flood lavas, in which large volumes of magma are erupted
rapidly from fissures and spread out as sheets on the surface
[e.g., Head, 1976]. The Moon seems to lack the very large
shield volcanoes [BVSP, 1981; Head and Wilson, 1991] that
typify some of the mountains of Earth, Mars, and Venus [Pike,
1978; BVSP, 1981; Plescia, 2004; Herrick et al., 2005].
Or does it?
[4] Shield volcanoes are positive-relief, central vent
structures that are broader than they are high, so they have
relatively low, average positive slopes [Whitford-Stark,
1975]. The term was first coined to characterize the shape
of certain lava constructs on Earth made up principally
of low viscosity, basaltic lava that builds up a broad,
shield-shaped construct. The bulk of the volcano is made
of lava flows, although pyroclastic activity may occur in
minor amounts, particularly during late stage eruptions.
Many shield volcanoes display a summit crater (caldera)
resulting from collapse of the surface over a drained or
depleted magma chamber, but some shields do not have a
summit crater [Whitford-Stark, 1975; Herrick et al., 2005]
and such is not required for the edifice to be classified as a
shield volcano. Shield volcanoes typically have both radial
and circumferential fissure zones, which serve as pathways
for magma to get to the surface and erupt a continuing
supply of lava. Parasitical cone and dome building often
occurs near the summit and on the flanks of such features
during the latter stages of shield growth [e.g., McDonald
and Abbott, 1970].
[5] Until recently, regional topographic information for
the Moon was sparse and non-contiguous. Nonetheless,
substantial regional slopes were evident in the Clementine
Introduction
[2] Although basaltic volcanism is a common process on
the terrestrial planets, it manifests itself with differing styles
and intensities on different planetary bodies. Volcanism on
the Moon is manifested largely by the presence of extensive
plains of basaltic lava. The dark smooth lunar maria are
composed of basaltic lava flows that were largely emplaced
through fissure-fed, flood-style eruptions. This style of
volcanism is also common on the other terrestrial planets;
both Venus and Mars show vast plains made of basaltic lava
in addition to their massive, central-vent shields. On Earth,
continental flood basalt eruptions are considered the best
analog for mare volcanism, with high-effusion rate flows
of fluid lava creating vast plateaus of basalt [e.g., Swanson
and Wright, 1978].
[3] Central vent, shield-building volcanism is common on
Earth, Venus, Mars, and Io and may also have occurred on
Mercury. Small shield and dome volcanoes have been
observed and mapped on the Moon for many years, but
typically are very small (2–10 km diameter) and occur in
groups or clusters within selected areas of the maria
[McCauley, 1964; Greeley, 1971; Guest, 1971; Whitford-Stark
and Head, 1977]. The vast bulk of lunar volcanic deposits is
1
Lunar and Planetary Institute, Houston, Texas, USA.
Corresponding author: P. D. Spudis, Lunar and Planetary Institute, 3600
Bay Area Blvd., Houston, TX 77058, USA. ([email protected])
©2013. American Geophysical Union. All Rights Reserved.
2169-9097/13/10.1002/jgre.20059
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SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
global altimetry [Zuber et al., 1994] for the relatively “flat”
maria of the Moon. Such slopes tend to conform to the
configuration of the containing impact basins, but it was
noted that some igneous centers in the maria occur on
topographic rises. Specifically, the lunar Marius Hills
complex was found to occur on the summit of a broad,
gentle topographic swell, leading to the supposition that this
complex might be the lunar manifestation of a basaltic shield
volcano, a couple of hundred kilometers across and several
hundred meters high [Spudis, 1996]. The Clementine
topography was of low resolution and could not resolve
features within the lunar maria with high precision.
However, new global data from the LRO laser altimeter
[Smith et al., 2010] give us a high-resolution view of lunar
topography. Moreover, global stereo images from the LRO
camera have been processed into a global topographic map
[Scholten et al., 2012]. Thus, it is an appropriate time to
re-visit the topographic character of volcanic complexes
in the lunar maria and address the question: Do shield
volcanoes exist on the Moon?
sinuous rilles, a common feature of these eruptive centers
and are interpreted as vent systems and their associated lava
channels and tube systems.
[8] We have used the basic classification and mapping of
Guest and Murray [1976], including their distinction
between shields (or “low domes”) with and without summit
pit craters. In the feature maps presented in this paper, we
recognize low domes (shields), with and without summit
pits, cones, collapse pits, sinuous rilles, and chains of cinder
cones (interpreted as fissure vents) [Guest and Murray,
1976]. Additionally, we mapped eruptive vent centers where
recognized (indicated by an irregular crater or landform
associated with dark mantling materials). We have plotted
the locations of the most prominent features in these areas
on a shaded relief base (created from the GLD100
topographic map) [Scholten et al., 2012] for each proposed
shield volcano. Associated topographic profiles of each
shield volcano were extracted from the GLD100 database
using the profiling tool of the Quickmap LROC global
basemap (http://target.lroc.asu.edu/da/qmap.html).
2.
3. Topography of Volcanic Complexes in the
Lunar Maria
Data Sources and Approach
[6] The new global topographic map of the Moon
obtained by the Lunar Reconnaissance Orbiter (LRO) is
the principal source of topographic information used in this
study. The GLD100 global map [Scholten et al., 2012] is a
stereo-model based on LRO Camera Wide Angle stereo
image data. It has a resolution of 100 m/pixel, covers the
Moon between !79" latitude, and has been determined to
have vertical accuracy of about 18 m compared to the LRO
laser altimeter data set. The laser altimeter data [Smith
et al., 2010] complete the global topographic maps for
latitudes greater than 79" [Scholten et al., 2012]. The
features studied in this paper all fall within the boundaries
of the GLD100 map and have dimensions of hundreds of
kilometers and heights greater than 1000 m, much larger
than the scale of this high-resolution topographic data.
[7] Small volcanic features in the maria have been mapped
for many years [e.g., McCauley, 1964; Guest, 1971;
Wilhelms and McCauley, 1971] and we have used this
previous mapping to locate clusters of such small features
in relation to our larger landforms. Specifically, we have
used the landform classification and map of Guest and
Murray [1976] to show the correspondence of small
volcanic features with our larger shields. Guest and Murray
[1976] recognized several distinct landforms, including
rilles, domes, pits, and cones. Low domes (small shields)
are smooth, convex-shaped positive relief landforms with
side slopes of 2–3" and sizes of a few kilometers diameter
[Guest and Murray, 1976]; some of these features have
summit craters while others do not. Steep domes have more
prominent topography and are comparable in size to shields,
but have steeper sides with slopes of 7–20" [Guest and
Murray, 1976]. Cones are small features (2-3 km across),
often occur on top of a broader shield (over 40 of these are
found in the Marius Hills) or aligned along a linear vent
system, and tend to have steep sides (>20" ). Collapse craters
(or pits) are common throughout the maria and many are
found in association with the other landforms; they tend to
be small (a few kilometers across or less) and shallow (tens
to hundreds meters). Many collapse pits are associated with
[9] The new global topographic map of the Moon reveals
many new relationships on the lunar surface. Although these
data validate the conventional wisdom that mare deposits
occupy low-lying areas of the Moon, several broad
topographic highs are found in both the eastern and western
near-side maria (Figure 1). These topographic bulges are
tens to hundreds of kilometers across and from 600 to over
2200 m high. We have identified six major and two minor
topographic swells (Table 1) that occur within the near-side
lunar maria. Interestingly, all of these rises correspond to
high concentrations of small (kilometer-scale) volcanic
features as mapped over the entire near-side by Guest and
Murray [1976], although it appears that the styles of
eruption and nature of the dominant landform varies by
location. The correspondence of volcanic landforms with
topography not only encompasses such long-familiar mare
volcanic “complexes” as Mons Rümker, the Marius Hills,
and the Aristarchus plateau, but also includes some lesser
known eruptive centers, such as Hortensius and Cauchy.
Because our new interpretation of these areas is so radical,
we here describe the geology of each volcanic center and
its regional geological and topographic setting.
3.1. Marius Hills
[10] This complex has long been known as a center of
intense volcanic activity (Figure 2), displaying over 300
small cones and domes and numerous sinuous rilles and
collapse pits [McCauley, 1967, 1968; Greeley, 1971; Guest,
1971; Weitz and Head, 1999; Heather et al. 2003]. The
Marius Hills volcanic complex (Figures 3 and 4) occurs
within Oceanus Procellarum, the most extensive maria on
the Moon and the site of some of the youngest lunar lava
flows [Schultz and Spudis, 1983; Hiesinger et al., 2003].
The cones and domes range in plan from a few kilometers
to almost 20 km across and from 200 to over 600 m in
height. Numerous sinuous rilles are found in the area,
emanating from irregular or elongate source vents [McCauley,
1967, 1968; Greeley, 1971; Guest, 1971]. Pit craters and
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SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 1. Hemisphere view of topographic data from GLD100 [Scholten et al., 2012] for the near-side of
the Moon, centered on 0" , 20" W showing location of proposed large lunar shield volcanoes. Color map
has contour interval of ~250 m. Outlines of shield boundaries are approximate.
Table 1. Large Shield Volcanoes on the Moona
Diameter
(km)
Height
(km)
Average
slope (" )
66
70
1.2
1.6
2.4
2.6
1,400
2,100
3.4–3.6
~3.8
166
240
0.8*
2.0*
0.5
0.9
5,800
30,100
8" N, 38" W
13" N, 29" W
14" N, 52" W
2.1–3.6
3.1–3.5
1.1–3.3
270
300
330
0.6*
1.2
2.2
0.3
0.4
0.8
12,300
28,300
62,700
8" N, 35" E
3.6–3.7
560
1.8
0.1
148,000
Shield
Summit
"
Rümker
Gardner
Prinz
Aristarchus
Kepler
Hortensius
Marius
Hills
Cauchy
"
41 N, 59 W
16.1" N,
34.1" E
26" N, 43" W
25.4" N, 50" W
Age
(Ga)
>3.4
>3.8
Volume
(km3)
Comments
Built on highland block
Similar to Rümker; on northern
flank of Cauchy shield
Built on highland block
Partly developed; built on highland
block
Very low slopes; few volcanic features
Asymmetric; built on Montes Carpatus
Fully developed shield
Largest shield
a
Age estimates taken from the literature (see text). Asterisk indicates that non-shield impact topography was deleted from estimate of edifice height.
Measurements of diameter and height were made on the LROC-LOLA Digital Terrain Model GLD 100 [Scholten et al., 2012]. Volumes are computed using
an approximation of a simple conical segment of radius (1/2D) and height shown.
Figure 2. The Marius Hills shield. At left, topographic image shows abrupt boundary at northern edge of
shield (arrows). This boundary is clearly seen in the Kaguya high-definition television view (right, top and
bottom) of the edge of the Marius Hills shield. Kaguya view is looking south while flying over about
18" N, 52" W. Field of view is about 200 km.
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SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 3. Volcanic features of the Marius Hills. Left, sinuous rilles and vent craters on western half of
shield. Center, collapse pits (caldera?) near summit of Marius Hills shield. Right, Pit craters, sinuous rilles
and low domes near eastern edge of Marius Hills shield.
(Table 1). Images from the orbiting Kaguya HDTV imager
clearly show the shield-like morphology of the structure
(Figure 2) and it is also evident in topographic profiles taken
from the new global DTM (Figure 5). The cones, domes,
and rilles that make up the volcanic complex are all
superposed on the shield in a manner similar to the
numerous late-stage cones and eruptive vents of the Mauna
Kea shield on the island of Hawaii [MacDonald and Abbott,
1970]. On the basis of the broad, low-relief shape of this
topographic bulge seen in Clementine topography, the
Marius Hills were proposed to be the lunar equivalent of a
basaltic shield volcano by Spudis [1996].
[12] The precise age of the Marius Hills construct is
uncertain, but most workers agree that it is relatively young.
It was mapped as Eratosthenian in age by McCauley [1967]
and Wilhelms and McCauley [1971]. Whitford-Stark and
Head [1980] mapped the lava flows of Oceanus Procellarum
collapse features are common, including a recently discovered
skylight within an apparent lava tube [Haruyama et al., 2009].
The cones and domes of the Marius Hills do not appear to be
compositionally distinct from either the surrounding mare
plains or the inter-volcano plains that make up the surface of
the Marius Hills construct [Weitz and Head, 1999; Heather
et al., 2003; Besse et al., 2011]. However, the decimeter-scale
surface texture of the domes indicates that at least some of the
constructs are rougher than the average mare surface, possibly
indicating that clinkery aa lava, pasty eruptive spatter, and/or
interbedded pyroclastics make up at least some of these edifices
[Campbell et al., 2009; Lawrence et al., 2013].
[11] The Marius Hills complex occurs on an elongated,
elliptical topographic rise approximately 330 km in extent.
It is broadly shaped like a shield, with the summit near
14" N, 52" W, about 40 km northwest of the crater Marius,
and it rises about 2.2 km above the surrounding mare plain
Figure 4. Volcanic features map of Marius Hills, simplified from McCauley [1968] and Guest and
Murray [1976]. Symbols for volcanic landforms used here are the same as on all subsequent maps
(landform definition and classification from Guest and Murray [1976]; see text for discussion).
4
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 5. Topographic profiles of the Marius Hills shield; all topographic information for the features
described in this paper is taken from the LROC stereo DTM GLD 100 [Scholten et al., 2012]. The typical
broad shield profile is evident, showing a feature about 330 km across and 2.2 km high.
one 100–140 km in extent, south of the main part of the
Marius Hills shield. The two structures are connected by a
narrower line of dense material. The geophysical evidence
of a large, dense subsurface feature centered under the
topographic bulge is consistent with the Marius Hills being
made up of a single subsurface magmatic system and
supports our interpretation of it as a large basaltic shield
volcano. If the anomaly is caused by dispersed intrusive in
the crustal pore space rather than being concentrated in a
classical magma chamber, this would inhibit the crustal
collapse needed to form a large, significant summit caldera
(although a chain of collapse features (center image,
Figure 3) near the summit is evident). Because impactinduced porosity is probably widespread in the upper
crust across the Moon, this may provide an explanation
for the general absence of calderas on the lunar volcanic
complexes described here.
[14] The predominant volcanic landform of the Marius
Hills is the relatively steep-sided cone (Figure 4) [see also
and found that the surface flows around the Marius Hills
included lavas from the uppermost sequence of flows, the
Sharp and Hermann Formations; more recent studies have
mapped these flows on the surface of the Marius Hills shield
and have estimated ages of 2.5 and 3.3 Ga for the two
principal flow series [Heather and Dunkin, 2002; Heather
et al., 2003]. Most recently, Huang et al. [2011] propose
very young ages for the surface lavas of the Marius Hills,
ranging from 0.8 to 1.1 Ga.
[13] The free-air gravity anomaly at the Marius Hills is
about 200–250 km across [Konopliv et al., 2001] and
requires the presence of a significant volume of dense
subsurface material, which likely takes the form of either a
laccolithic intrusion or the infilling by basalt of the impactinduced pore space in the uppermost crust. Kiefer [2013]
models this gravity feature as being produced by two dense,
subsurface bodies: a northern one 160–180 km across,
corresponding to the Marius Hills bulge (including most of
the domes and cones within its boundary) and a smaller
Figure 6. Rümker, a volcanic shield in northern Oceanus Procellarum. This relatively small feature
consists of a broad shield and several overlapping low shields. WAC view of Rümker structure (left);
low-sun view of the overlap of shields of the Rümker shield (right).
5
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 7. Volcanic features map of Rümker. Low shields, probably of basaltic composition, predominate.
(resulting in the production of abundant spatter, degassing,
and clinkery aa lava flows) [Weitz and Head, 1999; Heather
et al., 2003].
McCauley, 1968; Guest and Murray, 1976], most of which
are found at the summits of broad, low shields. Both
collapse pits and sinuous rilles are also common [e.g.,
Greeley, 1971] as are larger collapse pits near the summit
(Figure 3). Small shield-like volcanoes without capping
cones are much less common here, although they are abundant at some of the other complexes we discuss (see below).
The dominance of certain landforms and the paucity of
others at different complexes probably indicate differing
styles of the predominant eruption and evolutionary paths
of the various complexes [Whitford-Stark and Head,
1977]. The near-exclusive presence of cones at Marius Hills
suggests a protracted volcanic evolution, with the eruption
of many, relatively volatile-rich, partly crystallized magmas
3.2. Rümker
[15] The Rümker complex (Figure 6) in northern Oceanus
Procellarum (~70 km in extent, centered at 40" N, 58" W) was
recognized as a volcanic center early in lunar geological
studies [McCauley, 1968; Guest, 1971; Scott and Eggleton,
1973; Smith, 1973, 1974]. It consists of a broadly elevated
cluster of more than a dozen (up to 30 according to Smith
[1974]) blister-like landforms (Figure 7), built on top of a
kipuka of Imbrium basin ejecta, the Fra Mauro Formation
[Guest, 1971; Scott and Eggleton, 1973]. The thin mare
Figure 8. Topographic profiles of the Rümker shield, showing a construct 66 km across and about
1.2 km high.
6
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 9. Volcanic vents and channels of the Prinz shield (~150 km in extent; 26" N 43" W). Collapse
pits and rilles near rim of crater Prinz (left); caldera-like pit and vent area near middle of structure (right).
1976, 1982]. A large crater at the north end of the complex
may be a collapse pit (Figure 6). There is no evidence for
sinuous rilles or other vent structures, although such features
could be covered by the younger mare basalts of the
surrounding plain.
flows of northern Procellarum lap up and partly cover the
lower portions of the edifice, suggesting that the complex
pre-dates the ~3.4 Ga old surface mare basalts in this region;
very young mare basalts (<1.5 Ga old) lap over the complex
to the northeast [Hiesinger et al., 2003]. In profile, Rümker
displays the bulbous shape of an elongate shield (Figure 8),
with a typical relief of 1000–1200 m above the surrounding
mare surface.
[16] In some ways, Mons Rümker appears to be a
miniature version of the Marius Hills shield, but much
smaller and less well developed [Smith, 1974]. However,
here at Rümker, the low shield is the dominant landform
(Figure 7). Some of the overlapping shield constructs may
be thinly mantled uplands, but the characteristic shield shape
of these features argues instead that they are small, central
volcanoes, similar to basaltic shields found elsewhere on
the Moon and the other terrestrial planets [e.g., Greeley,
3.3. Prinz
[17] The Prinz volcanic complex (Figures 9 and 10;
~150 km in extent; 26" N 43" W) is built upon a block of
highlands material (Montes Harbinger) that is probably
related to the Imbrium basin [Strain and El-Baz, 1977].
Unlike Rümker (but similar to its neighbor, Aristarchus),
the Prinz complex is notable as the source region for several
sinuous rilles (lava channels) that supply the mare deposits
north and west of the plateau (Figure 10). Some of the
dome-like features in the Prinz area could be volcanic
constructs, particularly one that appears to be a breached
Figure 10. Volcanic features map of Prinz shield. Pit craters and sinuous rilles are the dominant landform.
7
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 11. Topographic profiles of the Prinz shield, about 160 km across and slightly less than 1 km high.
superposed, unrelated impact crater Kepler. Highland
materials in this area are facies of Imbrium basin ejecta,
primarily the knobby Alpes Formation [Wilhelms and
McCauley, 1971]. This region is not commonly thought
of as a mare volcanic complex, but close examination
reveals that sinuous rilles, irregular volcanic craters and
associated dark mantle (pyroclastic) materials occur
throughout the area (Figures 12 and 13). Mare lavas on
the Kepler shield have not been dated directly, but two
mare units 3.6 and 2.1 Ga make up part of the western
and southern edges of the shield [Hiesinger et al., 2003].
The Kepler feature is similar in developmental state to the
Prinz structure described above; it is built on top of
highlands material, mostly ejecta from the Imbrium basin,
which is thinly covered by a veneer of basaltic lava and
pyroclastic deposits. Although relatively low in overall
relief (~0.6 km in height; Figure 14), this rise is too tall
and wide to be attributed exclusively to impact causes,
either from the crater Kepler or the regional Imbrium
basin back slope topography and thus we classify it as a
shield (Table 1). Rough topography near the summit of
the Kepler shield (partially obscured by impact ejecta
from the crater Kepler) could be remnants of additional
igneous activity.
cone that might have served as a lava source (arrow in
Figure 9) [see also “E” in Figure 1 of Strain and El Baz,
1977]. The Prinz complex as a whole displays relief of about
600 m (Figure 11); this low value, in conjunction with the
exposure of many highlands units near and within the
construct as well as the principal manifestation of rilles as
the main landform here, suggests incomplete development
as a volcanic complex. Prinz appears to have been a
significant eruptive center, but most of its lava products were
supplied to the surrounding mare plains of Oceanus
Procellarum and Mare Imbrium. Although Prinz is adjacent
to the Aristarchus plateau, there is no obvious direct
connection between the two complexes; each features
similar landforms, but the relative importance of the various
types differ. The basalts of the Prinz shield appear to have
ages between 3.4 and 3.6 Ga [Strain and El-Baz, 1977; Zisk
et al., 1977], indicating an Imbrian age for the construct.
3.4. Kepler
[18] The Kepler volcanic complex (~270 km; 7" N, 38" W)
is newly identified in this work, although its presence
peripheral to the Imbrium basin is predicted by the work of
McGovern and Litherland [2011]. It consists of a very low
relief topographic rise near and south of the younger
Figure 12. Volcanic features of the Kepler shield (~270 km; 7" N, 38" W). Collapse pits and rilles (arrow;
left) near Maestlin R; elongate vent and associated dark pyroclastics (arrow) NE of Encke (center);
sinuous rille complex (arrow) on eastern shield edge (right).
8
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 13. Volcanic features map of Kepler shield. Pit craters, sinuous rilles, and pyroclastic vents are
the principal features of this shield.
3.5. Hortensius
[19] The Hortensius-Tobias Mayer area (approximately
150 by 350 km; 12" N, 27" W; Figures 15 and 16) has been
long known for its high concentration of small volcanic
landforms, including small shields, rilles, and cones
[Shoemaker, 1962; Schmitt et al., 1967; Smith, 1973;
Schultz, 1976; Phillips, 1989; Wood, 2003; Wöhler et al.,
2006]. The northern edge of this structure contains a series
of vents and pyroclastic deposits that are associated with
the eruption of the famous late Imbrium flows, the long,
striking lobate lava flows that cover Mare Imbrium
[Schaber, 1973; Schaber et al., 1976]. This shield is similar
to both the Prinz and Kepler shields in that altimetry data
show that it is built upon the main ring/rim of the Imbrium
basin, but both volcanic development and the diversity of
landforms at Hortensius are much greater than in either Prinz
or Kepler. Imbrium ejecta crop out on the surface as knobby
Alpes Fm. [Wilhelms and McCauley, 1971] along with the
occasional basin massif, but volcanic shields and structures
are abundant and clearly distinguishable from the highlands
units upon which they are built (Figure 15). The small
shields of Hortensius are well-developed miniature
volcanoes [Schultz, 1976; Head and Gifford, 1980; Wöhler
et al., 2006], similar to basaltic shields that are found in
Figure 14. Topographic profiles of the Kepler shield. Feature is about 270 km across and 500–600 m high.
9
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 15. Volcanic features of the Hortensius shield (approximately 150 by 350 km; 12" N, 27" W).
Vent and associated pyroclastic deposits (top left); dark halo crater (vent, yellow arrow) and line of spatter
cones along fissure vent (green arrow) (top right); cluster of small shields (bottom left); shields and low
domes on western edge of Hortensius shield (bottom right).
2003; Wöhler et al., 2006]. Topographic data reveal that
the eastern half of Mare Tranquillitatis is a broad, low rise
about 560 km across and over 1.8 km high (Figure 20). This
shield has been the source of multiple flows and eruptive
events emplaced between 3.6 and 3.7 Ga [Hiesinger et al.,
2000; Rajmon and Spudis, 2004]. The existence of this
volcanic shield may explain the apparent lack of topographic
evidence for the putative Tranquillitatis impact basin, for
which there is clear morphological evidence [Wilhelms
and McCauley, 1971; Wilhelms, 1987]—the Cauchy shield
is built on the floor of the Tranquillitatis basin, creating
a topographic high in its eastern half, where a low would
be expected.
[22] The Cauchy shield displays most of the volcanic
landforms seen in other lunar shields. The predominant
feature is a low shield, usually with a summit pit (Figure 18).
The unusual Rimae Cauchy I and II appear to be
combinations of linear graben and sinuous rilles in different
portions of the features. Rima Cauchy I terminates in a
couple of collapse pits (Figure 18), apparently the source
vents for the basalts that created the sinuous rille parts of
the feature (Figure 19). The Gardner “megadome” shield
(described below) occurs on the northern margin of the
Cauchy shield, but as with Prinz and Aristarchus, there
appears to be no direct genetic connection between the
two features.
“plains volcanic terrains” elsewhere on the Moon and on
other terrestrial planets [Greeley, 1976; 1982]. Linear vents,
lines of spatter cones, and discontinuous pyroclastic deposits
(Figures 15 and 16) [see also Gustafson et al., 2012] are also
abundant, particularly at the northern edge of the shield, the
probable source vents for the spectacular late Imbrium lava
flows [Schaber, 1973; Schaber et al., 1976].
[20] The Hortensius shield is one of the largest identified
in this study, extending almost 350 km along its NW/SE axis
(Figure 16). Topographic profiles show that the shield is
about 1200 m or less in total height (Figure 17 and Table 1),
making it a very low relief structure. Clusters of small
shields are found mostly along the margins of the shield,
particularly in the southwest (Figure 16). The eastern margin
of the shield is partly masked by ejecta from the crater
Copernicus. Mare basalts on the western and southern edge
of the shield have estimated ages of 3.5 and 3.1 Ga,
respectively [Hiesinger et al., 2003].
3.6. Cauchy
[21] The largest of the newly detected volcanic shields is
in eastern Mare Tranquillitatis, centered near the crater
Cauchy (~560 km; 8" N, 35" E; Figures 18 and 19). This area
has long been known as a locus of small volcanic features,
including numerous cones, low shields, and sinuous rilles
[e.g., Wilhelms, 1972; Guest and Murray, 1976; Wood,
10
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 16. Map of volcanic features of Hortensius shield. After Guest and Murray [1976].
3.7. Gardner
[23] At the northern end of the Cauchy structure is a
smaller feature, identified as the Gardner “megadome” by
Wood [2003, 2004]. This small (~70 km; 16" N, 34" E;
Figures 21 and 22) topographic blister displays several
smaller, overlapping shields and sinuous rilles. Its summit
displays a series of irregular depressions that may constitute
a caldera complex. A similar sequence of collapse pits are
found near the summit of the Marius Hills shield [Guest,
1971; Figure 3], but in this case, the collapse structure
covers most of the summit of Gardner (Figure 22). The
surface composition of Gardner seems to be less mafic
than the surrounding lavas of Mare Tranquillitatis, with
~14–16 wt.% FeO content, values that while broadly basaltic
are lower than typical mare material (e.g., the surrounding
Mare Tranquillitatis basalts have FeO content of ~20 wt.
%). The low FeO content of the surface of Gardner could
indicate either the eruption of a less mafic type of volcanic
magma (e.g., high-alumina mare basalts) [BVSP, 1981] or
that the surface materials are at least partly of highlands
Figure 17. Topographic profile of Hortensius shield showing feature about 300 km across and over
1 km high.
11
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 18. Selected volcanic features of the Cauchy shield (560 km; 8" N, 35" E). Flooded craters and
collapse pits near Jansen (left); source pit craters and co-mingled sinuous rilles of Rima Cauchy I (center);
low shield with summit pit near Cauchy (right).
features, the Gardner structure strongly resembles the
Rümker shield, which in turn is a miniature version of the
Marius Hills shield. Surface features of the Gardner shield
are mostly smooth, overlapping shield-like domes, some of
which have collapse pits (Figure 22). Thus, there appears
to be a continuous sequence of size in lunar shield volcanoes
over at least an order of magnitude.
composition and the Gardner shield is the surface expression
of predominantly intrusive activity, such as a laccolith [e.g.,
see Wöhler et al., 2006]. The Gardner shield is relatively
small, but shows relief of about 1.6 km (Figure 23), thus,
as at Rümker, it has higher than typical average slope
compared to other large shield volcanoes. The age of the
feature is undetermined, but both its heavily cratered
appearance and the apparent superposition of basin
radial texture (Figure 21) suggest that it is old, perhaps
older than 3.8 Ga.
[24] Based in part on its alignment with the volcanic area
near the crater Jansen, Wood et al. [2005] proposed that
Gardner is the northern terminus of an elongate quasi-linear,
volcano-tectonic structure. We suggest instead that Jansen is
part of the Cauchy shield (Figure 19) and that Gardner is a
possible parasitic shield of Cauchy, located on its periphery
(Figure 1). In terms of size, morphology, and distribution of
3.8. Aristarchus
[25] Among the lunar volcanic structures described here, the
Aristarchus plateau (~250 km; 25" N, 50" W; Figures 24, 25)
seems to be a special case. For the plateau, topographic prominence is caused principally by the uplifting of a structural
block associated with the formation of the Imbrium basin
and is enhanced only partly by the overplating of erupted lava
[Moore, 1967; Zisk et al., 1977; McEwen et al., 1994]. The
highland block that makes up the bulk of the plateau shows
Figure 19. Map of the volcanic features of the Cauchy shield. Most features are low shields and sinuous rilles.
12
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 20. Topographic profiles of the Cauchy shield.
[Gaddis et al., 2003]. These eruptions of both lava and ash
have partly covered the pre-existing plateau but seem to have
ended before a significant shield-shaped construct could be
built. Zisk et al. [1977] estimate the age of plateau materials
to be Orientale-contemporaneous (about 3.8 Ga) but very
young lavas are found south of the shield (1–1.5 Ga)
[Hiesinger et al., 2003] and at least some of these flows
originated from vents on the Aristarchus shield itself
(Figure 26).
[26] The dominant landform on the Aristarchus shield is
similar to the neighboring Prinz shield, collapse craters,
and sinuous rilles (Figure 26), along with the substantial
pyroclastic deposits mentioned above. Some of the smaller
hills might be volcanic constructs, but the majority appears
to be outcrops of the underlying highlands block upon which
the plateau lavas have been erupted [Zisk et al., 1977]. The
very young mare basalts that lap up onto the plateau [e.g.,
Hiesinger et al., 2003] partly obscure the relations of the
rille termini around the eruptive center. These relations suggest that the Aristarchus plateau is a proto-shield, currently
exposed in an arrested state of development whereas the
Marius Hills construct (Figure 2) is a fully developed lunar
shield volcano. Nevertheless, topographic profiles of the
Aristarchus shield (Figure 27) show a broad, shield-like
shape, 240 km across and up to 2 km high (Table 1). The
blister-like morphology of the Aristarchus shield is also
evident in the low sun angle mosaic (Figure 25).
[27] Many of the lunar shields have long been recognized
as volcanic complexes [McCauley, 1967; Guest, 1971;
Guest and Murray, 1976] or eruptive centers [WhitfordStark and Head, 1977], but their topographic nature
(Figure 1 and Table 1) has been only mentioned in passing
or has not been known. Both Marius Hills and Rümker were
long known to occur on topographic highs and data from the
Apollo metric camera demonstrated that the Prinz complex
is associated with a fragment of the rim massifs of the
Imbrium basin (Montes Harbinger) [Strain and El-Baz,
1977]. Most workers consider the volcanic activity of these
eruptive centers to have been minor adjuncts of the main
phase of mare volcanism, which was largely characterized
by voluminous, fissure-fed, flat-lying eruptions of flood
Figure 21. The Gardner “megadome” shield [Wood, 2003,
2004]. Northern edge of shield grades into highlands related
to Serenitatis basin. Large collapse pit (caldera?) evident,
centered on top of shield.
clear control by radial structures of the Imbrium basin and
ejecta from that event is exposed in the rugged terra of the plateau (Figure 25). However, massive eruption of lava both onto
and away from the plateau is indicated by the presence of
many large sinuous rilles, including the enormous Vallis
Schröteri (165 km long) and numerous other rilles (Figure 26).
Some rilles show clear evidence of at least two-phases of
eruption (e.g., the highly sinuous rille within the broader,
graben-like rille of V. Schröteri; Figure 24), suggesting a
prolonged, multi-phased volcanic evolution [e.g., Moore,
1967; Schultz, 1976]. The Aristarchus plateau is also the
source of dark red, 49,000 km2 regional pyroclastic deposits,
five times larger in areal extent than any other on the Moon
13
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 22. Volcanic features map of the Gardner shield. The predominant landform expression is a
series of interfingered low shields and the collapse pit at its summit.
grouping (Marius, Prinz, Hortensius, Rümker, Aristarchus,
and Kepler) is distributed along the southern and western periphery of the Imbrium basin, within the large ProcellarumKREEP terrane [Haskin et al., 2000], with its anomalously
high-Th content [Lawrence et al., 2007], while the largest
shield (Cauchy) sits in the midst of a cluster of mare basins,
including Serenitatis, Nectaris, and Crisium. These shields
all lie within predicted annular zones of enhanced magma
ascent produced by stresses from loading of mare units inside these basins [Litherland and McGovern, 2009; McGovern and Litherland, 2011]. The mechanism for enhancing
ascent of magma stems from a combination of the
lithosphere’s flexural and membrane responses to initial
basin-filling mare loads that creates favorable principal
lavas [e.g., Wilhelms, 1987]. However, Whitford-Stark and
Head [1977] suggested that much of the basalt of Oceanus
Procellarum may have been emplaced as eruptives from a
few volcanic centers, including both the Marius Hills
and Aristarchus structures. We concur with this latter
interpretation at least in part, as rilles (lava channels)
originating on the lunar shields have clearly supplied lava
to the surrounding maria.
4. Distribution and Morphometry of Large
Lunar Shields
[28] The distribution of these proposed lunar shield
volcanoes is decidedly non-random (Figure 1). The largest
Figure 23. Topographic profiles of the Gardner shield.
14
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 24. Volcanic features of the Aristarchus shield (~250 km; 25" N, 50" W). Pit craters and sinuous
rilles of the northern margin of the plateau (left); Cobra Head pit crater source vent and Vallis Schröteri, a
nested sinuous rille indicating multi-phase eruption history (center); collapse pits and sinuous rilles of the
NE section of the plateau (right).
Mars, which range from 1012 to more than 1015 m3 [Plescia,
2004]. It is evident that the size, shape, and volume of these
lunar features (Figures 28 and 29 and Table 1) are comparable in morphometry to unequivocal basaltic shield volcanoes
on other terrestrial planets, including Earth [Pike, 1978],
Mars [Plescia, 2004], and Venus [Herrick et al., 2005].
[30] With the possible exceptions of Gardner and Marius
Hills, most of the newly described lunar shields do not have
summit pits or calderas. However, some shields on the other
terrestrial planets likewise do not have summit craters and
the absence of such does not negate the classification of either these or the lunar features as shield volcanoes. Many
shield volcanoes on Venus do not display summit craters
but the existence of these volcanoes is evident by concentrations of eruptive landforms, radiating flows, and broad, low
relief topographic swells. The absence of a caldera is consistent with the mode of mantle-to-surface magma transport
predicted by the models of McGovern and Litherland
[2011]: dikes are aligned with regional (basin-loading)
stresses. Evidence for this is particularly strong at the
Cauchy shield, which is topped by a set of large graben/lava
fissures (Rimae Cauchy I and II) that are radial to the Serenitatis basin.
[31] Scenarios for the development of the proposed
Cauchy shield can be constrained by remote sensing and
geological mapping. Cauchy lacks the strong free-air gravity
high characteristic of flexurally supported shield volcanoes
such as Marius Hills [Kiefer, 2013], but joint analysis of
gravity and topography indicates a local crustal thickness
maximum beneath the structure [Neumann et al., 1996],
consistent with an isostatically compensated rise of material
with the density of crustal rocks. Mapping and interpretation
of multispectral imaging data indicate that the surface basalts
in eastern Mare Tranquillitatis are on the order of several
hundred meters in thickness [Rajmon and Spudis, 2004].
This value is a fraction of the observed 1.8 km relief of the
Cauchy shield. Perhaps the basalts of the Cauchy shield
comprise a thin carapace covering a pre-existing crustal
block (similar to the developmental scenario proposed above
for Aristarchus). Alternatively, the topographic rise could
have been built up by substantial amounts of intrusion and
underplating of moderate-density magmas, augmented by
late-stage eruptions of denser cumulate-rich magmas aided
by favorable stresses in the lithosphere [McGovern and
Litherland, 2011]. The former case would be inconsistent
stress orientations [Anderson, 1936] and tectonic stress gradients [Rubin, 1995] for vertical transport of magma in dikes
with orientations radial to the basins.
[29] The sizes of these lunar shield volcanoes are comparable to other broad, low-relief basaltic shields on the Earth,
Mars, and Venus (Figures 28 and 29). For example, the Kali
Mons volcano on Venus has diameter and flank slope distributions similar to those of the proposed Marius Hills shield
(Figure 29). Typical slopes for the flanks of these shields are
extremely low, on the order of less than 1" (Table 1). However, the slopes of the two smallest features, Rümker and
Gardner, are about 2.5" ; these features also have unique
populations of small overlapping volcanic domes and low
shields. These relations may indicate slightly different processes at work in the construction of these smaller shields.
The estimated volumes of the large lunar shields range from
about 1012 to 1014 m3 (Table 1). These values fall among the
lower range of estimates of volumes for shield volcanoes on
Figure 25. The LROC WAC mosaic at low sun illumination of the Aristarchus region, showing bulbous, blister-like
shape of the topography of the plateau. Such morphology is
consistent with the interpretation of the Aristarchus plateau
as a proto-shield volcano. Mosaic made by Maurice Collins
(http://lpod.wikispaces.com/August+20%2C+2012).
15
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 26. Map of the volcanic features of the Aristarchus shield (~250 km; 25" N, 50" W). The principal
landform is the sinuous rille and pit crater, many of which have emplaced the surrounding mare deposits.
Abundant pyroclastic materials discontinuously cover the plateau lavas, making determination of the ages
of surface units uncertain.
mantle were required; these would produce sheets of flood
lavas rather than central-vent edifices [Head and Wilson,
1991]. Although there is reason to question the negative
buoyancy of mare basalt magmas [e.g., Wieczorek et al.,
2001], our observations that large shields do exist suggest
that lunar eruptions probably spanned a range of volumes
and mass eruption rates, allowing both shield-building and
flood-type eruptions. The shield-building eruptions could
come directly from the mantle, driven by basin loading-induced stress in dikes [McGovern and Litherland, 2011],
from shallow magma bodies created by intrusion-trapping
loading stresses [e.g., Solomon and Head, 1980; Galgana
et al., 2011] or filling of pore space in an impact-processed
and fractured upper crust [Kiefer, 2013].
with the postulated presence of a Tranquillitatis impact basin
[e.g., Wilhelms, 1987], while the latter allows it.
[32] The recognition of the existence of large shield volcanoes on the Moon invites a re-examination of scenarios for
lunar magma ascent. It has been previously held that the
apparent absence of shield volcanoes on the Moon was
consistent with buoyancy-controlled ascent of magmas
through the lunar crust [e.g., Head and Wilson, 1991]. In this
view, the presumed negative buoyancy of basaltic magmas
in an anorthositic crust precludes the creation of shallow
reservoirs at neutral buoyancy horizons, from which relatively low- and moderate-volume shield-building eruptions
could emanate. Instead, high-volume eruptions from dikes
long enough to benefit from positive buoyancy deep in the
Figure 27. Topographic profiles of the Aristarchus shield.
16
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Figure 28. Height-diameter relations for shield volcanoes on several terrestrial planets; plotted “trends”
are informal and diagrammatic. The newly described lunar structures (stars) fall within the ranges of sizes
and heights of large, low-relief basaltic shields on Venus and Mars. Data for Earth (green squares) [Pike,
1978], Mars (red triangles) [Plescia, 2004], and Venus (blue circles) [Herrick et al., 2005].
numerical constant. Based on this simple model, one would
expect that planets with smaller g have larger values of h/d.
Figure 28 shows that this simple gravity scaling relation
does not work. For example, based simply on gravity
scaling, Earth and Venus should have very similar values
of h/d, with Mars and the Moon having increasingly larger
values of h/d. In fact, Figure 28 shows that Earth has the
largest values of h/d. Other factors, such as the magma
viscosity and the magma supply rate, must exert stronger
controls on shield morphology than does a simple gravity
scaling relationship.
[35] In the case of the Moon, the magma composition and
viscosity are different from basaltic magmas on Earth; lunar
basalts are richer in FeO and TiO2 and deficient in SiO2
relative to terrestrial basalts [Neal and Taylor, 1992;
Wieczorek et al., 2006]. The lower SiO2 results in significantly
lower magma viscosities on the Moon relative to Earth [Weill
et al., 1971; Bottinga and Weill, 1972], which more than offsets the differences in gravitational acceleration. Differences
[33] Figure 28 shows that the height/diameter ratios for the
lunar volcanic shields fall within the general range defined
by shield volcanoes on Earth, Mars, and Venus. Although
some broad trends are evident, there is considerable scatter
in the observations for all four objects. One can use a model
of the spreading of an unconfined viscous surface sheet flow
to gain some insight into the factors that are likely to control
the large-scale morphology of these volcanoes [Turcotte and
Schubert, 2002; Baratoux et al., 2009]. Equations (9-99) and
(9-101) of Turcotte and Schubert [2002] can be combined to
find that:
!
"
h
mQ 3=4
¼C
ðV Þ&1=2
d
krg
[34] where h/d is the height/diameter ratio of the volcano,
m is the magma viscosity, r is the magma density, Q is the
magma supply rate, g is the gravitational acceleration, k is
the permeability, V is the volcano volume, and C is a
Figure 29. Cross-sections of topography (solid black line, left y axis) and slope (red boxes, right y axis)
for proposed lunar shield volcano Marius Hills (left, from LOLA) [Smith et al., 2010] and for the Kali
Mons edifice on Venus (right, from Magellan) [Ford and Pettengill, 1992]. Azimuths from feature center
are indicated above each profile. Slope measurements are calculated using a least squares fit to a plane
over an 80 km wide baseline centered on each point.
17
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
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well constrained observationally. Assuming such supply rate
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in magma supply may also be important. For example, volcanic flows from the Marius Hills have been mapped as sources
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Conclusions
[37] We correlate large topographic prominences in the lunar maria with concentrations of small volcanic features
such as domes, pit craters, small shields, cones, and rilles.
We interpret these large, broad topographic features as
shield volcanoes, a previously unrecognized style of lunar
volcanic activity. Shield building occurred during the main
phases of mare volcanism on the Moon, between 3.9 and
3.0 Ga ago. The lunar shields show a variety of developmental states, ranging from nearly complete shield development
(e.g., Marius Hills) to proto-shield, highland block volcanic
resurfacing (e.g., Aristarchus). The features studied in this
work are comparable in size and shape to basaltic shield
volcanoes on other terrestrial planets, supporting our interpretation that they too are volcanic shields. Shields are found
proximate to the large, lava-filled impact basins Imbrium
and Serenitatis and structural features associated with the
shields tend to be radial to these basins. The recognition
of shield volcanoes on the Moon affirms that this style of
volcanism has been ubiquitous on the terrestrial planets of
our Solar System.
[38] Acknowledgments. This paper is Lunar and Planetary Institute
contribution 1720. This work was supported in part by NASA grant
NNX07AF95G. We thank Aileen Yingst, Lisa Gaddis, and two anonymous
reviewers for helpful comments on an earlier version of this paper.
18
SPUDIS ET AL.: LUNAR SHIELD VOLCANOES
Lawrence, D. J., R. C. Puetter, R. C. Elphic, W. C. Feldman, J. J. Hagerty,
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