1995 eruptions of Cerro Negro volcano, Nicaragua,
and risk assessment for future eruptions
Brittain E. Hill*
Charles B. Connor
Mark S. Jarzemba
Peter C. La Femina
}
M. Navarro
W. Strauch
} Instituto Nicaragüense de Estudios Territoriales, Apartado 2110, Managua, Nicaragua
ABSTRACT
,
,
Center for Nuclear Waste Regulatory Analyses, 6220 Culebra Road,
San Antonio, Texas 78238-5166
Cerro Negro volcano, Nicaragua, continued
a 147-yr-long duration of cinder-cone activity
with a major eruption in 1995. Small, phreatically driven eruptions began in May 1995
and continued for 79 days. Following a 95 day
repose, the main eruption produced 8 × 106 m3
of basalt from Cerro Negro over 13 days of activity and deposited 5 mm of ash in the city of
León. Although the damage from the 1995
eruptions was fortunately minor, previous
tephra falls from Cerro Negro have produced
significant crop damage and multiple deaths
through building collapse. In spite of its apparent longevity for a historically active cinder
cone, Cerro Negro has mass-flow rates typical
of arc-related basaltic cinder cone volcanoes.
Volcanic hazards beyond 3 km from Cerro Negro consist of tephra falls. Few models are
available to calculate tephra-fall risks from
basaltic volcanoes such as Cerro Negro, and
none have been applied to dispersive cinder
cone eruptions. A convective-dispersive model
of Suzuki is modified and evaluated using detailed data from the 1995 Cerro Negro eruption and is found to reasonably calculate
tephra-fall thickness between 8 and 30 km
from the vent. This model is used with detailed
data from previous Cerro Negro eruptions in a
tephra-fall hazard assessment. Cerro Negro
also appears to have had a steady-state eruption rate since about A.D. 1900, which is used to
estimate the timing of the next eruption as before A.D. 2006. The potential tephra fall from
*E-mail: [email protected].
,
,,
,
,,,
,,,
,,,
,,
,,,
,,,,
,,
,,,
,,,,
,,,,
,,
,,,,
,,,,
North
Figure 1. Distribution of tephra-fall deposits for November–December 1995 Cerro Negro
eruption, based on field measurements by M. Kesseler (University of Geneva). Isopachs in centimeters. Dashed line indicates uncertainty in location of 0.1 cm isopach. Ruled pattern—older
Cerro Negro basaltic lavas; black—1995 lavas; stippled pattern—cone. Topographic contours
(100 m) derived from 93-m-resolution digital elevation data. Coordinates in Universal Transverse Mercator, Zone 16, WGS-84 spheroid. Inset map shows locations of major Nicaraguan
volcanoes (open triangles).
Cerro Negro in León, Nicaragua, is calculated
as 2.2 mm/yr until 2006, with 95% confidence
that deposits will be <11 cm thick.
INTRODUCTION
More than 200 000 people in and around the
city of León have been affected by previous
Data Repository item 9870 contains additional material related to this article.
GSA Bulletin, October 1998; v. 110; no. 10; p. 1231–1241; 8 figures; 4 tables.
1231
tephra falls from Cerro Negro volcano, Nicaragua
(Fig. 1). Although not as devastating as large
eruptions from stratovolcanoes such as Mount
Pinatubo, basaltic cinder cones such as Cerro Negro present tangible hazards to surrounding inhabitants. Fortunately, these hazards can be mitigated with planning based on probabilistic risk
assessments. Cerro Negro is a relatively long-
HILL ET AL.
lived basaltic cinder cone; there have been 21
documented eruptions between 1850 and 1992
(Simkin and Siebert, 1994). Because of its recurring activity, Cerro Negro provides an excellent
opportunity to evaluate models traditionally used
in volcanic risk assessments for applicability to
small-volume basaltic volcanoes. This evaluation
is necessary because long-term risk assessments
for some critical facilities—such as the proposed
high-level radioactive waste repository site at
Yucca Mountain, Nevada—need to consider
tephra fall hazards from basaltic volcanoes.
Following the relatively large eruption of April
1992 (e.g., Connor et al., 1993), Cerro Negro remained dormant for 3.13 yr. Renewed activity began on May 29, 1995, with a 79 day period of sporadic phreatically driven eruptive bursts that
culminated in the effusion of a small, intracrater
lava. Following this activity, Cerro Negro was
volcanically and seismically quiet until November 19, 1995, when a dominantly magmatic eruption occurred for 13 days. Fortunately, the hazards
from the 1995 eruptions were relatively small and
consisted primarily of thin fall deposits that produced minor crop destruction. In contrast, the
April 1992 eruption killed nine people through
building collapse (P. Baxter, 1996, personal commun.), disrupted León’s water supply system, and
caused extensive crop damage. A fundamental assumption in volcanic risk analyses is that the past
character of eruptions reflects the likely range of
future eruptions. Data from the 1995 eruptions are
evaluated in context of historical activity at Cerro
Negro to better understand the range of likely future hazards for this volcano.
SUMMARY OF THE 1995 ERUPTION
The 1995 eruption of Cerro Negro began with
increased seismic tremor beneath the volcano on
May 24, followed by small, phreatically driven
eruptive bursts on May 29. The bursts produced
dilute ash clouds that rose to altitudes of <1000 m
and quickly dissipated in prevailing easterly
winds. Size and frequency of the bursts peaked in
early June and gradually declined to the end of
the eruption on August 16 (Global Volcanism
Network, 1995a). Details of the May–August
1995 activity and associated eruption products
are included in the GSA Data Repository1 associated with this article.
Renewed activity at Cerro Negro began at
11:45 on November 19, 1995, with an increase in
seismicity that corresponded with local observations of eruptive activity (Global Volcanism Net1GSA Data Repository item 9870, additional tables and figures, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO
80301. E-mail: [email protected].
1232
work, 1995b). Low-energy Strombolian eruptions
of incandescent bombs and scoria with minor ash
began to construct a small cinder cone within the
1992 eruption crater. A lava flow issued from the
west base of the new cone and began to flow
down the north flank of Cerro Negro on November 22 (Global Volcanism Network, 1995b). Lava
output increased between November 23 and 28,
with the lava advancing to 2 km north of Cerro
Negro by November 28. During this time, eruption intensity increased substantially. By November 29, the eruption sustained a continuous tephra
column and ash began to accumulate in León
(Fig. 1). Rhythmic pulses of magma occurred
every 2–7 s to sustain the eruption column; no explosions were audible at an observation point 500
m south and 200 m below the rim of the new
cone. Incandescent ejecta columns commonly
reached altitudes of 200–400 m above the cone
rim and were accompanied by roiling black
clouds of cooled basaltic tephra. A maximum incandescent column height of 810 ± 20 m above
the cone was measured at 17:03 on December 1.
Prevailing winds of 8–10 m s–1 deflected the rising eruption column to the west-southwest, where
it reached a maximum altitude between 2–2.5 km
above ground level within about 5 km of the vent.
Lava output appeared to remain constant during
the period of a sustained column, with flows migrating north and slightly east (Fig. 1). The continuous phase of the eruption ceased on December 2 at 6:30 and was marked by a return of
seismicity to near background levels.
Eruption activity between December 2 and 6
consisted of 5–30-min-long periods of small
eruptive bursts, which propelled relatively dense
clouds of juvenile and reworked basaltic tephra to
altitudes of 1–2 km. Tephra clouds quickly dissipated in the prevailing easterly winds, producing
trace deposits between Cerro Negro and León. A
single burst following a 26 hr hiatus in activity
marked the end of the eruption at 10:40 on December 6. Lava effusion occurred at a very low
rate between December 2 and 6, and northward
movement of the main lava flow ceased on December 8 (Global Volcanism Network, 1995b).
PHYSICAL CHARACTERISTICS
OF 1995 DEPOSITS
Tephra deposit thicknesses were measured
immediately after the eruption and are shown in
Figure 1. Isopach areas were calculated using a
geographic information system (GIS) coverage
developed for the Cerro Negro region. Simple
addition of the volumes represented by the
isopachs yielded a minimum fall volume of 2.16
× 106 m3. Volume of the fall deposit also was
calculated using the isopach areas and the twoslope line method of Fierstein and Nathenson
(1992). This method extrapolates for proximal
and distal deposits that are not measured directly and results in a tephra-fall volume of 2.87
× 106 m3. Average tephra-fall deposit densities
measured in situ during the eruption were
1200 kg m–3 at 1 km, which likely increase with
time through compaction and infiltration (cf.
McKnight and Williams, 1997). On the basis of
geochemical and petrographic data presented in
the accompanying GSA Data Repository (see
footnote 1), 1995 Cerro Negro basaltic magma
has a density of 2600 kg m–3. Thus, 1.33 ×
106 m3 dense-rock equivalent of basalt was
erupted as fall deposits. Appreciable tephra deposits formed only during the last 4 days of the
sustained eruption, which yields dense-rock
equivalent mass eruption rates of 3.9 m3 s–1. In
comparison, dense-rock equivalent tephra eruption rates are 3.1 m3 s–1 for the 26 days of the
Eldfell eruption at Vestmannaeyjar, Iceland
(i.e., Self et al., 1974), 27 m3 s–1 for March 18 to
June 9, 1943, activity at Parícutin, Mexico (i.e.,
Luhr and Simkin, 1993), and 16 m3 s–1 for the
northern breakout of the 1975 great Tolbachik
fissure eruption (i.e., Fedotov et al., 1984).
Tephra-fall deposits from the November
19–25 activity are restricted to within about
5 km of Cerro Negro, where they constitute
19%–32% of the total fall deposit section. These
early deposits contain noticeable amounts of
finely pulverized basalt, giving the deposit a
gray, ashy appearance. Granulometric analysis
shows that these deposits are slightly finer
grained and fines enriched relative to overlying
fall deposits (Fig. 2A). Main fall deposits extend
to León, consist solely of juvenile basaltic
tephra, and lack the unusual fines enrichment of
the underlying early deposits (Fig. 2A). Bulk deposit granulometric characteristics were calculated for six sampled sections along the main
distribution axis (Fig. 2B). Weighting these bulk
values with the isopach volumes derived from
Figure 1 gives a deposit median diameter of 0.5
φ (0.7 mm) with simple sorting at 0.3 φ. The
bulk tephra sections show an expected, overall
exponential decrease in median diameter with
distance from the vent (e.g., Walker, 1971). The
median diameter measured at 20.1 km, however,
is an order of magnitude higher than predicted
by a simple exponential fit to these data. Cerro
Negro 1995 fall deposits further emphasize the
need for caution in extrapolating proximal to
distal deposit characteristics using simple functions (Fisher and Schminke, 1984), which is often necessary to model processes associated with
ancient volcanoes.
Volcanic bombs and blocks to 2 m diameter
were ejected from Cerro Negro during the 1995
eruption. Roughly 10% of these ejecta impacted
beyond the base of Cerro Negro, to a maximum
Geological Society of America Bulletin, October 1998
1995 ERUPTIONS OF CERRO NEGRO VOLCANO
observed distance of 1.0 km from the vent (i.e.,
500 m from base of the cone). Numerous blocks
and bombs impacting the sides of Cerro Negro,
however, continued to roll for several hundred
meters past this point. Rolling blocks thus present
a serious hazard that extends significantly beyond
the maximum flight distance for large ejecta. This
hazard may not be intuitive at relatively smallvolume cinder cone eruptions, where few blocks
are commonly ejected beyond the base of the
cone. Ballistically ejected bombs approximately
1 m in diameter reached the top of 200–300-mhigh parabolic flight paths in 3–4 s, indicating
maximum eruption muzzle velocities >75 m s–1.
The 1992 Cerro Negro eruption enlarged the
summit crater to 370 m diameter with an average depth of 90 m (Connor et al., 1993). The
cone from the November–December 1995 eruptions filled approximately 70% of the old crater
and increased the cone height to 240 m on the
northern rim (Fig. 3). An estimated volume for
the 1995 cone is 2.9 × 106 m3, which corrects to
2.5 × 106 m3 dense-rock equivalent using a deposit density of 2200 kg m–3 (McKnight and
Williams, 1997). Cone construction occurred
over a 13 day period, giving an average denserock equivalent production rate of 2.2 m3 s–1.
Lavas from the 1995 eruption covered a 0.65
km2 area primarily north of Cerro Negro (Fig. 4),
with average thicknesses of individual flow lobes
between 2 and 10 m. Lavas also formed extensive ramps and levees on the northern flank of
Cerro Negro, which extended 75 m beyond the
base of the 1992 cone. Using a lava-flow density
of 2400 kg m–3 (e.g., McKnight and Williams,
1997), 3.7 × 106 m3 of basalt was erupted as lava
during 1995 activity. Most of the lava effusion
occurred between November 22 and December
2, giving an average production rate of 3.9 m3 s–1.
The 1995 eruption of Cerro Negro thus added
7.5 × 106 m3 of basalt to the volcano over a period of 13 days. Falls, cone, and lavas formed
over different intervals of time during this eruption, resulting in a maximum combined denserock equivalent mass-flow rate of 10.0 m3 s–1. On
the basis of 1995 eruptions, volcanic hazards
proximal to Cerro Negro are presumed to be
slowly advancing blocky lava flows, tephra falls,
ballistically transported bombs and blocks, and
small pyroclastic flows during phreatically dominated eruption. Volcanic hazards more than several kilometers from Cerro Negro consist solely
of tephra falls.
COMPARISON WITH
EARLIER ERUPTIONS
Calculating the likelihood and character of future eruptions from Cerro Negro is predicated
upon accurately determining the volume and du-
Figure 2. (A) Grain-size distribution for representative December 1995 Cerro Negro tephrafall section 3.2 km west-southwest of the vent. Basal (2 cm thick) and lower (1.1 cm thick) ashy
lapilli layers from November 19–28 activity, main deposits (10 cm thick) from November 28 to
December 2 activity. (B) Bulk-deposit granulometry for Cerro Negro fall deposits, sampled
along the main axis of dispersal.
ration of previous eruptions. Cerro Negro has
erupted at least 22 times since A.D. 1850 (Simkin
and Siebert, 1994). Summaries of Cerro Negro
eruption data are presented in Table 1. Detailed
explanations on data sources, including isopach
maps, are included in the GSA Data Repository
(see footnote 1) associated with this article.
Quantitative modeling of tephra dispersion requires accurate determination of mass-flow rates
(e.g., Suzuki, 1983). Empirical relationships are
commonly used to relate column height to massflow rate (Wilson et al., 1978; Walker et al.,
1984). Although the total duration of Cerro Negro eruptions is available from historical accounts, most of the tephra is usually produced in
a relatively short interval that often has not been
reported. Observers generally record maximum
column heights and little information is available regarding column height through time.
Within these limitations, however, there is generally good agreement between reported and calculated column heights for main tephra-producing eruptions of Cerro Negro (Table 2). Previous
eruptions of Cerro Negro have produced tephra
dense-rock equivalent mass-flow rates between
1 and 172 m3 s–1 over periods of 1–50 days.
These ranges in mass-flow parameters are used
for the ensuing risk analyses.
Petrographic characteristics and variations between 1995 and 1992 Cerro Negro basalt provide
useful insights into processes relevant to risk
analyses. The 1995 fall deposit section is petro-
Geological Society of America Bulletin, October 1998
1233
HILL ET AL.
1385000 mN
Figure 3. Enhanced oblique aerial photograph of Cerro Negro volcano, Nicaragua, taken December 6, 1995, about 20 km from the northeast. 1995 eruption deposits are outlined in black.
Note the new cone partially fills the 1992 crater.
300
Cerro La Mula
1383000 mN
500
1995 Lava
1995 Cone
1992 Cone
1968 Lava
1960 Lava
1957 Lava
1947 Lava
1923 Lava
1850 Lava
Contour Interval
20 m
1384000 mN
CERRO NEGRO
Nicaragua
50
0
500
1 km
531000 mE
532000 mE
Las
Pilas
533000 mE
1381000 mN
1382000 mN
North
Figure 4. Cerro Negro lavas, modified from Viramonte and Di Scalia (1970). Tephra-fall deposits that mantle the western lavas are not shown. Summit elevation of Cerro La Mula is 590 m,
base is from 1:50 000 topographic maps.
1234
graphically uniform and there are no significant
differences in phenocryst modes between falls and
lava. In contrast, 1992 fall deposits are less vesicular, and the upper part of the fall section contains
about twice as much olivine and augite as the base.
Phenocrysts from 1995 and 1992 basalt are characteristically large (2 mm common, ranging to 6
mm). Plagioclase phenocrysts have a weak normal
compositional zonation from about An80 to An70
(Michel Lévy and Carlsbad-albite methods), with
most of the zonation occurring in the outer quarter
of the crystals. Plagioclase phenocrysts ³2 mm
from 1995 basalt commonly contain a zone of
abundant glass inclusions in the outer third of the
crystal, in addition to moderately sieved interiors.
In contrast, plagioclase from 1992 basalt contains
relatively diffuse glass inclusions that are generally in the inner two-thirds of the phenocrysts.
Abundant glass inclusions are commonly thought
to indicate magma mixing (e.g., Ussler and
Glazner, 1989); however, these Cerro Negro
basalts lack the abrupt compositional changes that
generally accompany transitions between sieved
and nonsieved plagioclase zones (i.e., Lofgren,
1980; Corrigan, 1982). Roggensack et al. (1997)
observed systematic variations in the volatile contents of phenocryst melt inclusions for 1995 and
1992 basalts. They concluded that 1992 basalt ascended to midcrustal levels before eruption,
whereas the 1995 basalt ascended to shallow
crustal levels and underwent a significant volatile
loss before erupting. Observed patterns in plagioclase compositional and glass-inclusion zoning, in
addition to the lesser degrees of vesiculation in the
1992 tephra, support the magma ascent mechanisms concluded by Roggensack et al. (1997).
Basalt from the 1995 Cerro Negro eruption is
compositionally similar to basalt from previous
eruptions, although there are petrogenetic variations within and between some older units
(Walker and Carr, 1986; Carr and Walker, 1987;
McKnight, 1995). There are no significant compositional differences with time during the 1995
eruption. In contrast, basaltic lapilli from the top
of the 1992 fall deposit are significantly more
mafic and less aluminous than lapilli from the
base of the unit. Using representative 1968 mineral compositions from Walker and Carr (1986),
addition of 7% augite and 5% olivine to early
1992 basalt through simple least-squares modeling results in late 1992 basalt compositions (sum
of squared residuals is 0.08). This addition of
augite and olivine is consistent with measured increases in modal abundances between early and
late 1992 Cerro Negro basalt. Although early
1992 and 1995 basalt have the same modal mineral abundances, 1995 basalt is slightly less
evolved than early 1992 basalt. The addition of
about 2% augite and 1% each of plagioclase and
olivine to early 1992 basalt through simple least-
Geological Society of America Bulletin, October 1998
1995 ERUPTIONS OF CERRO NEGRO VOLCANO
TABLE 1. CERRO NEGRO ERUPTION DATA
Year
(A.D.)
Duration
(days)
1850
1867
1899
1914
1919
1923
1929
1947
1948
1949
1950
1954
1957
1960
1961
1962
1963
1968
1969
1971
1992
1995
1995
Total
10
16
7
6
10
49
19
13
1+
1+
26
1+
20
89
1+
2
1+
48
10
10.6
3.6
79.0
13.0
436
Repose
(years)
0.00
17.5
32.0
14.9
4.6
4.3
5.2
18.4
0.7
1.2
1.5
3.1
3.6
3.0
0.8
0.4
1.0
5.7
1.0
1.1
21.2
3.1
0.3
144.6
Cone
height*
(m)
50
60
50
unkn
unkn
≈200
unkn
60–130
unkn
unkn
130–230
unkn
unkn
250
unkn
unkn
unkn
230
unkn
160–246
90–210
tr
90–240
N.A.
Basal
diameter
(m)
100
200
240
unkn
unkn
550
unkn
550
unkn
unkn
600
unkn
unkn
775
unkn
unkn
unkn
875
unkn
1000
1050
tr
1050
N.A.
Crater
diameter
(m)
20
60
100
unkn
unkn
70
unkn
250
unkn
unkn
150
unkn
unkn
130
unkn
unkn
unkn
160
unkn
300
400
tr
150
N.A.
Crater
depth
(m)
unkn
unkn
40
unkn
unkn
50
unkn
75
unkn
unkn
75
unkn
unkn
50
unkn
unkn
unkn
60
unkn
75
90
tr
50
N.A.
Cone
volume
(km3)
0.0001
0.0006
0.0008
unkn
unkn
0.0090
unkn
-0.0032
unkn
unkn
0.0164
unkn
unkn
0.0153
unkn
unkn
unkn
0.0082
unkn
0.0129
-0.0011
tr
0.0025
0.0615
Fall
volume
(km3)
0.0002
0.0034
tr
0.0013
tr
0.0077
tr
0.0110
tr
tr
0.0013
tr
0.0013
0.0005
tr
tr
tr
0.0045
tr
0.0139
0.0110
tr
0.0013
0.0574
Lava
volume
(km3)
0.0054
N.A.
N.A.
N.A.
N.A.
0.0100
0.0001
0.0038
N.A.
N.A.
0.0001
N.A.
0.0045
0.0052
N.A.
N.A.
N.A.
0.0069
N.A.
tr
N.A.
tr
0.0037
0.0397
Total
volume
(km3)
0.006
0.010
0.011
0.012
0.012
0.039
0.039
0.051
0.051
0.051
0.068
0.068
0.074
0.095
0.095
0.096
0.096
0.115
0.115
0.142
0.152
0.152
0.160
N.A.
Note: Volumes in dense-rock equivalent (DRE). N.A.—not applicable; unkn—variations likely but not reported; tr = little appreciable variation
occurred or was likely.
*Cone height with two values indicates asymmetric cone with minimum height in northeast quadrant and maximum height in southwest quadrant.
squares modeling results in 1995 basalt compositions (sum of squared residuals is 0.05). These results confirm that short-term petrogenetic variations at Cerro Negro are readily explained by
minor variations in phenocryst accumulation
(Walker and Carr, 1986; Carr and Walker, 1987;
McKnight, 1995).
In summary, the 1995 Cerro Negro eruption
was similar to previous eruptions in character and
products. Several months of small, phreatically
driven eruptions in early 1995, however, lasted
longer than other historical eruptions of Cerro
Negro that produced no preservable deposits.
Additional magma ascent followed several
months of inactivity, followed by a typical eruption about 10 days long. This punctuated magma
ascent likely allowed for more shallow-level degassing of 1995 magma than occurred in the
1992 eruption, leading to a less explosive eruption in 1995 (i.e., Roggensack et al., 1997).
The addition of 7.5 × 106 m3 dense-rock
equivalent of basalt from the 1995 eruption
brings the total erupted volume of Cerro Negro to
0.160 km3 dense-rock equivalent over 436 days
of activity (Table 1), which averages 3.7 × 105 m3
day–1. This eruption rate is comparable to other
historically observed basaltic cinder cone eruptions. For example, the 1943–1952 Parícutin
eruption (e.g., Luhr and Simkin, 1993) had an average dense-rock equivalent eruption rate of 2.8
× 105 m3 day–1, for a total volume of 0.92 km3.
The 157 days of eruptive activity at Eldfell volcano in 1973 (Self et al., 1974; Thorarinsson et
al., 1973) produced 0.18 km3 of basalt at a rate of
1.2 × 106 m3 day–1. The northern breakout of the
great Tolbachik fissure eruption (e.g., Fedotov et
al., 1984) had an average dense-rock equivalent
eruption rate of 1.3 × 107 m3 day–1, for a total volume of 0.91 km3. Cerro Negro may have transitory morphometric characteristics reminiscent
of composite volcanoes (e.g., McKnight and
Williams, 1997); however, mass-flow rates are a
more important parameter in evaluating volcanic
hazards, and Cerro Negro clearly is a typical
basaltic cinder cone in this regard.
TEPHRA-FALL HAZARDS ASSESSMENT
Previous eruptions show that tephra falls represent the only volcanic hazard at distances beyond
several kilometers from Cerro Negro. As the nearest permanent inhabitants are located more than
3 km from Cerro Negro, the ensuing analysis will
only examine tephra-fall hazards. A variety of
models are available to simulate the aerial transport of material from erupting volcanoes (e.g.,
Suzuki, 1983; Carey and Sparks, 1986; Woods,
1988; Armienti et al., 1988; Bursik et al., 1992).
These models have been developed primarily for
large-volume Plinian eruption columns. Smallvolume basaltic volcanoes such as Cerro Negro
often produce sustained, convective columns during eruptions with significantly lower mass-flow
rates than silicic Plinian eruptions. In addition,
basaltic tephra likely are coarser grained than fineash–dominated silicic tephra (e.g., Walker, 1973).
On the basis of these clear differences between
silicic and basaltic eruptions, tephra-dispersal
models need to be evaluated before application to
Cerro Negro–type basaltic eruptions.
The Suzuki (1983) model is commonly used to
calculate fall-deposit thicknesses for silicic eruptions (e.g., Glaze and Self, 1991). Suzuki (1983)
used a two-dimensional diffusion model that is
suitable for essentially continuous-release eruptions with particle sizes >15 µm. This model assumes a turbulent dispersal of particles, governed
by advection and diffusion of the wind-blown
plume. Observed eruptions of Cerro Negro and
other analogous basaltic volcanoes are characterized by low-altitude turbulent columns (<10 km)
that are laterally advected by prevailing winds. Although the Suzuki model is attractive because it is
computationally straightforward, it does make
simplifying assumptions not required by thermofluid-dynamic models (Woods, 1988). Therefore
the Suzuki (1983) model was examined in detail
for suitability in Cerro Negro hazards assessments
by comparison with the 1995 eruption.
A fundamental assumption of Suzuki (1983) is
that particle velocity is maximum at the vent and
decreases linearly to zero at the top of the eruption column, in marked contrast to other models
(e.g., Woods, 1988, 1995) that show nonlinear
change in upward particle velocity. After adding
a term to model diffusion in the eruption column
as a function of height, Suzuki (1983) derived a
probability density function (PDF) for diffusion
of mass out of the eruption column within dz
about height z as
Geological Society of America Bulletin, October 1998
P(z) = AY(z)exp–Y(z),
(1)
1235
HILL ET AL.
TABLE 2. COLUMN HEIGHT DATA FOR CERRO NEGRO ERUPTIONS
Year
(A.D.)
Fall
duration
(days)
1867
1923
1947
1950
1957
1968
1971
1992
1995
2.8
49
0.8
26
15
42
7
0.7
4
Fall
eruption
rate*
(m3 s–1)
14
1.8
161
0.6
1.0
1.2
23
172
3.9
Observed
column
height
(km)
>1
>0.3
4–6.5
>0.3
2
1–1.5
6
3–7
2–2.5
Calculated†
column
height
(km)
3.4
2.0
6.3
1.5
1.8
1.9
3.8
6.4
2.5
Calculated§
column
height
(km)
3.3
2.0
6.1
1.5
1.7
1.8
3.8
6.2
2.4
*Calculated from dense-rock equivalent (DRE) volumes and eruption durations in
Table 1.
†Calculated from equations 2–3 in Wilson et al. (1978) and a bulk density of
2600 kg m–3, specific heat of 1.1 × 103 J kg–1 K–1, 1055 K temperature change, and a
0.7 thermal efficiency factor.
§Calculated using equation 1 of Walker et al. (1984) and eruption rates in Table 1.
where
Y (z) =
[
]
β W ( z ) – V0 .
V0
(2)
β = a constant controlling diffusion of particles in
the eruption column; W(z) = particle upward velocity as a function of height = W0(1 – z/H); W0 =
particle upward velocity at zero height;
H = column height, V0 = particle terminal velocity
at sea level, and A = a normalization constant
found by integrating P(z) over the column height.
Suzuki (1983) presented A as
A=
βW0
, (3)

– Y0 ) 
(
V0 H 1 – 1 + Y0 exp



(
)
where Y0 is equal to Y(z = 0). The variable Y(z)
should be redefined as Y(z) = βW(z)/V0 for two
reasons: (1) with the definition of Y(z) as given in
Suzuki (1983), P(z) is negative at heights approaching the top of the column, and (2) the expression for A published in Suzuki (1983) is realized only with the redefined Y(z). By instituting
this change, mass is conserved in the model
where it previously was not.
The Suzuki (1983) model is implemented in
this study by using eruption column height and
duration to derive mass-flow input parameters,
using equations 2–3 in Wilson et al. (1978) and
equation 1 of Walker et al. (1984). Details of this
implementation were described in Jarzemba
(1997). Data from the 1995 Cerro Negro eruption
are used to test the Suzuki (1983) model for precision and accuracy in calculating tephra-fall deposit thicknesses for a small-volume basaltic
eruption. Model parameters are given in Table 3.
Deposit thicknesses between 8 and 20 km are well
represented by the modified Suzuki (1983) model
(Fig. 5), using the eruption parameters in Table 3.
These distances are of primary interest for the en-
1236
TABLE 3. COMPILATION OF 1995
CERRO NEGRO ERUPTION PARAMETERS
Parameter
Magma density
Mass-flow rate
Specific heat of basalt
Magma temperature
Final temperature
Thermal efficiency
Wind speed
Column height
Tephra column duration
Tephra mass (DRE)
Average clast diameter
Average clast sorting
Average clast density
Clast shape
Constant β
Constant C
suing risk analyses. Proximal deposit (i.e., <8 km)
thicknesses, however, are somewhat overestimated. Variations in wind speed and direction
would likely redistribute this additional tephra to
more closely approximate observed shapes of the
1995 isopachs (Fig. 5). The location of the 0.1 cm
isopach may be inaccurate on the order of 5 km,
because of the difficulty in measuring thin deposits under field conditions (Fig. 1). Linear extrapolation of the 6–20 km fall deposit thicknesses to
30 km suggests that 1995 fall deposits were 0.2
cm rather than 0.1 cm thick at this location, which
corresponds to deposit thicknesses calculated by
the modified Suzuki (1983) model.
The modified Suzuki (1983) model was examined for sensitivity to variations in key parameters, which may significantly affect calculated
deposit thicknesses during probabilistic risk
analyses. Suzuki (1983) used a constant (β) to
control the mass diffusion from the eruption column as a function of grain size and settling velocity. Greater values of β broaden the ash blanket and increase down-wind ash accumulation.
Varying β between 0.02 and 0.5 for the large
Plinian eruption conditions in Suzuki (1983) resulted in marked differences in calculated deposit
thicknesses. In addition, larger values of β for
these conditions shifted the maximum deposit
thickness farther from the vent. For 1995 Cerro
Negro eruption conditions, however, varying β
between 0.001 and 10 produced only minor variations in calculated deposit thicknesses for a
given wind speed (Fig. 6A). Maximum deposit
thicknesses also remained at the vent for all values of β between 0.001 and 10 (Fig. 6A), in contrast to the original model in which deposit maxima were located away from the vent for large
values of β (Suzuki, 1983). The relative insensitivity of β for 1995 Cerro Negro likely results
from a small eruption-column height. Most particles with settling velocities <0.5 m s–1 diffuse out
near the top of the eruption column for heights
less than 5 km and 0.02 < β < 10. This is in rea-
Value
2600 kg m–3
3.9 m3 s–1
1100 J kg–1 K–1
1325 K
270 K
0.7
9 m s–1
2.4 km
3.46 × 105 s
1.32 × 106 m3
0.7 mm
0.9 mm
1200 kg m–3
0.5
10
400
Note: DRE—dense-rock equivalent.
sonable agreement with thermo-fluid-dynamic
models (Woods, 1988, 1995), which indicate a
rapid drop in upward particle velocity near the
top of short (<5 km) eruption columns. For example, the Woods (1988) model indicates that
upward velocities drop below 10 m s–1 only in
the upper 200 m for a 4-km-high column (initial
velocity = 50 m s–1, vent radius = 10 m, initial gas
fraction = 0.01).
Wind speed (Fig. 6B) strongly affects modeled
deposit thicknesses and ash thickness is most sensitive to this parameter in the modified Suzuki
(1983) model. Wind speed for the 1995 Cerro Negro eruption is well constrained at 8–10 m s–1, on
the basis of ground and satellite measurements
(Global Volcanism Network, 1995b). Lowering
the wind speed to 4.5 m s–1 improves the model fit
for proximal deposits, but significantly underestimates distal deposit thicknesses and is inconsistent
with the wind-speed data. Increasing wind speed
overestimates deposit thicknesses at all locations
(Fig. 6B). The minor discrepancies between modeled and measured deposit thicknesses may partially result from advection of the eruption column
downwind as it rises buoyantly. For example, applying the model of Woods (1988, 1995) yields
rise times for pyroclasts in the convecting column
of 160–190 s. In a 9 m s–1 wind, the centerline of
the column is deflected on the order of 1500 m
downwind and the column spreads laterally to a
radius on the order of 1 km as particles decelerate
and approach a level of neutral buoyancy. These
effects, however, will tend to increase ash thickness downwind compared to the results obtained
using the modified Suzuki (1983) model.
Reasonable variations in eruption column
height produce minor variations in calculated deposit thicknesses for the 1995 Cerro Negro eruption. Column height is constrained, however, by
the total mass in the system (e.g., Wilson et al.,
1978). Any increase in column height must be
accompanied by a decrease in eruption duration
to conserve mass. An increase in column height
Geological Society of America Bulletin, October 1998
1995 ERUPTIONS OF CERRO NEGRO VOLCANO
Figure 5. Comparison of fall-deposit thicknesses calculated using the modified Suzuki (1983) model with measured isopachs. Model parameters are from Table 3. Inset shows deposit thicknesses measured along main axis of dispersion, for reference with subsequent figures.
from 2.4 to 3 km for the 1995 Cerro Negro eruption would limit column duration to 1.6 days,
which contradicts observation of a sustained column for 4 days. Increasing column height from
2.4 km to 3 km with a 9 m s–1 wind speed only results in a small decrease in proximal and a small
increase in distal deposit thicknesses (Fig. 6C).
Decreasing column height to 2 km increases
proximal thicknesses and decreases distal thicknesses slightly (Fig. 6C), but increases column
duration to 8.1 days. Thus, if the total mass of the
small-volume tephra eruption is reasonably constrained, minor variations in column heights calculated from tephra mass (e.g., Walker et al.,
1984) will only produce minor variations in deposit thicknesses calculated using the modified
Suzuki (1983) convective-dispersive model.
Variations in particle density, diameter, sorting, and shape have minor to negligible effects on
calculated ash-deposit thicknesses. Decreasing
clast density to 800 kg m–3 with a 9 m s–1 wind
speed increases deposit thicknesses slightly (Fig.
6D). This low clast density, however, is difficult
to support for basaltic tephra containing 44%
vesicles, 33% groundmass, and 23% phenocrysts, but may be possible for phenocrystpoor ash particles. Increasing clast density to
1700 kg m–3 decreases deposit thicknesses
slightly (Fig. 6D). Reasonable variations in average particle diameter produce minor variations in
calculated deposited thicknesses. Decreasing average diameter to 0.4 mm results in a small increase in distal deposit thicknesses (Fig. 6E),
whereas increasing average diameter to 2 mm decreases distal deposit thicknesses. Changing the
shape of particles (Suzuki’s parameter F) between 0.25 and 0.75 produced barely discernable
variations in calculated deposit thicknesses. Variations in particle sorting, however, produced potentially significant differences in calculated deposit thicknesses. Decreasing the sorting to 0.5
overestimates deposit thicknesses greatly (Fig.
6F), although this effect decreases in magnitude
with distance from the vent. Increasing sorting to
1.5 provides a relatively good fit to observed
proximal deposit thicknesses, but underestimates
distal deposits somewhat (Fig. 6F). This poor degree of sorting also is difficult to reconcile with
measured grain-size distributions (Fig. 2).
In summary, the modified Suzuki (1983) model
reasonably calculates 1995 Cerro Negro tephrafall–deposit thicknesses for distances between 8
and 20 km from the vent and is probably accurate
to at least 30 km for these eruption conditions.
Proximal deposit thicknesses are somewhat overestimated, but model fits may be improved proximally by varying wind speed and direction to distribute the excess mass over a slightly broader
area. Although the original Suzuki (1983) model
has been successfully applied to numerous larger
volume silicic eruptions (e.g., Suzuki, 1985; Glaze
and Self, 1991), application to the 1995 Cerro Negro eruption resulted in large underestimations of
distal (i.e., > 8 km) deposit thicknesses. This limitation only became apparent for a low mass-flow,
low column-height eruption, where processes
within the eruption column, such as buoyant acceleration of particles above the gas-thrust region
and downwind advection of the column during
buoyant rise, probably account for some of this
discrepancy. On the basis of comparison with the
1995 Cerro Negro eruption, the modified Suzuki
(1983) model corrects this underestimation of deposit thickness. The modified Suzuki (1983)
model is thus useful in hazard analysis as it is reasonably accurate, the main parameters are readily
derived from volcanological observations, and the
simplifying assumptions of the model make computations rapid, allowing for a large number of
simulations during probabilistic analysis.
The hazards analysis for tephra fall in León,
Nicaragua, was made for future Cerro Negro eruptions using the modified Suzuki (1983) dispersal
model and parameters derived from the character
of past eruptions. A total of 1000 Monte Carlo
simulations of future eruptions were calculated,
with parameters sampled stochastically from uniform distributions (shown in braces) derived from
data in Table 1. Column height (2 km, 6 km) and
eruption duration (0.5 days, 40 days) were sam-
Geological Society of America Bulletin, October 1998
1237
HILL ET AL.
Figure 6. Sensitivity analysis for the modified Suzuki (1983) tephra dispersion model to variations in (A) column shape, (B) wind speed, (C) column height, (D) particle density, (E) particle
diameter, and (F) sorting. Parameters in bold type represent 1995 Cerro Negro eruption conditions. Note variations due to wind speed are significantly greater than variations caused by reasonable changes in other model parameters.
pled stochastically and conditioned so that denserock equivalent tephra volume varied between
0.0001 km3 and 0.05 km3 (Table 1). Wind speed,
(5 m s–1, 15 m s–1), wind direction (40°, 90° from
north), average particle diameter (0.05 cm, 0.1
cm), and sorting (0.75, 1.25) also were sampled
stochastically, with other parameters from Table 3.
On the basis of these parameter distributions, the
complementary cumulative distribution function
(Fig. 7) gives the probability of ash thickness exceeding a given value in the center of León,
Nicaragua. Future eruptions have a 50% chance of
depositing more than 0.2 cm of tephra fall in León,
with 95% confidence that future fall deposits will
be less than 11 cm thick (Fig. 7).
1238
TIMING OF FUTURE ERUPTIONS
Cerro Negro’s repeated eruptions over the
past 148 yr suggest that future eruptions are
likely from this volcano. Basaltic cinder cones
commonly are active for 1 yr or less (Wood,
1980), although punctuated eruptions may occur for at least a decade at cogenetic complexes
such as Jorullo, Mexico (Luhr and Carmichael,
1985), and possibly for centuries at Sunset
Crater, Arizona (Holm, 1987). To date, no reliable method has been developed to determine
the extinction of a basaltic volcano, except for a
prolonged absence of eruptions. Thus, the conservative assumption for Cerro Negro risk as-
sessment is that the timing and character of past
eruptions provide the best available indicators
of future eruptions.
Volume-predictable models have been developed for relatively large volume volcanic systems
(Wadge, 1982; Bacon, 1982; Kuntz et al., 1986;
King, 1989; Stieltjes and Moutou, 1989). These
volcanic systems have a characteristic period of
steady-state activity, in which eruption volume is
relatively constant through time, generally preceded by a period of waxing activity. Since about
A.D. 1900, Cerro Negro has had a relatively steady
eruption rate of 1.75 × 10–3 km3 yr–1 (Fig. 8A). Using methodology developed in Wadge (1982), a
95% confidence interval about the cumulative volume curve constrains the likely maximum volume
for future Cerro Negro eruptions to 0.05 km3. Assuming that Cerro Negro activity is steady state,
there is a maximum repose time of 29 yr until the
next eruption, at a 95% confidence interval.
The timing of Cerro Negro’s past large eruptions may more precisely constrain the timing of
the next eruption than the maximum repose period. Since A.D. 1900, there have been nine relatively large eruptions at Cerro Negro that control
the cumulative volume curve in Figure 8B. Qualitatively, larger eruptions of Cerro Negro have
been followed by relatively long periods of quiescence, whereas smaller-volume eruptions produce shorter periods of quiescence. Following
the methods used by Bacon (1982), cumulative
volume is plotted at the time of the succeeding
large-volume eruption (Fig. 8B). For steady-state
volcanoes such as Cerro Negro, the timing of future eruptions should be governed by the volume
of the preceding eruption. The 1995 eruption
brought the cumulative volume of Cerro Negro
to 0.16 km3. Recovery of the system to steadystate conditions is most likely through an eruption occurring before A.D. 2006, on the basis of
the linear regression and 95% confidence interval
in Figure 8B.
Other probability models assume that the timing of past eruptions is a Poisson process (e.g.,
Wickman, 1965) and that the probability (P) of
future eruptions is calculated by
Pn
–n
λt )
(
=
exp – λt
(4)
n!
where n is the number of eruptions in the time period of interest, λ is the annual recurrence rate,
and t is the time period of interest. The time-volume relationship for Cerro Negro strongly indicates that eruptions are to some extent dependent
on the character of the preceding eruption and
thus are not truly Poissonian. The steady-state
trend, however, may be fortuitous, or the pattern
of Cerro Negro eruptions may be Poissonian over
relatively short periods (cf. McBirney, 1992).
Geological Society of America Bulletin, October 1998
1995 ERUPTIONS OF CERRO NEGRO VOLCANO
Figure 7. Complementary cumulative distribution function for ashfall-deposit thicknesses in León,
Nicaragua, calculated using the
modified dispersion model of Suzuki
(1983) and stochastic sampling of
Cerro Negro eruption parameters.
Exceedance probability
1
Considering the nine largest eruptions between
A.D. 1923 and 1995, Cerro Negro has an average recurrence rate of 0.125 large eruptions per
year. This recurrence rate increases slightly to
0.15 eruptions per year if all documented eruptions (Table 1) are considered. Assuming that
eruptions occur independently through time at
Cerro Negro, the Poisson-process model indicates a 67% probability of a relatively large
Cerro Negro eruption and 74% probability of
any Cerro Negro eruption before A.D. 2006. In
contrast, the steady-state model predicts that
the next eruption will likely occur with an annual probability of 0.10 per year in the same 10
yr period.
50% chance
thickness > 0.2 cm
0.1
10% chance
thickness > 6 cm
5% chance
thickness > 11 cm
0.01
0.001 0.01
RISK ASSESSMENT
i =1
1
t
Ni
0.16
(5)
0.14
0.12
Linear regression 1899-1995
r 2 = 0.92
m = 1.75x10-3 km3/yr
V2 (max vol) = 0.05 km 3
Max repose (V 2/m) = 29 yr
0.10
0.08
95%
Prediction
interval
0.06
0.04
0.02
where R = annual risk of tephra accumulation expressed in millimeters, P(E) = annual probability
of eruption from steady-state model (0.10/yr), ti =
ash thickness calculated for each realization (i),
and N = number of realizations. Using the calculated thicknesses in Figure 7, the annual risk R
for tephra fall in León is calculated as 2.2 mm/yr
until A.D. 2006.
DISCUSSION
Forecasting eruptions without precursory activity is an inexact science. Although some volcanoes
have quasiperiodic eruption rates over short intervals of time, reliable interpretation of eruption recurrence remains elusive (e.g., De La Cruz-Reyna,
1996). For Cerro Negro volcano, where sufficient
data exist to support the assumption of a steadystate eruption rate, the timing of future eruptions
appears reasonably correlated with the volume of
the preceding significant eruption. The accuracy
of using the cumulative volume curve for Cerro
Negro eruption forecasting is evaluated by removing successively older eruptions from the data set
and calculating the timing of the next major eruption from the resulting cumulative volume curve.
0.00
0.16
Cumulative volume (km 3)
N
R = P( E )∑
0.18
Cumulative volume (km 3)
Volcanic risks are commonly represented as a
composite of the expected effects of an eruption
and the likelihood of their occurrence (e.g., Booth,
1979). Complete volcanic risk assessments often
assess the vulnerability of people, buildings, infrastructure and economic activity in the affected region (e.g., Blong, 1996). These vulnerabilities
need to be assessed by Nicaraguan authorities in
order to complete the risk assessment for León and
surrounding areas. Neglecting the societal and
economic judgments, the annual risk of tephra-fall
in León, Nicaragua, can be calculated by:
0.1
1
10
Thickness (cm)
0.14
0.12
0.10
0.08
Steady-state recurrence
r 2 = 0.970
y = 2.068x10-3 x -3.975
1995, volume 0.160 km3 thus
x = 2000
x = 1995-2005 at 95% CI
Time of main eruption
Cumulative volume
0.06
0.04
95%
Confidence
interval
0.02
0.00
1840 1860 1880 1900 1920 1940 1960 1980 2000
Eruption year (A.D.)
Figure 8. (A) Cumulative volume curve for Cerro Negro volcano, Nicaragua, using data in
Table 1. Since about A.D. 1900, Cerro Negro has had a relatively steady state eruption rate of 1.75
× 10–3 km3 yr–1. Maximum repose period is calculated from Wadge (1982). (B) Volume-predictable model for Cerro Negro, using methods from Bacon (1982). Cumulative volumes are
projected to the times of succeeding, large-volume eruptions. The added volume of the 1995
Cerro Negro eruption indicates that the next eruption will probably occur before A.D. 2006.
Geological Society of America Bulletin, October 1998
1239
HILL ET AL.
TABLE 4. EVALUATION OF THE CERRO NEGRO TIME-VOLUME PREDICTION MODEL
Year
(A.D.)
Volume*
(km3)
Correlation
coefficient† (r2)
Regression
slope†
Regression
intercept†
Predicted
eruption (A.D.)
1992
1971
1968
1960
0.152
0.142
0.115
0.096
0.957
0.926
0.946
0.947
2.046 × 10–3
2.075 × 10–3
1.839 × 10–3
1.652 × 10–3
–3.933
–3.990
–3.531
–3.168
1996.6
1991
1983
1976
95% confidence Actual eruption
interval
(A.D.)
1992–2005
1981–2008
1973–1996
1967–1994
1995.9
1992
1971
1968
Preceding
eruptions
1
0
1
3
*Total eruption volume, dense-rock equivalent (DRE) basalt (Table 1).
†Linear regression on cumulative volume curve.
Results of this accuracy analysis are presented in
Table 4. This model successfully predicts the 1992
and 1995 eruptions to within 1 yr of their occurrence. Cumulative volume relationships in 1968,
however, indicated that the next eruption should
have occurred in 1983 rather than 1971. The 1971
eruption occurred sooner than expected, outside
the 95% confidence interval on the cumulative
volume curve (Table 4). The 1968 eruption also
occurred somewhat earlier than predicted but still
within the 95% confidence interval of 1967–1994.
Before 1960, there are insufficient eruptions to
constrain a reasonably narrow (i.e., < 30 yr) confidence interval about the regression. Intervening
small-volume eruptions appear to trigger major
eruptions sooner than predicted by the cumulative
volume curve. In the absence of intervening smallvolume eruptions, cumulative volume relationships appear to forecast recent Cerro Negro eruptions accurately. These relationships indicate that
the next large-volume eruption of Cerro Negro is
most likely to occur before A.D. 2006, with the
maximum likelihood being A.D. 2000 ± 1. Cerro
Negro volcano, in addition to other active volcanoes in Nicaragua, is monitored for seismic activity in addition to regular observation by Instituto
Nicaragüense de Estudios Territoriales scientists.
Probability models presented herein support the
need for continued monitoring of Cerro Negro.
An inherent limitation in the volume-predictable model is that characteristics of the next
eruption cannot be predicted, but only can be
constrained by past activity. There is a tendency
for the more dispersive Cerro Negro eruptions
(i.e., A.D. 1947, 1992) to follow a relatively
large-volume eruption and ensuing long hiatus
in activity (McKnight, 1995), although the 1971
eruption did not follow this tendency. A possible
explanation for this relationship is that larger
volume eruptions require more extensive replenishment from deeper, less-degassed magmas
(e.g., Roggensack et al., 1997). The highly dispersive 1971 eruption, however, only followed a
3 yr hiatus in significant activity and would not
have been predicted on the basis of these qualitative relationships. Further research may quantify relationships between melt-inclusion volatile
contents, eruption dispersivity, and patterns in
magma ascent history for use in risk assessments. Current data and models preclude pre-
1240
dicting the ascent pathways (i.e., eruption dispersivity) for new basaltic magma that is likely
replenishing the Cerro Negro system. Thus, a
conservative approach to risk assessment for the
next Cerro Negro eruption is to bound that activity by previous ranges in observed eruption dispersivity, rather than impose nonconservative
process-level constraints.
Basaltic cinder cone volcanoes span a surprisingly wide range of eruptive styles. Typically,
basaltic cinder cone eruptions in oceanic-island
settings produce minor tephra falls that are restricted to within several kilometers of the vent, in
addition to lava flows (e.g., Walker, 1973, 1981).
Relatively small variations in magma flux and
magmatic gas pressure commonly limit the fragmentation and dispersal capabilities of typical
basaltic Strombolian eruptions (e.g., Blackburn
et al., 1976; Wadge, 1981; Head and Wilson,
1989). As a result, basaltic cinder cone volcanoes
in oceanic-island settings commonly present minor tephra-fall hazards beyond 2–5 km from the
vent. Available historical eruptions of arc-related
cinder cones such as Cerro Negro, however, show
that these basaltic volcanoes present tephra-fall
hazards as far as tens of kilometers from the vent.
Tephra dispersion models have had limited application to assessing these hazards. Results of the
current analysis show the modified Suzuki (1983)
model appears to be suitable for modeling tephra
dispersal from “violent strombolian” (Walker,
1973) basaltic volcanoes, which have some important similarities to Plinian-type eruptions. The
results of the risk analysis indicate that only 1 in
10 eruptions, or approximately 1 eruption every
100 yr, from Cerro Negro would result in more
than 6 cm of tephra fall in León. There is an annual probability of 1:200 of an eruption resulting
in more than 11 cm of tephra accumulation in
León. These values are useful for evaluation and
design of buildings and facilities in León.
Detailed eruption forecasting is important for
extremely long-term hazard analysis. A proposed
high-level radioactive waste repository site at
Yucca Mountain, Nevada, is located within 20 km
of five Quaternary basaltic cinder cone volcanoes
(e.g., Crowe et al., 1982). One concern in the
Yucca Mountain risk assessment is the ability of
basaltic cinder cone volcanoes to transport material tens of kilometers into the accessible environ-
ment during the next 10 000 yr. Erosion at these
volcanoes has removed most of the features used
to derive parameters for dispersal modeling. Data
from Cerro Negro tephra falls are being used to
evaluate the reliability of tephra transport models
for use in Yucca Mountain risk assessments (e.g.,
Jarzemba, 1997), in addition to providing examples of the dispersal capabilities of some smallvolume basaltic volcanoes. The unusual duration
of activity and variations in cone morphology at
Cerro Negro (Table 1) also can aid in the interpretation of controversial morphological features at
inactive basaltic cinder cones, including those
found around Yucca Mountain (e.g., Wells et al.,
1990, 1992; Turrin et al., 1991, 1992).
The duration of activity at Cerro Negro is
somewhat long for a basaltic cinder cone, although there only have been about 436 days of
eruptive activity over the past 148 yr (Table 1).
The record of historical arc-related cinder cone
eruptions, however, is sparse (Wood, 1980) and
Cerro Negro may merely represent the upper
range of activity duration for this type of volcano.
Mass-flow rates, not total longevity, govern the
types of hazards associated with a basaltic volcano (e.g., Head and Wilson, 1989). Past eruptive
activity at Cerro Negro has been characterized by
mass-flow rates typical for well-documented cinder cone eruptions, and hazards beyond 3 km
from the vent have consisted solely of tephra falls.
Although it is interesting to speculate that longterm activity may lead to the formation of a large
basaltic composite volcano and associated pyroclastic flow hazards (McKnight and Williams,
1997), Cerro Negro hazards of immediate concern are those typically associated with a violent
Strombolian basaltic cinder cone.
ACKNOWLEDGMENTS
Our understanding of Cerro Negro has been
greatly improved through discussions with
Michael Conway, Benjamin van Wyk de Vries,
Steve Jordi, and Kurt Roggensack. Markus
Kesseler generously provided the data used in the
1995 isopach map. Careful reviews by Michael
Conway, Wes Patrick, John Trapp, William Rose,
and an anonymous reviewer have significantly
improved this manuscript. We also thank Oscar
Cañales, Pedro Perez, Michael Conway, and the
Geological Society of America Bulletin, October 1998
1995 ERUPTIONS OF CERRO NEGRO VOLCANO
Instituto Nicaragüense de Estudios Territoriales
for their assistance with the field studies, Randy
Korotev (Instrumental Neutron Activation
Analyses) and Rex Couture (X-ray fluorescence
analyses) for their geochemical expertise, and
Ron Martin for geographic information systems
assistance. The work reported here was supported by U.S. Nuclear Regulatory Commission
contract NRC-02-97-009. This work is an independent product of the Center for Nuclear Waste
Regulatory Analyses and does not necessarily reflect the views or regulatory position of the Nuclear Regulatory Commission.
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REVISED MANUSCRIPT RECEIVED JANUARY 17, 1998
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