The voluminous Acajutla debris avalanche from Santa

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Geological Society of America Special Paper 375 2004

The voluminous Acajutla debris avalanche from Santa Ana volcano, western El Salvador, and comparison with other Central American edifice-failure events Lee Siebert* Paul Kimberly Smithsonian Institution, Global Volcanism Program, Washington, D.C. 20013-7012, USA Carlos R. Pullinger Servicio Nacional de Estudios Territoriales (SNET), San Salvador, El Salvador

ABSTRACT Collapse of Santa Ana volcano during the late Pleistocene produced the voluminous and extremely mobile Acajutla debris avalanche, which traveled ~50 km south into the Pacific Ocean, forming the broad Acajutla Peninsula. The subaerial deposit covers ~390 km2; inclusion of a possible additional ~150 km2 submarine component gives an estimated volume of 16 ± 5 km3. Hummocks are present to beyond the coastline but are most prominent in four clusters corresponding to the location of buried bedrock ridges. Bulking in distal portions incorporated accessory Tertiary-to-Quaternary volcaniclastic rocks and ignimbrites. Modern Santa Ana volcano was constructed within the collapse scarp, visible only on its northwest side, following an apparent transition in eruptive style. More than 286,000 people, the country’s main port, and important agricultural land now overlie the Acajutla debris-avalanche deposit, which is one of only a few in Central America to exceed 10 km3 in size. Because major edifice failures are high-impact, low-frequency events, the probability of a future Acajutlascale collapse is very low. However, a collapse even an order of magnitude smaller in volume from modern Santa Ana volcano would impact heavily populated areas. The Acajutla failure was perpendicular to a NW-trending fissure system cutting across Santa Ana volcano, which may also influence future failure orientations. The current structure of Santa Ana volcano suggests that future collapses are most likely to the southwest, but the possibility of northward failures cannot be excluded. Keywords: Santa Ana volcano, edifice failure, debris avalanche, volcanic hazards. INTRODUCTION

voluminous volcaniclastic deposit that extends from the Santa Ana volcanic complex in the main volcanic front of western El Salvador to the Pacific Ocean. The avalanche extended the otherwise linear Pacific Ocean shoreline, forming a broad 20 km wide delta, and a significant submarine component extends offshore. The subaerial component of this deposit was for many years mapped both as part of the regional Pliocene Bálsamo Formation and overlying Quaternary alluvial deposits (Weber and Weisemann, 1978). Pullinger (1998) noted the debris-avalanche origin of this deposit during a study of the Santa Ana volcanic

Large-scale volcanic edifice collapse is a common process at volcanoes in a wide variety of tectonic settings. Although steepsided continental-margin stratovolcanoes are particularly susceptible to edifice failure, which has consequently been identified at many volcanoes in México and Central America, little attention has been devoted to this topic in El Salvador. We report here on a *E-mail: [email protected]

Siebert, L., Kimberly, P., and Pullinger, C.R., 2004, The voluminous Acajutla debris avalanche from Santa Ana volcano, western El Salvador, and comparison with other Central American edifice-failure events, in Rose, W.I., Bommer, J.J., López, D.L., Carr, M.J., and Major, J.J., eds., Natural hazards in El Salvador: Boulder, Colorado, Geological Society of America Special Paper 375, p. 5–23. For permission to copy, contact [email protected]. © 2004 Geological Society of America

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L. Siebert, P. Kimberly, and C.R. Pullinger

complex. More detailed mapping during the present study was undertaken to further delineate the extent of the deposit and its characteristics, to determine the impact of the edifice-failure event on the eruptive style and subsequent constructional history of the Santa Ana volcanic complex, and to assess the volcanic hazard implications of this voluminous edifice failure, one of the largest in Central America. Seismically induced small-volume slope failures have caused extensive devastation in El Salvador (see papers in this volume by Evans and Bent, Chapter 3; Jibson et al., Chapter 6; and Konagai et al., Chapter 4); this study underscores the potential hazards in El Salvador from landslides several orders of magnitude larger in volume. REGIONAL GEOLOGY El Salvador occupies the west-central portion of the Chortis block of the Caribbean plate. A roughly E-W–trending chain of volcanoes extends from the México/Guatemala border across El Salvador to the Gulf of Fonseca, ~150 km inboard of the Middle American Trench (Fig. 1). Crustal thickness drops from ~50 km at either end to ~32–37 km over a broad portion of the center of the Central American arc. Sinistral movement of the plate-bounding Motagua-Polochic fault system has caused extensional faulting in southeastern Guatemala and western El Salvador (Burkhart and Self, 1985), and in contrast to the extremities of the arc, volcanoes in El Salvador and Nicaragua are not constructed over topographic highs, but rise in isolation above broad down-dropped crustal blocks. Santa Ana volcano lies adjacent to a plate segment boundary proposed by Stoiber and Carr (1973), although later work (Burbach et al., 1984) questioned micro-segmentation of the descending Cocos plate. The Santa Ana volcanic complex in western El Salvador is situated at the southern margin of the Median Trough that transects the country and lies between San Salvador volcano and the E-W–trending cluster of small stratovolcanoes and underlying silicic calderas known as the Sierra de Apaneca or Ahuachapán Range (Fig. 1). The 165 km3 (Carr, 1984) Santa Ana volcanic complex (Fig. 2) forms the largest and highest stratovolcano in El Salvador. The broad flanks of the 2381 m high basaltic-to-andesitic volcano extend north to the city of Santa Ana and south to Sonsonate and are home to more than a million inhabitants, 15% of the population of El Salvador (Pullinger, 1998). Basement rocks underlying the Santa Ana complex (Fig. 3) are older than ~200,000 yr (Pullinger, 1998) and exposed southward toward the coast. They consist of the Miocene-Pliocene Bálsamo Formation and the basal part of the Quaternary San Salvador Formation (Weber and Weisemann, 1978; Baxter, 1984). Three broad benches and a series of nested craters, the youngest of which contains an acidic lake, form the 1.5 km wide, flat-topped summit of the volcano. The lake and surrounding crater walls are fumarolically active, and the crater has been the source of phreatomagmatic eruptions in historical time, most recently in 1920. A 30 km long, NW-SE–trending fissure system that transects the

volcano from its summit to lower flanks (Fig. 1) has been active since the late Pleistocene and is the locus of vents ranging from phreatomagmatic craters on the NW end to historically active cinder cones on the SE (Pullinger, 1998). The dramatic unvegetated cone of Izalco volcano was constructed since 1770 on the southern flank of Santa Ana (Carr and Pontier, 1981). Frequent strombolian eruptions visible from the coast caused it to be called the “Lighthouse of the Pacific.” Coatepeque caldera partially truncates the eastern side of ancestral Santa Ana volcano. The 7 × 10 km wide caldera, now largely filled by a scenic lake, was formed during two major late-Pleistocene rhyodacitic pyroclastic-flow-producing eruptions. The Arce pyroclastic-flow deposit, distinguishable by its large biotite content, has been dated using the high-precision 40Ar/39Ar method at 72,000 ± 2000 yr B.P. (Rose et al., 1999). The Congo eruption was dated using a high-sensitivity 14C procedure at 56,900 +2800 –2100 yr B.P. (Rose et al., 1999). Post-caldera basaltic cinder cones and lava flows were erupted along the caldera rim, and a half dozen rhyodacitic lava domes were emplaced along a NESW line on the caldera floor near the lake margins. ACAJUTLA DEBRIS–AVALANCHE DEPOSIT Distribution and Morphology The Acajutla debris-avalanche deposit (Fig. 4) is broadly exposed SSW of Santa Ana volcano to the coast, where it forms the Acajutla Peninsula. The deposit is named after the peninsula and the overlying coastal city of Acajutla. In proximal areas, the deposit is overlain by lava flows and tephra deposits from modern Santa Ana and Izalco volcanoes that extend as far as 18 km from Santa Ana volcano, but exposures of the avalanche deposit have been found to within less than 10 km of the current summit of Santa Ana. On the east, the deposit is bounded by the rugged hills of the Bálsamo Range, and on the west it laps onto erosionally dissected pyroclastic-flow deposits of the basal San Salvador Formation from the Sierra de Apaneca. The indistinct margins of the deposit on the NE and western sides are approximately inferred from scattered outcrops, deflection of drainages, and topographic smoothing of dissected valleys by infill of the deposit. The medial portion of the deposit south of the mantling lava flows from Santa Ana volcano contains abundant hummocks prominent on topographic maps. Hummocks in this area, ~25–30 km from the source, reach to ~60 m high and are typically ~100–300 m in longest dimension. Some apparent hummocks are instead kipukas of Bálsamo Formation rocks in line with NE-SW–trending Bálsamo ridges to the east (Fig. 5). Hummock density is highest between ~15 and 30 km from the volcano, but hummocks are found to the current shoreline and offshore. The NE boundary of the avalanche deposit in the flat-lying area north of the Bálsamo Range is uncertain because the deposit is largely obscured by overlying younger lava flows. The avalanche deposit was exposed in a now-filled utility trench under construction in 2002 about one km southeast of the village of


The voluminous Acajutla debris avalanche from Santa Ana volcano

NORTH

7

AMERICAN PLATE

BELIZE

MEXICO

GUATEMALA Polochic Fault

HONDURAS

CARIBBEAN PLATE

HONDURAS EL SALVADOR

Pacific Ocean

N

Metapán

Gulf of Fonseca

L. de Guija COCOS PLATE 0

150 km

Texistepeque CA

-12

GUATEMALA Chalchuapa

14°

Santa Ana

NW vents

Sierra de Apaneca Ahuachapán

N LV Cu LR

CA

Ap

LN

Ag

Coatepeque MC SA IZ

La Hachadura

Quezaltepeque

CA-8

CA -2

Sonsonate

Nueva San Salvador (Santa Tecla)

San Salvador

CA-2

Pacific Ocean

90°

SS

Armenia

Acajutla debrisavalanche deposit

Acajutla

Apopa

SE vents

Caluco

La Libertad 0

10

20 km

Figure 1. Location map. Triangles denote Quaternary volcanoes: CA—Concepción de Ataco caldera, Ap—Apaneca, N— La Ninfas, LV—Laguna Verde, Cu—Cuyanausol, LR—Las Ranas, Ag—El Aguila, LN—Los Naranjos, MC—Malacara, SA—Santa Ana, IZ—Izalco, SS—San Salvador. Inset map of northern Central America shows tectonic setting; teeth are on upthrust plate; open triangles represent volcanoes. Light-gray shaded area marks approximate subaerial extent of Acajutla debris-avalanche deposit. After Pullinger (1998).


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L. Siebert, P. Kimberly, and C.R. Pullinger LS ER

MN

MC

PH

Sierra de Apaneca

Coatepeque caldera

Santa Ana CV

Izalco

EA EC LO

SM CC

N Santa Ana complex lava flows

Figure 2. Shaded-relief digital elevation model view of the Santa Ana massif. Vents of the NW-trending fissure zone cutting across the volcano and extending ~7 km north of the top of the image are labeled from north to south: LS—Laguna Seca–El Hoyo, ER—El Retiro, MN—Montañita, MC—Malacara, PH—Plan de Hoyo, CV—Cerro Verde, EC—El Conejal, EA—El Astillero, LO—La Olla, CC—Cerro Chino, SM—San Marcelino.

5 km

Figure 3. Generalized stratigraphic column of the Santa Ana area, modified from Weber and Weisemann (1978) and Baxter (1984). K-Ar dates are from Rose et al. (1999).

Caluco (Fig. 4); this is near the inferred eastern margin of the deposit. Two exposures of the Arce pyroclastic-flow deposit from Coatepeque volcano dated at 72,000 yr B.P. (Rose et al., 1999) are present nearby at the Hacienda Comalapa 2.5 km south of Caluco and along the Río Chiquihuat south of the Hacienda Victoria 3 km SE of Caluco, but Arce deposits are not exposed farther west and are considered to underlie the avalanche deposit. Small conical hills resembling debris-avalanche hummocks are exposed ~10 km east of the inferred deposit margin ~3 km north of the Sonsonate–San Salvador highway (CA-8 on Fig. 1) on both sides of the road to Coatepeque caldera. Excavations have

shown that these hills, however, are Mayan archaeological sites (Sheets, 1983). Farther south, the eastern margin of the Acajutla deposit is buried by flat-lying sediments from Bálsamo Range river valleys that were likely initially the site of ephemeral avalanche-dammed lakes. Although these tributary valleys intersect the deposit at an angle oblique to the avalanche travel direction, the buried avalanche deposit is arbitrarily inferred to extend halfway up the flat-lying portion of these valleys due to lateral spreading of the avalanche by analogy with the 1980 Mount St. Helens debris avalanche, which extended at least 3.5 km up Coldwater Creek


The voluminous Acajutla debris avalanche from Santa Ana volcano

9

N

Santa Ana Sierra de Apaneca

Coatepeque caldera

LN 65

Izalco 49

NA

76

IZ

CA

Sonsonate

San Salvador Fm. Ignimbrites

RC 4

LF

HV

HC

Ch

Gr

an

Rí

de

o

de

ReC

iq

So

ui

ns

hu

at

on

at

e

Bálsamo Range

o

10

Rí

RSP

21 20

Acajutla

16 s dera Ban Río

RM

15 PC

Punta Remedios

5 km

Figure 4. Shaded-relief digital elevation model of western El Salvador showing extent of Acajutla debris-avalanche deposit. Dashed and solid lines mark inferred extent of Acajutla debris-avalanche deposit, hachured line marks exposed margin of avalanche source area, dots indicate inferred buried margin. Dashed-dot line lies on crest of arcuate escarpment south of Izalco volcano. CA—Caluco, HC—Hacienda Comalapa, HV—Hacienda Victoria, IZ—town of Izalco, LF—maximum extent of lava flows from modern Santa Ana volcano, LN—Los Naranjos, NA—Nahuizalco, PC—Playa Los Cóbonos, RC—Río Chiquihuat, ReC—Río el Coyol, RM—Río Mandinga, RSP—Río San Pedro. Numbers mark locations of samples used for sedimentological or geochemical analyses.


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Kipuka

Hummocks

in a direction 140° oblique to that of the avalanche travel path (Glicken, 1986). The southeasternmost margin of the deposit is considered to lie near the Río Mandinga, whose southwestward trajectory is deflected to the SE after the river extends beyond the confining Bálsamo ridges. On the western side, the Acajutla avalanche deposit underlies the city of Sonsonate and is exposed in quarries on the western side of the city, where it directly overlies ignimbrite deposits containing abundant black, glassy blocks. Farther south, the avalanche deposit feathers out just west of the Río San Pedro (Fig. 4). North of here the avalanche deposit laps onto ignimbrite deposits forming steep-walled gorges along the Río el Coyol, ~3 km north of Highway 2 (Fig. 1), and SW-trending topography here oblique to the roughly N-S–trending ridges below the Sierra de Apaneca is considered to be underlain by the avalanche deposit. The Río Grande de Sonsonate (also known as the Río Sensunapán) occupies the topographic inflection point below where the western margin of the avalanche lapped onto the ignimbrite ridges.

Figure 5. Coarse blocks litter the hummock surface in the foreground, and a hummock cluster can be seen to the right of the larger hill at the left, a kipuka of Bálsamo Formation material that served as a barrier to avalanche transport. Santa Ana volcano (center skyline) lies 30 km away.

whose western side has been eroded by longshore currents forming vertical cliffs ~15 m high at the port of Acajutla (Fig. 6). Semi-indurated, normally graded beach deposits lap onto the avalanche deposit at the tip of the peninsula at Punta Remedios (Fig. 4), and post-avalanche deposition has smoothed the shoreline to the east. A 1:25,000-scale bathymetric map prepared to identify shipping channels for Acajutla, the country’s largest port, delineates submarine topography of the western part of the deposit. The map extends eastward to cover an area beyond the southernmost subaerial extent of the deposit at Punta Remedios (Fig. 4).

Submarine Extent The Acajutla Peninsula forms the largest topographic irregularity along a 900 km long stretch of the Pacific coast between the Gulf of Tehuantepec off the coast of Oaxaca, México, and the Gulf of Fonseca at the southeastern tip of El Salvador. The ~20 km wide peninsula extends up to 7 km off the former coastline. Vigorous longshore drift prevents the formation of deltas off the Salvadoran coast; thus, a large delta is not likely to have existed prior to the collapse, and the pre-collapse shoreline would likely have paralleled the current coastline. The tip of the asymmetrical peninsula lies west of the axis of the avalanche deposit,

Figure 6. Irregular coastal cliffs at the city of Acajutla expose the debris-avalanche deposit and contrast with the low, linear, beach-fringed shoreline in the distance west of the deposit.


The voluminous Acajutla debris avalanche from Santa Ana volcano Small rocky shoals and closed submarine contours indicate that hummocky topography continues offshore. The abundant map notations of “muchas rocas” (“many rocks”) and “no sonedos” (“no soundings”) distinguish the rocky avalanche deposit from offshore areas on either side. Irregular terrain with hummocks ranging to ~5 m in height is present to ~5 km offshore at the southernmost extent of detailed bathymetry. The submarine extent of the avalanche deposit is inferred from the inflection point of otherwise shore-parallel bathymetric contours. The 35 m contour deflects, while the 40 m contour does not, suggesting that the avalanche deposit extends to a depth of just over 35 m ~5 km SW of the tip of the Acajutla Peninsula. Although comparable bathymetry is not available to the east, a 1:250,000 map shows 6-, 10- and 20-fathom contours. A pronounced deflection of the 20-fathom (37 m) submarine contour south of the mouth of Río Mandinga (Fig. 4) may correspond to the eastern deposit margin. If this inference is correct, the submarine component of the Acajutla debris-avalanche deposit extends up to 11 km from the eastern shoreline, and the currently submarine component of the deposit would be nearly 200 km2. The lack of detailed bathymetry leaves remaining uncertainty regarding the eastern submarine margin of the avalanche. Although detailed bathymetric contours on the western side suggest that the deposit extends to ~35 m depth at its terminus WSW of Punta Remedios, it is possible that the deflected 20-fathom bathymetric contour is underlain by beach-sand deposits banked up against the avalanche deposit margin by longshore currents, and thus the avalanche may not have reached this depth along its full terminus. Using a more conservative extrapolated 15-fathom contour to reflect the deposit margin results in an estimated currently submarine area of ~150 km2. This implies that ~215 km2 of the avalanche deposit lies beyond the former shoreline and that the terminus of the deposit lies a minimum of 46 and a maximum of 50 km from the volcano. Impact of an avalanche of this magnitude into the Pacific would almost certainly have produced a tsunami, although tsunami deposits this old would likely not be preserved. The largest slope-failure-related tsunamis have occurred when the basal failure plane extended below sea level and the edifice failure itself had a submarine component, such as at Ritter Island in Melanesia in 1888 (Johnson, 1987) and Oshima-Oshima Island off Hokkaido in 1742 (Satake and Kato, 2001). This was not the case at Santa Ana; however, significant tsunamis elsewhere have resulted from the sudden impact of debris avalanches into the sea (Siebert et al., 1987). Textural Characteristics Despite the massive size of the Acajutla debris-avalanche deposit, few natural cross-sectional exposures are available. Scattered aggregate quarries provided ephemeral exposures critical to determining the textural characteristics of the deposit (Fig. 7). Block-facies material (Fig. 7A) is abundant in avalanche hummocks, where quarries expose spectacular sections of color-mottled units of the former edifice transported

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relatively intact from their source on the volcano. The bestexposed outcrop was at Cerro Jícaro, ~20 km from the volcano (sample #4 on Figure 4), where quarrying operations revealed a colorful assemblage of slightly sheared and faulted units from Santa Ana volcano. Individual clasts include andesitic, basaltic andesite, and olivine basaltic lithologies (Pullinger, 1998). The juxtaposition of multicolored units highlighted evidence of normal faulting and horst-and-graben formation on the scale of up to a few meters offset. Large clasts up to 6 m in diameter were exposed, and jigsaw blocks were present. Some individual units could be traced several tens of meters across the quarry wall. A quarry in another hummock south of Highway 2 (sample #21, Fig. 4) ~30 km from the volcano exposed large segments of essentially intact bedded ash-and-lapilli layers transported without significant disruption (Fig. 7B). Tephra layers up to several tens of cm thick were offset only slightly along normal faults and appeared almost as pristine as an outcrop on the flanks of the volcano; smaller segments of tephra packages were found on the coast. A wide variety of relatively undisturbed segments of the Santa Ana edifice were observed in this hummock, including interbedded tephra layers, massive and brecciated lava flows, and a 20 × 30 m segment of a basaltic (52.4% SiO2) block-and-ashflow deposit with abundant juvenile clasts. Mixed-facies or transitional material in intrahummock areas, within which block-facies material was transported, constitutes the volumetrically dominant component of the deposit. This material consists of a homogenized heterolithologic matrix containing clasts from Santa Ana volcano mixed at the margins of the deposit with significant amounts of accessory material from the Bálsamo Formation on the east and Quaternary ignimbrites on the west. Ripped-up soil clasts are also present (Fig. 7C). Mixed-facies material is exposed only in stream banks of rivers superposed on the surface of the avalanche deposit. Fieldwork in 1998 benefited from heavy rains associated with Hurricane Mitch that scoured riverbeds and produced unusually pristine exposures of mixed-facies material not readily apparent during a later visit. This revealed large segments of block-facies material transported within the mixed facies that could easily be overlooked when streambeds and banks are more vegetated and obscured. Clastic dikes (Fig. 7D) of mixed-facies material intruding into megaclasts of block-facies material are common. These clastic dikes are planar to irregular in form and can be traced for many meters into block-facies material several tens of meters in dimension. Blockfacies material transported within the mixed facies ranged up to several tens of meters in size in medial portions of the avalanche deposit along the Río Chiquihuat ESE of Sonsonate. A wave-cut terrace beyond the farthest subaerial extent of the avalanche displays an extensive lag deposit of course boulders up to several m in size from which fine-grained matrix has been removed. Sedimentological Characteristics Preliminary study of the sedimentological properties of the Acajutla debris-avalanche deposit shows similarities with those of other debris-avalanche deposits. Size analyses of lahar and


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Figure 7. Textural characteristics. A: Block facies in quarry 30 km from Santa Ana, showing fault-bounded contact between lava-flow segments (right) and tephra layers (left). Large block at lower right is ~1 m in diameter. B: Bedded lapilli-and-ash layers offset along small normal faults at same quarry, scale bar is 2 m long. C: Light-colored soil clast (upper right) and small block-facies clast (left) transported within mixed facies at Río Banderas, 32 km from the volcano; trowel is 26 cm long. D: Light-colored clastic dikes intrude block-facies megaclasts at same location as C. Hammer (see arrow) is 31 cm long.

debris-avalanche deposits suffer from underrepresentation of clasts at the coarse end of the size spectrum. The coarse fraction is sometimes incorporated by field point counting (Major and Voight, 1986) or photographic analysis (Glicken, 1986, 1996; Siebert et al., 1989) of coarser clasts, but even these techniques underrepresent very coarse clasts. We analyzed data from lahars at Mount St. Helens (Major and Voight, 1986), where both techniques were used for samples from the same deposits, to investigate the degree of variability of the two procedures. Calculation of the average median size (Mdø) and sorting (σø) parameters for 20 samples using the sieving technique only and for 13 samples supplemented by coarse-clast data show that the two procedures had more of an effect on median grain size than sorting, but that both parameters showed relatively small effects. The average of Mdø and σø for each technique varied by only 0.10 ø. The coarseend shift that would be expected using the point-count analysis increased to 0.48 ø when three outlier samples used for the sieving-only procedure were discarded because they were signifi-

cantly coarser than any of the samples used for coarse-tail point counting; however, the median sorting differential in this case decreased to only 0.03. Although these data are not necessarily representative for lahar and debris-avalanche deposits worldwide, they provide perspective for comparison of data using the two techniques. Size-fraction histograms (Fig. 8) of block-facies material from the Acajutla deposit (Table 1) do not show as pronounced a bimodal distribution as those from Mount St. Helens (Glicken, 1986, 1996) and Augustine (Siebert et al., 1989), although they lie within the range of Mount St. Helens and other Cascade Range samples (Siebert et al., 1989). This reflects in part the broadening of the histogram distribution using the sieving-only procedure (Major and Voight, 1986). Preliminary data suggest that mixed- or lahar-facies material displays significantly higher silt and clay contents. Sorting and size parameters (Inman, 1952) of the Acajutla deposit are compared with those of other avalanche deposits around the world and with pyroclastic-flow and


The voluminous Acajutla debris avalanche from Santa Ana volcano 25

25 20

Weight (%)

15 10 5

10 5

0

0 -6

-4

-2

0

2

Grain Size (φ)

4

6

-6

25

-4

-2

0

2

Grain Size (φ)

4

6

25 AC-4b

AC-10

20

15 10 5

15

7 10

FØ)

20

Weight (%)

Weight (%)

15

5

0

0 -6

-4

-2

0

2

Grain Size (φ)

4

6

Cummulative Weight (%)

25 AC-15

20 15 10 5 0 -6

-4

-2

0

2

Grain Size (φ)

-6

4

6

-4

-2

0

2

Grain Size (φ)

4

6

99 98 95 90 80 70 60 50 40 30 20 10 5 2 1

Sorting Coefficient (

Weight (%)

AC-4a

AC-49

20

Weight (%)

13

Debris-Avalanche & Lahar Deposits

6

Santa Ana Augustine St. Helens Cascades Japan Indonesia Lahars Lahars-MSH

5 4 3 2 1 0 -5

-4

-3

-2

-1

0

1

2

3

4

5

6

Median Grain Size (MdØ) -6

-4

-2

0

2

Grain Size (φ)

4

Figure 8. Grain-size plots of debris-avalanche samples; bottom axis is size fraction in ø units. Histogram bar to right of 4ø includes silt- and clay-sized fractions. Distance from source: AC-49, 10 km; AC-4a and 4b, 19 km; AC-10, 33 km; AC-15, 42 km. Normal probability plot (lower right) compares Acajutla samples (solid lines) with range of Mount St. Helens (1980) samples (dashed lines) from Glicken (1986).

pyroclastic-fall fields established by Walker (1971). The Acajutla samples are coarser and more poorly sorted than pyroclastic-fall deposits and lie at the coarse end or outside of the field of pyroclastic-flow deposits (Fig. 9). As with other debris-avalanche deposits, there is considerable overlap with the size and sorting characteristics of lahar deposits. Source of the Acajutla Debris Avalanche The geometry of the Acajutla debris-avalanche deposit allows three possible source areas—the Sierra de Apaneca, Santa Ana volcano, and Coatepeque caldera. The small volume of individual volcanoes forming the Sierra de Apaneca massif precludes the Apaneca Range as a source of the massive Acajutla debris avalanche and requires an origin from the Santa Ana–Coatepeque complex.

Figure 9. Size parameters of debris-avalanche deposits (solid symbols) compared with those of lahars (open symbols): Santa Ana data (this study, gray large diamond symbols); Augustine (Siebert et al., 1989); Mount St. Helens (Glicken, 1986, 1996); Cascade Range and Indonesia (Siebert et al., 1989); Japan (Murai, 1961); lahar data (Walker, 1971); Mount St. Helens southwest flank lahars (Major and Voight, 1986). Augustine and Mount St. Helens avalanche data corrected for coarse fraction; other data by sieving only. Solid and dashed lines outline, respectively, the pyroclastic-flow and plinian fall fields of Walker (1971).

Debris-avalanche clasts, including eight from one hummock at Cerro Jícaro (sample number 4 on Fig. 4) were analyzed chemically (Table 2) and compared with analyses of rocks from Santa Ana and Coatepeque volcanoes. The most common rock type was basaltic andesite, but basaltic, andesitic, and dacitic rocks were also present (Fig. 10), all with high TiO2 values. Both Santa Ana and debris-avalanche deposit samples show relatively high TiO2 values dissimilar to most rocks from the adjacent, more silicic Coatepeque caldera (Fig. 11), and the chemistry of Acajutla avalanche deposit samples is consistent with a source from Santa Ana volcano (Pullinger, 1998). Although modern Santa Ana volcano has buried most of the avalanche scarp, part of the failure scarp is exposed on the NW side (Fig. 12). An arcuate ridge displays a steep south-fac-


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L. Siebert, P. Kimberly, and C.R. Pullinger TABLE 2. MAJOR AND TRACE-ELEMENT GEOCHEMISTRY

Sample # SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Sum

AC-1

AC-2

AC-3

AC-4

AC-5

AC-6

AC-7

AC-8

AC-15

AC-20

AC-21

AC-65

AC-76

54.09 1.15 19.54 n.a. 8.53 0.19 3.33 8.27 4.08 1.17 0.30 n.a. 100.65

51.80 1.14 19.99 n.a. 9.61 0.19 3.95 8.86 3.61 1.03 0.26 n.a. 100.44

65.45 0.76 16.71 n.a. 5.63 0.12 1.52 3.80 3.66 3.81 0.32 n.a. 101.78

52.86 1.17 18.30 n.a. 10.61 0.20 4.37 8.49 3.29 1.83 0.37 n.a. 101.49

53.76 1.17 17.68 n.a. 10.46 0.18 4.11 8.15 3.38 1.87 0.34 n.a. 101.10

65.35 0.74 16.07 n.a. 5.59 0.12 1.55 3.83 4.02 3.81 0.33 n.a. 101.41

54.29 0.92 20.99 n.a. 8.51 0.17 2.66 8.98 3.62 0.84 0.27 n.a. 101.25

53.82 0.89 20.55 n.a. 8.23 0.18 2.8 8.92 3.79 1.01 0.26 n.a. 100.45

52.80 1.03 19.40 5.83 3.42 0.20 2.87 8.21 4.09 1.39 0.29 0.32 99.85

53.05 0.94 19.79 3.48 5.23 0.18 3.03 8.54 3.96 1.20 0.28 –0.08 99.6

51.33 1.03 20.23 2.35 7.03 0.19 3.33 9.56 3.55 1.13 0.26 –0.18 99.82

52.43 0.95 18.89 3.39 6.29 0.18 3.99 8.47 3.58 1.45 0.29 –0.27 99.64

60.36 0.78 16.23 2.57 3.68 0.15 1.22 3.22 3.18 3.00 0.32 4.81 99.52

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.






147 1] –

Volume 3 (km )

#

3

Alvarado and Vega, 2002 Alvarado and Vega, 2002 Alvarado and Vega, 2002 Alvarado and Vega, 2002 Alvarado and Vega, 2002 Alvarado and Carr, 1993 Alvarado and Carr, 1993 Alvarado and Vega, 2002 Alvarado and Vega, 2002 Alvarado and Vega, 2002 Soto, 1988; Alvarado and Vega, 2002 Alvarado and Vega, 2002

Van Wyk de Vries and Borgia, 1996 Ui, 1972; Van Wyk de Vries and Francis, 1997; this study Ui, 1972; Van Wyk de Vries and Francis, 1997 Van Wyk de Vries and Francis, 1997

Pullinger, 1998; this study This study Major et al., 2004; this study Pullinger, 2002, pers. commun.

Mercado and Rose, 1992; this study Vallance et al., 1995 Vallance et al., 1995 Conway et al., 1992 Vallance et al., 1995; Siebert et al., 1994; Basset, 1996 Vallance et al., 1995; Siebert et al., 1994 Vallance et al., 1995; Siebert et al., 1994 Duffield et al., 1989 Reynolds, 1987; Siebert et al., 1994

References

PANAMÁ (>10) Cerro Colorado Río Chiriquí Viejo South – – – IRHE, 1987; this study (25) Barú Hato del Volcán WSW – – – IRHE, 1987; this study – Barú ? – SW? – – – This study *Volcano names followed by a “?” represent edifice-collapse events inferred from source area or deposit morphology on maps or aerial photographs, but without field identification of deposits. † Length is distance from summit or headwall scarp to terminus of debris-avalanche deposit. § H/L is ratio between vertical drop and travel length; in some cases may include lahar facies of edifice-collapse event. # Volume data in parentheses are source area volumes; figures in square brackets are order-of-magnitude volume assessments.

COSTA RICA Cerro Cacao Rincón de la Vieja Miravalles Miravalles Miravalles Tenorio Platanar Barva? Irazú Irazú Turrialba Turrialba

El Chonco Las Isletas South Crater –

Acajutla Modern Santa Ana Tecoluca Lolotique

EL SALVADOR Santa Ana Santa Ana? San Vicente Unknown source

NNE South South NW SSW SSE SSW SE SE

Direction

§

20

NICARAGUA San Cristóbal Mombacho Mombacho Mombacho

Río Las Majadas – – Cerro Quemado La Democracia Escuintla Río Metapa Miraflores Los Achiotes

Avalanche

GUATEMALA Tacaná? Tajumulco? Siete Orejas? Almolonga Acatenango Fuego Pacaya Tecuamburro Ixtahuan?

Volcano*

†

TABLE 3. CENTRAL AMERICAN DEBRIS AVALANCHES (>0.1 km )




The voluminous Acajutla debris avalanche from Santa Ana volcano Hato del Volcán debris-avalanche deposit, which extends beyond the Pan-American Highway. Restoration of inferred pre-failure contours suggests that as much as 30 km3 may have collapsed (Siebert, 2002b). About 5 km3 of this volume remained within the horseshoe-shaped caldera in the form of large toreva blocks, distinct from the post-collapse lava dome complex that later grew to a height above the caldera rim. The debris-avalanche deposit from Barú volcano overlaps another avalanche deposit from Pleistocene Cerro Colorado volcano and may have formed during late-Holocene time. The basal sediment layer cored from a lake characterized by Behling (2000) as a volcanic crater (but which is more likely a pond formed within a closed depression on the surface of the avalanche deposit) was dated at 2860 ± 50 yr B.P. and may provide a limiting age for the collapse. Cerro Colorado, located immediately NW of Barú, has a 7 km wide horseshoe-shaped caldera (now partially filled by post-collapse lava domes) whose area is comparable to that of the Barú caldera and whose original volume thus also likely exceeded 10 km3. Examination of Quaternary edifice failures in Central America the size of the Acajutla deposit suggests that although such catastrophic collapses are not unique, their probability is very low. Failures one to two orders of magnitude smaller in volume, although still of low frequency, are more common (Table 3) and would be sufficiently mobile to impact heavily populated areas. Hazards associated with Central American slope failures also include associated eruptions and tsunamis. In addition to the possible collapse-related explosive eruption at Santa Ana, the collapse of Cerro Quemado volcano in Guatemala triggered a lateral blast (Conway et al., 1992) an order of magnitude smaller than at Mount St. Helens in 1980. Pyroclastic surges accompanying collapse of Pacaya volcano in Guatemala reached as far as 10 km (Vallance et al., 1995). The Acajutla avalanche is the only Pacific coastal plain avalanche in Central America known to have reached the sea and likely to have produced a tsunami. Lacustrine tsunamis, however, would probably have occurred during collapses of Mombacho volcano into Lake Nicaragua and could also occur at Lake Coatepeque during a northward collapse of Santa Ana. Regional tectonic factors have influenced failure directions of volcanoes on both margins of the Caribbean plate. In contrast to West Indies arc slope failures, which occur largely in a direction opposite that of the ocean-floor trench, most of the Central American slope failures occur toward the trench side of the arc (Siebert, 2002b). These preferred failure directions have been attributed to the construction of volcanoes on regional basement slopes. In the West Indies, this reflects the higher slope of islands toward the deep backarc Grenada Basin (Boudon et al., 2002). In Guatemala, this has been attributed to the construction of volcanoes over inclined basements at the trenchward margin of the arc (Vallance et al., 1995). A similar topographic effect is seen in the Cofre de Perote–Citlaltépetl Range in the eastern Mexican Volcanic Belt. All slope failures in this massif, constructed above the margin of the Altiplano where it drops dramatically toward the coastal plain, have occurred to the east (Carrasco-Núñez et

21

al., 2002). In contrast, the central and eastern sides of the Chortis block of the Caribbean plate in El Salvador and Nicaragua are characterized by extensional grabens, where the basement topographic effect is less pronounced, and collapses here have had more variable orientations. Future collapses at Santa Ana to the north, although perhaps less likely, thus should not be ruled out. CONCLUSIONS A voluminous volcaniclastic deposit extending from the volcanic front in western El Salvador to the Pacific Ocean was formed during the largest slope-failure event known in El Salvador. The geometry of the debris-avalanche deposit allows three possible source areas, but the geochemical affinity of rocks within the debris-avalanche deposit with those from Santa Ana volcano indicates its origin. The 16 ± 5 km3 avalanche extended the former shoreline by 7 km, forming the Acajutla Peninsula. The subaerial portion of the deposit covers an area of ~390 km2, and bathymetry implies an additional submarine component of 150 km2. The collapse is younger than the ~57,000 yr B.P. deposits associated with the formation of Coatepeque caldera. Avalanche hummocks are most abundant in the medial portion of the deposit but are found to the coast and offshore. Significant bulking of the deposit during emplacement incorporated material from the Bálsamo Formation on the east and from Tertiary mafic ignimbrites on the west. Stratigraphy in the collapse scarp wall suggests that the massive edifice collapse marked a transition in the eruptive style at Santa Ana. The collapse occurred perpendicular to a NW-SE–trending fissure system cutting across Santa Ana, and extension and elevated pore pressures related to dike intrusion may have contributed to the collapse. In contrast to volcanoes at the western and eastern ends of Central America that were constructed over inclined basement substrates and failed preferentially in the direction of regional slope, Santa Ana is a free-standing edifice overlying the Median Trough of El Salvador, and future failure to the north, although perhaps less likely, is also possible. ACKNOWLEDGMENTS We thank Bill Rose for his long-term promotion of collaborative research projects in El Salvador and his support of field sessions for this project in 1998 and 2002 through his National Science Foundation grant. Jim Vallance initially identified the debris-avalanche origin of the Acajutla deposit in the field and provided a tentative soil-thickness age for the deposit. Tim Rose and Tim Gooding of the Smithsonian Institution provided help with sample preparation for geochemical analyses, and Heather Njo performed XRF analyses. Tom Jorstad and Bill Boykins of the Smithsonian’s sedimentology laboratory provided guidance for size analyses. Steve Schilling of the U.S. Geological Survey developed the DEM files used in several of the figures. Jon Major, Nancy Riggs, and Jim Vallance provided valuable comments in review.


22


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Geological Society of America Special Papers The voluminous Acajutla debris avalanche from Santa Ana volcano, western El Salvador, and comparison with other Central American edifice-failure events Lee Siebert, Paul Kimberly and Carlos R. Pullinger Geological Society of America Special Papers 2004;375; 5-24 doi:10.1130/0-8137-2375-2.5

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