Rincón de la Vieja volcano, Guanacaste province, Costa Rica: geology of the southwestern flank and hazards implications

ELSEVIER

Journal of Volcanology

and Geothermal

Research 71 (1996) 109-127

Rinc6n de la Vieja volcano, Guanacaste province, Costa Rica: geology of the southwestern flank and hazards implications Kirt A. Kempter aY *, Shawn G. Benner a, Stanley N. Williams b~l a Department of Geological Sciences, University of Texas, Austin, TX, USA b Department of Geology, Arizona State University, Tempe, AZ, USA Received

14 April 1995; accepted 6 October 1995

Abstract Acid rain and ongoing eruptive activity at Rincdn de la Vieja volcano in northwestern Costa Rica have created a triangular, deeply eroded “dead zone” west-southwest of the Active Crater. The barren, steep-walled canyons in this area expose one of the best internal stratigraphic profiles of any active or dormant volcano in Costa Rica. Geologic mapping along the southwestern flank of the volcano reveals over 300 m of prehistoric volcanic stratigraphy, dominated by tephra deposits and two-pyroxene andesite lavas. Dense tropical forests and poor access preclude mapping elsewhere on the volcano. In the “dead zone” four stratigraphic groups are distinguished by their relative proportions of lava and tephra. In general, early volcanism was dominated by voluminous lava emissions, with explosive plinian eruptions becoming increasingly more dominant with time. Numerous phreatic eruptions have occurred in historic times, all emanating from the Active Crater. The stratigraphic sequence is capped by the Rio Blanc0 tephra deposit, erupted at approximately 3500 yr B.P. Approximately 0.25 km3 (0.1 km3 DRE) of tephra was deposited in a highly asymmetrical dispersal pattern west-southwest of the source vent, indicating strong prevailing winds from the east and east-northeast at the time of the eruption. Grain-size studies of the deposit reveal that the eruption was subplinian, attaining an estimated column height of 16 km. A qualitative hazards assessment at Rincdn de la Vieja indicates that future eruptions are likely to be explosive in style, with the zone of greatest hazard extending several kilometers north from the Active Crater. 1. Introduction Rinccin de la Vieja, a composite stratovolcano in northwestern Costa Rica, forms a NW-trending ridge consisting of several eruptive centers that have coalesced through time (Fig. 1). The volume of the massif is estimated as 130 km3 (Cur, 1984), with

* Corresponding author. E-m ail address: [email protected]. Fax: (512) 471-9425. Rinc6n de la Vieja web site: ttp://www.utexas.edu/cons/geo/barker/kempter/~ncon.html ’ e-mail address: [email protected]. 0377-0273/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0377-0273(95)00072-O

elevations of individual cones ranging from 1670 to 1920 m. Although nine craters are readily identified by their topographic expression, historical activity at the volcano has been limited to the Active Crater. The crater contains a highly acidic lake from which numerous phreatic eruptions have occurred since 185 1 (Tristan, 1921; Barquero and Segura, 1983), and phreatomagmatic eruptions in 1967 and 1991 (Alvarado, 1993). Historical accounts of eruptions over the past two centuries and descriptions of the summit crater morphology by Tristti (1921) indicate that conditions at the summit have changed little during this time.

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Fig. 1. Location of study area and view looking north at the Rincdn de la Vieja volcanic massif. The SW quadrant of the massif (left side in photograph) is devoid of vegetation due to volcanically derived acid rain. The Active Crater lies approximately midway along the summit ridge, near the boundary between the “dead zone” and zone of vegetation.

An extraordinary feature of Rincon de la Vieja is a triangular “dead zone” along the southwestern flank of the volcano formed by recent eruptions and volcanically derived acid rain (Fig. 2). Barren. steep-walled canyons in this area provide unusual access to the prehistoric eruptive deposits of the volcano. The acid rain results from steady prevailing east-northeasterly trade winds which mix with acidic vapors rising from the Active Crater. The high degree of erosion in the “dead zone” and historical records of activity argue that this process has been ongoing for several centuries. The primary goal of this study is to map and characterize the volcanic deposits in the “dead zone” in order to assess the volcanic evolution and eruptive history of Rincon de la Vieja. The exposed stratigraphy is comprised predominantly of intercalated lava and tephra deposits and has been subdivided into

four groups. Unfortunately, dense cloud and tropical forests cover three quarters of the volcano’s flanks, precluding detailed geologic mapping. Therefore, the stratigraphic record presented in this study is undoubtedly incomplete. Nonetheless, the excellent exposures in the “dead zone” represent a significant chapter in the volcano’s eruptive history and provide key insights into the past and future eruptive behavior of the volcano. Because of the imminent risk of future eruptions, particularly close attention was given to the last major eruption involving juvenile magma at Rincon de la Vieja. The deposit produced from this eruption, termed the Rio Blanc0 tephra, extends west-southwest from the Active Crater and caps the volcanic sequence in the “dead zone”. Dynamics of this eruption have been reconstructed through detailed field and grain-size analyses. Data from the Rio

K.A. Kempter et al. /Journal

of Volcanology and Geothermal Research 71 (1996) 109-127

111

Fig. 2. Composite aerial photograph of the western summit of Rinc6n de la Vieja. East-northeasterly trade winds and volcanically derived acid rain have created a zone of devegetation (dead zone) southwest of the Active Crater. The high degree of erosion in this area exposes over 300 m of prehistoric volcanic stratigraphy. The Rio Blanco tephra, erupted from the Active Crater at approximately 3500 yr B.P., blankets the Von Seebach cone, the West Crater and caps most of the ridges in the dead zone. Dense vegetation precludes geologic mapping in other sectors of the volcano.

Blanc0 tephra and older deposits are then integrated to provide a qualitative hazards assessment for future eruptions at Rinc6n de la Vieja volcano. Within zones of moderate to high hazard are two villages, over 300 km* of National Park land, and numerous cattle and agrarian ranches. In this report, site locations are referenced using the 1:50,000 CurubandC topographic map and UTM (kilometer) grid system.

2. Geologic

setting and prior volcanic

history

Voluminous ( > 500 km31 caldera-related volcanism characterized much of northwestern Costa Rica

during the Plio-Pleistocene, forming a broad ignimbrite plateau between the Caribbean and Pacific coasts. Although several major ignimbrites were erupted during this episode of volcanism (Dengo, 1962a; Tournon, 1984; Setser, 19941, the only obvious vestige occurs southeast of Rincon de la Vieja at Miravalles volcano, where a scallop-edged caldera scarp encircles a large portion of the younger volcano (Chiesa et al., 1987; Protti, 1988). The lack of remnant caldera structures at the surface, and the distribution and thicknesses of ignimbrite deposits in the region, suggest that these Pleistocene calderas are closely coincident in space with the modern volcanic

chain, and have been buried by younger volcanoes and eruptive products. A geologic map of major volcanic units south and southwest of Rincon de la Vieja is presented in Fig. 3. The oldest volcanic units in this region (QTpf) consist of poorly welded ignimbrite deposits, fluvial and lacustrine sediments, diatomites and intermediate composition lava flows. Neighboring lacustrine deposits crop out northwest and southwest of the map area, indicating that numerous lakes, possibly related to caldera subsidence structures, existed prior to the modern chain of volcanoes. Stratigraphically above deposits of QTpf are voluminous, high-aspect-ratio, silicic lava flows and associated ignimbrite deposits which are moderately to densely welded (QTat).

Undifferentiated deposits from de la Vieja volcano.

Rinc6n

Consolidated gravel and conglomerate deposits derived from Rlnc6n de la Vieja volcano.

EEI m a@

These lavas exhibit contorted flow banding characteristic of high-emission-rate, degassed lavas (Henry et al., 1988) which, similar to the lacustrine deposits, may have filled in a preexisting caldera structure. Above the QTal lavas is the Liberia tuff (Qlt>, possibly the most voluminous rhyolitic ignimbrite in the region, and interpreted by Dengo (1962b) and Healy (1969) to have erupted from a source at Rincon de la Vieja. The deposit is white to beige. poorly welded, and exposed over a broad portion of the Pacific coastal plains south and west of Rincon de la Vieja (Fig. 4a). The original extent of this deposit has been estimated by Chiesa (1991) to be 3500-4000 km’, with an erupted volume of 25 km’. The ignimbrite is crystal-vitric and is often distin-

now5

Andesite lavas and small volume lgnimbdte deposits.

Liberia tuif - crystal vitric poorly welded ignlmbrite deposit.

Voluminous silicic lava and moderately to densely welded ignimbrites.

Caldera-fill ignimbrite deposits, poorly welded.

Hornblende and biotite-bearing lava flows and domes.

Poorly welded ignimbritas. lava?., diatomites, fluvial and lake deposits.

Fiy. 3. Major volcanic units southwest ol the Active Crater at Rinc6n de la Vieja volcano

K.A. Kempter et al./ Journal of Volcanology and Geothermal Research 71 (1996) 109-127

guished from other ignimbrite deposits in northwestern Costa Rica by its high content of biotite phenocrysts. Where exposed, the base of this unit typically exhibits 2-3 m of pre-ignimbrite surge and plinian airfall deposits (a roadside quarry exposing these deposits is located at N-98.5, E-76.0 on the CurubandC topographic map). The southern margin of a caldera structure, believed to be related to the eruption of the Liberia tuff, has been identified near the base of Rincon de la Vieja (Fig. 3). The name Guachipelin caldera is proposed for this caldera, after a large cattle ranch south-southeast of the Las Pailas geothermal site. Although most of the caldera structure has been buried by younger deposits, the southern topographic margin of this caldera is preserved, and coincides with a marked break in the volcanic stratigraphy. The association between the Liberia tuff and the

113

Guachipelin caldera is supported by (1) the radial distribution of the Liberia tuff peripheral to the caldera margin, and (2) an increase in deposit thickness and the size of incorporated lithic clasts approaching the caldera margin. Within the caldera margin intracaldera Liberia tuff is interpreted to be buried by younger ignimbrites and deposits from the modem volcano. Recent K-Ar ages (Alvarado et al., 1992) indicate that the Liberia tuff was emplaced at approximately 1.6 f 0.1 Ma. Four rhyodacitic lava domes [CaHas Dulces hills (Qcd)] were emplaced prior to the Liberia tuff, perhaps as leaks from an evolving batholithic magma that later gave rise to the voluminous rhyolitic ignimbrite. Three smaller-volume ignimbrites are exposed within the Guachipelin caldera and appear to have filled the caldera depression (Qgf), as they are not found outside the caldera margin. Two of these

Fig. 4. (a) Liberia tuff (Qlt) exposed 9 km northwest of Liberia in the Rio Colorado. Two main cooling units of this ignimbrite are expc Ised in tlre canyon (arrow at cooling break). The Liberia Tuff was erupted from a caldera structure, now largely buried by deposits from the mod lem volcano. (b) Consolidated conglomerate (Qbg) in Rio Salitral (Curubandt, N-9.0, E-75.3). Boulders and cobbles in these depc Isits andesite lava and may record the volcano’s early eruptive history. cons ;ist of andesite-basaltic

ignimbrite deposits are exposed along Rio Colorado south of Hacienda Guachipelin (Curubande. N-4.0. E-88.0). Toward the north, the ignimbrite deposits are unconformably overlain by Quaternary lahar deposits and lavas of Rincdn de la Vieja. One or more of these ignimbrites may be related to a 5 km’ diameter caldera proposed by Carr et al. ( 1985). The periphery of this proposed caldera is defined by an ellipse connecting the Las Pailas geothermal site. the volcanic cones Santa Maria, Rincon de la Vieja. Active Crater. Von Seebach. and an arcuate ridge west of Rio Blanco. At present. timing of the onset of Quaternary andesitic volcanism at Rincon de la Vieja is poorly constrained. However, fluvial boulder-bed deposits (Qbg) rest unconformably above ignimbrite deposits immediately outside the caldera margin (exposed in canyons along Rio Colorado, Rio Salitral. and Quebrada DOS Quebradas-Curubande. N-00. E-83.5. N9.3, E-75.4) and may record this early activity (Fig. 4b). Lavas and lahars from the modern volcano (Qrvu) lie stratigraphically above the boulder-bed conglomerates and the aforementioned caldera-fill ignimbrites (Qgf). Several geothermal manifestations occur along a NW-trending lineation at the base of Rincon de la Vieja (Azufrales, Las Pailas, Las Hornillas, Borinquen). Soil mercury studies by Lescinsky et al. (1987) indicate that a fault parallel to the volcanic front underlies and connects the geothermal sites. Gravity and magnetic studies in the Las Pailas area by Quesada (1989) identify negative and positive anomalies associated with distinct volcanic lithologies and support the existence of two NNE-SSWtrending faults that delimit the area of thermal activity.

3. Rincdn de la Vieja volcanic stratigraphy The volcanic sequence exposed along the southwestern flank of Rinc6n de la Vieja volcano has been subdivided into four stratigraphic groups based primarily on the relative proportions of lava and tephra. The only stratigraphic division distinguished by a widespread unconformity is between groups 1 and 2. Other unconformities exist, but could not be correlated over a broad area. A geologic map show-

ing the stratigraphic groups is presented in Fig. 5. The oldest deposits, Group 4, are dominated by voluminous two-pyroxene andesitic lavas. Lavas are increasingly less voluminous in Group 3 while intercalated tephra deposits increase in thickness. Tephra deposits dominate Group 2 volcanic products, although intercalated small-volume lavas are common. Group 1 deposits include the last major tephra deposit, the Rio Blanc0 tephra, and all subsequent tephra deposits erupted from the Active Crater. The base of the volcano, including the Las Pailas geothermal site, consists primarily of lahar deposits. The lower to mid-slopes are comprised of thick andesitic lava flows, with interbedded tephra and lahar deposits. The summit region of the volcano consists primarily of thick, stacked tephra deposits. In general, the exposed volcanic succession along the southwestern flank of Rincon de la Vieja suggests that early volcanism was dominated by voluminous non-explosive lava emissions, with explosive plinian-style eruptions becoming increasingly more dominant with time. A schematic profile of the stratigraphic section southwest of the Active Crater is portrayed in Fig. 6.

4. Oldest deposits: Group 4 (Hornillas lava series) The oldest volcanic rocks in the study area form the Hornillas hills, the southwestern base of the volcano. Locally, remnant lava flow morphology is still preserved, with steep-sided lava toes evident near the base of the volcano. Quebrada Agria. a steep-walled river canyon in this area, offers superb exposures of at least six major flows from this Group. Although the flows vary in thickness from 4 to 30 m, they are commonly lo- 15 m thick and exhibit brecciated basal and surficial horizons. Intercalated with the flows are minor tephra and lahar deposits, exposed only in river canyons. The westernmost, partially eroded crater along the summit ridge of the volcano (West Crater in Fig. 5) may have been a source for some of these flows. Another potential source is a preserved volcanic neck which has been exhumed near the upper tributaries of Rio Blanc0 (Curubande, N- 10.7, E-88.2). Petrographically, the lava flows appear very similar: modal percentages of phenocrysts vary only

K.A. Kempter et al. / Journal of Volcanology and Geothermal Research 71 (1996) 109-127

115

1 km 1

I

Explanation of Volcanic Units

El Group

1

Lapilli pumice airfall deposit of the Rfo Blanc0 tephra and younger tephra deposits in the vicinfty of the Active Crater. SW of the Active Crater the Rfo Blanco tephra exceeds 20 meters in thickness and consists of mixed light gray (95%) and dark gray (5%) pumices. Brown to beige tephra deposits varytng in thickness from -zl meter to >20 meters and containing abundant lithic fragments (up to 1 meter in diameter). Relatively minor andesite lava flows (cl 0 meters thick) are intercalated with tephra deposits

e3

Location of carbon-14 sample dated at 27,000 ybp.

Fig. 5. Geologic

Intercalated brown to yellowishbrown tephra deposits and gray andesite lavas. Tephra deposits vary in thickness from cl meter to >30 meters and have undergone pervasive meteoric alteration. Similarly altered andesite lava flows vary in thickness from 5 - 15 meters. Dark gray to purplish gray andesite lava flows with some flows exceeding 30 meters in thickness. Brecciated flow horizons common near the base and top of individual flows.

. : :&ii:. . . . . . . I

map in the zone of devegetation

slightly from the following phenocryst proportions: 6 plagioclase:2 augite: 1 hypersthene: 1 Fe-Ti oxides. Two lava flows are exceptional, however, in that hypersthene is the dominant pyroxene. Texturally, these lavas are typically seriate, glomeroporphyritic

Undifferentiated eruptive deposits including lavas, tephras, lahars, and pyroclastic flow deposits.

southwest of the Active Crater,

and pilotaxitic. Orthopyroxene phenocrysts are commonly resorbed or rimmed by augite, suggesting that prior to eruption, magmas that produced the Homillas lavas were not in equilibrium with hypersthene. Locally, the Homillas lavas have undergone hy-

300m

2OOm

1 OOm

Om

?

?

?

Inferred subsurface -

q

.$%C Poorly welded r;;‘:~‘; ignimbrite deposits

Andesite lavas

_ cl

Tephra deposits

t@ L.2

Block and ashflow deposits

Fig. 6. Schematic htratieraphic profile southwest oi’ the Active Crater In general. the volume of tephra relative to lava increases up-section. evidence for the Rio Age constraints include a “‘C analysis in lower Group 7- tephra deposit\ (27.000 + 560 yr B.P.) and ‘“C/archaeological Blanco tephra deposit ( < 3500 yr B.P.). The stratigraphy suggest\ that lava emissions (andesitic-basaltic andesite) characterize the growth of the volcano prior to 30,000 yr B.P.. whereas more explosive tcphra eruptions (andesite-dacite) characterize its eruptive behavior since.

drothermal alteration and sulfide mineralization. At location N- 10.7. E-88.2 on the CurubandC map a strongly altered zone occurs where Hornillas lavas overlie 3 m of white to gray ignimbrite. The zone of alteration occurs over a 0.5 km’ area, roughly circular in its w-ficial expression. Disseminated pyrite occurs in both the lavas and the ignimbrite, more prevalent in the latter due to its greater pore space. The ignimbrite is exposed only in this area and its lateral extent is unknown. Despite the alteration, small (< I cm) pumices and phenocrysts of plagioclase and pyroxene are distinguishable. The ignimbrue is significant in that it may represent (I 1 a

caldera-forming event during the growth of the volcano. and/or (2) a correlative with ignimbrite deposits exposed at the base of the volcano (Qgf, Fig. 3). Unfortunately, the degree of alteration precludes chemical analysis of this deposit.

5. Group 3 Group 3 deposits occur stratigraphically above the Homillas lavas along the Quebrada Agria ridge (Nuevo Mundo site-CurubandC, N-9.5, E-87.5). These deposits consist of alternating lava flows and

K.A. Kempter et al./ Journal of Volcanology and Geothermal Research 71 (1996) 109-127

tephras, and reach a maximum thickness of about 150 m in the upper tributaries of Rio Blanco. At least five major flows belong to this group, although their relative stratigraphic relationships are unclear (only one lava flow from this group was correlated across the study area>. In general, these lava flows are less extensive and voluminous than the Hornillas lavas, and originated from undetermined sources. Locally, minor block and ash-flow deposits occur between lavas and tephra units. Many of the Group 3 deposits dip slightly to the north (best recognized by bedding attitudes within tephra deposits), into the present site of the Von Seebach cone. This implies that the Von Seebach cone formed subsequent to their deposition, and that a topographic valley existed east of the West Crater during this episode of volcanism (Fig. 2). Two thick individual tephra units (up to 20 m) are well preserved in this area, and appear to thicken in the direction of Santa Maria cone. Unfortunately, all tephras in this group have undergone pervasive meteoric alteration and are unsuitable for petrographic/chemical analysis. Lavas from this episode of volcanism exhibit the greatest petrographic variation amongst the classified volcanic groups. Rare hornblende is present in two of the lavas, accompanied by an increase in the modal ratio of hypersthene:augite. Of the seven petrographic sections from lavas in this group, three exhibit 2: 1 ratios of augite:hypersthene, two sections have roughly equivalent amounts of each pyroxene, and two have hypersthene > augite. Textures are similarly variable. It is notable that the three hypersthene-rich samples exhibit sieve-textured and resorbed plagioclases and reaction rims around augite phenocrysts. These “more evolved” lavas appear to have been in greater disequilibrium with their phenocrysts prior to eruption than the augite-rich lavas. 6. Group 2 This group consists primarily of stacked tephra deposits with lesser intercalated andesite lavas. Maximum thickness of these deposits approaches 100 m in eroded canyons just south of the Von Seebach cone. The uppermost 25-35 m of exposed tephra in this group appears to have constructed much of the Von Seebach cone. At least two small lava flows

117

also emanated from the cone during this eruptive episode. The tephra units thin rapidly away from the cone, however, and are only a few meters thick on the Quebrada Agria ridge (Curubande, N-10.7, E87.6). Abundant carbonized wood fragments occur in a 0.6 m tephra horizon near the base of the Group 2 at this location (Fig. 5). Radiocarbon dating on a 2.6 g sample of this material at the University of Texas Radiocarbon Lab yielded an age of 27,000 &-560 yr B.P. (Kempter and Benner, 1989). Because younger deposits at Rinc6n de la Vieja are primarily tephras, this age suggests that volcanic activity has been dominated by explosive, tephra-producing volcanism since that time. Low-volume lavas erupted during this episode are typically < 10 m thick and are often brecciated throughout. Six flows appear to have been erupted from sources at or near the Von Seebach cone. Petrographically, augite is the dominant pyroxene in each of the lavas. The overall phenocryst content varies significantly between samples, although relative phenocryst proportions are roughly similar ( N 6 plagioclase:2 augite: 1 hypersthene: < 1 opaques). Commonly observed textural features include glomeroporphyritic, poikiloblastic pyroxenes, pilotaxitic groundmass, fritted plagioclases and resorbed pyroxenes. Disequilibrium textures are common among plagioclase and pyroxene phenocrysts. 7. Group 1. The Rio Blanc0 tephra deposit Group 1 consists of the Rio Blanco tephra deposit and younger deposits erupted from the Active Crater. An age of 3490 f 105 yr B.P. was obtained from a small, 870 mg sample of amber in the paleosol just below the tephra (Melson, 1988), using radiocarbon dating methods at a Swiss laboratory, through Beta Analytic, Inc. More recent archaeological evidence presented by Alvarado (1993) supports an age of approximately 3500 yr B.P. for the eruption. The Rio Blanc0 tephra blankets most of the exposed volcanic sequence southwest of the Active Crater, although it has been heavily dissected by erosion. As the last major eruption at the volcano involving juvenile magma, the interpretation of this deposit is crucial for predicting the style of future eruptions. Since the Rio Blanc0 tephra eruption, explosive phreatic and phreatomagmatic eruptions from the Active Crater

have only deposited minor amounts of tephra and ash in the vicinity of the Active Crater. The Rio Blanc0 tephra was deposited in a highly

asymmetrical dispersal pattern west-southwest of the source vent, indicating a strong prevailing wind from the east-northeast at the time of the eruption. The

K.A. Kempter et al. / Journal of Volcanology and Geothermal Research 71 (1996) 109-127

tephra is a well-sorted, lithic-poor lapilli airfall deposit that ranges in thickness from a few centimeters to > 20 m near source (Fig. 7). The deposit is dominantly composed of pumice with phenocrysts of plagioclase, orthopyroxene, and rare hornblende. A pyroclastic flow associated with the tephra extends north of the Active Crater for several kilometers. Preliminary chemical and microprobe results indicate the pumices are dacitic, ranging in SiO, content from 62.5 to 66.5%. Interestingly, a mafic juvenile component of andesitic composition (SiO, N 57.5%) is also present, suggesting that magma mixing occurred and possibly triggered the eruption. The matic pumices comprise < 5% of the tephra but are slightly more abundant in the ignimbrite deposit north of the volcano. More detailed REE and other trace-element analyses from these distinct pumices are currently in progress. In general, the deposit is lithic poor, implying that significant vent erosion did not occur as the eruption progressed. The tephra rests unconformably above all older volcanic units at the volcano, suggesting that a significant period of erosion and volcanic quiescence preceded the tephra eruption. At least twenty distinct layers can be identified in the nearsource tephra deposits, each representing changes in eruption intensity. With increasing distance from source, individual layers within the deposit coalesce. Maximum pumice and lithic clasts typically occur in the middle and upper horizons of the deposit, giving the appearance of two inversely graded fallout units. However, correlation of the coarse middle horizon with near-source deposits indicates that the coarse horizon is the result of a distinct strong pulse during the eruption and not a gradual increase in eruption intensity as the eruption progressed. 7.1. Source vent The tephra isopach map of the Rio Blanc0 tephra deposit indicates that deposit thickness increases ex-

119

ponentially towards the Active Crater (Fig. 8). Other evidence that supports the Active Crater as the source vent for the Rio Blanco tephra include: (1) the Von Seebach cone is blanketed by the Rio Blanc0 tephra, with thicker tephra deposits on the side of the cone nearest the Active Crater; (2) the maximum thickness of the Rio Blanc0 tephra ( > 20 m) is exposed approximately 1.3 km southwest (downwind) of the Active Crater; and (3) the Von Seebach cone shows no sign of geothermal activity or alteration of the Rio Blanc0 tephra deposit. Some degree of posteruption geothermal activity should have affected the vent site, thereby altering tephra within the crater. Given the exponential thickening of the deposit towards the Active Crater, an estimated 95 m of tephra should have been deposited at the vent (not accounting for displacement of the eruption column by wind). Initial geologic observations from this study suggested that tephra and lava stratigraphy exposed in the wall of the Active Crater are older than the Rio Blanc0 tephra deposit. However, more recent observations of the Active Crater site by G. Sot0 (pers. commun., 1995) argue that the crater stratigraphy is younger than the Rio Blanc0 tephra, and that the original Rio Blanco tephra cone was destroyed by subsequent explosive eruptions. Further study of the summit region is needed to clarify the stratigraphic relationship between the Rio Blanc0 tephra and the stratigraphy exposed at the Active Crater. 7.2. Dispersal, MP and ML parameters of the tephra In the past decade, several numerical models have been presented which attempt to reconstruct the behavior of volcanic eruption columns based on the physical characteristics of the tephra fall deposits (Sparks, 1986; Pyle, 1989). The dispersal of the Rio Blanc0 tephra deposit is shown by the isopach map of thickness (Fig. 8) and isopleth maps of maximum

Fig. 7. (a) Profile of the Rio Blanco tephra deposit exposed near Cerro Fortuna (Curubande, N-10.8, E-76.3). The tephra is lithic poor and became tines depleted with distance from source as elutriation of ash by strong westerly winds occurred. Maximum pumice and lithic clasts, representing maximum eruption intensity, are typically found in the middle and uppermost horizons of the deposit. (b) Profile of the Rio Blanco tephra deposit 1.2 km southwest of the Active Crater. This avalanche scarp exposes the thickest deposit of the Rio Blanc0 tephra observed in the study area (see person for scale). These near source deposits are poorly sorted and contain bombs and blocks that exceed 1 m in diameter. The tephra rests in erosional unconformity upon all older volcanic units at the volcano, suggesting that a significant period of erosion preceded the tephra eruption.

r (2nd Geothermul

pumice (MP) and maximum lithic (ML) fragments (Fig. 9a and b). In the field, MP and ML were taken by averaging the maximum diameters of the three largest pumice and lithic fragments at each site. Sieve analyses were later performed to derive the mean diameter (Md) cumulative curves for bulk samples. Although 150 sites were observed in the field, less than one third of these sites were interpreted as primary deposits, showing no reworking of the deposit. The tephra isopach map indicates that a curved dispersal axis characterizes the thickness isopachs of the deposit. Near-source, the dispersal axis trends southwest; with distance the axis bends progressively westward. These data suggest that two wind components affected the dispersal of the tephra; a component trending to the southwest and one trending due west. Wind direction studies over Costa Rica’s central valley (Zarate. 1988) show that westerly trade winds occur throughout the year at altitudes below 5

Reseurch 7/ 119961 109-127

km. but are more variable at higher altitudes (5-15 km), particularly during the wet season (May through October). During this study, consistent prevailing wind measurements of N50E f 5” were taken at the summit of the volcano throughout the year, whereas wind data collected by ICE (Instituto Costanicense de Electricidad) near Liberia (N 20 km south of the volcano) and Quebrada Grande (N 10 km due west of the volcano) indicate prevailing winds to be typically N85E k 5” (except for the months of September and October when a prevailing southeasterly wind is common). These measurements closely mimic the dispersal axis for the tephra deposit, and probably represent the two wind components which dictated dispersal of the Rio Blanc0 tephra. Most likely, tephra erupted high in the eruption column ( > 9 km) was carried by westerly winds. Tephra fall out during less intense pulses of the eruption (lower column height) or from the periphery of the eruption column, was primarily

Fig. 8. Isopach thickness map and dispersal axis for the Rio Blanc0 tephra deposit. The curved dispersal axis shows that two wind components, one southwesterly and one due west, dictated tephra dispersal. Tephra deposited near the vent was primarily transported by southwesterly winds. Tephra erupted higher in the eruption column (and farther from source), was transported and deposited by more westerly winds.

K.A. Kempter et al. /Journal

of Volcanolog) 1and Geothermal Research 71 (1996) 109-127

transported by southwesterly winds. This scenario would also account for the thick deposits of tephra observed 1.O- 1.5 km southwest of the Active Crater. Maximum pumice and lithic clast diameters and tephra thickness were measured at 47 outcrops, with 37 samples collected for grain-size analyses. Invariably, the maximum pumice and lithic clasts at each

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outcrop occur in either the middle or upper coarse horizons, presumably when episodes of vent widening and maximum eruption intensity occurred. MP and ML isopleth maps, based on the average measurements of the three largest pumice and lithic fragments, are shown in Fig. 9a and b. These diagrams show the size of both lithic and pumice

a. Maximum Pumice

Active Crater

0 Measurement in cm

b. Maximum Lithic

Actlve Crater

0 Measurement in cm m

~-1.2cm

q

>2.0 cm

n

a7.0 cm

n

0

5km

z-20.0cm

a

Fig. 9. Maximum pumice (a) and lithic (b) isopleth maps for the Rio Blanc0 tephra deposit. The isopleth contours are similar to the isopach contours, with near source isopleth contours showing a curvilinear (southwest to west) dispersal axis. Both the pumice and lithic distal isopleths document the increasing influence of the westerly wind component with distance from source.

122

0

5

0

Maximum pumice

A

Maximum lithic

10

15

20

(Isopleth area) ‘/2 km

O

10

20

30

Distance along dispersal axis (km) Fig. 10. (a) In(clast size) vs. isopleth area diagram for the Rio Blanc” tephra deposit. The change in slope (inflection point) fw both data sets is interpreted to represent overthickening of nearsource deposits (cone building) during less energetic pulses of the eruption. shedding of material from the periphery of the eruption column, or a combination of these factors. (b) Thickness, maximum pumice and lithic data vs. distance from vent along the dispersal axis.

fragments decreasing exponentially away from the vent. with lithic fragment size decreasing more rapidly than pumice fragment size. The dispersal axis of the lithic fragments is skewed slightly southwest from that of the pumices, possibly related to contrasts in density, inertia and aerodynamic properties (Papali and Rosi, 1993). MP and ML data from the Rio Blanc0 tephra plotted on a ln(clast size)-(area)‘/2 diagram, using the method of Pyle (1989) are shown in Fig. 10a. These data confirm the exponential decrease of these

parameters with distance from source. In addition the pumice and lithic data both plot as two lines of different slope. Plots of the log(thickness), MP and ML against distance from the vent along the dispersal axis (Fig. lob) also yield two lines of different slope (showing a sharp inflection point at about 4.0 f 0.8 km from source). The dramatic increase in thickness and clast size in the near-source deposits could also have resulted from (1) an eruptive column which varied in height throughout the eruption-with less energetic pulses causing overthickening of the tephra near the vent, and/or (2) shedding of material from the periphery of the eruption column, including ballistic pumice and lithics, surges and pyroclastic t‘lows. Field observations of the tephra stratigraphy show that the eruption column varied in intensity throughout the eruption. Most of the near-source deposits are well stratified, and several reversely graded sequences can be observed. The lower deposits contain the largest and most abundant ballistic pumice and lithic clasts (i.e.. less energetic eruption column), correlating with fine-grained basal distal deposits. Therefore, much of the overthickening probably occurred during the early phases of the eruption. The eruption column reached maximum intensity midway through the eruption, resulting in the broad distribution of the largest pumice and lithic fragments. Large pumices also occur in the uppermost horizon of the deposit. indicating a second peak of eruption intensity near the end of the eruption. 7.3. Grain-size analyses Thirty-seven sieved samples of the Rio Blanc0 tephra were used to obtain median grain-size and sorting information, using the methods described by Walker (1971). In general, these data show that efficient sorting of fallout occurred during the eruption, with the Inman sorting parameter averaging 1.42 for all samples. Sorting efficiency increases slightly with distance from source, reaching a minimum of 0.99 at a distance of 25 km. Generally, the sieved samples exhibit a lognormal size distribution, plotting as lines on probability paper (cumulative weight percent vs. median grain size). Representative analyses from proximal, medial and distal samples along the dispersal axis are shown

K.A. Kempter et al./Journal

of Volcanology and Geothermal Research 71 (1996) 109-127

Proximal tephra

vzi

60 50

64

E E

+ i?so

40

B

B g

30

fi *

20

g

16

L

10 0 -6

-4

-2

0

2

4

123

eating the poor sorting of these deposits (with a maximum weight % around -44). Medial and distal deposits, successively farther from source along the dispersal axis, illustrate a progressive decrease in median grain size, increase in sorting efficiency, and depletion of line material in the deposit. The sharp inflection point at 14 on the lognormal plots in the distal deposits is interpreted to represent the elutriation by wind of particles less than l/2 mm in diameter.

Phi

7.4. Volume and eruption dynamics

Medial tephra deoosits 60 I p

64

50

;

50

40

g

2

16

30

;

20

ij

s

g

10 0 -6

-4

-2

0

2

4

Phi

60 w 564

50

5

.E !?60

40

b 0

i

30

= z

20

d s

16

10 0 -6

-4

-2

0

2

4

Phi Fig. 11. Grain-size characteristics of the Rio Blanco tepbra along the dispersal axis. Two representative samples are shown for proximal, medial and distal deposits, all exhibiting lognormal size distributions (plotting as lines on a probability graph). A representative histogram is shown in the background. These data clearly illustrate a progressive decrease in Md grain size, increase in sorting efficiency, and depletion (elutriation) of tine material with increasing distance from source.

in Fig. 11. As a background plot superimposed on the lognormal size distribution, a histogram of one representative sample from each of the three groups is also shown. The proximal samples exhibit a broad size distribution and plot as gradational slopes, indi-

A tephra volume estimate was made using the method of Pyle (19891, applying the equation V = 13.08Z”b: + 6 to a ln(thickness)-(area)‘/Z plot of the deposit. In this equation b, is the thickness half-distance (i.e., thinning rate of the deposit), 6 is a correction factor for deposits with a break in slope in the thickness plot (i.e., overthickening of the deposit near source> and TO is the extrapolated maximum thickness of the deposit. Unlike other methods (e.g., Carey and Sparks, 19861, this method eliminates complexities caused by the distortion of isopach contours due to wind and secondary thickening processes. The Rio Blanc0 tephra yields an estimated cumulative volume of 0.25 km3, or 0.1 km3 DRE, and is classified as subplinian according to Pyle’s classilication. The S parameter included in this estimate is 0.08 km3, approximately one third of the overall volume. This value is believed to represent the volume of the cone-building, early phase of the eruption A slightly lower volume estimate of 0.24 km3 for the Rio Blanco tephra is obtained using a modification of the Pyle method proposed by Fierstein and Nathenson (1989). These should be considered as minimum estimates for the overall volume of erupted material, however, since they do not consider the volume of the pyroclastic flow deposited north of the Active Crater. Carey and Sparks (1986) provide a theoretical model that relates the geometry of lithic isopleths to maximum eruption column height and average wind speed. By estimating 1.6 and 3.2 cm lithic isopleths of the Rio Blanc0 tephra, a column height of approximately 16 km and wind speed of almost 110 km/h

are inferred (such wind speeds are commonly reported at the summit of Rincon). Assuming tropical temperatures at the time of the eruption and a magma temperature of 850°C a 16 km column height yields a mass discharge rate of about I .O X IO’ kg/s (Sparks, 1986). Furthermore, by dividing the total mass of the deposit by the rate of magma discharge. it is possible to estimate the duration of the eruption. Assuming that the maximum discharge rate of I .O X 10’ kg/s was maintained throughout the eruption, a minimum eruption duration of about 9 hours is estimated.

8. Implications

for volcanic

hazards

Although the purpose of this study was not to produce a hazards assessment of Rincon de la Vieja. the results enable us to make some qualitative statements regarding potential future hazards. Volcanic hazard evaluations are often based on detailed geologic and geochronologic studies of a volcano’s past activity, using the assumption that future eruptions will be of similar eruptive style and volume (Blong, 1984). The modern topography of the volcano is also an important consideration, as exemplified by the accurate prediction of lahar distributions during the 1985 Nevado de1 Ruiz eruption which killed approximately 23,000 people (Anonymous. 198.5; Williams, 1987). Based on the volcanic succession documented in this study, four eruption types at Rincon de la Vieja are briefly assessed, including deposits formed by ( 1) debris avalanches, (2) subplinian to plinian tephras. (3) pyroclastic flows and lahars and (4) lava flows. Evaluation of these eruption types, in conjunction with the present-day morphology of the volcano and its Active Crater, yields a preliminary hazards map for the volcano (Fig. 12). In general, areas immediately north of the Active Crater are in the highest hazard zone, primarily from Mars or debris/pyroelastic flows associated with explosive eruptions. Plinian-style eruptions would most likely affect large areas west and southwest of the volcano, however, in a zone roughly coincident with the Rio Blanc0 tephra distribution. At Rincon de la Vieja, the greatest potential hazards are to areas immediately north of the Active

Crater. where (I) historic phreatic eruptions have produced small, yet far-traveled lahars (e.g., Barquero and Segura, 1983). and (2) the eruption of the Rio Blanc0 tephra produced a northward directed pyroclastic flow. Stable volcanic deposits and topographic highs buttress the Active Crater on all but the northern flank, which slopes steeply towards the back arc plains (slopes exceed 30” approaching the crater rim). Siebert (1984) showed that volcanoes with slopes greater than 20” are highly susceptible to Bank failure, especially those with slopes > 28”. Given the analysis of Lopez and Williams (1992) concerning sector collapse of volcanoes due to hydrothermal alteration, this hazard is especially rele\:ant for the Active Crater, which has maintained an acidic crater lake throughout its historic activity (Tristan. 1921). Preliminary geochemical sampling of streams along the northern flank indicates that crater lake water is permeating through the crater wall rock and mixing with meteoric groundwater (G. Rowe and K. Kempter. unpubl. data). Leaching and alteration of the northern crater wall has therefore occurred, although it is not known to what extent or over what period of time. Given the steepness of the northern crater wall and its potential instability, sector collapse of this flank, associated with or without a volcanic eruption. represents the most serious threat to human lives, property and ecosystem. A debris avalanche or major lahar directed northward would channel into the Rio Cucaracho canyon, and potentially tlood the village of Birmania (Fig. 12). The uppermost volcanic sequence (< 27,000 yr B.P.) at Rincdn is dominated by tephra deposits. especially in the summit region. Therefore, if a significant volume of magma ( > 0.1 km’) is erupted in the near future, the eruption will probably be plinian, or at least vulcanian. in style. The zone which is likely to be covered by tephra is coincident to the dispersal pattern of the Rio Blanc0 tephra deposit, i.e., an asymmetric area to the west-southwest of the Active Crater. East-northeasterly prevailing trade winds occur throughout the year, and older tephra deposits also show a westward, asymmetric dispersal pattern (although most tephra deposits peripheral to Rincon have been removed by erosion). The small pueblos (< 3000 people) of Catias Dulces and Quebrada Grande, 19 and 17 km from the Active

K.A. Kempter et al. / Journal of Volcanology and Geothermal Research 71 (1996) 109-127

125

Pyroclastic Flows, Debris Avalanches and Lahars Moderate hazard High hazard

Tephra Eruptions Moderate hazard High hazard

IOkm

Fig. 12. Volcanic hazard map for future eruptions at Rincon de la Vieja volcano. Explosive eruptions that produce Mars, pyroclastic or debris avalanches pose the greatest threat to lives, property and National Park land. The zone of highest hazard extends north Active crater (horizontal lines) with moderate hazard zones (shaded areas) extending farther north and to the south. The stippled represent zones of moderate and high hazard for plinian-style, tephra producing eruptions. The wedge-shaped zone extending represents normal trade wind conditions (+ 10” north and south to accommodate variations in wind direction). The circular stippled consider no wind conditions.

Crater, respectively, are at greatest risk from a plinian-style eruption t w 30 cm of Rio Blanco tephra were deposited at Quebrada Grande). The largest town in the region, Liberia (N 30,000 habitants), lies N 25 km south-southwest of the Active Crater and may also be affected by a plinian eruption, especially if atypical wind conditions exist at the time of the eruption. In the event of a magmatic eruption, the potential for a pyroclastic flow to occur is strong, possibly

flows, of the areas west zones

associated with a plinian event. Pyroclastic flows and lahars related to eruptions from the Active Crater are primarily a threat to the northern flank of the volcano, where historic eruptions have repeatedly destroyed vegetation and produced acidic, fast-moving Mars in Quebrada Azufrosa and Rio Penjamo. Several large haciendas on the northern flank of the volcano, including Seltiverio, Buenos Aires and finca La Plot-, are in the zone of highest hazard from eruptions that produce pyroclastic flows and/or la-

126

hats. The topographic amphitheater southwest of the Active Crater (in the “dead zone”) is also at risk, but is somewhat protected by an elongate volcanic ridge that separates the amphitheater from the Active Crater (Fig. 5). Should a pyroclastic flow or lahar develop in this region, however, areas adjacent to the Rio Blanc0 and Rio Colorado are at greatest risk. especially the Las Pailas campground and the large haciendas Rincon de la Vieja and Guachipelin (Fig. 12). A major lava outpouring at Rincdn is highly unlikely. Lava flows intercalated with tephras in deposits younger than 27,000 yr B.P. are typically thin ( < 10 m) and of minor extent. Aerial photographic observations of unmapped regions on the volcano’s northern flank also fail to discern morphologies indicative of a major lava flow (e.g. flow lobes). If a major lava flow were to erupt from the Active Crater, however, the topography of the summit region dictates that it would travel northward down river canyon(s). Since volcanic activity appears to have migrated northward at Rincon (Kempter and Benner, 19891, a new vent may open on the northern flank, similar to the Arena1 example (Borgia et al.. 1988). Still, lava emission at Rincon should only damage the dense jungle vegetation on the northern flank and is not considered a serious hazard at Rincon.

9. Conclusions (1) The southwestern flank of Rincon de la Vieja exposes a significant portion of the volcano’s eruptive sequence, dominated by plinian/subplinian tephra deposits and two-pyroxene andesite lavas. The oldest deposits consist of thick, superimposed Bows of andesite lava, locally affected by subsequent hydrothermal alteration. In general, the volume of tephra relative to lava increases up section. with the past 27,000 years of activity recording more explosive, plinian-style eruptions than lava eruptions. (2) The last eruption involving significant juvenile magma at Rincon occurred at about 3500 yr B.P. (Alvarado et al., 19921, producing the Rio Blanc0 tephra deposit. Approximately 0.25 km’ of tephra was deposited in a highly asymmetrical dispersal pattern west-southwest of the source vent, indicating

strong prevailing winds from the east and east-northeast at the time of the eruption. Overthickening of the deposit near source ( > 20 m) reflects a low column-height eruption (_ 16 km) which varied in intensity throughout the eruption. Grain-size analyses of the deposit indicate that the overall eruption was subplinian with tephra sorting efficiency increasing with distance from the vent. In distal deposits, the tephra is strongly fines depleted, as small particles t < 24) were elutriated by strong westerly winds. (3) A preliminary volcanic hazard assessment at Rincon, based on the volcano’s eruptive history and the modern geologic setting of the Active Crater, indicates that the zone of greatest risk to human lives and property extends north of the Active Crater. Future eruptions involving significant juvenile magma ( > 0.1 km31 will probably be explosive in style (vulcanian to plinian), endangering a wide zone west-southwest of the volcano, and north of the volcano in the event of a pyroclastic flow. Lava flows are not considered a serious hazard.

Acknowledgements We are greatly indebted to many people and institutions who enabled us to accomplish this work. Our sincerest thanks are offered to the staff at the Observatorio de Vulcanologia at the National University in Heredia, Costa Rica. Jorge Barquero and Erik Fernandez are especially thanked for providing logistical and laboratorial support throughout this project. We also thank Dan Barker and the University of Texas at Austin, Dan Janzen, Colorado College, Louisiana State University, Gerard0 Soto and Guillermo Alvarado at the Instituto Costarricense de Electricidad, Bruce Loeffler, Bill Melson, Zeph Vamvakias, Alvaro and Evelyn Wiessel, Gary Byerly, the Hernandez Herrera family and “Austin” for their respective support and assistance. Financial support was provided by the Fulbright Foundation, the Nature Conservancy and the Associated Colleges of the Midwest program in Costa Rica.

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