DESCRIPTION:
Costa Rica Volcanoes and Volcanics

[Map,15K,InlineGIF]
Major Volcanoes of Costa Rica

Arenal Volcano, Costa Rica
Location: Costa Rica
Latitude: 10.463 N
Longitude: 84.703 W
Height: 1,657 meters (5,437 feet)
Type: Stratovolcano
Lastest Eruptions: Currently Active
Remarks: The volcano lies directly adjacent to Lake Arenal, a dammed reservoir for generating hydroelectric power. The volcano has been watched by many tourists from a mountain lodge 2.8 kilometers (1.75 miles) south of the vent that enables visitors to hear, to see, and occasionally to smell its dynamism.

The conical Volcan Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1,657-meter-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal was constructed to the northwest of the older Chato volcano, which contains a 500-meter-wide summit crater. The activity of Chato ended about 3,500 years ago, and the oldest known Arenal rocks are only 2,900 years old. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. Arenal's most recent eruptive period began with a major explosive eruption in 1968. Continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows has occurred since then from vents at the summit and on the upper western flank.

Costa Rica's highest volcano, Irazú has a broad summit, vegetated flanks, and a history of frequent eruptions going back to 1723. Its last eruption, 1963-65, sent tephra and secondary mudflows into cultivated areas, caused at least 20 deaths, and destroyed 400 houses and some factories. ...

Irazú, Costa Rica's highest volcano and one of its most active, rises to 3,432 meters immediately east of the capital city of San Jose. The massive volcano is vegetated to within a few hundred meters of its broad summit crater complex. At least 10 satellitic cones are located on the southern flank of Irazú. No lava flows have been identified from Irazú since the eruption of the massive Cervantes lava flows from south-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the west towards the historically active crater, which contains a small lake. The first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from its last major eruption during 1963-65 caused significant disruption to San Jose and surrounding areas.

In a Strombolian-type eruption observed during the 1965 activity of Irazú Volcano in Costa Rica, huge clots of molten lava burst from the summit crater to form luminous arcs through the sky. Collecting on the flanks of the cone, lava clots combined to stream down the slopes in fiery rivulets.

Miravalles (2,208 meters) is an andesitic stratovolcano that is one of five post-caldera cones along a northeast-trending line within the Pleistocene, 15 x 20 kilometer Miravalles caldera. Morphologically youthful lava flows cover the western and SW flanks of the volcano. The only reported historical eruptive activity was a small steam explosion in 1946, although high heat flow remains and a geothermal field is located within the caldera.

Miravalles is situated along the volcanic front of Central America, just southeast of a postulated boundary between eastern Nicaragua and Costa Rican segments of the Cocos-Caribbean subduction zone (Stoiber and Carr, 1973).

Miravalles Caldera formed approximately 500,000 +/- 150,000 years ago (Tournon, 1984), after eruption of voluminous silicic ash flows. An andesitic stratovolcano of the same name grew within the caldera (Healy, 1969). At least five cones are recognizable in the summit area, distributed along a northeast-trending line (probable fault). The modern intracaldera cone may be the second of two; a previous postcaldera cone was destroyed (R. Fournier, oral commun., 1987). Melson and others (1986) note a preliminary age of 8,000 (?) years B.P. for a major plinian eruption from Miravalles, and an age of 7,000 (?) years B.P. for a large debris avalanche from Miravalles. A geothermal field is situated between the caldera scarp and Miravalles stratovolcano.

No significant eruption has occurred at Miravalles in historical time. However, on 14 September 1946, a small steam eruption occurred high on the south flank of the volcano, near the summit crater. Weak earthquakes were felt at the time. An explosion crater 20 meters in diameter formed, and steam, mud, and ash were thrown 100 meters high (Hantke, 1951, citing a letter from Sigismund von Preussen of the town of La Barranca). The hydrothermal field is a hot-water-dominated system, with a vapor-dominated cap (few tens of meters thick) in some parts of the geothermal field. Changes in the vapor-dominated cap can give rise to surface hydrologic changes and small blowouts. Fumarolic activity has reportedly decreased in recent years (as of late 1987) (R. Fournier, oral commun., 1987).

Poás, one of the most active volcanoes of Costa Rica, is a broad, well-vegetated volcano with a summit area containing three craters along a north-south line. The frequently visited multi-hued summit craters of one of Costa Rica's most prominent natural landmarks are accessible by vehicle. The 2,708-meter-high complex stratovolcano is constructed within eroded remnants of nested 7- and 3-kilometer-wide calderas. A north-south-trending fissure extending to the lower north flank has produced the Congo stratovolcano and several maars. The southernmost of two summit crater lakes, Botos, is cold and clear, and last erupted about 7,500 years ago. The other is warm and acid and has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Poás eruptions often include geyser-like ejection of crater lake water.

Poás is situated near the southeastern end of the Central American arc, above the Cocos-Caribbean subduction zone. Cinder cones are strongly aligned in a north-south direction.

Two calderas, one nested within the other, developed in the summit area of Poás (Thorpe and others, 1981). The younger, smaller collapse occurred less than 40,000 years B.P. but more than 7,540 years ago (a carbon-14 date on the postcaldera Botos cone) (Prosser, 1983, 1985; Prosser and Carr, 1987). The modern cone of Poás has grown within and nearly obscured the two calderas; only the flat-topped morphology of Poás belies its calderas. The summit of the modern cone has a main crater that may contain a sulfur-rich lake with a layer of molten sulfur at its base (Bennett and Raccichini, 1978; Francis and others, 1980).

The magma erupted to form the younger caldera has been tentatively identified as basaltic andesite (Prosser and Carr, 1987); magma associated with the earlier caldera-forming eruption is not known. A mafic composition is consistent with the lack of a major gravity low in the area of the caldera and the existence of a local gravity high at Poás's summit. Tournon (1984) presented analyses of basalt, andesite, and low-silica dacite -- samples that may span the time(s) of caldera formation.

Most historical eruptions of Poás have been small, phreatic or magmatic, sometimes including molten sulfur.

Rincon de la Vieja, the largest volcano in northwest Costa Rica, is a remote volcanic complex in the Guanacaste Range consisting of an elongated, arcuate NNW-SSE-trending ridge. At least nine eruptive vents are located within an older 15-kilometer-wide caldera remnant, with activity migrating to the southeast, where the youngest-looking craters are located. The twin cone of 1,916-meter-high Santa Maria volcano, the highest peak of the Rincon complex, is located at the eastern end of a smaller, 5-kiloeter-wide caldera and has a 500-meter-wide crater. A Plinian eruption producing the 0.25 cubic kilometers Rio Blanca tephra about 3,500 years ago was the last major magmatic eruption from the volcano. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the Active Crater, which contains a 500-meter-wide crater lake.

Rincón de la Vieja, is situated along the volcanic front of Central America, at or just southeast of a postulated boundary between eastern Nicaragua and Costa Rican segments of teh Cocos-Caribbean subduction zone (Stoiber and Carr, 1983).

Rincón de la Vieja, the largest volcano in northern Costa Rica, consists of six volcanic centers that form an elongate ridge, built on a shield of ignimbrites. Healy (1969) outlined a large, 15-20 kilometer diameter caldera within which the present ridge-shaped volcano has grown. Carr and others (1986) concurred with this interpretation and outlined another nested, 5-kilometer-diameter caldera on the Rincón de la Vieja edifice.

Most historical eruptions of Rincón de la Vieja have been vulcanian, Strombolian, or phreatic explosions from the central crater of the complex.

Turrialba, (3,340 meters) the southeast-most of Costa Rica's Holocene volcanoes, is a large vegetation-covered stratovolcano located to the northeast of Irazu volcano. Three well-defined craters occur at the upper end of a broad summit depression that is breached to the northeast. Turrialba has been quiescent since a series of explosive eruptions in the 19th century that were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the summit craters.

Return to:
[Costa Rica Volcanoes and Volcanics Menu] ...