Poás Volcano, Costa Rica

Volcano Type: Stratovolcano
Volcano Status: Historical
Last Known Eruption: 2010
Summit Elevation: 2708 m 8,884 feet
Latitude: 10.20°N  10°12’0″N
Longitude: 84.233°W 84°13’58″W

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 Volcano

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.

Poás Crater Lake

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).

Poás Volcano

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.

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 N-S line. The 2708-m-high volcano is constructed within eroded remnants of two nested calderas. This photo from the east shows the lake-filled Botos crater (left center) and a steam plume rising from the historically active summit crater, which has been the site of frequent phreatic and phreatomagmatic eruptions since 1828. Eruptions often include geyser-like ejection of crater lake water.

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

Poás Volcano Sulfur Lake Caldera (Steven Depolo)

Originally posted 2010-07-06 10:58:34.