Volcanism during the past 84 ka at Atitlan caldera, Guatemala
Abstract
During the ∼84 ka Los Chocoyos eruption, Atitlan caldera erupted 250 to 300 km3 of magma (DRE). The plinian-fall deposit covered 6 x 106 km2, including significant parts of three ocean basins. Pyroclastic flow deposits extend more than 100 km to the NE and NW filling valleys to depths of 200m. The Los Chocoyos ignimbrite deposit also extends S to the Pacific Ocean. Since 84 ka, Atitlan caldera has produced at least 4 explosive, rhyolitic sequences, numerous andesitic tephras, and constructed 3 stratovolcanoes. Shortly after the Los Chocoyos eruption, an explosive andesitic eruption produced a 1-12m-thick fall deposit with 10-100cm blocks at the W end of the caldera, perhaps representing the first product of San Pedro volcano. Two rhyolitic eruptions of Atitlan caldera occurred between 84 ka and about 60 ka, and two more eruptions, closely spaced in time, occurred about 40 ka. The first tephra, the San Pablo ash, comprises a plinian-fall deposit with accretionary lapilli-bearing fine ash deposits above and below. The second deposit, the voluminous Las Canoas ash, contains a sequence of phreatomagmatic, plinian, and pyroclastic-flow deposits, and thickens toward its likely source at the E end of Lake Atitlan. The San Pablo and Las Canoas deposits are bracketed between 84+/-5 ka and 68+/-7 ka, and plagioclase from Las Canoas yields a provisional 86+/-16 ka 40Ar/39Ar age. The youngest two rhyolitic tephra sequences, separated by a thin soil, have plinian fall deposits that are coarsest on the slopes of San Pedro volcano on the S caldera margin. The lower unit yields 40Ar/39Ar plagioclase ages of 38+/-9 ka and 43+/-9 ka. Both tephras overlie even the youngest San Pedro lavas (43+/-16 ka), but predate exposed Toliman and Atitlan lavas. Since the major caldera-forming event of 84 ka, three stratovolcanoes, Atitlan, Toliman and San Pedro, have formed within the caldera. Toliman and San Pedro have produced thick lava flows or domes and some tephra layers, whereas Atitlan has erupted more frequently and explosively to form pyroclastic flows and widespread tephras in addition to lavas. Although eruptions at the 3 volcanoes likely overlapped, San Pedro grew to about its present size prior to 40 ka. Toliman probably remains active and began growing after about 40 ka and Atitlan grew almost entirely in the last 10 ka.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.V32D1050V
- Keywords:
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- 8404 Ash deposits