The 1968 andesitic lateral blast eruption at Arenal volcano, Costa Rica

Abstract

The magmatic andesitic eruption of Arenal volcano on July 29–31, 1968, after centuries of dormancy, produced three new fissural craters (A, B and C) on its western flank and a multilayered pyroclastic deposit emplaced by complex transport mechanisms. The explosions were initially triggered by a volatile oversaturated (4–7 wt.% H 2O) magma. Several lines of evidences suggest a small blast surge, where a wood-rich pyroclastic deposit was emplaced as a ground layer, followed by several units of coarse-grained (Md Φ between − 0.65 and − 5.40) tephra deposits (LU: lapilli units, DAU: double ash units). LU-1, -2, -3, DAU-1 and -2 consist of unconsolidated and well- to poorly sorted vesiculated bombs and lapilli of andesite, some blocks, ash and shredded wood. The individual units are possibly correlated with the major explosions of July 29. The thickness of the deposits decreases with the distance from the volcano from 5.6 m to a few centimeters. On average, 90% of the components are juvenile (10% dense andesite and 90% vesicular). These coarse-grained beds were deposited in rapid succession by a complex transport process, involving normal fallout, strong ballistic trajectories with a lateral hot (∼ 400 °C) blast surge (LU, equivalent to A 1). Ballistic and coarse tephra sprayed in a narrow (85°) area within about 5.5 km from the lowest crater, and a high (ca. 10 km) eruption column dispersed airfall fine lapilli-ash > 100 km from the volcano. Ash-cloud forming explosions, producing thin pyroclastic surge and muddy phreatomagmatic fallout deposits (FLAU, equivalent to A 2 and A 3), closed the blast surge sequence. The successive explosions on July 30–31 mainly produced block and ash flows, and widely dispersed ash fall. The total volume of pyroclastic material is calculated as 25.8 ± 5.5 × 10 6 m 3 (9.4 ± 2.0 × 10 6 m 3 DRE). A model is proposed to explain the peculiarities of the formation, transportation and emplacement of the blast deposits. The intrusion of the presumed andesitic cryptodome possibly happened through an active thrust fault, favoring not only the formation of the lowest crater A, but also the low-angle explosive events. Prior to the eruption, several minerals were settling to the bottom of the magma chamber as is suggested by the increase of incompatible elements towards the bottom of the stratigraphic section. The major elements indicate that some crystal redistribution occurred and the maximum concentration of Al 2O 3, and Eu, and Sr support plagioclase enrichment in early phases of the eruption (top of LU-1 and DAU-1). From the about 20 recognized prehistoric and historic blast deposits in the world, approximately half were produced by sector collapse of the volcano and the other half by sudden decompression of cryptodomes or lava-dome collapses. The recent blasts (1888–1990s) elsewhere have an apparent recurrence of one event/decade, compared to just a dozen described for the previous 50 ka. Therefore, the adequate recognizers of the blast facies in the cone-building lithofacies, especially for small stratocones as described here, can help in understanding other historic and prehistoric cases, and their related hazards.