Abstract

On March 28, 1907, a sequence of explosive eruptions within the Ksudach Caldera Complex, located in the southern part of the Kamchatka Peninsula, Russia, partially destroyed Shtyubel' Volcano, leaving a tuff cone with a 1.5 km in diameter horseshoe-shaped crater filled with a deep lake. The total estimated volume of erupted products is 2.4 km3, of which 1.9 km3 consists of juvenile andesitic to rhyolitic material. The eruption can be subdivided into four phases. The first phase opened the vent and dispersed about 0.02 km3 of pyroclastic fallout within Ksudach Caldera. The second phase produced a Plinian column that distributed about 1.5 km3 juvenile pyroclastic fallout to north-northeast and southeast. The third phase was characterized by a powerful blast explosion toward the north-northeast in a 60 sector. The blast cloud traveled at least 100–110 ms−1 to surmount topographic barriers 500–600 m high. Other explosions produced pyroclastic flows, surges, and breccias that cover about 115 km2 with a volume of 0.15 km3. The fourth phase consisted of a series of shallow hydromagmatic eruptions that emplaced five pyroclastic surge deposits with ash cloud surge beds at their tops to the west-northwest. Two or three of these surge clouds must have exceeded velocities of 100 ms−1 in order to surmount Ksudach Caldera walls. These deposits cover an area of 160 km2 with a volume of 0.2 km3. Analyses of deposit structures, particle textures, and fragment components indicate that the first two phases were magmatically driven. However, the last two phases were increasingly hydromagmatic. Lack of erosional contacts between the deposits, abundant impact sags at horizon tops, and absence of reworked materials within the sequence suggest that the eruption lasted only a few hours to a few days. Juvenile products range from basaltic andesite to rhyolite in composition. Based on the variation of chemistry and texture of juvenile components, we hypothesize that there was a zoned magma chamber below Shtyubel' Volcano just prior to the 1907 event.

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