The main trends in the dynamics of vegetation on the territory affected by the catastrophic eruption of Bezymyanny Volcano on March 30, 1956 (Kamchatka)

Abstract

The transformation of the vegetation cover in the impact zone of the 1956 eruption, in territories covered by various deposits, is considered. As a result of a gigantic eruption (VEI 5), vegetation was exposed to a series of different volcanic impacts. Five main categories of events are distinguished: the movement of material of a huge volume of volcano edifice over a large distance as a result of a giant clastic avalanch, the pyroclastic surge of a direct blast, the pyroclastic flows, the formation of a giant eruptive cloud and ashfalls, as well as the lahars. The volume of erupted (initially high-temperature) deposits was, according to various estimates, in the amount of 1.35-1.5 km3, the volume of cold deposits of a clastic avalanche was 0.5-0.8 km3. The volume of lahar was 0.5 km3. The area covered by the pyroclastic wave of the directed explosion was about 500 km2. Within this lesion zone, deposits of pyroclastic flows have occupied 30-40 km2, and clastic avalanche deposits from 35 to 60 km2. Below 900 m above sea level (a.s.l.) these deposits buried cover of subalpine dwarf alder (dominant species is Alnus fruticosa) and mountain meadow vegetation, as well as forest vegetation (dominant species is Betula ermanii) at its upper limit. Forest and partially dwarf alder vegetation was destroyed on a vast territory mainly under the influence of a pyroclastic wave (in the altitude range from 700-800 to 200 m a.s.l.), as well as lahars (in the range of 250-50 m a.s.l.). Primary successions occur in the alpine and partially subalpine zone on avalanche deposits and pyroclastic flows deposits, as well as in the upper part of the zone impacted by pyroclastic surge of the direct blast (40-45 km2). In part of the territories where thick deposits of the lahars were formed, primary successions also probably occurred. In the zone of primary successions, deposits of a clastic avalanche are settled by plants most slowly due to not-favourable edaphic factors. The process is somewhat more efficient on the deposits of pyroclastic flows (the same ratio was noted on the Shiveluch Volcano). The surface overlapped by deposits of the pyroclastic surge is populated relatively quickly. Secondary succession occurs in the zone of damage to the forest and dwarf trees by the influence of a pyroclastic wave, as well as in the zone of passage of the lahars. Restoring of vegetation to its previous state will take from 50 to ~500 years on different deposits and in different parts of an impact zone.