Deposits associated with the 1990–1995 eruption of Unzen volcano, Japan

The 1990–1995 eruption of Unzen volcano in Japan was characterized by lava dome growth and block-and-ash flows triggered by dome collapse. The repeated dome collapse events produced block-and-ash flow, ash-cloud surge and fine-grained ash-fall deposits. Block-and-ash flow deposits, which are confined to valleys, are massive to reversely graded, very poorly sorted and contain non-vesiculated to poorly vesiculated blocks up to 10 m across. The deposits often display finer-grained basal parts. Coarse-clast-depleted flow units, which have intermediate grain size distribution between main flow and surge deposits, occur as marginal part of block-and-ash flow deposits. Ash-cloud surge deposits, which spread more widely, are finer grained and better sorted than block-and-ash flow deposits. Ash-fall deposits, which were derived from ash clouds of block-and-ash flows and were dispersed around the volcano, are very fine grained and are as well sorted as the surge deposits. Lahars resulting from remobilization of pyroclastic debris devastated the base of the volcano and their deposits are moderately fines-depleted and occasionally display two different lahar facies: debris flow and hyperconcentrated flow units.