I'd like to recommend this article for its explanation of the type of eruption scientists believe occurred (a steam or phreatic eruption).
-- Kirk
http://the-japan-news.com/news/article/0003269764
Boiling water likely cause of Mt. Aso blast
An eruption at Mt. Aso in Kumamoto Prefecture early Saturday morning was likely a phreatic eruption, in which underground water in the volcano was heated by magma to cause an explosion, a volcanic expert said.