Three maars or craters, Itinome-gata, Ninome- gata and Sannome-gata,all of which formed about 10,000years ago,iire locatedon the west coast of northeasternJapan(Fig. 1).Thebasementunderly- ing the Itinome-gatadistrict consistsof a complex of Cretaceousto Paleogenesalic plutonic and vol- canicrocks and Neogenevolcanicsand sediments. According to the field work of Katsui et al. (1979), eruptiontook placein threedistinctstagesseparat- ed by intercalated soil zones. The first stagebegan with the destruction of near-surface rocks and openingof the Itinome-gataand Ninome-gatavents by steamexplosions.This activity wasfollowedby the eruption of gray- to white-colored andesitic pumices and abundant accidental upper crustal
fragments,which are representedby mud flow and base-surgedeposits.All xenolithsderivedfrom the upper mantle and lower crust occur as ellipsoidal bombs or as loose blocks. within the andesitic pumices,up to 30 cm in diameter.Megacrystsof diopside(lessthan5cmin size),whichmaycontain euhedral forsterite crystals, occur also as crystal lapilli or as "xenocrysts" in the host rocks. In the third stage,basaltic scoria falls were erupted from the Sannome-gatacrater; thesealso contain small amountsof deep-seated xenoliths.