Abstract
As a result of various investigations, it has been found that Niijima Island in the Izu Archipelago consists of several volcanic masses which are mainly composed of liparitic lavas and ejecta erupted in the Quaternary.
The present writer newly defined the relationship between these lavas and ejecta in the formation of several Homa-Tholoides. We find three of them now existing at Mt. Miyatsuka, Mt. Atchi and Mt. Mukai. Almost all of the Homate of Mt. Miyatsuka have been eroded and destroyed, but both Mt. Atchi and Mt. Mukai still retain their Homates well preserved. (See Fig. 3 and Fig. 6)
A deposit of basaltic lapilli in the northernmost part of the island makes a sea cliff about 50 m above sea level. I. Friedlender considered that the lappili had been ejected from Udone Island (about 4km off the northern point of Niijima Island) which consists of basaltic lava and lapilli or scoria.
On the other hand, Tarô Tsujimura and Hiromichi Tsuya thought that the lapilli had come from “a craterlike hollow” which was inside the island (craters 1 and 2 in Fig .3). But the present writer believes that it was ejected from Craters 3 and 6 in Fig. 3 on the basis of its depositional feature. That means Craters 1, 2, 4, and 5 are a group of small Homates relating to Mt. Atchi.
It has also been discussed whether these ejecta-beds, both basaltic and liparitic, are of submarine or subaerial formation……the former was asserted by Nobuyo Fukuchi and Hisashi Satô, but the latter by Tsujimura and Tsuya.
The present writer concludes that these ejecta-beds are of subaerial formation because of new findings that the Mt. Daisan Tuff Breccia, lying immediately below the Shiromama (one of the most widespread deposits of liparitic ash and pumice) shows some features of fluvial erosion not retainable in the sea, and just below the sea cliff of the basaltic lapilli at Kadobara there can be seen a small exposure of a talus composed of liparitic materials. (_??_, _??_, in Fig. 4)
The above, coupled with the fact that all of these ejecta have been originated from Niijima itself, and that there are, in these ejecta, so many Tertiary materials ground in the sea…… (which are thought as fragments from the base of Niijima Island)……may possibly mean that some depression of the island happened after the last eruption, and, at least, that may confirm a common view of volcanology that we seldom experience the uplift after the eruption.
Reference: Hiromichi Tsuya's report in the Bulletin of the Earthquake Research Institute University of Tokyo, Vol. 15, published in 1937 in English, pp. 309-316 “On the Volcanism of Huzi Volcanic Zone, with Special Reference to the Geology and Petrology of Idu and Southern Islands.”
