Ring-shaped Cone Formed During the 1983 Miyake-jima Eruption

Abstract

A ring-shaped cone with the diameter of the base and the crater rim 400 and 150 meters, respectively, was formed during the Oct. 3, 1983 Miyake-jima Eruption by shallow submarine eruption around Nippana coast, marking the southern end of the 1983 fissure. The cone consists of well-bedded surge deposits showing nearly circular distribution with the center of the S crater. The author investigated the process of the formation of the cone through geological and volcanological observations of outcrops, VTR and photographs of the eruption and grain-size analysis. The results are as follows : (1) The ring-shaped cone comprises three units of deposits ; S-1, S-2 and S-3, in ascending order. S-1 represents the opening stage, rich in breccia with a massive matrix of fine sideromelane fragments. S-2 makes up the main portion of the cone and is surge deposits which are composed mainly of stratified sideromelane fragments accompanied with lithic fragments associated with block sag structure, water-chilled bombs, and fragmented water-chilled scoria. S-3 is a thin layer of surge deposits showing the reactivation of the S crater in the waning stage. (2) S-2 is characterized, as a whole, by planar bedding. Its lithological features change vertically : the lower deposits (S-2l) rich in water-chilled bombs, block sag structure and thin layers of mud, are dominated by planar bedding, lack material finer than 4 phi and are distributed only in the proximal area. The upper (S-2u) lacking water-chilled bombs except for the lowest horizon, are characterized by increase in fine material, intercalations of vesicular scoria from the R crater, occasional occurrences of dune bedding and wider distribution. (3) Block sag structure, water-chilled bombs and lens of water-chilled scoria are found as ballistic deposits only within 300 meters from the center of the S crater. In the distal area, planar bedded and partly dune-bedded surge deposits are dominant, showing the dominance of laterally moving cloud. It suggests that two different processes of emplacement, cock's tail jets and base surge cloud, existed simultaneously. (4) Ring-shaped surge cloud and cock’s tail jets around the S crater and lava fountain from the R crater in the vicinity were recorded in VTR and photographs of the eruption at about 17 : 30. Mixed processes of these cloud and jets produced the ring-shaped cone. (5) The mode of eruption resulted in the ring-shaped cone changed from wet one characterized by the dominance of cock’s tail jets, gradually to drier one dominated with base surge cloud.