Mode of eruption of Miyake-jima volcano in historic times

Abstract

Miyake-jima, an island volcano, resting on the Izu-Mariana arc is a basaltic stratovolcano dotted with many parasitic cones and craters. Literatures tell us thirteen eruptions which took place in A.D. 1085, 1154, 1469, 1535, 1595, 1643, 1712–1714, 1763–1769, 1811, 1835, 1874, 1940 and 1962. The last two eruptions were studied in detail by using modern scientific techniques after the surface activities had almost died down. Of the thirteen eruptions, the earlier five are only described as «eruption» and no detailed accounts are written in a document. The eruptions in and after 1643 all occurred from parasitic craters or fissure vents and are classified into the following two types: Most of these eruptions are simply flank activity, but some are preceded or followed by summit activity. In either type of eruption, the activity begins with the destruction and ejection of pre-existing rocks and then the violent ejection of juvenile scoriae follows. Spindle-shaped bombs seem to be thrown during the later, rather calm stage of the scoria ejection or from lava-filled vent or vents different from the center or centers of the scoria ejection. During the culminating or declining stage of the activity, lava flows down along valley as a torrent and often reaches the sea. In case of the historic eruptions of this volcano, time interval between the start of the activity and the lava outflow is very short, generally less than several hours. The activity abruptly comes to an end soon after the lava outflow. Total volume of the materials erupted in each activity which took place every twenty to seventy years is of the order of 0.01 to 0.02 cubic kilometers. The case history of this volcano as revealed by investigations of the literatures describing the volcanic activity and the basic field survey will give us what a proper means of the prediction of volcanic eruption is.