Cooling History of the Lava of 1983, Miyake-jima, in these Three Years with the Formation of the Hydrothermal System

Abstract

The October 1983 eruption of Miyake-jima, about 180 km south of Tokyo, left aa lava flow of 5-lOm thick, which covered the Ako district. The thickness of the upper clinker part of the lava flow, in which air and water circulated easily, ranges from 0.5 m to more than 3 m. Temperatures in the lava flow were monitored by means of thermocouples in steel pipes inserted in the upper clinker part starting 50 days after the eruption and in a drill hole, Miyake-GS-DH-1, penetrating the flow starting 116 days after the eruption. Both monitorings were continued until 3 years later. One year after the eruption, the maximum observed temperature in the pipe holes was 403°C. 81/2 months later, it fell to less than 100°C, Rainfall, which ranged from 2.4 to 3.0 m/year, greately increased the rate of cooling of the lava flow. But the degree of cooling effect by water varied because of the hydrothermal convection system. There are two convection systems in the lava. One is in the narrow cooling joint, generated largely by thermal contraction in the massive part of the lava, and the other is in the upper clinker part just above the previous one. Three dimensional temperature distribution in the latter one is observed directly through pipe holes. The convection system was mantled by the 100°C water saturated zone. The surface of the massive part near the cooling joint wass not affected by the cooling water while it was roofed by the 100°C zone. At the site of Miyake-GS-DH-1, a 5.5 m thick lava flow, which was divided into upper clinker (2.2 m), massive (2.5 m) and lower clinker part (0.8 m), covered the previous ground with 1 m vacant space between them. The maximum temperature was found in the massive part just after the starting of the measurement. And then it fell down to 0.3 m depth in the previous ground at 71/2 months after the eruption and to 1 m depth, 2 months later.