三宅島 1983 年溶岩の冷却過程

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Temperature measurements of the lava of 1983 in Miyake-jima in the Ako district were started fifty days after the eruption and have been continued since then. The following three kinds of temperature data have been obtained (Fig. 2). 1. Temperatures at 20 cm depth along a graveled temporary road on the clinkery surface of the lava using mercury and alcohol thermometers. 2. Temperatures at 0.5 to 2.5 m depth in iron pipes inserted into the clinker layer using thermocouples and mercury thermometers. The pipe holes were distributed along the temporary road and at scattered stations on the surface of the lava. 3. We drilled a borehole (DH-1) which penetrates through 5.5 m-thick lava into the previous ground. Temperature was measured at 10 points in the hole using thermocouples. For comparison, similar measurements in the Awabe district were made in pipes with depths up to 2.5 m (Fig. 3). These pipes were buried in the holes dug into the massive part of the lava for electric poles. The temperature data at 20 cm depth and in the pipe holes (Figs. 5-10) indicate that isothermal surfaces in the clinker layer are very complicated. This complexity is explained by rising plumes of hot vapor irregularly present in the lava field. The vapor is produced by degassing process in the massive part of the lava and comes up through newly formed cooling joints. Once a cooling joint is formed, the temperature of the massive part of the lava around the joint fell rapidly because a gas plume effectively transports the heat from the massive part to the surface. But the rate of temperature decrease varies greatly from one station to another. New plumes were formed sporadically and the temperatures of the new plumes were much higher than the decreased temperatures of the older plumes. Some older plumes died out because degassing process ended or the joints were self sealed by sublimates. It is necessary to arrange a number of observation stations and to add stations timely in order to reveal a cooling history of aa lava like the lava of 1983 in Miyake-jima. Around a plume, a convection cell was identified in the clinker layer (Figs. 16-18), which is similar to a hydrothermal convection system usually found in geothermal areas. The change of the temperature-depth profile of DH-1 with time (Figs. 11, 12) clearly shows that the lava heated the underlying previous ground. The peak shape of the profile has become broader and the depth of the maximum temperature has steadily fallen. The change of the temperature-depth profile also suggests that the upper clinker layer prevented rainfall from effective cooling of the massive part of the lava for the first 250 days. During that time, raindrops were evaporated in the clinker layer and did not reach the massive part below the clinker layer. Difference of cooling rate between Awabe lava and Ako lava may be due to the difference of the thickness of the clinker layers (Figs. 15, 19).
特定非営利活動法人日本火山学会の論文
1984-12-28

三宅島 1983 年溶岩の冷却過程

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