DISTRIBUTIONS OF TEMPERATURE IN AN UNDERGROUND STRUCTURE ALTERED BY HYDROTHERMAL CONDITIONS

Abstract

Core samples are obtained from places from the surface to about forty meters deep during drilling works of test holes distributed on the slope of the upper side of Myoban steaming ground, where dry steam with hydrogen sulfide is ejected from a number of cracks in the ground. The method of X-rays diffraction is supplied to these samples bacause they show that the upper stratum of forty meters in thickness which rests on unaltered andesite have been almost perfectly altered by geothermal activity. The result makes it possible to distinguish two layers in the altered zone, one of which is called the Montmorillonite layer and the other of which is known as the Kaoline layer. The former layer is limited to the lower side of the cliff only and occupies relatively deeper region. Temperature in this layer is considerably high and some of the holes tapping this discharge steam. In contrast, temperature in the Kaoline layer is rather low and becomes lower higher up the slope of the cliff. This agreement between distributions of ground temperature and clay minerals supports the theory that water infiltration through a Kaoline layer changes the stratum more permeably by acid alteration under hydrothermal condition, and the temperature in the Montmorillonite layer which is occupied by steam is gradually reduced so that the layer becomes saturated by liquid water. It is assumed that such a process of reducing steaming ground may be applied to the problem as to how the area controled only by volcanic heat in ancient times has changed to the present hydrothermal system separating recharge and discharge zones as studied in the usual hot spring districts.