Volcanological and Paleomagnetic Studies of Welded Tuff from Sikotu Caldera, Hokkaido

Abstract

Sikotu Caldera near Sapporo has been studied from the viewpoints of the distribution of its welded tuff, the subsurface structure, and the origin of its formation. The age of the caldera formation was already determined by means of ¹⁴C as 32, 000 years B. P.. In this paper, magnetic properties of the welded tuff from the caldera was discussed from the standpoints of volcanology and paleomagnetism. The specimens were collected mainly at an outcrop 20 meters high at a distance of 18 km from the caldera. First, paleomagnetic intensities of geomagnetic field at that period were determined by Thellier's method, as about 1.4 times the present one. Second, the temperatures of deposition of the ejecta at various depths of the same outcrop were deduced by comparison between the natural remanent magnetization and the partial thermal remanent magnetization. The middle part of the outcrop might arrive at the spot with the temperature higher than Curie point and then cooled acquiring the N. R. M. while the upper and lower parts were near 500℃. Such distribution of the temperatures of deposition is not due to the effects of cooling at the upper and lower boundaries but may be due to the changes of the original temperatures of the ejecta within the volcano, or due to the differences in cooling of the ejecta during the travels, at each stage of the eruption.