1966 年 75 巻 2 号 p. 93-104
Taal Volcano, situated at SW part of Luzon Island, Philippines, erupted during 28th to 30th, September 1965 after fifty-four years of quiescence.
As an example of a magmatophreatic eruption of basaltic volcano, this paper deals with the course and the ejecta of the eruption with special reference to the horizontal blasts, based both on a paper by MOORE et al. and on the writers field observation. Topography of the island and historical records of eruption are examined from the same viewpoint by literature survey.
Taal Volcano is an island in Taal Lake and is 25 km2 in area with a summit crater 2 km in diameter. The volcano island is composed almost of pyroclastic materials which are the product of past magmatophreatic eruptions. Sixteen craters are found on the slope of the main flat cone and they are wide in proportion to their height suggesting their origin to be explosive eruptions.
No historical record describes quiet effusion of lava flows, but it indicates explosive nature of eruption.
Essential materials responsible for the present eruption was titanaugie-olivine-basalt. The eruption continued for about 60 hours and no lava flow issued. The area of about 60 km2 was covered by new pyroclastic ejecta more than 25 cm thick. New elongate explosion crater was opened by the present fissure explosion on the southwestern slope of the main cone.
From the base of explosively rising eruption clouds, horizontal blasts spread out in all directions. The blast is turbulent mixture of mud, lapilli, blocks and crept on the surface of the ground with hurricane velocity and near the crater it spilled over a ridge of 300 m in relative height (summit crater rim). But it tended to spread on lower places as it proceeded further away from the crater. Parts of the blast crossed the lake surface over 2 km and caused much damage to the villages at opposite lakeside. The blasts left giant ripples on the surface of the deposits within 2 km from the crater. The wave-length of the ripples decreases regularly from 15 m to 3 m as the distance from the crater increases.
The blasts are regarded to be a low temperature pyroclastic flows. The mobility of the flow is thought to be maintained only by the initial formation of a fluidized system. Because, no delayed vesiculation is expected since the essential material is basaltic in composition, and also because the internal turbulence caused by envelopment of cold air is not great enough, since the temperature of the flow was only about 100°C or below. Magmatophreatic explosion through layers of pyroclastics seems to have been a favourable condition for the initial formation of the fluidized system.