Geog 1202: February 2012

Plate tectonics are responsible for the formation of the island arc of Japan. The Philippine and the Pacific plates continuously move toward and eventually under (a process known as subduction) the Eurasian and North American plates creating the islands of Japan.

This style of convergence forces the subducting plates into the athenosphere generating friction. The partially molten athenosphere and the solid ocean crust become completely molten in areas and that molten material rises to the surface of the ocean crust. The molten material cools as it rises, creating islands. Since converging plates and not hot spots, like Hawaii, create these islands they should continue building as long as the plates move in this fashion.

In the case of Japan, the subduction zone allows for an abundance of active volcanos. Beppu as well as Mount Aso, an active composite volcano, reside on Kyushu. Mt. Aso is not a single peak, but actually composed of Mt. Taka, Mt. Naka, Mt. Eboshi, Mt. Kijima, and Mt. Neko. Pyroclastic flows from four different explosions supposedly formed each peak- the oldest about 270,000 years ago (“Description”). To this day, the crater of Mt. Naka continues realeasing volcanic ash made of incredibly tiny pyroclastics, also known as tephra.

Mt. Naka Crater

Mt. Aso Caldera

One of the world's largest calderas, measuring 11x15 miles, surrounds the five peaks (“Mt. Aso”). Although a composite volcano like Mt. Aso encourages explosive eruptions, this caldera formed nonexplosively. Instead of volcanos collapsing in on themselves after erupting, this area of the continental plate "floated" on chambers of molten material. When the volcanos contained in the area exploded, the magma released from the underground chambers causing the structure to fall. A vast expanse of fertile land capable of sustaining over 50,000 people remained ("Mt. Aso).