Izu-Oshima Island as a GEOPARK

is a volcanic island about 110km SSW of Tokyo. It is about 15 km

long and 9 km wide elongated in NNW_SSE direction,and 758 m high.

The island is mostly occupied by a basaltic stratovolcano topped by a doubble

caldera and a central cone called Mihara-yama. The stratovolcano called “Izu-

Oshima Volcano”is one of the most active volcanoes in Japan on the volcanic

front along the Izu-Ogasawara trench on the Philippine Sea Plate. The latest

eruption was in 1986,when both central and lateral eruptions occurred issuing

pyroclastic falls and lava flows that forced about 10,000 inhabitants to

evacuate outside the island in only one night. Such sort of middle class

eruptions with several ten million tons of magma occur every 35 years,

however large eruptions more than 10E8 tons of magma generally occur in

every 130-150 years.The latest large eruption was in 1777-1792 when the

central cone Mihara-yama was formed.

Izu-Oshima Volcano displays variety of young volcanic materials(variety of

basaltic pyroclastic products including scoria,spatter,bimb,agglutinate and ash,

and aa and pahoehoe lava flows) together with modes of emplacement of

materials and volcanic landforms.They are quite accessible;the summit of the

central cone Mihara-yama across the caldera floor.Along the trail many

observation points are distributed.On the summit of the central cone,a trail

goes around the crater about 800 m across that has a pit crater about 300 m

across and 250 m deep,giving spectacular scenery on both sides of the trail.

On the middle slope of the volcano,there are many lateral cones that are

aligned mostly on the NNW-SSE direction which corresponds with the regional

stress field in this northen edge area of the Philippine Sea Plate.Middle and

lower slopes are mostly covered by alternations of scoria falls and ash layers

together with lava flows in low portions. Alarge road-cut exposure on the SW

lower slope gives a beautiful alternation of air-fall scoria layers. On the low

altitude area along the sea side ring road,various hydromagmatic products and

related topographies are observed. There are nice tuff rings and explosion

crater on the SE tip of the island. On the eastern cliff,dissected older

volcanoes are exposed showing radial dikes and a central vent which is now

a pinnacle in the sea.

IZU-Oshima island has much to be enjoyed and studied,quite suitable

for a
GEOPARK. The island is easily reached from Tokyo by airplane(25

minutes) and by high-speed boat(1h45m),and from Atami(Izu Peninsula,45min)

A committee on the island is now making an effort to produce a nice

GEOPARK there!                      (by Osamu OSHIMA,Ph.D)

inserted by FC2 system