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• Shikinejima

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Daily Journal

Day 6 | July 23, 2005

Oshima Island

Collecting rocks on Oshima

Our last full day on Oshima was taken up by visiting outcrops around the island to complete our collection of rocks. One of the highlights of the day was our visit to the Museum of Volcanoes which is a must-see attraction for visitors - particularly for a group of volcanologists. The museum was magnificent - with detailed explanations of the geological history of Oshima volcano illustrated by rock specimens and spectacular photography. There were also displays of other volcanoes of Japan plus a section dedicated to volcanoes of the world.

Some of the photographs on display were taken by the Kraffts - a husband and wife team of French volcanologists. Ironically, the Kraffts were killed in Japan in 1991 when a pyroclastic flow engulfed them and about 40 other people at Unzen volcano in the southwest of the country. Much of Oshima is covered by pyroclastic deposits and the museum is built on a so-called 'cold' pyroclastic flow as shown by the preservation of pieces of wood in the rock matrix.

'Cold' pyroclastic flow

The volcanic history of Oshima volcano is beautifully displayed in a series of alternating layers of air-fall deposits of scoria and ash. These layers repeat about 120 times over the past 20,000 years . The initial blast of an eruption lays down scoria - pebble-sized pieces of solidified rock. This is followed by fine-grained ash. After about 20-50 years or so the eruption finishes and the ash starts to weather into soil. The next eruption repeats the cycle laying down the next layer of scoria on the weathered ash. The outcrop we observed today nicely illustrates the cycle and allows volcanologists to estimate how often a major eruption will occur. For Oshima, a major eruption occurs about every 150 years.

Alternating layers of air-fall deposits of scoria and ash

As we continued around the island, we were reminded of the power of the subduction zone - which produces these volcanoes - as we were gently rocked for about 2 seconds by an earthquake. We later found out that it was a 5.7 magnitude event on the mainland of Honshu, not too far from Tokyo. No wonder that there are reinforced concrete shelters dotted all over the island.

dave and nia

Reinforced concrete shelter