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Daily JournalDay 6 | July 23, 2005Oshima Island
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.
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
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.
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