Unzen is a complex of stratovolcanoes on the island of Kyushu. It is active and has a long history of eruptions; the oldest eruptions date to 6 million years, and the youngest, well, are still in progress. The latest severe activity reported by the USGS consists of a series of pyroclastic flows (ash eruptions and lahars) that occurred between 1991 and 1995. The worst, however, was in 1792 when the 4000 year old Mayu-yama lava dome erupted with a series of dacitic lavas. Silica-rich dacites are among the most dangerous types of lavas: they are hard viscous lavas (the more silica in a lava, the greater its tendency to form strong silicate molecules bonding its internal structure and making flow difficult), and rather than flow as do the basalts of Hawaii, they tend to cause explosive eruptions. The eruption instigated a massive landslide that devastated Shimabara in its passing, continued into the sea, where it then generated a massive tsunami: 15,000 people were killed in the debris landslide and subsequent tsunami, making it Japan’s worst volcano- related catastrophe.
This image is a map of the Unzen Volcano documenting flows and destruction published as a book plate in 1822. Its author, Issaac Titsingh, was a member of the Royal Society, a surgeon, and a lawyer. While a member of the East Indian Company (between about 1765 and 1799), he worked in Japan and has become known as a “Japanologist.” His map is the closest thing to a scientific eye-witness account of the region following the 1792 eruption and catastrophic tsunami.
And his geologic map – it’s a piece of art!
Image from “"Illustrations of Japan consisting of private memoirs and anecdotes of the reigning dynasty of the Djogouns, or sovereigns of Japan..." by Isaac Titsingh, London: R. Ackermann, 1822”