Fugendake

Mount Unzen
Mt. Unzen, showing extensive pyroclastic flow and lahar deposits
Highest point
Elevation	1,500 m (4,900 ft)
Coordinates	32°45′24″N 130°17′40″E / 32.75667°N 130.29444°E / 32.75667; 130.29444Coordinates: 32°45′24″N 130°17′40″E / 32.75667°N 130.29444°E / 32.75667; 130.29444
Geography
Mount Unzen Kyūshū, Japan
Geology
Age of rock	Oldest 500 kyr
Mountain type	Complex stratovolcano
Last eruption	February to May 1996

Mount Unzen (雲仙岳, Unzen-dake) is an active volcanic group of several overlapping stratovolcanoes, near the city of Shimabara, Nagasaki Prefecture, on the island of Kyūshū, Japan's southernmost main island.

In 1792, the collapse of one of its several lava domes triggered a megatsunami that killed 14,524 people in Japan's worst-ever volcanic-related disaster. The volcano was most recently active from 1990 to 1995, and a large eruption in 1991 generated a pyroclastic flow that killed 43 people, including three volcanologists.

Currently its highest peaks are Fugen-dake (普賢岳) at 1,359 metres (4,459 ft) and Heisei-shinzan (平成新山) at 1,486 metres (4,875 ft). The latter emerged during the eruptions of the early, eponymous Heisei era (1989–).

Mount Unzen is part of Shimabara Peninsula, which has seen extensive volcanism over millions of years. The oldest volcanic deposits in the region date from over 6 million years ago, and extensive eruptions occurred over the whole peninsula between 2.5 and 0.5 million years ago.

The origins of the Unzen complex are traced to the formation of a graben through crustal faulting. This caused parts of the peninsula to subside by up to 1,000 metres (3,300 ft) below sea level and may have caused eruptive activity to localize at one site inside the graben. Eruptions of dacitic lava began from a site slightly to the south of today's Mount Unzen and migrated north over time.

The volcano rapidly grew during its first 200,000 years, forming a large cone. Later eruptions over the following 150,000 years filled in much of the graben. Initially, activity was dominated by blocky andesitic lava and ash flows, changing to dacitic pumice flows and airfall deposits from 500,000 to 400,000 years ago. The period from 400,000 to 300,000 years ago saw the emplacement of large areas of pyroclastic flow and lahar deposits; these form the major part of the volcanic fan surrounding the volcano. Beginning 300,000 to 150,000 years ago, thick phreatomagmatic deposits were laid down, suggesting the subsidence of the volcano into its graben was rapid during this period.