A Year: Day to Day Men: 21st of May
Opposites: Torrid and Frigid
May 21, 1792 is the date of Japan’s Mount Unzen’s deadliest volcanic eruption.
Mount Unzen consists of a group of composite volcanic domes located on Japan’s Shimabara Peninsula east of Nagasaki. This area 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.
Unzen’s deadliest recorded eruption occurred on May 21 in 1792, with a large igneous rock lava flow coming from the Fugen-dake dome. The east flank of the Mayu-yama dome collapsed unexpectedly following a post-eruption earthquake, creating a landslide into Ariake Bay. This caused a mega-tsunami that reached a height of 330 feet. The wave surge devastated nearby areas, causing further widespread damage and death. Most of the estimated 15,000 deaths caused by the event are believed to have resulted from the landslide and the tsunami. The scar created from the Mayuyama landslide remains visible today.
After 1792, the volcano remained dormant until November 1989 when an earthquake swarm, a series of earthquakes in a short time, began about 12 miles underneath and 6 miles west of the Fugen-dake dome. Over the following year, earthquakes continued, their hypo-centers gradually migrating towards the summit. The first steam blast eruptions began in November 1990, and after inflation of the summit area, fresh lava began to emerge on May 20, 1991.
The threat of further disastrous events prompted authorities to evacuate 12,000 residents from their homes. On June 3, 1991, the volcano erupted violently, possibly as a result of depressurization of the magma column after a landslide in the crater. A pyroclastic flow triggered by the collapse of a lava dome reached 3 miles from the crater and claimed the lives of 43 scientists and journalists, including volcanologist Katia and Maurice Krafft and Harry Glicken.
From 1993 onward, the rate of lava effusion gradually decreased, and eruptions came to an end in 1995. Since then heavy rains have frequently caused pyroclastic material, rocky debris and water to flow down the slopes. Dikes have been constructed in several river valleys to channel the lava flows away from vulnerable areas.