A dome of destruction
After 198 years of quiet, Mount Unzen reawakened in November 1990. The initial phreatic, or steam-blast, eruptions were small, but by May 1991, thick, viscous dacite lava began to squeeze out of the summit of Fugendake peak. This high-silica lava was too thick to flow far, so it piled up, creating a mound of hot rock called a lava dome. This dome grew unstably on the mountain's shoulder. As more lava pushed from below, the dome's steep flanks would frequently crumble under gravity.
On June 3, 1991, a larger section of the dome collapsed. This triggered a block-and-ash flow, a type of pyroclastic flow, that accelerated down the mountain. This avalanche of hot rock, ash, and gas reached 4.5 km from the crater and claimed the lives of 43 people, including volcanologists Katia and Maurice Krafft and Harry Glicken. Over the course of the eruption, which lasted until 1995, the volcano produced nearly 10,000 such pyroclastic flows, destroying more than 2,000 buildings in its path. The total volume of erupted magma was 210 million cubic meters, with about half of that volume collapsing into pyroclastic flows.
An avalanche of ground rock
The pyroclastic flows at Unzen were a specific type known to geologists as a nuée ardente, or "glowing cloud." These are generated by the mechanical collapse of a lava dome rather than the explosive disintegration of magma in an eruption column. The material in the Unzen flows consists of dense, blocky fragments from the pre-existing dome, mixed into a matrix of finer ash.
Detailed scientific analysis of the ash deposits from the Unzen flows revealed a surprising origin. A significant portion of the fine ash particles were not created by the explosive expansion of gas in magma. They were the product of mechanical grinding. As the massive, hot blocks of the collapsing dome tumbled and crashed against each other in the avalanche, they milled themselves down into fine dust through a process called comminution. This self-grinding of solid rock is what produced much of the ash. This discovery showed that dome-collapse pyroclastic flows are fundamentally different from those of Plinian eruptions, where ash is formed primarily by the fragmentation of frothy, gas-rich pumice. The 1990-1995 eruption was one of the most thoroughly documented lava dome eruptions in history, providing data on these hazardous processes.