This massive boulder (top photo) was carried down the Biwasawa valley on the east side of Bandai volcano in a mudflow during an eruption in 1888.
The identical boulder now forms part of the landscaping of a house in the town of Inawashiro.
Top photo by Fukushima Minposha Newspaper, 1888; bottom photo by Lee Siebert, 1988 (Smithsonian Institution).
A hot lahar sweeps down a channel on the SW flank of Mayon volcano in the Philippines on September 14, 1984, five days after the onset of an eruption.
Stratovolcanoes Shield Volcanoes Calderas Craters Fissure Vents Pyroclastic Cones Lava Domes Pyroclastic Fall Pyroclastic Flows Magma meets Water Submarine Eruptions Lava Flows Lahars (mud flows) Volcanic Landslides Geothermal Activity Volcano Monitoring Volcanoes and Humans Mudflows are somewhat of a misnomer, because these volcanic flows include not only mud, but debris ranging up to boulder size.
The term, however, has been commonly applied to water-saturated volcanic flows and is well entrenched in the literature.
The Indonesian word lahar refers to mudflows in volcanic terrain.
Lahars can occur both during an eruption and as secondary flows long after an eruption is over, as rainfall remobilizes volcanic ash deposits.
Lahars have lower velocities than debris avalanches formed by volcanic landslides, but can travel long distances beyond a volcano, inundating large areas in low-lying terrain.
The most catastrophic lahar in historical time took place on 13 November 1985 at Nevado del Ruiz volcano in Colombia, when a lahar struck the city of Armero in the dark of night, catching people unawares and causing more than 21,000 fatalities.