In 1985, lahars triggered by melting of glaciers near the summit of Nevada del Ruiz in Colombia destroyed and buried the town of Armero, killing 25,000. In the Philippines, in 1992, Mt. Pinatubo let loose some enormous lahars, but due to good warnings, there were very few deaths. Lahars can also occur after an eruption, as the loose deposits of ash form ashy avalanches or are mobilized by rain. A pyroclastic flow killed 29,000 in 1902 when Mt. Pelee erupted on the Caribbean island of Martinique . The hot gases reduce friction and allow the fast motion.
Phreatic explosions occur when volcanic heat makes steam.
A powerful phreatic explosion occurred two months before the big eruption at Mt. St. Helens. It blew when magma heated groundwater that flashed to steamn and sent ash several thousand feet into the air. On the flanks of Mauna Loa, lava flows run downhill to the ocean, forming new land at the edge of the sea. Mauna Loa on Hawaii, standing 10 kilometers from ocean floor to summit, is the classic shield volcano. Seamounts are underwater volcanoes that appear over a mid-ocean ridge or a hot spot.