Volcanic mudflows are known as lahars, some of
which have killed tens of thousands of people in single
events. When pyroclastic flows and nuée ardentes move into
large rivers, they quickly cool and mix with water, becoming
fast-moving mudflows. Lahars may also result from the
extremely rapid melting of icecaps on volcanoes. A type of
lahar in which ash, blocks of rock, trees, and other material
is chaotically mixed together is known as a debris flow. Such
lahars and mudflows were responsible for much of the burial
of buildings and deaths from the trapping of automobiles
during the 1980 eruption of Mount Saint Helens. Big river
valleys were also filled by lahars and mudflows during the
1991 eruption of Mount Pinatubo in the Philippines, resulting
in extensive property damage and loss of life. One of the
greatest volcanic disasters of the 20th century resulted from
the generation of a huge lahar when the icecap on the volcano
Nevado del Ruiz in Colombia catastrophically melted
during the 1985 eruption. Nevado del Ruiz entered an active
phase in November 1984 and began to show harmonic
tremors on November 10, 1985. At 9:37 P.M. that night, a
large Plinian eruption sent an ash cloud several miles into the
atmosphere, and this ash settled onto the ice cap on top of
the mountain. This ash, together with volcanic steam, quickly
melted large amounts of the ice, which mixed with the ash
and formed giant lahars down the east side of the mountain
into the village of Chinchina, killing 1,800 people. The eruption
continued and melted more ice that mixed with more ash
and sent additional larger lahars westward. Some of these
lahars moved nearly 30 miles (48 km) at nearly 30 miles per
hour (48 km/hr) and buried the town of Armero under 26
feet (8 m) of mud, killing 23,000 people in Armero and surrounding
communities.
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