Lahars / Volcanic Mud Flows
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A lahar(Indonesian) or volcanic mudflow is a mixture of volcanic debris and either hot or cold water which flows down the side of a volcano. Lahars occur when large amounts of water quickly mix with either newly erupted volcanic material or old material resting on slope.
The water comes from either melted ice,breached crater lake, an engulfed river or rainfall.It resembles flowing concrete with particles from clay to boulder size, and gains new material as it erodes its bed. A lahar can move very quickly from a few meters/second to 10s of meters /second- faster than a man can run. These mudflows can also travel 10s of kilometres from their origin moving alon established river valleys.
Lahars occur mainly on steep sided stratovolcanoes where the combination of steep slopes,unconsolidated slopes,explosive eruptions,crater lakes and high altitude allowing ice formation, provide ideal conditions for lahar formation.Nevado del Ruiz 1985: Armero: Colombian Andes
On 13th November 1985 Nevado del Ruiz erupted dactic tephra 19 miles into the atmpsphere. Pyroclastic flows melted ice and snow on the summit and created 4 lahars which sped down the volcano and into the valleys below. The unsuspecting town of Armero,30 miles away,was engulfed and buried by the mudflow killing around 20,000 people.23,000 were killed overall by the eruption. Armero was built on an old mudflow, scientists were aware of the imminent danger but government officials refused to take action to evacuate.
Mt Rainier: Cascades USA
Mt.Rainier is considered one of the most dangerous volcanoes in the world. Located in the Cascade range between MtSt Helens and Seattle.The volcano has the most extensive glaciers in the lower 48 states and the underlying rocks have been weakened by hydrothermal activity.
If an eruption occured massive lahars could travel north as far as Seattle and the Puget Sound.