>>tHe cAuSes oF LaHar



This mudline left behind on the trees on the banks of the Muddy River after the 1980 eruption of Mount St. Helens shows how high the lahars reached here.

Lahars have several possible causes:

• Snow and glaciers can be melted by lava or a pyroclastic flow during an eruption
• A flood caused by a glacier, lake breakout, or heavy rainfall can release a lahar, also called glacier run or jökulhlaup
• Water from a crater lake, combined with volcanic material in an eruption

In particular, although lahars are typically associated with the effects of volcanic activity, lahars can occur even without any current volcanic activity, as long as the conditions are right to cause the collapse and movement of mud originating from existing volcanic ash deposits.

• Earthquakes underneath or close to the volcano can shake material loose and cause it to collapse triggering a lahar avalanche.
• Rainfall or typhoons can cause the still-hanging slabs of solidified mud to come rushing down the slopes at a speed of more than 30 Kilometers per hour, causing devastating results.