Hi. I noticed in your answer to the question regarding predicting eruptions that a "lahar" killed many people in South America. What is a lahar? Is it a sort of land-based tidal wave? The word is not in my dictionary!

rocky April Marine

Hi April,

I didn't answer the original question but yours was passed along to me. A lahar is an Indonesian word for a volcanic mudflow. Some people only use it for hot mudflows, those associated with an actual eruption, but other people use Lahar to describe any mudflow on a volcano. This means that heavy rains or earthquakes can cause lahars (which would of course be cold), even if there isn't an eruption going on.
There are all kinds of technical sub-types depending on the sediment:water ratio but in general a lahar is a fast-moving sediment-laden flood of water. They travel very quickly and cannot be out-run. Because they carry so much sediment and debris they leave thick deposits of mud, sand, boulders, and other debris behind, often many meters thick. Of course they are deadly if they go through populated areas and the 1985 lahars off of Nevado del Ruiz in Colombia killed something like 23,000 people.
A good reference is: Smith GA, Lowe DR (1991). Lahars: volcano-hydrologic events and deposition in the debris flow--hyperconcentrated flow continuum. in Sedimentation in Volcanic Settings. Society for Sedimentary Geology Special Publication #45, RV Fisher and GA Smith, editors.

Good Luck!

Scott Rowland, University of Hawaii 

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