Lahars are called jökulhlaups in Iceland, or mudflows in other regions of the world. What are they? A lahar is a combination of water with volcanic ash and lots of other debris.
The necesarry water can come out of many sources:
- Glaciers are one source: IngeB wrote a really nice post on Icelandic glacier runs. When glaciers start running 1, Author IngeB, January 9th.
- Snow can also be melted by volcanic action and start a lahar.
- A lake can provide the necessary water too. This also happened with the latest eruption of Mount Saint Helens.
- But lahars can also be created when torrential rains pour down on fresh or even old volcanic ash. This happens frequently in the tropics like in the Philippines, Indonesia or when a hurricane strikes Central America. This means a lahar can even happen quite some time after 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.”
This video was taken about a year after Merapi´s eruption.
The name originates in the Javanese language used in Java ( surprise ;) ), Indonesia, where it means lava or lava flow. In Sumeria people worshiped a god called Lahar, he was a god for cattle so I do not really see a connection with this volcanic hazard.
The video shows a dramatic lahar in Japan.
Lahars have the consistency, viscosity and approximate density of concrete. Impossible to escape if someone gets trapped by it. Lahars can be huge. The Osceola lahar produced by Mount Rainier (Washington) some 5,600 years ago resulted in a wall of mud 140 meters deep in the White River canyon, which covered an area of over 330 square kilometers for a total volume of 2.3 cubic kilometres.
Another problem is that a lahar tends to solidify once it comes to a halt. With the potential to flow at speeds up to 100 kilometers per hour, and flow distances of more than 300 kilometers, a lahar can cause catastrophic destruction in its path.
A Jökulhlaup during the Eyjafjallajökull eruption 2010.
http://all-geo.org/volcan01010/2011/12/colima-lahar-videos/ a very informative link describing two different types of lahar, with videos, supplied by Sherine France. (schtevie edit)
Spica (a volcanoholic, but a layman)