Volcanic eruptions are among the natural dangers with which mankind has had to deal since there have been people. They are also among the very small number of disastrous events that can have positive (agriculturally rich soils) as well as negative effects (destruction of life and property). The positive effects have encouraged people to live in the shadow of disaster or the chance of disaster. Just think of the population centers in the area of Vesuvius (Italy), Mt. Pinatubo (Philippines) and Mt. St. Helens (USA).
Among the most dangerous of volcanic hazards is an event known as a lahar. Lahars are similar to pyroclastic flows, and may even start out as a pyroclastic flow. One major difference between lahars and pyroclastic flows is the presence of water in the mix. Another difference is the fact that there does not need to be a volcanic eruption in progress, just volcanic debris available for an addition of water to move.
The word “lahar” comes from Indonesian, and is the term that was used for a rapidly flowing mix of volcanic debris and water. It is often defined now as “volcanic mudflow.”
While a volcanic eruption is not always present for a lahar occurrence, it is the most common generator, being the proximate (close in time) cause in at least 50% of lahar events. In these cases, ice and snow are melted, usually by pyroclastic flows (which become part of the lahar) or lava flows. Natural dams and crater lakes can be sources of the water for a lahar. If the dam or one of the banks of the lake has a weak spot, leading to a release of water, a lahar can occur. Other initiating events for lahars have also included rainfall on areas of loose ash.
Lahars can be small, covering only a few centimeters in depth and width, to huge in scope. The Osceola Lahar, which occurred about 5700 years ago at Mt. Rainier (Washington) is one of the largest in the world. It flowed over 100 kilometers down the White River valley from the source at Rainier, covers over 300 square kilometers, and is up to 140 meters deep. The flow of ash, rock, debris and water have been measured at rates of up to 85 kilometers per hour. They always flow downhill and down valley, so the best protection for humans in the area of an active lahar event is to move uphill, perpendicular to the flow.
Volcanoes with well-known documented lahar flows include Mt. Pinatubo, Mt. Rainier, Nevado del Ruiz (Colombia), Mt. Ruapehu (New Zealand) and Galunggung (Indonesia).
Sites for more detailed information and for further links include:
http://www.geo.mtu.edu/volcanoes/hazards/primer/lahar.html http://vulcan.wr.usgs.gov/Glossary/Lahars/description_lahars.html http://volcanoes.usgs.gov/images/pglossary/lahar.php http://www.geology.sdsu.edu/how_volcanoes_work/Lahars.html http://en.wikipedia.org/wiki/Lahar