Lahars are a form of mudslide or landslide that doesn’t necessary need to be triggered by volcanism. The name lahar derives from the Javanese Hindi language in Indonesia. The volcanic mud-flow in Hindi means wave. Lahars can effect all of the mountains of the world; but not all of them are prone to lahars. The appearance of lahars from the pyroclastic result create a cone shape top that covers the top of the crater that are often snow covered or contains water in a crater lake.
Lahars can result from volcanic eruption or by melting glaciers that produce flooding in the summit crater, or by heavy rainfall, or ice in the crater. In addition, water deposits that produce lahars can occur at (or) beneath the crater of the mountain. Thereby, producing glacial outburst (s) from the flooding activity, or by a volcano eruption. The eruption of pyroclastic density can flow out tens of meters per second and spread several kilometers away.
Lahars are superheated to temperatures that can range up to 1250 degrees. The shape and size it takes on varies. It depends on the clay deposit ingredients of the consistency. Lahar can build up its consistency of hot gas, ash, and rock to 10 meters, or more. The flowing material of pyroclastic resembles a bed of hot liquid concrete rushing by. Their speed and makeup depends on the texture and consistency of the clay content. They can either be cohesive or non-cohesive. The cohesive lahar picks up debris in is way and can reach heights of 100 meters or more.
They can range from ten to hundreds of kilometers downstream and take out anything in its way depending on the clay content. Cohesive texture contains more than 3 to 5 percent clay which allows the flow to stay at its consistency as it rushes down the mountain into the lower lying valley entering into nearby streams and rivers. If the sediment content reaches 60 percent it can be a very violent construction of deadly moving force that will take out any bridges, building, and anything else that is in its path. Lahars can travel up to 100 miles an hour and take anything in its path down if it is cohesive.
If the flow is non-cohesive it contains 1 to 5 percent clay and is diluted as it flows with little consistency that outruns the sediment. This type will lose speed as it sheds sediment; making its flow slow down and continue to decrease in size and shape. The mountains that are known for producing lahars are high threat areas such as: USA at Mount Rainier, New Zealand’s Mount Ruapehu, and Indonesia’s Galunggung mountain. Some towns near Mount Rainer are built on top of lahar. The towns are Orting, Washington State, and the Puyallup River Valley, in Washington State have been constructed atop lahars.
Mount Rainer has been investigated by scientists because of the high risk factor involved with the future eruption from this mountain. Mt. Rainier explodes every 500 to 1,000 years according to scientists and it has been 550 years since the mountain erupted. The valleys that would be hit would be
Mount Saint Helen in Washington State is another hot spot but not considered as high of a risk of eruption. In 1980 the volcano erupted producing cohesive pyroclastic content that wiped out 27 bridges, destroyed 200 homes, and took out 185 miles of roadway, along with 15 miles of railway train tracks.
When this mountain erupts again in the future scientists have predicted that it will hit the valleys surrounding area of the mountain on the North of Puget Sound. Area. Also, Orting, Sumner Ashford , Elbe, Packwood, Randle, Greenwater, South Hill Section of Puyallup, Milton, Auburn Commencement Bay, Kent, Renton will be in its way.
In addition, Tocoma, Buckley, and Enuclaw will all experience some effects when an eruption there produces
pyroclastic material that is cohesive. The least likely areas to be hit by any lahars from this mount would be Northward, and Southward sides of Mount Saint Helens because it is blocked by other mountains.
It is estimated that Mount Rainier is the most likely mountain to erupt in Washington State in the future in regards to the two mountains. Scientists have estimated that the end result of an eruption on Mount Rainer would be restricted to the valley floor closest to the vicinity of Mount Rainier National Park. In addition, the data estimates that the slow moving tephra eruption would allow people enough time to safely leave the danger area scientists conclude.
It is good to know that the mountains are monitored for any likely activity and the technical gadgets used are examined often to make sure they are in working order. If lahar activity or volcanic activity was detected at a serious level sounding alarms will go off. Giving communities time to escape from the fury. At this time scientists do not know when an eruptions will occur or the magnitude of the impact on the surrounding areas..
Reference
Volcano Slides and Flows
http://faculty.weber.edu/bdattilo/shknbk/notes/vlchzrds.htm
USGS
http://vulcan.wr.usgs.gov/Glossary/Hydrology/description_hydrologic_hazards.html
Channel 5 News report on the ‘Sleeping Giant’
http://www.king5.com/news/specials/volcano/stories/NW_050505WABmtrainierEL.2469135af.html
