A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley.
Lahars are extremely destructive: they can flow tens of meters per second (22 mph or more), be 140 meters (460 ft) deep, and destroy any structures in their path.
A lahar is a volcanic mudflow or debris flow. Lahars have the consistency, viscosity and approximate density of wet concrete: fluid when moving, solid at rest. Lahars can be huge.
A lahar of sufficient size and intensity can erase virtually any structure in its path, and is capable of carving its own pathway, making the prediction of its course difficult. Conversely, a lahar quickly loses force when it leaves the channel of its flow: even frail huts may remain standing, while at the same time being buried to the roof line in mud. A lahar’s viscosity decreases with time, and can be further thinned by rain, but it nevertheless solidifies quickly when coming to a stop.
Lahars vary in size and speed. Small lahars less than a few meters wide and several centimetres deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second (22 mph or more): much too fast for people to outrun. With the potential to flow at speeds up to 100 kilometers per hour (60 mph), and flow distances of more than 300 kilometers (190 mi), a lahar can cause catastrophic destruction in its path.
Lahars have several possible causes:
- Snow and glaciers can be melted by lava or pyroclastic flows during an eruption.
- Lava flows out of open vents and can mix with wet soil and mud on the slope of the volcano making a very viscous, high energy lahar. (The higher up the slope of the volcano the more gravitational potential energy the flow will have.)
- 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.
- Heavy rainfall on unconsolidated pyroclastic deposits.
- Volcanic landslides.
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.
- Snow and glaciers can melt during periods of mild weather
- Earthquakes underneath or close to the volcano can shake material loose and cause it to collapse triggering a lahar avalanche.
- Rainfall 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 (20 mph), causing devastating results.
Damavand volcano is the tallest volcano in Iran and Asia (5671m.). This volcano is currently a semi-active volcano. Smoke and steam come out from the volcano, but it has not erupted for years. Its last volcanic activity was 38,500 years ago (based on age determination by carbon 14). The mountain range is covered by frequent lava streams that have been flooded with peaks from the summit or sub-cones, as well as pyroclastic materials such as Pumice, Tuff and Lahar.
Lahar deposits accumulated around the city of Rudehen (near Tehran) are shown in the image. These deposits were flowing down the slopes of the Damavand volcano in ancient times. These deposits consist of fine grained deposits (ash) and large and small boulder(lava).
Keywords: Iran Geology , Iran Geotourism , Lahar , Rudehen , Tehran province , zamingasht