Why does lahar occur

Figure 1. Photograph by R. Janda on December 18, Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and or river valleys.

When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10 m in diameter.

Lahars vary in size and speed. Small lahars less than a few meters wide and several centimeters 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—much too fast for people to outrun.

As a lahar rushes downstream from a volcano, its size, speed, and the amount of water and rock debris it carries constantly change. The beginning surge of water and rock debris often erodes rocks and vegetation from the side of a volcano and along the river valley it enters. This initial flow can also incorporate water from melting snow and ice if present and the river it overruns. By eroding rock debris and incorporating additional water, lahars can easily grow to more than 10 times their initial size.

But as a lahar moves farther away from a volcano, it will eventually begin to lose its heavy load of sediment and decrease in size. Eruptions may trigger one or more lahars directly by quickly melting snow and ice on a volcano or ejecting water from a crater lake. More often, lahars are formed by intense rainfall during or after an eruption—rainwater can easily erode loose volcanic rock and soil on hillsides and in river valleys. Some of the largest lahars begin as landslides of saturated and hydrothermally altered rock on the flank of a volcano or adjacent hillslopes.

Landslides are triggered by eruptions, earthquakes, precipitation, or the unceasing pull of gravity on the volcano. Lahars almost always occur on or near stratovolcanoes because these volcanoes tend to erupt explosively and their tall, steep cones are either snow covered, topped with a crater lake, constructed of weakly consolidated rock debris that is easily eroded, or internally weakened by hot hydrothermal fluids.

Lahars are also common from the snow- and ice-covered shield volcanoes in Iceland where eruptions of fluid basalt lava frequently occur beneath huge glaciers.

Ice-clad volcanoes are common locations for the initiation of small debris flows, owing to their steep slopes, abundant loose rocks and surface water, and diurnal, seasonal, and long-term variability of surface water flow. These events have some seasonality—one population of events occurs after the snowpack has diminished during mid-to late summer, and the other is triggered by early-season intense rains of winter before the snowpack has accumulated.

The Cascades Volcano Observatory offers additional information about these events. To understand lahars, scientists observe and quantify what happens when these events occur naturally, derive equations to describe lahar behavior, and perform controlled experiments in a foot m long debris flow flume. The results help us to understand flow dynamics and develop improved technologies for mitigating the destructive effects of lahars and other debris flows. Because of the danger presented by lahars, scientists are ready to deploy lahar-detection systems when eruptions are imminent.

Officials near Mount Rainier in Washington State maintain a permanent lahar-detection system and accompanying public notification system. Skip to main content. Search Search. Volcano Hazards Program. Lahars move rapidly down valleys like rivers of concrete. Lahar devastation after the eruption of Mount Pinatubo, Philippines. Emergency Management. Survey Manual. Lahars from Mount St. Helens carried this large boulder downstream as it ripped down trees and left a thick mud flow deposit after the May 18, eruption.

Muddy River, Washington. Lahars, along with debris flows and debris avalanches, are masses of rock, mud and water that travel rapidly downslope and downstream under the action of gravity. Volcanoes are a perfect setting for these events because of an abundance of steep, rocky rubble and a ready source of water in the form of rain, snow or ice.

Lahars can flow many miles downstream from the volcano, making this the most threatening hazard in the Cascades. Lahar is an Indonesian word describing a mudflow or debris flow that originates on the slopes of a volcano. Pinatubo lahar, Philippines Lahars are generated by a variety of mechanisms. The majority are produced by intense rainfall during or after an eruption. A tragic example of such an event was the eruption of Mt.

Pinatubo in the Philippines, which was contemporaneous with the arrival of a major hurricane. An estimated people died from buiral by the ensuing lahars, together with the collapse of structures benearth the wet ash. As demonstrated by the graph below, lahars can also be generated directly from a volcanic eruption as massive amounts of water are generated either by the rapid melting of ice and snow, or by the disruption of crater lakes.