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Define Lahars

What are lahars?

The word lahars is an Indonesian word for rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflow or debris flows.

Pyroclastic material primarily is composed of volcanic material.

What kinds of hazards are associated with volcanic eruptions?

Debris flows, or lahars, are slurries of muddy debris and water caused by mixing of solid debris with water, melted snow, or ice. Lahars destroyed houses, bridges, and logging trucks during the May 1980 eruption of Mount St. Helens. Lehars can flow for hours and destroy everything in their path.

LaharsTephra is made up of ash and coarser debris and composed of fragments of magma or rock blown apart by gas expansion. Tephra can cause roofs to collapse, endanger people with respiratory problems, and damage machinery. Multi-band radios are thus important to have to keep updated incase of power outages. Tools such as work gloves and survival shovels are also important to have in case a collapsed roof.

Pyroclastic surges and flows are hot, turbulent clouds of tephra, or dense, turbulent mixtures of tephra and gas. Pyroclastic flows and surges can travel more than a hundred miles per hour and incinerate or crush most objects in their path.

The risk of mudflows formed this way is especially high along rivers downstream from Mount Rainier, because of the large population on floodplains, the huge weakened edifice of the volcano, and a long history of large flows that occurred when the volcano was otherwise dormant.

How to prepare for future eruptions

Residents can obtain copies of USGS volcano-hazard reports to determine whether they live or work in areas at risk from volcanic activity. Everyone should plan how they and their family will respond to a natural disaster, including unrest or eruptive activity at nearby volcanoes. Preparation might include knowing where to go when family members are separated, where to go for emergency housing, what emergency supplies to keep on hand, and how to be self sufficient for several days, as recommended by local emergency management agencies. Residents who live within 100 miles of a volcano should also find out what their local officials are doing to prepare their community for the possibility of renewed volcanic activity

How does the USGS provide eruption warnings?

The USGS volcano observatories post updates about volcanic activity on our web site. Information about our alert system is available online.

If activity at a volcano increases, we provide hazards-zone maps and other information about the frequency of eruptions and extent of specific hazards to public officials, land-use planners, and emergency-management agencies. The USGS works with the Federal Aviation Administration and National Weather Service to provide airline pilots with timely information about hazardous volcanic ash clouds.

When communities are at risk, scientists give hazard information directly to public officials to help them make decisions about land-use or evacuations. Unlike what is often portrayed in movies, warnings are delivered only after a thorough analysis of all existing information and careful consultation among members of the USGS response team.

How many active volcanoes are there in the United States?

There are about 65 volcanoes in the United States that scientists consider active. Most of these are located in Alaska, where eruptions occur virtually every year. Others are located throughout the west and in Hawaii (see our Volcano Activity Map for their locations). Kilauea volcano in Hawaii is one of the most active volcanoes on Earth.

How many active volcanoes are there on Earth?

There are about 1500 potentially active volcanoes worldwide, aside from the continuous belt of volcanoes on the ocean floor. About 500 of these have erupted in historical time. Many of these are located along the Pacific Rim in what is known as the "Ring of Fire." In the U.S., volcanoes in the Cascade Range and Alaska (Aleutian volcanic chain) are part of the Ring, while Hawaiian volcanoes form over a “hot spot” near the center of the Ring.