Puyallup is vulnerable to a lahar if Mount Rainier erupts. It's a type of mudflow which would flow down a river valley powered by melting glacial ice.

PUYALLUP, Wash. — Students from both public and private schools participated in the largest evacuation drill in the Puyallup's history. The reason is literally a powerful one, as Mount Rainier, considered one of the world's most dangerous volcanoes, threatens to send a wall of mud down the Puyallup River into the town during an eruption.

This kind of mud flow is fueled by melting glaciers which would quickly turn to water in the heat and explosion of an eruption. Adding to the glacial melt, rain which is often part of an eruption mixes in massive amounts of ash, creating the mud. That mud is often described as wet concrete, and includes rocks, dirt and other material.

It is not a hypothetical problem, as lahar's came down the Toutle river during the May 18th eruption from Mount St. Helens, also in Washington state.

The City of Puyallup has one advantage, it has some time. A lahar is not expected to reach the growing city for up to four hours after an eruption, according to the Cascades Volcano Observatory, part of the U.S. Geological Survey and based in Vancouver, Washington.

At the city of Orting further up the river, it would take a lahar less than an hour to reach the town, according to scientists with CVO.

Pierce County Emergency Management and CVO are also in the middle of upgrading the lahar warning system, including trip wires attached to things like logs which would signal that mud is on the move as the logs started to move.