Location: Pierce County, WA
Elevation: 4,392 m; 14,410 ft
Nearby towns: Orting, Seattle, Tacoma, Yakima
Mount Rainier produces andesitic and dacitic lava flows, pumice, and lahars.
The areas prone to lahars are determined in part by figuring out where lahars traveled in the past. Evidence of massive lahars is still abundant in many of the valleys that drain Mount Rainier. The figure at right shows the distribution of lava flows and lahars mapped at the surface compared to hazard zones (gray shaded areas). Much of the volcanic deposits have been either eroded or buried by rivers, glaciers, and human development.
ERUPTION AND LAHAR HISTORY
Modern Mount Rainier started erupting only 500,000 years ago with intermittent eruptions and mudflows thereafter.
Mount Rainier still issues steam and gases from fumaroles near the summit crater, which melt the snow and ice at the crater, as well as melt the summit icecap, forming caves beneath the ice.
Researchers study earthquake activity in the Mount Rainier area to learn about the background seismicity, or the small day-to-day earthquakes, that occur in the crust as magma below the volcano shifts and faults in the area move to accommodate fluids and gasses produced by the magma.
By studying the earthquakes, geologists can monitor increases in seismicity (earthquakes) to hopefully be able to tell if the volcano is about to erupt. Geologists are particularly interested in a large north-trending fault zone west of Mount Rainier, called the Western Rainier Seismic Zone, which is an area of dense and shallow earthquakes.
Osceola and Electron Mudflows
5,600 years ago, a massive debris avalanche, called the Osceola Mudflow, poured down from the summit of Mount Rainier, picking up sediment and anything else in its path as it traveled down the White River valley and into the Puget Sound. The mudflow filled valleys with ~400 feet of sediment and moved at speeds of 40 to 50 miles an hour. Following the Osceola Mudflow, many smaller volcanic eruptions and lahars occurred as the volcano continued to show signs of unrest.
The last major mudflow, called the Electron Mudflow, began as a part of a crater collapse and traveled down the Puyallup River into Sumner in ~1503.
It is estimated that Mount Rainier has generated about 60 of these lahars in the last 10,000 years. Many of the communities between Mount Rainier and the Puget Sound are built right on top of these deposits.
Are You Volcano Ready?
- Get to know your local volcano’s hazards
- Register for notifications about the volcano’s activity
- Make a plan to prepare your entire family for an emergency
Visit the Opportunities for Educators webpage by clicking the image at left. To access USGS volcano teaching materials or posters click here.
Visit the Pierce County website for more information on how to be volcano ready, interpretive signs, and lahar evacuation routes.
Click on the image (left) for a link to the poster.