Mount Rainier (14,410 ft) has lately attracted a small amount of attention because of what is considered by some an increase in seismic activity there, so I thought it might be nice to get a baseline description of this volcano for those of you interested in such things. For scientifically accurate information and interesting discussion on the mountain, keep an eye on Eruptions Blog.
Mount Rainier is a stratovolcano. This means that it is made up of strata of lava, tuff/tephra, ash, and so on that has been pushed out into a conical pattern. The lava from stratovolcanoes is usually viscous enough that it does not spread very far, which contributes to the build up of the cone. Many of the world's famous volcanoes are of this type (Fuji is a good example). As this volcano grows over time, it also collapses here and there, with avalanches (some of which are called "lahars") redistributing material outward from the cone. These lahars are fast moving rivers of mud and volcanic stuff that would destroy forest, buildings, whatever was in their path and that can go a considerable distance from the cone of the volcano.
One of the largest lahars recorded by geologists is actually the Osceola lahar from Mount Rainier, about 5,600 years ago. That event covered about 330 square kilometers of landcape. Historically, one of the deadliest lahars was in Columbia, which Nevado del Ruiz produced on in 1985. About 23,000 people died in that event.
There are about three million people who live in the vicinity of Mount Rainier, so if something really big happened there it could have a significant human impact. About 150,000 people currently live on top of old mudflow/lahar sediments. Various models of possible mudflow patterns from Mount Rainier indicate effects quite a long way from the mountain, including heavily settled areas of and near Tacoma.
If you want to monitor the seismic activity at Mt. Reinier, you can check out the USGS Cascade Range Current Update. At present we are getting these messages from the update
"All volcanoes in the Cascade Range are at normal levels of background seismicity. These include Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams in Washington State; Mount Hood, Mount Jefferson, Three Sisters, Newberry Volcano, and Crater Lake, in Oregon; and Medicine Lake volcano, Mount Shasta, and Lassen Peak in northern California."
"A swarm of small, shallow earthquakes occurred beneath the summit of Mount Rainier on Sunday, September 20, 2009, starting at ~0920 PDT. According to the Pacific Northwest Seismic Network (PNSN), the largest event at 0945 PDT has a preliminary magnitude of M 2.2 with a depth of 1.8 km, located 1 km NNE of the summit. The swarm also featured many small events that were too small to be located, and lasted for several hours before gradually waning. Such swarms have happened several times in the past decade. One example is the swarm on November 7, 2004, that included a M 3.2 event. We consider the September 20, 2009, swarm to be within the realm of normal activity at Mount Rainier."
The USGS also has a seismic network. This page has a map with Mount Rainier on it. Keep an eye on that and if anything happens let us know!