While browsing a used book store a little over 2 years ago, I picked up an interesting book, "Volcano Cowboys", by Dick Thompson.
It is about the quantum leaps made in understanding volcanoes, especially the explosive composite volcanoes, between the time of the Mount St. Helens eruption (1980) and the Mount Pinatubo eruption (1991). Having skimmed through it and then reading through most of the first chapter, I have already learned several new things, including learning about the Osceola Mudflow.
As composite volcanoes are the tallest of the volcanoes, it is not unusual for them to have glacier-capped peaks. When an eruption suddenly melts tons of ice and snow and it mixes with the previously-erupted ash and broken rock, it can be a recipe for disaster, producing what we call lahar flows (volcanic mudflows). A lahar killed an estimated 23,000 to 25,000 people in Armero, Colombia, in the middle 1980s. An "average" lahar may have a consistency of wet concrete and carry with it boulders the size of cars and downed trees.
Mapping by USGS geologists revealed that the Osceola Mudflow was produced by a partial collapse of a portion of Mt. Rainier, about 5700 years ago. It traveled a reported 60 miles from Mt. Rainier to Puget Sound. The worst part is that today, five towns have been built on the volcanic sediments of the Osceola Mudflow. If it happened once, it is likely to happen again.
There could well have been a few human inhabitants of the Puget Sound area at that time.
Just something to consider if you ever get a job offer from the Seattle/Tacoma area.