Southern flanks of Mount Rainier as viewed from McClure Rock. Photo taken near benchmark at the 7,380 feet elevation, showing Nisqually Glacier on the left, and Gibralter Rock high on the right. A USGS crew is in the foreground.
The last significant eruptive activity at Mount Rainier was between 120 and 160 years ago, and several minor eruptions were reported during the late 1800's. A careful stratigraphic study of the volcano's recent products led scientists to conclude that "Mount Rainier will almost surely erupt again sometime within the next few hundred years," and that future eruptions might endanger the livelihood of thousands of people. Hence, the need for careful monitoring of this loftiest peak in the Cascade Range.
The Nisqually Glacier is one of the most documented and accessible glaciers on Mount Rainier. Annual measurements of Nisqually Glacier's terminus position were begun in 1918 by National Park Service (NPS) personnel and are currently made by U.S. Geological Survey (USGS) personnel. Whereas 25,000 years ago the glacier extended some 30 miles downvalley, today the glacier is the 6th largest glacier on Mount Rainier and covers about 1.8 square miles. The Nisqually Glacier has advanced and retreated three times between 1965 and 1992. The most recent period of retreat occurred between 1985 and 1991 during which time the glacier thinned by 52 feet in the region immediately west of Glacier Vista. The retreat that has been occurring since the late 1980's may be slowing.