Geologists call Mount Rainier an 'episodically active' volcano, meaning one that will erupt again some time in the future. The mountain formed between half a million and a million years ago when it began to grow at a weak spot in the earth's crust, where molten rock could reach the surface. The first lava flows moved down the deep valleys of the Cascades as far as 15 miles from the central vent. Later flows were smaller and thinner and did not flow as far from the vent. They gradually built the high main cone. While Mount Rainier grew here, several other volcanoes that can be seen from the park were also forming, notably Mount Baker, Glacier Peak, Mount Adams, and Mount Hood.

Today there are three distinct summits, or high points, at the top of Mount Rainier. The lower two, Liberty Cap and Point Success, are remnants of the sides of an old, higher cone. The third and highest summit, Columbia Crest (14,410 feet) lies in the rim of a small recent lava cone. This cone is indented by two craters, the larger of which is about 1/4 mile in diameter. Both craters are nearly filled with snow and ice, into which a system of tunnels and caves are melted by volcanic heat and steam.

Geologic evidence shows that Mount Rainier has erupted at least four times in the last 4000 years. The most recent eruptions were in the 1840s, and produced clouds of steam and ash. When will Mount Rainier erupt again? Geologists cannot predict exactly when this volcano will erupt again, but they can watch for symptoms that the mountain is beginning a period of renewed activity. Earth scientists at the University of Washington and the United States Geological Survey are constantly monitoring their instruments for increased earthquake activity or changes in the shape of the mountain.

Mount Rainier is not the only volcano in the Northwest. From Lassen Peak in California to Mt. Garibaldi in British Columbia, there are 14 volcanoes in the Cascade Range that have erupted in the last 4000 years. Mount St. Helens began a series of eruptions in 1980. The volcanoes dominate our skylines, but they affect our lives in other ways, too. Ancient eruptions and mud flows produced rich soils that today's farmers raise crops on, and millions of people visit our area each year to see Mount Rainier, Mt. Mazama (now filled by Crater Lake), Mt. St. Helens, or the other Cascade volcanoes.

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