Mount Rainier National Park - Birth of a Volcano

Rainier's impressive height, reaching nearly three miles above sea level, is testimony to the awesome mountain-building power of volcanic forces. And the sulfurous fumes still issuing from its summit vents are reminders that its internal fires continue to smolder; for though Rainier seems dormant, it is by no means dead. On the contrary, geologists speculate that sputterings of renewed volcanic life could conceivably occur within the next few years, most likely in the form of small ash eruptions and occasional mudflows. Nor can the possibility of a catastrophic outburst be discounted. Mount Rainier's volcanic pulse seems to beat in a 3,000-year cycle -- and the last major eruption occurred approximately 2,500 years ago.

But that was only the most recent in a long chain of events, for the mountain's enormous bulk took a long time to build. Geologists believe it first began to form about 12 million years ago, when a mass of molten rock welled up from the earth's interior and, for the most part, cooled without breaking through to the surface. This rock forms the foundation for the cone itself.

Sometime within the last million years, new upwellings began bursting through the surface and spewing out streams of lava. Periodically, too, violent explosions shot out masses of pumice -- molten rock made frothy by trapped gas bubbles. Layer upon layer, the lava and debris piled up, building a great volcanic cone. By the time the major eruptions had ceased, Rainier stood as the highest peak in the Pacific Northwest.

In fact, it was even higher than it is today. Instead of the classic pointed top of a volcanic cone, Rainier's summit is a broad dome. Judging from the angle of lava flows preserved on upper rock ledges, and from various other clues, scientists estimate that Rainier is missing at least 1,000 feet, compared with its height immediately after the main cone finished building.

Geologists used to think that the summit blew off in a spectacular eruption such as the one that in May 1980 demolished the top of Mount St. Helens, 50 miles southwest of Rainier. Now they believe the former summit was lost in a massive mudflow. Changes in Mount Rainier's interior volcanic "plumbing" probably set events in motion. About 5,000 years ago the mountain's slumbering fires apparently stirred to life, and their increased heat melted summit ice and weakened the volcanic rock. A minor eruption then shook the mountain and sent the unstable summit sliding down on the surrounding lowlands. As the muddy torrent swept down the slopes, it carried along tons of silt and rubble as well as the previously solid rock that had been softened by hot gases and superheated water. Snow and ice torn from the glaciers added to the flow, boulders rode the moving mass, and cliffs undercut by the force of the flowing mud collapsed into it. When the cataclysm ended, the land at the base of Rainier lay buried beneath an apron of viscous mud and debris some 20 miles long and up to 10 miles wide. And the old mountaintop was gone forever.