Figure 1: Location of glaciers, campgrounds, streams, and visitor centers at Mount Rainier National Park.

Regional Setting
Towering 1.5 miles (2.4 km) above the surrounding mountains and about 3 miles (5 km) above the lowlands to the west, Mount Rainier is the second highest peak in the conterminous United States at 14,410 ft (4393 m). Mount Rainier also has the distinction of having the greatest single- peak glacial system in the United States. Over 35 square miles (91 sq km) of snow and ice cover this active volcano. Glaciers radiate from its summit so that its rocky, ice- mantled slopes above timberline contrast vividly with the green, verdant forests that include old growth trees. The snowfields, alpine tundra with vivid alpine flowers, and dense forests provide the park visitor with a variety of visitor experiences. In 1899, 235,625 acres were set aside as Mount Rainier National Park and today, 97% of this area is designated wilderness (Figure 1).

The Native Americans know the mountain as Tahoma or Takhoma or Ta- co- bet, names meaning �Snow Mountain� or the �Mountain that was God� or �Place where the Waters Begin�. These names are more descriptive and perhaps capture the mountain�s impact on the surrounding region better than the European moniker �Rainier�, a name bestowed on the mountain by Captain Vancouver in 1792 in honor of his friend and superior officer, Rear Admiral Peter Rainier.

In terms of its potential impact on human populations, this �mountain of fire and ice� is the most dangerous volcano in the Cascade Range. In the shadow of Mount Rainier, live over 1.5 million people. The glaciers on Mount Rainier provide a steady flow of water for hydroelectric power in the region, regardless of the season. However, should an eruption occur, these 35 acres of ice and snow could melt and send debris flows towards Puget Sound and the Seattle/Tacoma metropolitan area.

Figure 2: Geologic time scale. Red lines indicate major unconformities between eras. Absolute ages shown are in millions in years.
Scale is from the U.S.G.S.

General Geology
Compared to the total age of the earth, the rocks exposed within Mount Rainier National Park are relatively young. Earth formed about 4600 million years ago (4.6 billion years ago), but the rocks in the park formed within the last 60 million years, during the Tertiary and Quaternary Periods (Figure 2). By the time Mount Rainier formed, the dinosaurs were extinct and primitive ancestors of many of today�s familiar mammals roamed the Earth. The trees and plants growing in Tertiary forests were clearly forerunners of today�s forests.

Mount Rainier is a stratovolcano, a composite volcano created through successive eruptions of lava and pyroclastic flows. It is one of more than a dozen stratovolcanoes perched on older rocks of the Cascade Range (Figure 3). The impressive eruption of Mount St. Helens in 1980, demonstrated the explosive nature of these types of volcanoes.

The Cascade volcanoes are aligned in a north- south direction that roughly parallels the Pacific coastline for about 500 miles (800 km). The volcanic mountains of the Cascade Range are a small segment of the circum- Pacific volcanic belt, or �Ring of Fire,� that encircles the Pacific Ocean. Volcanoes in the Ring of Fire mark converging lithospheric plate boundaries where dense, oceanic crust is subducted beneath the less dense, overriding continental crust or an island arc. The volcanoes in Washington and Oregon are the result of the collision between the oceanic Juan de Fucca plate, traveling northeast, and the continental North American plate, moving westward.

Figure 3: Volcanoes on the west coast of the conterminous United States. Green areas on the map represent units of the National Park Service.

The prevailing rock type of the Cascade volcanoes is andesite, an igneous rock of intermediate composition between light- colored, silica- rich rocks (e.g., rhyolite) and dark, basaltic rocks that contain very little silica. Andesite is typically dark gray or greenish black in color and is composed of approximately equal amounts of light- colored minerals like plagioclase feldspars and dark minerals like hornblende, olivine, and pyroxenes.

The north- south trend of Cascade volcanoes is believed to be located above the zone of melting deep within the crust where rocks in the subducting plate begin to melt. Magma rises from this zone to erupt at the surface as lava.

Lithospheric plates moving past one another generate earthquakes. Few large- scale earthquakes have occurred in historic time along the Pacific Northwest coastal area, but severe earthquakes accompanied by tsunamis have occurred in prehistoric time. The Cascade subduction zone may be storing strain energy that, when released, could result in catastrophic earthquakes and tsunamis along the Washington and Oregon coasts.

The landscape of Mount Rainier is complex, but its origins are simple: fire and ice. The mountain first erupted about half a million years ago and as recently as in the 1840s. Two large eruptions took place, first about 2,300 years ago and another 1,000 years ago. About 90 percent of Mount Rainier�s eruptions have been in the form of lava flows, which is unusual for a composite cone (Harris et al., 1995). In contrast, most of the other Cascade volcanoes, including Mount St. Helens, have had a more violent history with few lava flows and a high volume of pyroclastics. Much of the ash and pumice on Mount Rainier�s slopes came from Mount St. Helens during explosive episodes like the 1980 eruption.

The mountain�s great height and northerly location allowed glaciers to cut deeply into its volcanic deposits. Today, steam from the volcano creates ice caves near the summit of the volcano, and in the past, devastating debris flows and mudflows were triggered by lava and rock debris from Mount Rainier�s eruptions. The consistency of these mudflows on Mount Rainier is like wet cement. The collapse of unstable parts of the volcano has led to additional debris flows. At one time, the summit rose perhaps 2,000 ft (600 m) higher than it does today. About 5,700 years ago, an eruption took off the top of the mountain and left a depression 1.25 miles (2.01 km) in diameter. The mountain is a history of lava flows, lahars, mudflows, pyroclastic explosions, and ash falls mixed with glacial debris, glacial outwash floods, and rockfalls.

References:

Harris, A.G., Tuttle, E., Tuttle, S.D., 1995, Geology of National Parks: Kendall/Hunt Publishing Company, Dubuque, IA., p. 436- 449.