Geology of Mount Rainier National Park, Washington

Part of the NPS Geodiversity Atlas, which describes each park's significant geologic features and processes. Geodiversity is the full variety of a park's natural geologic resources — rocks, minerals, sediments, fossils, landforms and physical processes — and its soils. The atlas comes from the NPS Geologic Resources Inventory.

The park's Geologic Resource Evaluation Report. Image from the NPS's page.

The park

Mount Rainier National Park lies on the western slope of the Cascade Range, about 75 km (47 mi) southeast of Seattle in Pierce and Lewis Counties. Established on March 2, 1899, it covers 92,462 hectares (228,480 acres) of rugged peaks and valleys, protecting Mount Rainier, a glaciated volcano, with its natural and cultural resources and dynamic processes.

The volcano's base covers more than 259 km² (100 mi²), and its summit rises more than 2,745 m (9,000 ft) above the valley floor. It carries more than 29 major glaciers, the greatest single-peak glacial system in the contiguous United States. The park also has 470 mapped rivers and streams, 382 mapped lakes and ponds, subalpine meadows, dense forest, more than 1,214 hectares (3,000 acres) of wetland, waterfalls and mineral springs — and more than 1,200 plant species and about 300 species of native birds, mammals, reptiles, amphibians and fish.

Geologic setting

Most of the park's rock formed within the last 60 million years. Mount Rainier is one of more than a dozen stratovolcanoes in the Cascades, built of successive lava and pyroclastic flows. It is the second most seismically active volcano in the range, and a regional hazard with a history of lava flows, lahars, mudflows, pyroclastic explosions, and ash falls mixed with glacial debris, outwash floods and rockfalls. It first erupted in the Pleistocene, about 500,000 years ago, and as recently as 1894.

The Cascade volcanoes began forming in the Eocene, from an oblique collision of the Juan de Fuca and North American plates. Mapped rock units in the park include:

• the Eocene Ohanapecosh Formation, basaltic andesite (a main rock of the Cascade volcanoes) and rhyolite;
• the Oligocene–Miocene Stevens Ridge and Fifes Peak Formations;
• Miocene–Pliocene intrusive rocks and the welded tuff of the Palisades;
• Pleistocene–Holocene volcanic rocks and andesite flows of Mount Rainier.

Surficial geologic map of the park, from the Geologic Resources Inventory. NPS.

Glaciers

Rainier's glaciers are the largest glacier system in the conterminous United States and feed several rivers. Some begin at the summit; others flow down the flanks from cirques in a high-precipitation zone at middle elevations. Glacial erosion has carved horns, cirques, U-shaped valleys, moraines and arêtes — sharp ridges between valleys, called cleavers here. The Willis Wall, a vertical headwall 3,600 feet (1,100 m) high, marks the Carbon Glacier cirque.

The best-known glaciers are the Emmons — the largest in the contiguous United States — Cowlitz-Ingraham, Paradise, Nisqually, Tahoma, Carbon, Kautz and Winthrop.

Fossils, caves and soils

Fossils are documented from the Ohanapecosh Formation (late Eocene–Oligocene), the Stevens Ridge Formation (Oligocene–Miocene), Holocene mass-wasting deposits, and modern pollen and lake deposits. All NPS fossil resources are protected under the Paleontological Resources Preservation Act of 2009.

The mountain holds large volcanic ice caves. Near the summit, the caves and fumaroles form where groundwater reacts with gas and steam rising from the magma system below; other caves are carved by glacial meltwater. There are at least five caves, always changing and possibly unstable, and carbon dioxide-rich volcanic gas can endanger anyone entering the summit caves. All NPS caves are protected under the Federal Cave Resources Protection Act of 1988.

Cryptobiotic soil crusts matter to the high-elevation ecosystems: they bind soil particles, and help water soak in and nutrients reach plants. The high slopes are solid rock and talus with almost no topsoil; lower down is glacial till, and valley floors hold mixed rock and benches of silt left by streams and glaciers. Subalpine meadow soils are shallow and loose, and easily eroded by feet and hooves.

Hazards

• Earthquakes. Rainier is among the Cascades' most seismically active volcanoes, averaging 1–2 high-frequency volcano-tectonic earthquakes a month directly beneath the summit. A strong one could trigger edifice collapse — the volcano's cone giving way — avalanches, debris flows and rockfalls.
• Volcanic hazards include ash, projectiles, pyroclastic flows and surges, gases, lava flows and lateral blasts. Lahars are the most destructive, because of their speed and size; lahars and debris flows from an eruption could do major damage in the drainages of the Seattle–Tacoma lowland.
• Between eruptions, weakened rock walls can give way in snow and debris avalanches, rockfalls and landslides, and daily and seasonal temperature swings bring frequent rockfall on the upper mountain.
• Floods can come in any season, but are likeliest in summer as snow and ice melt; rain, volcanic activity and glacial outbursts can also cause them.
• Abandoned mines — adits, shafts, quarries, prospects, waste rock, old equipment, cabins and roads — can be cultural resources and habitat, but many are dangerous and harm water and landscapes. Stay out and stay alive.

Silver Forest Trail. Image from the NPS's page.

Maps and reports

Mount Rainier belongs to the Cascade–Sierra Mountains physiographic province. The Geologic Resources Inventory makes digital geologic maps, reports, posters and scoping summaries for more than 270 natural resource parks; Mount Rainier's are in the NPS DataStore. A Soil Resources Inventory project has been started.

More: the park's pages on glaciers, volcanoes and seismicity; the USGS Volcano Hazards Program and Cascades Volcano Observatory.

Sources

Based on "NPS Geodiversity Atlas—Mount Rainier National Park, Washington," National Park Service; a work of the United States government in the public domain. It cites Moran and Malone (2000) for the earthquake rate. A misspelling in the source ("freequent") is corrected.