Mount Rainierrising to 14,410 feet and surrounded by a dramatic landscape of volcanic ridges and glacier-carved valleysis an icon on the horizon in the Pacific Northwest. Look closerand this familiar landscape contains the origin story of this area. It has been covered in swamps and ancient seasflowing lavaexplosive ashand thick sheets of ice. Mountains rose and eroded away. The features described on this webpage provide an outline of Mount Rainier’s geologic pastbut it does not cover every formation you may observe while exploring the park.

For a detailed geologic history of Mount Rainiervisit:
Geology of Mount Rainier - U.S. Geologic Survey
Roadside Geology of Mount Rainier National Park and Vicinity – Patrick T. PringleWashington Division of Geology and Earth ResourcesInformation Circular 107June 2008.

Pre-Mount Rainier Geology

Formed over the last 500,000 yearsmodern Mount Rainier is a relatively young volcano standing on a foundation of rocks millions of years old. Some of these pre-Mount Rainier formations include:

Puget Group40+ million years ago
Over 40 million years agolowland swamps left behind layers of sandstoneshaleand coal. This coal was later mined near the Carbon River areaoutside the northwest corner of the park.

Ohanapecosh Formation36-28 million years ago
Ancient Ohanapecosh Formation volcanoes erupted underwaterforming layers sandstone and breccia that have been compressed and folded over millions of years. These layers are visible between Cayuse Pass and Tipsoo Lake along State Route 410.

Stevens Ridge Formation26 million years ago
Early volcanoes erupted pumice and welded tuff. Visible in cliffs west of Box Canyon along Stevens Canyon Road.

Tatoosh Granodiorite18-14 million years ago
A large mass of molten rock slowly cooled under the surfaceforming granodioritea rock similar to granite. This granodiorite underlies the Tatoosh RangeWhite RiverCarbon Riverand Nisqually River Valleys. View it up-close along White River Road near the White River Campground.

The Cascade Range
The Cascade Range starts to rise through subduction uplifting 10 million years agocompleting the geologic foundation of the Mount Rainier.

Formation of Mount Rainier

A subduction zone is an area where one tectonic plate is being forced underneath another into the earth's mantle. Approximately 50 miles downchemical changes lower the melting point of rockcausing the subducting slab to partially meltcreating magma. Some magma bubbles to the surface of the earth and erupts out of volcanoes as lava. An ancestral volcano stood in the same place as modern Mount Rainier from 1-2 million years ago. It largely eroded during a period of low volcanic activity before modern Mount Rainier started to rebuild.

Modern Mount Rainier was built by numerous eruptions over the last 500,000 yearsalternating between periods of high volume and low volume eruptions. These eruptions built up layer after layer of lava and loose rubbleeventually forming the tall cone that characterizes composite volcanoes. Some of these eruptions produced notable features on Mount Rainier that are still visible today.

Burroughs Mountain500-420 thousand years ago
Built during a very active eruptive period and some of the oldest lava flows in the park. Visible from the Sunrise Area.

Rampart Ridge380 thousand years ago
Began as a lava flow that formed against a wall of glacier icecreating the steep lava cliffs visible from Longmire today.

Little Tahoma Peak160-40 thousand years ago
Formed from dikes and vents feeding lava to the side of Mount Rainierinstead of erupting from the summit. This prominent feature on Mount Rainier's eastern slopes can be viewed from many locations in the park.

Gibraltar Rock40-14 thousand years ago
Rocks of many ages are exposed in the upper mountain features of Liberty RidgeWillis WallSuccess Cleaverand Gibraltar Rock.

Emmons Glacier Lava Flows2.2 thousand years ago
Mount Rainier’s youngest features are the summit craters built around 1,000 years agoand small surface lava flows under the Emmons and Winthrop Glaciers. Edges of these lava flows are visible breaking through the Emmons Glacier.

Volcanic Ash
Mount Rainier has erupted numerous times. Ashboth from Mount Rainier and neighboring volcanoes like Mount St. Helenscreated layers interspersed by rocks from debris flows and glaciers. These strata layers visible throughout the park and are a detailed timeline of Mount Rainier’s relatively recent past.