This picture shows part of the Tatoosh Range, just south of Mt. Rainier in Mt. Rainier National Park. The lower parts of the mountains consist of light-gray granodiorite. The steeper, upper parts of the peaks are red to dark-gray volcanic rock. The Tatoosh Range provides a view into the history of the Cascade Range volcanic arc, which shifted from slightly farther west and became established in its current location after about 25 million years ago.

The Cascade Range volcanic arc has been dominated by high-volume volcanic eruptions at the surface and intrusions, mainly of granodiorite, within the crust. Uplift and erosion of the range has been continuous. It has already brought some of the intrusions up to the surface, where their relations with the volcanic rocks can be scrutinized.

Tatoosh Granodiorite

This is a close-up of granodiorite of the Tatoosh Pluton. Most of the rock consists of white plagioclase, with lesser amounts of gray (actually clear) quartz and pink-to-white orthoclase. Biotite is the most abundant black mineral in this particular rock. The two dark-gray inclusions in the rock might have come from a more mafic magma. Inclusions can form when a more mafic magma that comes from deep in the crust or upper mantle mixes with a more felsic magma, such as the granodiorite of the Tatoosh Pluton. Select the image to see a larger view. Use your browser's back button to return to this page.

Such dark inclusions are common in granodiorites that form within volcanic arcs. Several studies have concluded that most dark inclusions of this type originate as mafic magmas that rose from the mantle and gave rise to the felsic magmas by causing continental crust to melt. The Tatoosh Pluton has radiometric ages that range from 26 to 14 million years, which suggests that it is predominantly of Miocene age.

Stevens Ridge

The outcrop at the back of the meadow in this photo consists of volcanic rock of the Stevens Ridge Formation. This is near the top of one of the higher peaks in the Tatoosh Range, above the exposures of Tatoosh Pluton granodiorite. Measurements of radiometric isotopes in rocks collected from the Stevens Ridge Formation yield ages ranging from 30 to 20 Ma. Thus the Stevens Ridge Formation is probably only slightly older than, or overlaps in age with, the Tatoosh Pluton. The pile of Stevens Ridge volcanic rock was intruded from below by the Tatoosh Pluton. This was followed by uplift and erosion, exposing the gray rock of the Tatoosh Pluton in the lowest parts of the Tatoosh Range. Select the image to see a larger view. Use your browser's back button to return to this page.

Rainier Andesite Flows

In this photograph looking north from the Tatoosh Range you can see many andesite flows on the southeast side of Mt. Rainer. At the pass between the Tatoosh Range and the slopes of Mt. Rainier, andesite flows lap over and cover the Tatoosh Pluton. The sides of the andesite flows have been exposed by glacial erosion. Many of the glaciers have retreated from this scene since late in the Pleistocene epoch (the last Ice Age). Select the image to see a larger view. Use your browser's back button to return to this page.

The Tatoosh Pluton crops out on the north and northeast sides of Mt. Rainer as well as here in the Tatoosh Range on Rainier's south side. As a result, geologists think that Mt. Rainier magma rose up through the middle of the pluton and built the volcano on top of the eroded surface of the pluton. Before Mt. Rainier started forming, approximately 1 million years ago, the Tatoosh Pluton must have already been exposed by millions of years of uplift and erosion.

Rainier Andesite

The main rock in this photograph is andesite of Mt. Rainier, sitting not far above its contact with the granodiorite of the Tatoosh Pluton. The andesite has a porphyritic texture, a mixture of visible crystals against a fine-grained background. Select the image to see a larger view. Use your browser's back button to return to this page.

Mt. Rainier consists mainly of andesite. The same is true of nearly all the composite cones associated with all the subduction zones in the world. Mt. Rainier is part of the modern Cascade volcanic arc and is associated with the Cascadia Subduction Zone. Plate tectonic theory suggests that the same subduction processes that formed Mt. Rainier also caused the Stevens Ridge volcanoes to erupt and the Tatoosh Pluton to intrude during earlier stages of the Cascade volcanic arc.

Glossary terms that appear on this page: granodiorite; volcanic arc; intrusion; plagioclase; quartz; orthoclase; biotite; mafic; magma; crust; mantle; radiometric; andesite; porphyritic; composite cone