Cascade Range and Volcanic Overview

Mount Rainier is a part of the larger Cascade Mountain Range. The range spans 700 miles across Washington and Oregon and contains 20 different volcanoes. Mount Rainier is the second most active of the volcanoes. Other prominent volcanoes in the range include Mount St. Helens and Mount Hood. Mount Rainier is a stratovolcano, which means it has a conelike shape, is made up of layers of cooled lava and volcanic debris and has explosive eruptions. Stratovolcanoes are common in subduction zones. Subduction zones are the result of two tectonic plates converging, the heavier one being forced underneath the other. This causes mountains and volcanoes to form.

Tectonic Activity

The subduction zone that caused the Cascadia Mountain range (including Mount Rainier) to form is the Cascadia Subduction Zone. The Juan De Fuca plate off of the coast of Washington and Oregon is converging with the North American plate. The Juan De Fuca plate is heavier, so it is pushed underneath the North American Plate, which creates magma. This magma rises to the surface, causing eruptions. Overtime, lava flows and debris built up in layers. This is how Mount Rainier and Other volcanoes in the Cascadia range formed. The Cascadia Range is shaped by tectonic activity, which contributes to much of the mountain landscape in the Pacific Northwest.

Cross-section view of how the Juan De Fuca, North American, and Pacific Plates interact

How Stratovolcanoes Erupt

Underground, the heat from earth's mantle melts rocks from the crust into magma. Because it's from the crust the magma is high in silica, making it thick and sticky. This slows the magmas movement and traps gasses. Pressure builds from the gasses, and as the magma gets close to the surface the pressure releases causing explosive eruptions. These eruptions make stratovolcanoes some of the most dangerous.

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