Geological Sciences

Cascade Volcanoes

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Volcano Web Links Volcano World USGS Cascade Volcano Observatory Hawaiian Volcano Observatory Smithsonian Global Volcanism Program	Only Mt. Garibaldi in British Columbia lies further north than Mt. Baker. Mt. Baker is a composite andesite volcano.

The Cascade volcanoes rest above a subduction zone. This seafloor gravity map shows the Gorda and Juan de Fuca rises extending north from the Mendocino Escarpment. The microplates created at these rises are subducted beneath the Pacific Northwest.	Glacier Peak, in the north central Cascade Mountains, is comprised mainly of dacite lava flows that have erupted in the last 770,000 years. The volcano has been inactive during the last 12,000 years. This view of the north side is from Image Lake.

Mt. Rainier, at 14,410 feet, is the highest of the Cascade volcanoes. This view from the north side shows that Mt. Rainier was once significantly higher. Notice that the lava flows on both the east and west sides project to a much higher summit that was destroyed in an eruption over 4,000 years ago. Also notice the symmetry and youth of the modern summit.	Mt. St. Helens in eruption is shown in the above photo. Mt. St. Helens had a nearly perfect symmetrical form previous to the 1980 eruption. This indicates the youth of this volcano. St. Helens was very active during the 19th century but remained quiet until 1980.

The 1980 eruption of Mt. St. Helens opened on March 27th. Ash was erupted onto the summit area and the activity was immediately picked up by seismic equipment in the area.	St. Helens was very active during April but was relatively quiet for about 2 weeks before the ultimate eruption on May 18. this photo was taken in April.

The May 18th eruption opened with a 5.1 magnitude earthquake and 2.1 cubic kilometers of the volcano collapsed into Spirit Lake. A pyroclastic flow traveling as fast as 250km/hr killed 62 people and devastated the north side of the volcano.	An even more devastating explosion occurred 6,600 years ago when Mt. Mazama collapsed to produce the caldera known as Crater Lake. Mt. Mazama, comprised of andesite and dacite flows, may have stood 12,000 feet high. Approximately 62 cubic kilometers of the volcano collapsed and spread Mazama Ash across the Pacific Northwest.

Mt. Shasta stands nearly as tall as Mt. Rainier and like Mt. Rainier it poses great dangers for the surrounding towns. Mt. Shasta is actually comprised of 4 volcanoes. The modern summit and Shastina on the west flank are easily distinguished in this photo. Shasta has been the source of numerous large debris avalanches and pyroclastic flows.	Mt. Lassen is the southern most of the Cascade volcanoes. It is positioned directly inland from the Mendocino Escarpment. The tectonic movements are convergent north of the Escarpment and strike-slip to the south. Lassen's history includes eruptions of both andesite and dacite. The modern volcano shown here is composed of dacite domes.

The pink material shown here is an ashflow tuff from Mt. Lassen shown in the background. Mt. Lassen's most recent eruption began on May 29, 1914 and activity was not totally completed until 1921. Lassen National Park now attracts tens of thousands of visitors each summer.	Lassen National park Visitor's Center is built on a rock avalanche deposit from the Chaos Crags. The deposit, known as the Jumbles, covers 4.5 square miles and may have formed only 300 years ago.

The Columbia River flood basalts are not part of the Cascade range but they certainly are a prominent feature of the Pacific Northwest. The basalts erupted in eastern Washington and spread west to form the massive flows you see here at Lake Lenore.	The Columns illustrate the massiveness of the flows within the Columbia River flood basalts. Almost 200,000 cubic kilometers of basalt were erupted during the Miocene.

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