7.9: Chapter Summary

7.1: The Cascadia Subduction Zone and the Cascade Continental Volcanic Arc

• The Cascade Volcanoes are produced by the subduction of the Juan de Fuca plate beneath the North American plate.
• Water released from the subducted Juan de Fuca slab causes flux melting in the mantle beneath the North American plate.
• The Cascades are a continental volcanic arc mountain range
• The Cascade Range extends from Northern California, through Oregon and Washington, and into British Columbia.

7.2: Rock Types of the Cascades

• The California Cascades include a variety of volcanic rocks.
• As magma diapirs rise from the mantle, some have more time to evolve into more felsic compositions, while others are less changed from the ultramafic mantle.
• Rock types include basalt, andesite, dacite and rhyolite, which are all named based on their composition.
• Other rocks such as obsidian, pumice and volcanic tuff are named for their unique textures and appearance.

7.3: The Variety of Volcanoes in the Cascades and the Modoc Plateau

• Subduction zones are often associated with composite volcanoes, but other types of volcanoes are present as well.
• Composite volcanoes and shield volcanoes are regional volcanoes and both are present in the California Cascades.
• Mount Shasta is a classic example of a composite volcano.
• Medicine Lake Volcano is sometimes classed as a shield volcano based on its shape, but its composition is atypical of a shield volcano.
• Cinder Cones and Lava Domes are examples of individual vent volcanoes and can be found throughout the California Cascades, both as part of larger regional volcanoes and on their own.

7.4: Volcanic Hazards

• Lava flows form when molten rock reaches the surface of the earth and flows across the ground or down the slopes of a volcano.
• Pyroclastic flows are a mixture of gas, ash and other debris that flows along the ground surface.
• Lahars are water-based mudflows composed of volcanic material.
• Lahars can form with or without a volcanic eruption.
• Lahars travel in river valleys and can reach distances further from the volcano than is typical for lava flows or pyroclastic flows.
• Ash and tephra are ejected in the air and can rain down far from the volcano. Ash in particular can travel hundreds of kilometers from the eruption.

7.5: Mount Shasta

• Mount Shasta is California’s iconic composite volcano.
• The modern edifice has been built over the past 250,000 years, through four cone-building episodes.
• Mount Shasta is ranked fifth in the U.S. Geological Survey’s National Volcanic Threat Assessment, meaning that there is a significant potential for a future eruption.
• Lahars and debris flows occur without eruptions, as hot summer temperatures result in rapid melting of glaciers.

7.6: Lassen Volcanic National Park

• The 1915 Lassen Peak eruption is one of the most famous volcanic eruptions in US History.
• Volcanic domes, cinder cones and shield volcanoes can all be found in the park.
• Mount Tehama, or Brokeoff Volcano, is the eroded remnant of a composite volcano.
• Hot springs and geysers are the result of hydrothermal heating from the magma chamber at depth.

7.7: Medicine Lake Volcano

• Medicine Lake Volcano is shaped much like a classic shield volcano, but while most shield volcanoes are made of exclusively basalt, Medicine Lake volcano consists of felsic rocks as well.
• Glass Mountain is an example of an obsidian dome.
• Lava Beds National Monument, known for its caves formed by lava tubes, is located on the flank of Medicine Lake Volcano where there are many basalt lava flows.

7.8: Glaciers in the Cascades

• During the Pleistocene ice ages, glaciers were extensive.
• Volcanic rock is often easily eroded by glaciers, and thus these glaciers significantly shaped the landscape of the Cascades.
• The only glaciers that remain in the California Cascades today are on Mount Shasta, but these too are threatened as the climate continues to warm.