Lava fountain erupting from Pu’u O’o cone. Forty-four episodes of such fountaining between 1983 and 1986 built the cone 255 meters tall. (Photograph by J.D. Griggs.)
Cascade volcanoes
Volcanoes of the Cascade Range erupt far less frequently than Kilauea and Mauna Loa, but they are more dangerous because of their violently explosive behavior and their proximity to populated and cultivated areas in Washington, Oregon, and California. The 1980 eruption of Mount St. Helens in southwest Washington dramatically illustrated the type of volcanic activity and destruction these volcanoes can produce. Scientific studies of the eruption of Mount St. Helens and the eruptive histories of other Cascade volcanoes continue to improve public awareness and understanding of these potentially dangerous peaks.
In contrast to Kilauea, Cascade volcanoes erupt a variety of magma types that generate a wide range of eruptive behavior and build steep-sided cones known as composite volcanoes. In addition to basalt, andesite and dacite magmas are common.
Cascade Volcanoes.
EXPLANATION Volcano active during past 2,000 years Potentially active volcano Area of potential volcanic activity Population centers 40,000 to 100,000 100,000 to 350,000 350,000 to 1,000,000 Greater than 1,000,000 PACIFIC OCEAN WASHINGTON Mount Baker Glacier Park Seattle Spokane Mount Rainier Mount St. Helens Mount Adams MONTANA Great Falls Billings IDAHO Boise Craters of the Moon OREGON Portland Mount Hood Mount Jefferson Three Sisters Eugene Newberry Crater Crater Lake WYOMING Yellowstone Casper Cheyenne CALIFORNIA Medicine Lake Mount Shasta Lassen Peak Clear Lake Sacramento San Francisco Long Valley Caldera Coso Los Angeles San Diego NEVADA Reno Las Vegas UTAH Salt Lake City COLORADO Denver ARIZONA San Francisco Field Phoenix Tucson NEW MEXICO Albuquerque Bandera Field
These magmas are so highly viscous, or sticky, that expanding volcanic gases cannot easily escape from them. This causes a tremendous build-up in pressure, often leading to extremely explosive eruptions. During such eruptions, magma is shattered into tiny fragments (chiefly ash and pumice) and ejected thousands of meters into the atmosphere or even the stratosphere. Under the force of gravity, sometimes these fragments sweep down a volcano’s flanks at speeds of more than 100 kilometers per hour, mixing with air and volcanic gases to form pyroclastic flows. Rock fragments can also mix with water in river valleys to form lahars (volcanic debris flows and mudflows) that destroy everything in their paths.
Andesite and dacite magmas also erupt to form lava flows. Because these lavas are more viscous (“stickier”) than basalt, they tend to form thicker flows that travel shorter distances from the vent; consequently, andesite and dacite lavas typically build tall cones with steep slopes of more than 20 degrees.