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Shasta/Lava Beds/Lassen The Cascade Volcanoes • Caused by subduction • Explorer, Juan de Fuca, and Gorda = oceanic plates • North American = continental plate Mt. Shasta • prominent landmark at an elevation of 4,317 meters (14,162 feet) • volume of nearly 500 cubic kilometers makes it the largest of the Cascade stratovolcanoes • Most recent eruption occurred 200 years ago • 7 main glaciers recognized on Shasta today • Seismically and geothermally active Rocks and Minerals • Rocks Basalt Andesite Dacite • Minerals Olivene Hypersthene Augite Hornblende Biotite Plagioclase Quartz Magnetite Why are Lavas Diverse? Magma chamber at depth under Shasta; basaltic lava rises from the mantle below. Magma can: a) differentiate; b) be modified by surrounding rock; and c) mix with other magma to produce different types of lavas seen at the surface. Glaciers and Glacial Erosion • 7 main glaciers on Shasta • Total volume of ice is about 140 million cubic meters • Cirques, aretes, and moraines are all seen on Shasta Streams and Debris Flows • Multiple debris flows have happened during historic time and will continue to occur • As water runs off of the slopes it may become 2 times as dense as water when volcanic sediment is incorporated • Slurries develop that are similar to concrete • Streams are able to buoy up and carry rocks much larger than normal • http://www.siskiyous.edu/Shasta/map/mp/wdf8-97.mpg Mass Movement • Downslope falls, slides, or flows of weathered rock due to gravity • 3 factors = steep slopes, weak rocks, and water • Rockfalls common • Debris flows common • Debris Avalanches uncommon, but do occur on Shasta Lava Beds National Monument • Formed from eruptions from Medicine Lake volcano • Main rock types = Basalt Aa and Pahoehoe Andesite Rhyolite Tuff Obsidian Pumice Volcanic Features • Lava Tube Caves • Cinder Cones • Spatter Cones • Craters • Fault Scarps Lassen Peak National Volcanic Park • Formed from a massive extrusion of dacite ~11,000 years ago • May have only erupted for a year or two • Mt. Maidu, Mt. Tehama, and Thousand Lakes volcano also formed landscape in greater area • Glaciers have played a major role in shaping the landscape around Lassen • Geothermal activity continues to date Volcanic & Geothermal Features • • • • • • • • Plug domes Cinder cones Craters Debris Avalanches Lava Flows Hot springs Fumaroles Boiling mud pots Conclusion • Whole region is formed by subduction • Some lavas are silica rich – pasty, sticky, explosive (Mt. Shasta, Lassen Peak) • Some lavas are silica poor – runny, less explosive/effusive (Medicine Lake/Lava Beds) • Glaciers have played a major role in shaping the landscape • Geologic hazards (mass movements, eruptions) continue to affect the region