shasta/lava beds/lassen. the cascade volcanoes caused by subduction explorer, juan de fuca, and...
TRANSCRIPT
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
• RocksBasaltAndesite Dacite
• MineralsOlivene
HyperstheneAugite
Hornblende Biotite
PlagioclaseQuartzMagnetite
Why are Lavas Diverse?
Magma chamber at depth underShasta; basaltic lava rises fromthe mantle below.
Magma can: a) differentiate; b) bemodified by surrounding rock; and c) mix with other magma to producedifferent types of lavas seen at thesurface.
Glaciers and Glacial Erosion
• 7 main glaciers on Shasta
• Total volume of ice isabout 140 million cubic meters
• Cirques, aretes, and moraines are all seenon 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 maybecome 2 times as dense as water whenvolcanic sediment is incorporated
• Slurries develop that are similar to concrete
• Streams are able to buoy up and carry rocksmuch 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 eruptionsfrom Medicine Lakevolcano
• Main rock types = Basalt
Aa andPahoehoe
AndesiteRhyoliteTuffObsidianPumice
Volcanic Features• Lava Tube Caves
• Cinder Cones
• Spatter Cones
• Craters
• Fault Scarps
Lassen PeakNational Volcanic Park
• Formed from a massiveextrusion 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