Geologic History - Northwestern U.S.A.
• The geologic history of the northwest for the last 20 million years
is a volcanic one. Why?
Geologic History - Northwestern U.S.A.
• Northwestern volcanic history
Distribution of Volcanic Activity
• Where does volcanic activity occur?
• Convergent and divergent plate boundaries, plus hot spots
Volcanic Eruptions
• Why do volcanoes erupt, or what forces drive the volcanic material out
of the ground?
• Gasses expanding and bubbling out of solution
• Less pressure near the surface
Volcanic Eruptions
• What determines how explosive a volcanic eruption will be?
• Magma viscosity
• Temperature, amount of water, composition
• Silica in Magma
• => more SiO2 -> high viscosity
• => less SiO2 -> low viscosity
• fluid versus explosive
Volcanic Eruptions
• What materials come out during a volcanic eruption?
• Lava
• Gasses (H2O, CO2, SO2 and H2S)
• Pyroclastics (bombs, cinders, ash)
Volcanic Anatomy
• Volcanic Architecture
• Magma chamber
• Chimney & vent
• Caldera
• Resurgent dome
Volcano Types
• Types = magma composition and viscosity
• Very low viscosity = fissure eruptions/flood basalts
Volcano Types
• Low viscosity = shield volcanoes
• Basaltic lava
• Largest on earth
Volcano Types
• High viscosity
• Cinder cones
• All pyroclastics - violent erutions
• Small, short lived, easily eroded
Volcano Types
• High viscosity
• Stratovolcanoes - very large dangerous mountains
• Layers of andesitic lava flows and volcanic ash flows
• Eruptive cycles
Volcano Types
• Very high viscosity
• Calderas - supervolcanoes
• bulge - fracture
• eruption - collapse
Mount Rainier National Park
• Tahoma - 14,411 ft
• George Vancouver - 1792
• First ascent - 1870
• Established - 1899
• 235,613 acres
The Cascades
• Western Washington
• Cascades
• Volcanic mountain chain
• Why is this chain of volcanoes here?The Cascades
• Convergent Boundary
• Subduction zone
• Jaun de Fuca plate and spreading center
• Why aren’t these volcanoes always erupting?
Mount Rainier National Park
• Conifer forests and alpine meadows
• Alpine glaciers
• Stream drainage
Mount Rainier National Park
• Why is Mount Rainier such a dangerous mountain?
Mount Rainier - Hazards
• Stratovolcano - eruptions
• Lava flows
• Ash Fall
• Ash flow
Mount Rainier - Hazards
• Hazards that do not require an eruption
• Sector collapse
– Slope failure
– Hydrothermal alteration of rock
• Water -36 square mi - ice and snow
• Glacial outburst flooding
Mount Rainier - Hazards
• The greatest hazard for people living within 100 miles of cascade volcanoes?
• Lahars - mudflows