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Explosivity of Volcanic Eruptions
- A.Viscosity-resistance to flow of lava or magma
- 1.Silica content influences viscosity
- a. The higer the silica content, the more viscoud the magma/ lava
- i.Basaltic lavas-50% silica
- ii. Andestitic lavas-60%
- iii.Rhyolitic lavas-70%
- 2.Temperature- the higher temperature, the less viscous the lava
- B.Importance of dissolved gases
- a.Gases are held in magma by confining pressure
- b.As magma moves up , the confining pressure is reduced and dissolved gases expand
- C.Basaltic eruptions tend to be less violent-less viscous lava, hotter , lower gas content
- D. Ryolititc eruptions tend to be more violent-more viscous lava, cooler, higher gas content
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What is Erupted during Volcanic Eruptions
- A. Lava Flows
- 1. Pahoehoe flows- fluid, ropy basaltic flows
- 2. AA flows- surface of rouch, jagged blocks, tend to be cool and thick
- 3. Pahoehoe lava flows may become aa flows as they cool
- B. Gases
- 1.Water Vapor- most abundant gas erupted
- C. Pyroclastic materials
- 1. Fragments ejected explosively from a volcano
- 2.May range in size from dust and ash to block, bombs
- 2.Carbon Dioxide-second most abundant gas erupted
- 3. Eruption volcanic gases formed earths atmosphere and hyrdrosphere
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Types of Volcanoes
- A. Features of volcanoes
- 1.Vent-opening in the crust through which material is ejected
- 2. Pipe-feeds magma from the magma chamber to the surface
- 3. Crater-steep-walled depression at the summit of a volcano, built through a successive eruptions
- 4. Caldera- large, circular depressions formed through collapse of the summit
- 5. Fumarole-vents that erupt only gases
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Types of Volcanoes
- B.1. Shield volcanoes
- a.Built by successive basaltic lava flows
- b. Broad, slightly domed shape
- c.Hawaiin Islands, Galapagos Islands
- 2. Cinder cones
- a. Built primarily by successive eruptions of pryoclastic materials
- b. Small, steep-sided , usually short lived
- c. May from parasitic cones on larger (composite cones)
- d. Paricutin, Mexico
- 3.Composite cones
- a. Built by alternating eruptions of andesitic lava flows and pyroclastic materials
- b. Large, symmertrical cones,
- c. Cascade Range, Andes Mountains.
- C. Volcanic hazards associated with composite cones
- 1. Nuee ardente
- a. Hot fiery cloud of gas and ash, often created by collapse of lava dome
- b. Mt Pelee, martinique
- 2. Lahar
- a. Volcanic mudflows
- b. Mt. St. Helens potential on Mt. Rainier for Lahars
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Other Volcanic Landforms
- A. Calderas
- 1. Large collapsed depressions with a diameter greater than one km
- 2. Crater Lake-type calderas
- a. Formed when Mt. Mazama violently erupted pyroclastic material and the cone suddenly collapsed
- b. The caldera eventually filled with water
- c. Crater Lake, Oregon
- 3. Hawaiin-type calderas
- a. Formed by gradual subsidence of the summit as magma slowly drained from magma chamber to a rift zone
- 4. Yellowstone- type calderas
- a. formed when ryholitic magma is emplaced near surface
- b. Fractures develop and provide a pathway to surface for gas charged magma, which cause a very explosive eruption
- c. Yellostone National Park is located in a caldera
- B.Fissure eruptions and lava plateaus
- 1. Very fluid basalt erupts from fissure (fractures in the crust)
- 2. Fissure eruptions create lava plateaus
- 3. Columbia River basalts created the columbia river plateau
- C. Volcanic pipes and Necks.
- 1. Volcanic pipes-connect magma chamber to the surface
- 2. Volcanic necks-eroded remnants of volcanic pipes
- a. Shiprock, NM
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Intrusive Igneous Activity
- A. Plutons- intrusive igneous bodies
- B. Classifying plutons
- 1. Shape-tabular (tabletop) or massive (no shape)
- 2. Orientation to surrounding rocks
- a. Discordant-cut across existing rocks
- b. Concordant-intrude between rock layers
- C. Types of Plutons
- 1. Dikes
- a. Tabular, discordant
- b. Often form when magma fills fractures
- 2. Sills- tabular, concordant
- 3.Laccoliths
- a. Mushroom-shaped, concordant
- b. Generally form at shallow depths
- 4. Batholiths
- a. Massive, discordant
- b. Greater than 40 square miles in surface extent
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