1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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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