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Unformatted text preview: GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111 Lecture Note Series Volcanoes and volcanic land forms Lecture Goals A) Volcanic land forms B) Volcanic eruptions (rock types, case studies) C) Measurements of activity Reference: Press et al., 2004, Chapters 5 and 6; Grotzinger et al., 2007, chapters 2, 4 and 12 A) Volcanic landforms Volcanoes form some of the most impressive land forms on the planet, but they aren’t always classic cone-shaped features. Volcanoes that are dead (or extinct; see part C below) very quickly lose their cone shape and eventually will completely erode away. Before that happens though, they can form near vertical volcanic necks . These features are the remnants of the volcanic pipe (see sketch 1 below) and had originally been filled by magma. When all volcanic activity stopped (the volcano went extinct) the magma cooled and crystallized (usually to a porphyry). This material is usually much harder and more resistant to erosion than the lava flows +/-ash that composed the flanks of the volcano. As a result, the pipes remain long after the volcano has vanished. Another prominent feature that hangs around long after a volcano is gone are dikes. Near the center of a volcano (surrounding the pipe), dikes often occur in swarms that radiate from the pipe almost like the spokes in a bicycle wheel. Not surprisingly, they are called radial dikes. Think that all volcanic land forms are positive topographic features (i.e., they are hills). Think again. When volcanoes collapse or blow up, they may leave behind a deep depression called a caldera (see image at lower right). If the collapse occurred as the result of a powerful explosion (e.g., Krakatoa), the caldera is said to be an explosive caldera . If the collapse occurred more passively as a result of an emptied magma chamber (the ground simply caves into a large hole below the Earth’s surface), the resulting hole is said to be a collapse caldera . Either way, calderas can be big (miles across). If the depression is later filled by water, it’s called a crater lake (a caldera lake would be a better term; see image on next page). The last landform that we will discuss today is a positive topographic feature that is composed of multiple volcanic cones. When oceanic lithosphere subducts below oceanic lithosphere (a convergent plate boundary such as this GY 111 Lecture Notes D. Haywick (2008-09) 2 marks the boundary between the Pacific Plate and the North American Plate west of Alaska), a series of volcanoes pops up along the edge of the subduction zone on the overriding plate (see sketch 2 below). Eventually, the volcanoes reach sea level and form a linear or curved string of islands. They are called island arcs and along with trenches , mark areas of subduction on the sea floor. Island arcs can be thin and sporadic (e.g., the Aleutian Islands of Alaska) or quite massive (e.g., Japan); however, either way, they are characterized by volcanoes that are usually explosive (see case studies below). are characterized by volcanoes that are usually explosive (see case studies below)....
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- Fall '11
- Geology, Volcano, D. Haywick