volclandforms - Volcanic Landforms Volcanoes and Plate...

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This page last updated on 28-Jan-2004 EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson Volcanic Landforms, Volcanoes and Plate Tectonics Volcanic Landforms Volcanic landforms are controlled by the geological processes that form them and act on them after they have formed. Thus, a given volcanic landform will be characteristic of the types of material it is made of, which in turn depends on the prior eruptive behavior of the volcano. Although later processes can modify the original landform, we should be able to find clues in the modified form that lead us to conclusions about the original formation process. Here we discuss the major volcanic landforms and how they are formed, and in some cases, later modified. Most of this material will be discussed with reference to slides shown in class that illustrate the essential features of each volcanic landform. Shield Volcanoes z A shield volcano is characterized by gentle upper slopes (about 5 o ) and somewhat steeper lower slopes (about 10 o ). z Shield volcanoes are composed almost entirely of relatively thin lava flows built up over a central vent. z Most shields were formed by low viscosity basaltic magma that flows easily down slope away form the summit vent. z The low viscosity of the magma allows the lava to travel down slope on a gentle slope, but as it cools and its viscosity increases, its thickness builds up on the lower slopes giving a somewhat steeper lower slope. z Most shield volcanoes have a roughly circular or oval shape in map view. z Very little pyroclastic material is found within a shield volcano, except near the eruptive vents, where small amounts of pyroclastic material accumulate as a result of fire fountaining events. z Shield volcanoes thus form by relatively non-explosive eruptions of low viscosity basaltic magma. Volcanic Landforms, Volcanoes and Plate Tectonics 1/28/2004 Page 1 of 12
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z Vents for most shield volcanoes are central vents, which are circular vents near the summit. Hawaiian shield volcanoes also have flank vents, which radiate from the summit and take the form of en-echelon fractures or fissures, called rift zones, from which lava flows are emitted. This gives Hawaiian shield volcanoes like Kilauea and Mauna Loa their characteristic oval shape in map view. Stratovolcanoes (also called Composite Volcanoes ) z Have steeper slopes than shield volcanoes, with slopes of 6 to 10 o low on the flanks to 30 o near the top. z The steep slope near the summit is due partly to thick, short viscous lava flows that do not travel far down slope from the vent. z The gentler slopes near the base are due to accumulations of material eroded from the volcano and to the accumulation of pyroclastic material. z Stratovolcanoes show inter-layering of lava flows and pyroclastic material, which is why they are sometimes called composite volcanoes. Pyroclastic material can make up over 50% of the volume of a stratovolcano.
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