chapter5 - Chapter 5 Volcanoes and Other Igneous Activity...

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    Chapter 5   Volcanoes and Other  Igneous Activity
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        Key Terms and Concepts Nature of volcanic eruptions: Viscosity, resistance to flow Controls on viscosity: composition, temperature, gas content Lavas and pyroclastic debris Types of volcanoes Shield Composite (stratovolcano) Cinder Other volcano-initiated flows ( Nuée ardente, lahar ) Intrusive terms (dike, sill, laccolith, batholith, volcanic pipes and necks) Location of volcanoes ( oceanic ridges, subduction zones, hot spots ) Melting mechanisms at plate boundaries Oceanic ridges, decompression melting of peridotite produces basalt Subduction zones, water-release melting produces mostly andesite, some basalt Examples
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    The nature of  volcanic eruptions Characteristics of a magma determine the “violence” or explosiveness of a volcanic eruption Composition Temperature Dissolved gases The above three factors actually control the viscosity of a given magma
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    The nature of  volcanic eruptions Viscosity is a measure of a material’s resistance to flow Viscosity increases with cooling (decreasing T) Low temperature = higher viscosity Viscosity increases with increasing silica (SiO 2 ) Higher silica content = higher viscosity rhyolite andesite basalt
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    The nature of  volcanic eruptions Dissolved gases Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure Gas pressure builds as crystallization take place The violence of an eruption is related to how easily gases escape from magma In summary Basalt, low gas, low viscosity = mild eruptions Rhyolite, high gas, high viscosity = explosive eruptions
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chapter5 - Chapter 5 Volcanoes and Other Igneous Activity...

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