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Volcanoes 12.10.07 / 12.11.07 Volcanic eruptions Factors that determine the violence of an eruption • Composition of the magma • Temperature of the magma • Dissolved gases in the magma Viscosity of magma • Viscosity is a measure of a material's resistance to flow Volcanic eruptions Viscosity of magma • Factors affecting viscosity • Temperature (hotter magmas are less viscous) • Composition (silica content) • High silica – high viscosity • Low silica – more fluid • Dissolved gases (volatiles) • mostly H2O and CO2 • Gases expand near the surface Volcanic eruptions Viscosity of magma • Factors affecting viscosity • Dissolved gases (volatiles) • Provide the force to extrude lava • Violence of an eruption is related to how easily gases escape from magma • Easy escape from fluid magma • Viscous magma produces a more violent eruption • Think of boiling water vs spaghetti sauce A low silica lava flow A high silica flow Pyroclastics • Pyroclastic material: fragments of rock formed during eruptions – Ash and dust: fine, glassy fragments – Lapilli: walnut sized rocks – Bombs: magma ejected into the air cools and forms rounded rocks – Blocks: ejected as large rocks Volcanoes General features • Conduit, or pipe carries gas-rich magma to the surface • Vent, the surface opening (connected to the magma chamber via a conduit) • Crater • Steep-walled depression at the summit • Caldera (a summit depression greater than 1 km diameter) Volcanoes Types of volcanoes • Shield volcano • Broad, slightly domed (like a shield) • Primarily made of fluid lava • Generally large size A shield volcano Volcanoes Types of volcanoes • Cinder cone • Built from ejected lava fragments • Steep slope angle • Relatively small size Sunset Crater – a cinder cone near Flagstaff, Arizona Volcanoes Types of volcanoes • Composite cone (or stratovolcano) • Most are adjacent to the Pacific Ocean (e.g., Mt. Rainier) • Large size • Interbedded lavas and pyroclastics • Most violent type of activity A composite volcano (stratovolcano) Mt. St. Helens – a typical composite volcano Mt. St. Helens after eruption A size comparison volcanoes Other volcanic landforms Calderas • • • • Steep walled depression at the summit Formed by collapse Nearly circular Size exceeds one kilometer in diameter Crater Lake, OR caldera Crater Lake in Oregon Plate tectonics Plate motions provide the mechanism by which mantle rocks melt to form magma • Convergent plate boundaries • • • • Deep-ocean trenches are generated Descending plate partially melts Magma slowly rises upward Rising magma can form • Volcanic island arcs in an ocean (Aleutian Islands) • Continental volcanic arcs (Andes Mountains) Convergent boundaries Plate tectonics Plate motions provide the mechanism by which mantle rocks melt to form magma • Divergent plate boundaries • The greatest volume of volcanic rock is produced along the oceanic ridge system Divergent boundaries Plate tectonics Plate motions provide the mechanism by which mantle rocks melt to form magma • Intraplate igneous activity • Activity within a rigid plate • Plumes of hot mantle material rise • Form localized volcanic regions called hot spots • One example is the Hawaiian Islands Hot Spots Locations of some of Earth’s major volcanoes r Continental and island volcanic arcs Review • Name and describe the three types of volcanoes. • Describe the different types of pyroclastics. • Explain the factors that affect the severity (violence) of an eruption. • Describe how continental and island arcs are formed.
