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Volcanoes

Volcanoes. MAGMA. All volcanoes are fueled by magma deep beneath Earth’s surface Magma is a mixture of molten rock, suspended mineral grains, and dissolved gases Depending on the composition rocks begin to melt at temperatures between 800°C and 1200°C. Factors affecting magma formation.

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Volcanoes

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  1. Volcanoes

  2. MAGMA • All volcanoes are fueled by magma deep beneath Earth’s surface • Magma is a mixture of molten rock, suspended mineral grains, and dissolved gases • Depending on the composition rocks begin to melt at temperatures between 800°C and 1200°C

  3. Factors affecting magma formation • Temperature • If rocks melt at temperatures found in the asthenosphere, and temperature increases with depth, then why isn’t the entire mantle liquid? • What other factors affect the formation?

  4. Pressure • Pressure increases with depth because of overlying rocks. • As pressure increases, the temperature at which a substance melts also increases • Most rocks in Earth’s lower crust and upper mantle do not melt to form magma even though the temperatures may be hot enough • Greater Pressure = Greater Melting Temperature

  5. Water Content • Water can be found in the pore spaces of some rocks and can be bound into the crystal structure of some minerals • A wet mineral or rock will melt at a lower temperature than the same mineral or rock under dry conditions • Greater water content = lower melting temperature

  6. Magma Composition • Many different factors determine the composition of magma • Viscosity • The internal resistance to flow • Gas Content • Silica Content • Source Material

  7. Magma Types • 3 Different types of magma • Basaltic • Forms when rocks in the upper mantle melt • Low viscosity • Contain small amounts of dissolved gases and silica • Eruptions are relatively quiet

  8. Andesitic • Found along subduction zones • Relatively high silica content • Has an intermediate viscosity • Volcanoes said to have intermediate eruptions

  9. Rhyolitic • Forms when molten material rises and mixes with the overlying silica and water rich continental crust • Highest viscosity • Resistant to flow • Large volume of trapped gas • Very explosive eruptions

  10. Viscosity • Viscosity of magma depends upon temperature and composition • Hotter the magma or lava, the lower the viscosity • Amount of silica in magma or lava also increases viscosity • High in silica = high in viscosity

  11. Intrusive Activity • Magma is less dense than surrounding rocks. • Moves upward and comes into contact with or intrudes the overlying crust • What happens deep in the magma chamber as the magma slowly cools? • As they cool they begin to form intrusive igneous rock bodies ranging in size • These intrusive igneous rock bodies are called plutons

  12. Batholiths & Stocks • Batholiths = largest plutons • Cover at least 100 km2 • Common in the interiors of major mountain chains • Composed primarily of granite • Stocks are irregular shaped and smaller in size than batholiths

  13. Laccoliths • A mushroom-shaped pluton with a round top and flat bottom • Relatively small in comparison to batholiths and stocks

  14. Sills • Sill = a pluton that forms when magma intrudes parallel to layers of rock • Range from only a few centimeters to hundreds of meters in thickness

  15. Dikes • Dike = a pluton that cuts across preexisting rocks • Form when magma invades cracks in surrounding rock bodies

  16. Anatomy of a Volcano • Volcanism produces various features that alter Earth’s landscape

  17. Vent • Opening in the crust through which lava erupts • Over time lava flowing from the vent can accumulate forming a mountain known as a volcano • Crater • A bowl shaped depression, around the vent, at the top of a volcano • Connected to the magma chamber by the vent

  18. Calderas • Craters are usually less than 1km in diameter • Larger depressions = caldera • Up to 50 km in diameter • Can form when volcano summit or side collapses into an empty magma chamber

  19. Types of Volcanoes • Shield • Broad, gently sloping sides • Nearly circular base • Formed when layers of lava accumulate • Non-explosive eruptions • Hawaiian Islands • Ex. Mauna Loa, Kilauea

  20. Cinder-Cone • Formed when material ejected into the air falls back to Earth and piles up around the vent • Steep sides • Small • Magma contains more water and silica = more viscous • Explosive eruptions

  21. Composite • Layers of volcanic fragments alternate with lava • Magma contains large amounts of silica, water, & gas • Much larger than cinder-cones • Violently explosive • Ex. Mount St. Helens

  22. Volcanic Material • Tephra = rock fragments thrown into the air during an eruption • Classified by size • Dust, ash, lapilli, volcanic blocks • Volcanic bombs

  23. Pyroclastic Flow • Clouds of gas, ash, and other tephra • Move at incredible speeds • 200km/h • Temperature can exceed 700°C • 1902 Mount Pelée

  24. Where Do Volcanoes Occur? • 80% occur along convergent boundaries • Subduction zones • Pacific Ring of Fire • 15% occur along divergent boundaries • Rift zones along ocean ridges • 5% occur away from boundaries

  25. Hot Spots • Hot Spots = unusually hot regions of Earth’s mantle • Plumes of material rise toward the surface • Magma melts through the crust to form volcanoes • Creates chains of volcanoes • Provide important information about plate motions • Rate and direction

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