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  1. Volcanoes

  2. A volcano is a mountian that forms when molten rock (magma) is forced to the earth’s surface

  3. What causes volcanoes? Tectonic Plate Boundaries!!! ~75% world’s active volcanoes in Ring of Fire

  4. Where do volcanoes occur? • Divergent boundaries • Convergent boundaries • Hot spots

  5. Volcanoes form around ventsthat release lava, which is the magma that reaches the earth’s surface.

  6. Non-explosive Eruptions • Quiet eruption in which magma flows easily • Usually gas dissolved in magma escapes easily • Thin, runny lava oozes quietly from vent • Mafic lava has low silica (silicon dioxide) content, so it is not viscous • Example: Hawaiian Islands

  7. Lava: Pohoehoe • Flows slowly, like wax dripping from a candle. • Forms glassy surface with rounded wrinkles ‘ropey’ appearance.

  8. Lava: A’a • A’a is slightly stiffer. • Pours out quickly and forms a brittle crust which is torn into jagged pieces as molten lava underneath continues to move.

  9. Lava: Block • Cooler and stiff • Oozes from volcano • Forms jumbled heaps of sharp-edged chunks. Mexico’s Colima Volcano

  10. Lava: Pillow • Forms when lava erupts underwater at sea floor spreading zones. • Forms rounded lumps the size and shape of pillows.

  11. Explosive Eruptions • Thick and sticky magma will not flow out of a volcano. Instead, it plugs the vent like a cork in wine bottle. • Felsic lava: high silica content results in high viscosity (resistance to flow) • Dissolved gasses and water cannot escape the thick magma, build up pressure that contributes to explosive eruptions. • Example: Mt. St. Helens

  12. Explosive Eruptions • Clouds of hot debris and gasses shoot out from the volcano, often at supersonic speeds. • Molten rock is blown into millions of pieces that harden in the air. • Dust-sized particles can circle the globe for years in the upper atmosphere. • Larger pieces fall closer to the volcano. • Can blast millions of tons of solid rock and, in just a few minutes, can demolish rock formations that took hundred or thousands of years to accumulate. • Volcanoes can actually shrink due to repeated eruptions.

  13. What Erupts from an explosive Volcano? • Pyroclastic material - rockfragments created by explosive eruptions • magma explodes from volcano and solidifies in the air • existing rock is shattered by powerful eruptions Volcanic ash Volcanic bombs Volcanic blocks Lapilli

  14. Pyroclastic Material: Size • Volcanic blocks: solid rock blasted out of volcano • Volcanic bombs: large blobs of magma that harden in air • Lapilli: “little stones” (Italian) – pebble-like bits of magma that become solid before hitting the ground. • Ash: forms when gases in stiff magma expand rapidly & walls of gas bubbles explode into tiny glasslike slivers.

  15. Types of Volcanoes Shield volcano Cinder cone volcano Composite volcano

  16. Shield Volcanoes are produced by the accumulation of basalt lava flows. • Broad, dome shape • Frequent non-explosive eruptions • low silica = low viscosity

  17. The Hawaiian Islands are hot spot volcanoes (shield)

  18. Composite Volcanoes • Switches between quiet eruptions of flowing lava and violent eruptions of thick, gas-rich lava (pyroclastic material). This type of volcano has the most powerful eruptions of all. • Composite volcanoes, also called stratovolcanoes, form tall conical mountains. • Example: Cascade Volcanoes.

  19. Cinder Cone Volcanoes • Built up from ashes, cinders, and rocks that burst from Earth during a violent eruption. • The rocks fall back to Earth near the opening, to form a cone. Ash covers a larger surrounding area. • Erupt for a short period of time, so most are not taller than 300m (about 1000 ft) • Occur in clusters or on sides of shield and composite volcanoes. • Erode quickly due to pyroclastic material not being cemented together by lava. Mt. Etna, Italy

  20. Craters, Calderas, and Lava Plateau • Crater • From explosions of material out of the vent and the collapse of material back into vent • Caldera • Much larger depression that forms when magma chamber empties and its roof collapses • Lava Plateau • Forms when lava erupts from long cracks, or fissures, and spreads out evenly (thousands of km)

  21. Crater Lake is an example of a caldera, created when Mt. Mazama erupted an collapsed around 5600 BC