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Volcanoes

Volcanoes. 11.1 What Causes Volcanoes?. Volcanoes and You Volcano – opening in Earth’s surface Often forms a mountain from layers of lava and volcanic ash Most are dormant, not currently active More than 600 currently active Mount St. Helens

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Volcanoes

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  1. Volcanoes 11.1 What Causes Volcanoes?

  2. Volcanoes and You • Volcano – opening in Earth’s surface • Often forms a mountain from layers of lava and volcanic ash • Most are dormant, not currently active • More than 600 currently active • Mount St. Helens • http://volcanoes.usgs.gov/volcanoes/st_helens/st_helens_geo_hist_102.html • http://www.history.com/videos/mount-st-helens-erupts • http://volcanoes.usgs.gov/volcanoes/st_helens/st_helens_geo_hist_100.html

  3. Active Volcanoes • Kilaua volcano in Hawaii • www.cbsnews.com/video/watch/?id=50133863n • www.youtube.com/watch?v=GX0yPGCEtLk • Iceland known for active volcanoes • http://www.cbsnews.com/video/watch/?id=6339052n • Eruptions of the Century • Mount Katmai, June 6, 1912 on the Alaska Peninsula • 30 times greater volume than Mount St. Helens • Mount Pinatubo, June 1991, Philippines • Killed nearly 900 people • Millions of tons of sulfur dioxide and ash into Earth’s upper atmosphere • Mount Unzen, June 1991, Japan • 44 people died

  4. How do Volcanoes Form? • Heat and pressure cause rock to melt and form magma. • Magma Forced Upward • Magma is less dense than rock, so it is pressed upward. • Vent – opening through which magma flows out • As lava flows out, it cools quickly and becomes sold, forming layers of igneous rock • Crater – steep-walled depression around a volcano’s vent

  5. Where do Volcanoes Occur? • Volcanoes are directly related to Plate movement and hot spots. • Plates Moving Apart • Iceland has volcanic activity because it sits on top of the Mid-Atlantic Ridge. • Mid-Atlantic Ridge – plates moving apart forming rifts • Magma flows from rifts and is cooled by seawater • As more lava flows, it builds up • It can form islands such as Iceland

  6. Plates Moving Together • Soufriere Hills volcano on the island of Montserrat • Formed because North American and South American Plates are being forced under the less dense Caribbean plate • Magma forms because the forced under plates get hot enough to partially melt • Magma is forced up forming the volcanoes of the Lesser Antilles • Hot Spots • Hawaii, not at a plate boundary • Some areas of mantle are hotter than others • Hypothesis is that hot spot magma forms at the boundary between the mantle and the outer core • Magma rises carrying heat from Earth’s core

  7. Hawaii formed when volcanoes rise above the water. • Islands in a straight line because plate is moving over the stationary hot spot. • Kauai is the oldest island. • As plate move, Kauai moved away from hot spot and became dormant. • Islands formed over about 5 million years.

  8. Chapter 11 Types of Volcanoes 11.2

  9. Styles of Eruption • Trapped Gases • Water vapor and carbon dioxide are trapped in magma by the pressure of surrounding magma and rock • As magma nears surface, there is less pressure. • Gas escapes easily during quiet eruptions • Gas that builds up to high pressures eventually causes explosive eruptions • Magma Composition • Low in silica means fluid lava, and quiet eruptions • Kilauea • Lava pours from vents, down the sides of a volcano • Trapped gasses can escape easily

  10. Silica-rich magma produces explosive eruptions • Sometimes forms where plates are moving together • Silica-rich magma is thick, and gases get trapped • When an explosive eruption occurs, gases expand rapidly, often carrying pieces of lava in the explosion • Magma Water Content • High water content causes explosive eruptions • Forms of Volcanoes • Depends on type of explosion and type of lava

  11. Shield Volcano • Quiet eruptions • Silica-poor lava • Flat layers • Shield Volcano – broad volcano with gently sloping sides • Hawaiian Islands

  12. Cinder Cone Volcano • Explosive eruptions • Tephra – rock or solidified lava dropped from the air • Varies in size • Cinder Cone Volcano – steep-sided volcano formed by loosely packed tephra

  13. Composite Volcano • Varies in explosions • Composite Volcano (Stratovolcano)– volcano formed from cycle of lava and tephra • Mostly found where plates come together • Mount St. Helens

  14. Volcanoes Igneous Rock Features 11.3

  15. Intrusive Features • Igneous rock – rock formed from fast cooling lava at or near Earth’s surface (extrusive igneous rock) and from slow-cooling magma deep inside Earth (intrusive igneous rock) • Batholiths • Batholiths – largest intrusive igneous rock bodies • Hundreds of kilometers wide and long, and several kilometers thick • Can be exposed by erosion • Granite domes of Yosemite National Park are the remains of a huge batholith that stretches across much of the length of California

  16. Dikes and Sills • Magma can squeeze into cracks in a rock below the surface • Dike – magma squeezed into a vertical crack • Sill – magma squeezed into a horizontal crack • Run a few meters to hundreds of meters long • Other Features • Volcanic Neck – solid igneous core left after cone erodes away • Caldera – large opening made when the top of a volcano collapses after an eruption • Igneous Features Exposed • Process of erosion wear down rock at the surface exposing features like batholiths, dikes, and sills.

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