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Volcanoes and Plate Boundaries

Volcanoes and Plate Boundaries. 6 th Grade. Do more volcanoes occur on land or in the ocean? . Volcanoes and Plate Boundaries. There are about 600 active volcanoes on land There are MANY MORE beneath the sea Volcanoes tend to occur in belts that extend across continents and oceans

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Volcanoes and Plate Boundaries

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  1. Volcanoes and Plate Boundaries 6th Grade

  2. Do more volcanoes occur on land or in the ocean?

  3. Volcanoes and Plate Boundaries • There are about 600 active volcanoes on land • There are MANY MORE beneath the sea • Volcanoes tend to occur in belts that extend across continents and oceans • Volcanic belts form along the boundaries of Earth’s plates. WHY is this?

  4. Volcanic belts form along the boundaries of Earth’s plates because… • At plate boundaries, huge pieces of the crust diverge (pull apart) or converge (push together) • These movements cause fractures in the crust that allows magma to reach the surface • Most volcanoes form along diverging plate boundaries such as mid-ocean ridges and converging plate boundaries where subduction takes place

  5. What is the Ring of Fire? • A major volcanic belt that is formed by many volcanoes that rim the Pacific Ocean

  6. Ring of Fire

  7. Ring of Fire

  8. Volcano at a Converging Plate Boundarye.g the Ring of Fire

  9. How can Subduction form volcanoes? • When 2 plates collide, the older, denser plate sinks back into the mantle • The rock from subducting plates melts and forms magma • Because magma is less dense than surrounding rock, it rises toward the surface • Eventually the magma breaks through the crust and creates a volcano

  10. Subduction and Volcanoes

  11. What is an island arc?

  12. Island Arcs: a string of islands formed by volcanoes along a deep-ocean trench

  13. Major island arcs include Japan, New Zealand, Indonesia, the Philippines, the Aleutians, the Caribbean islands

  14. Volcano at a Diverging Plate Boundary

  15. Note: Volcanoes do NOT occur along transform boundaries, only earthquakes occur there

  16. Hot Spot Volcanoes • Results from a “hot spot” in the Earth’s mantle • Hot spot : an area where material from deep within the mantle rises and then melts, forming magma • A volcano forms above a hot spot when magma erupts through the crust and reaches the surface

  17. Do hot spots have to occur along plate boundaries? • No! They CAN occur on or near plate boundaries but they do not have to • Hot spots can also form under oceanic floor or continents

  18. Hawaiian Islands formed one by one over millions of years as the Pacific plate drifted over a hot spot

  19. Yellowstone National Parkin Wyoming marks a hot spot under the North American plate –example of a hot spot that formed under the continents

  20. Lets look at the properties of magma/lava. Why doeslava exhibit different characteristics at different volcanoes?

  21. Viscosity: the physical property of liquids that determines the resistance of flow. • Because liquids differ in viscosity, some liquids flow more easily than others • The greater the viscosity, the more slowly lava flows • Which lava has a greater viscosity?

  22. Types of Lava • http://www.youtube.com/watch?v=Z9iW_oXMBB8 (pahoehoe) • http://www.youtube.com/watch?v=iyIV5fd1Aww (aa) • Which type of lava has a higher viscosity?

  23. Aa has a higher viscosity • Pahoehoe: fast-moving, hot lava with low viscosity • Looks like a solid mass of wrinkles, billows, ropelike coils • Aa: cooler, slow-moving lava with higher viscosity --when it hardens, it forms a rough surface consisting of jagged lava chunks

  24. Physical and Chemical Properties • Physical properties- any property that can be observed without changing the composition of a substance. (temperature, hardness, density, boiling point) • Chemical Properties- property that causes a change in composition. (the ability to burn or to combine or react with another substance)

  25. Why don’t all types of magma have the same viscosity? • The viscosity of magma depends upon its silica content(chemical property) and temperature (physical property). • Silica = a compound that is made up of oxygen and silicon • Silica content of magma ranges from 50-70 percent

  26. The more silica magma contains, the higher its viscosity. • Magma that is high is silica produces light-colored lava that is too sticky to flow very far. • When this type of lava cools, it forms rhyolite

  27. The less silica magma contains, the lower the viscosity. • Low-silica magma flows readily and produces dark-colored lava. • When this kind of lava cools, it forms basalt.

  28. Rhyolite

  29. Basalt

  30. Where does lava usually come from? --lava begins as magma, which usually forms in the asthenosphere --the materials of the asthenosphere are under great pressure --liquid magma is less dense than solid material around it, so magma flows upward into any cracks in the rocks above

  31. What pushes magma to the surface? --Like the carbon dioxide in soda pop, dissolved gases are trapped in magma --The dissolved gases are under tremendous pressure

  32. When a volcano erupts, the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent.

  33. When is an eruption over? • When the pressure eases after the gases bubble out of the magma

  34. What happens when the volcano stops erupting? The lava at the surface will have turned to rock, magma will have solidified in the vent and pipe, and some magma might remain in the chamber

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