1. Chapter 12 Section 4 - Volcanoes

2.  Movement along a fault causes a decrease in pressure – decompression  A decrease in pressure causes a decrease in melting point  The temperature remains the same

3. Magma from the Asthenosphere A buoyant force acts on magma that forms from rock surrounding it. Hot, nearly molten rock in Earth’s asthenosphere can change to a liquid by decompression melting.

4. Why Magma Rises Rising magma may reach Earth’s surface if pressure conditions allow and the rock has conduits (openings) through which it can flow.

5. Eruptive Products All solid materials expelled by a volcano are collectively called pyroclasts. Volcanoes release a broad variety of superheated gases, the most common of which is water vapor. In addition carbon dioxide and gases composed of sulfur compounds are expelled. Solids Gases

6. Viscosity is a measure of the resistance of a fluid to flow. High viscosity is thick like pancake syrup. Low viscosity is runny like water. Liquids Lavas can vary considerably in composition, which in turn affects their physical properties.

7. Plate Boundary Setting Most of Earth’s volcanoes lie in subduction zones where continental and oceanic materials are being mixed and partially melted.

8. Hot Spots Hot spots are volcanically active sites that arise in places where large quantities of magma move to the surface in large, column-like plumes. A hot spot under an oceanic plate forms volcanic island chains, such as the Hawaiian Islands. Yellowstone National Park is an example of a hot spot under a continental plate.

9. Moderate slopes, 20 degrees Made of pyroclasts (cinder, bombs) Rock type – basalt Relatively small in size Examples: Sunset Crater (AZ), Hawaii Types of Volcanoes Volcanoes are classified according to their size, shape, and the materials that compose them. Cinder Cone Volcanoes

10. Low slope angles, 2 to 8 degrees Long, fluid (basaltic) lava flows Rock type – basalt Can be very large Example: Mauna Loa, Hawaii Shield Volcanoes

11. Also called Stratovolcano Made of lava flows and pyroclastics (mostly ash) Moderate in size Rock type – andesite Examples: Mt. St. Helens, Fuji, Pinatubo Composite Volcanoes

12. Section Check Question 1 Where do most volcanoes occur? Answer Most volcanoes occur at plate boundaries where huge pieces of the crust pull apart or push together. As a result, the crust often fractures, allowing magma to reach the surface.

13. Section Check Question 2 What type of volcano is formed by an explosive eruption followed by a quiet eruption? A. cinder cone volcano B. composite volcano C. fissure eruption D. shield volcano

14. Section Check Answer The correct answer is B. Composite volcanoes erupt explosively releasing large quantities of gas and ash. They are followed by quieter eruptions that form a lava layer over the ash.

15. Section Check Question 3 How does a hot spot volcano form? Answer A volcano forms above a hot spot when magma erupts through the crust and reaches the surface. Hot spot volcanoes may lie in the middle of plates far from any plate boundaries or near or on plate boundaries.