1. Volcanoes

2. Volcanic activity takes place primarily at subduction boundaries, VOCABULARY How and Where Volcanoes Form Oceanic lithosphere Continental lithosphere Asthenosphere 1. Water in the subducted rock is released into the asthenosphere. 2. The water lowers the melting temperatures of materials in the asthenosphere, leading to magma formation. 3. The magma is less dense than its surroundings, so it rises. 4. Some of the magma reaches Earth’s surface, and volcanoes form on the overriding continental plate. volcano hot spot

3. Deep inside Earth, between the molten iron core and the thin crust at the surface, there is a solid body of rock called the mantle. When rock from the mantle melts, moves to the surface through the crust, and releases pent-up gases, volcanoes erupt. Extremely high temperature and pressure cause the rock to melt and become liquid rock or magma. When a large body of magma has formed, it rises thorugh the denser rock layers toward Earth's surface. Magma that has reached the surface is called lava.

4. The Hawaiian Islands formed over a hot spot. Volcanoes Magma and volcanoes also form at hot spots. VOCABULARY How and Where Volcanoes Form Direction of Plate Movement Kauai Oahu Molokai Lanai Maui Hawaii volcano hot spot

5. Volcanic Settings

6. Major Volcanoes Around the World

7. Gases escape easily from basaltic magma, generating relatively quiet eruptions. Hardened basaltic lava flows on land are characterized as pahoehoe or aa; if the lava cools underwater, it is characterized as pillow lava. Magma and Erupted Materials

8. VOCABULARY viscosity lava pahoehoe aa pillow lava pyroclastic material pyroclastic flow Magma and Erupted Materials Viscosity: Viscosity is a measure of how thick (viscous) and sticky a liquid is. Magma and Erupted Materials

9. Lava Flows Lava Flow - A 'stream' of molten rock 'aa' lava flow - jagged, rubbly, broken surface 'pahoehoe' lava flow - smooth, ropy surface

10. Silica Content Melting Temp. Gas Content Viscosity Type of Eruption Location Least (~50%) Basaltic Magma Least Least viscous Rarely explosive Highest Rifts, oceanic hot spots AndesiticMagma Intermediate (~60%) Intermediate Sometimes explosive Intermediate Subduction boundaries Rhyolitic Magma Most (~70%) Most Most viscous Usually explosive Lowest Continental hot spots Magma and Erupted Materials

11. Volcanic Landforms A volcano’s shape and structure depend on how it erupts and what materials are released. Shield volcanoes are formed by basaltic lava that flows long distances before hardening.

12. There are three types of volcanic cones: ______ – explosive eruptions, small but steep slopes, pyroclastics. _______ – non-explosive eruptions, fluid basaltic lava, gentle broad slopes. _________ – alternating between lava and pyroclastics, explosive and non-explosive eruptions, steep and tall. Cinder Shield Composite

13. Volcanoes A volcano’s shape and structure depend on how it erupts and what materials are released. VOCABULARY Volcanic Landforms Shield volcanoes are formed by basaltic lava that flows long distances before hardening. Shield Volcano shield volcano cinder cone composite volcano lahar caldera lava plateau

14. Shield and Composite Volcanoes

15. Volcanoes Cinder cones are formed when molten lava is thrown into the air from a vent and breaks into drops. These drops harden into cinders that form a steep cone around the vent. Volcanic Landforms Cinder Cone

16. Volcanoes Composite volcanoes are formed by layers of pyroclastic materials and lava that have erupted in the past. Volcanic Landforms Composite Volcano

17. Composite volcanoes