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Earthquakes and Volcanoes p. 346 - 353 What is This? Why do Earthquakes Occur? • Stress builds up at a fault • Stress is a force that adds potential energy to the rock • As soon as the rock slips, energy is released Why do Earthquakes Occur? • When there is sudden movement along a fault • The fault suddenly breaks and slides Focus and Epicenter • The focus of an earthquake is the area underground where rock moves. The focus is where waves are sent out • The epicenter of an earthquake is the location on the earth’s surface directly above the focus Focus and Epicenter Waves are Released seismic waves P and S waves • P: Primary Wave – causes the initial jolt of the earthquake. Compresses the rock as it travels • S: Secondary Wave – slower and stronger wave. Shakes the ground in many directions Volcanoes • A volcano is a weak spot in the crust where magma comes to the surface. • Magma that reaches earth’s surface is called lava. magma lava Lava - Lava cools to form solid rock - Volcanic activity builds mountains! Volcanoes at Convergent Boundaries Island Arcs • String of islands at an oceanic – oceanic convergent boundary • Japan, New Zealand, Indonesia, Caribbean islands Volcanoes at Divergent Boundaries Hot Spot Volcanoes • Hawaii was formed this way • Hot area under the surface – plate moves on top. As the plate moves, a chain of volcanoes is formed. • Watch the animation: http://esminfo.prenhall.com/ science/geoanimations/anima tions/35_VolcanicAct.html What is a Tsunami? Earthquake or volcanic activity under the ocean can lead to strong waves, creating a tsunami.