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Chapter 21 They occur mostly at plate boundaries, along fault lines Energy is released as seismic waves As the seismic waves travel through the Earth they create shaking Focus - the exact point within the Earth where an earthquake originates Epicenter - a point on the Earth’s surface immediately above the focus ◦ Damage from earthquakes are usually greatest at the epicenter Energy released is measured as shock waves Three types of waves generated: 1. longitudinal waves - originate from the focus and travel through rock faster than other waves Also called P-waves or primary waves They travel by compressing the crust in front of it and stretching the crust behind it These waves move more slowly than p-waves ◦ Known as secondary waves or, S-waves ◦ Transverse waves have peaks and valleys like ocean waves Moves only across the Earth’s surface Combination of up and down motion and back and forth motion Longer wavelengths and rolling action can cause most of surface damage to buildings Seismology is the study of earthquakes Seismograph machines measure ground motion during earthquakes and record data about p-waves, s-waves and surface waves Seismographs measure p-waves first, then swaves and finally the larger surface waves The difference in time from the arrival of pwaves and s-waves enables seismologists to determine the distance to the epicenter Using distances from three seismograph stations allows them to find the epicenter Because waves change speed and direction as they travel whenever the density changes, scientists have used seismograph information to create a model of the Earth’s interior Measure of the energy released by an earthquake Each step, on the scale represents a 30-fold increase in the energy released An earthquake of magnitude 8 releases 304, or 810,000 times as much energy as magnitude of 4 http://video.nationalgeographic.com/video/e nvironment/environment-naturaldisasters/earthquakes/earthquake-montage/ Distance from epicenter Type of bedrock Type of buildings present Population How deep the focus is http://www.bbc.co.uk/news/world-asiapacific-12709850 Any opening in the Earth’s crust through which magma reaches the surface ( vents ) Volcanoes release molten rock, ash, and a variety of gases When magma reaches the Earth’s surface , its physical behavior changes, and it is called lava http://video.nationalgeographic.com/video/e nvironment/environment-naturaldisasters/volcanoes/volcano-lava/ Mild eruptions Magma is rich in iron and magnesium, creates fluid lava The buildup of this kind of lava creates a gently sloping mountain known as a shield volcano Mauna Loa, in Hawaii is a shield volcano Composed of alternating layers of ash, cinders and lava Magma is rich in silica, is more viscous Gases are trapped in magma, causing eruptions that alternate between flows and explosive activity Japan’s Mount Fuji, Mount St. Helens, Mount Ranier, Mount Hood and Mount Shasta are examples http://www.bing.com/videos/search?q=earthquake+videos +discovery+channel&mid=149DB76B32D4742F389F149DB7 6B32D4742F389F&view=detail&FORM=VIRE3&adlt=strict Smallest and most abundant volcanoes Large amounts of gas trapped in magma cause more violent eruptions Cinders and ash are thrown from the vent Paricutin in Mexico is a cinder cone Seamounts – underwater volcano Most volcanoes occur along convergent plate boundaries 75% of active volcanoes are located in these zones Volcanoes around the Pacific Ocean lie in a zone known as the Ring of Fire These volcanoes often have explosive eruptions due to the highly viscous magma Underwater volcanoes occur at divergent plate boundaries Volcanic mountains of the central rift valley Iceland, in the North Atlantic Ocean, is on the Mid-Atlantic Ridge Some volcanoes occur in the middle of plates at hot spots Mantle-plumes of hot rock rise from deep inside the mantle, melt as they rise, and erupt to form volcanoes As the continental plate continues to move over the hot spot, a series of islands is created Hawaiian Islands are an example