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Objectives • Describe the formation and movement of magma. • Explain the relationship between volcanoes and plate tectonics. • Summarize the methods scientists use to predict volcanic eruptions. I. The Formation of Magma A. Pressure and Temperature Rock melts when its temperature increases or when the pressure on the rock decreases. B. Magma Formation in the Mantle Because the temperature of the mantle is fairly constant, a decrease in pressure is the most common cause of magma formation. http://www.geology.wisc.edu/courses/g115/volcano/images/voldgm.gif II. Where Volcanoes Form A. Location The locations of volcanoes give clues about how volcanoes form. The next slide shows the locations of major volcanoes around the world. B. Plate Boundaries About 80% of active volcanoes on land form where plates collide, and about 15% form where plates separate. The Location of Major Volcanoes III. When Tectonic Plates Separate A. Divergent Boundaries At a divergent boundary, tectonic plates move away from each other. B. Mid-Ocean Ridges Form at Divergent Boundaries Most volcanic activity on Earth occurs at mid-ocean ridges. The next slide shows how magma forms at divergent boundaries such as those found along midocean ridges. How Magma Forms at a Divergent Boundary IV. When Tectonic Plates Collide A. Convergent Boundary A convergent boundary is a place where tectonic plates collide. B. Subduction Produces Magma As the descending oceanic crust scrapes past the continental crust, the temperature and pressure increase. This causes the water contained in the oceanic crust to be released. The water mixes with the mantle rock, which lowers the rock’s melting point, causing it to melt into magma. This process is illustrated on the next slide. How Magma Forms at a Convergent Boundary V. Hot Spots A. What Are Hot Spots? Hot spots are volcanically active places on the Earth’s surface that are far from plate boundaries. B. What Do Hot Spots Form? A hot spot often produces a long chain of volcanoes. VI. Predicting Volcanic Eruptions A. Measuring Small Quakes and Volcanic Gases Most active volcanoes produce small earthquakes as the magma within them moves upward. Also the ratio of certain gases may be important in predicting eruptions. B. Measuring Slope and Temperature As magma moves upward prior to an eruption, it can cause the Earth’s surface to swell. Also, infrared satellite images record changes in the surface temperature. If the site is getting hotter, the magma below is probably rising. http://www.cnn.com/2012/08/30/us/wussupervolcanoes-yellowstone/index.html http://www.msnbc.msn.com/id/49037141/ns/ technology_and_science/t/yellowstonessupervolcano-where-lava-likely-erupt/