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CHAPTER 6: VOLCANOES: Magma: Viscosity, Sources & Composition Currently, more than 1,500 volcanoes around the world are classified as active. Just 20 active volcanoes worldwide have a complete network of monitoring instruments. The resistance to flow of a liquid is called viscosity; increases in temperature will reduce viscosity. Viscosity of magma is control by it composition and temperature. Silica-rich/iron-poor magma has high viscosity, while .silica-poor/iron-rich magma has low viscosity. More-violent eruptions occur where gases cannot escape easily, this happens due to high viscosity conditions. Most of the world’s active volcanoes are located along convergent plate boundaries. NOTE: divergent boundaries have a great deal of volcanic activity, but generally volcanoes do not form along them. Volcanoes may be located in the interior of tectonic plates above hot spots. Silica-rich minerals have a lower melting temperature than silica-poor (iron-rich) minerals. Limited melting, involving only some minerals, is termed partial melting. This leads to a more silicarich magma than was the original source rock. Different plate boundaries produce different magma compositions because each setting generates magma from melting a different source rock. Basaltic magmas are characteristic of divergent plate boundaries and hot spots. On May 18, 1980, Mount St. Helens in Washington State generated the most destructive volcanic eruption in U.S. history. Three reliable indicators prior to an eruption are: increasing frequency of earthquakes, rapid changes in the shape of the volcano, and large-scale emissions of common volcanic gases. In the days before the eruption, 50 to 100 earthquakes per day, several greater than magnitude 4, were common. The rising bulge caused the north flank of Mount St. Helens to grow by approximately 150 meters (490 feet) from its original elevation. Enormous blocks collapsed and slid down slope, merging to form a massive debris avalanche when Mount St. Helens erupted.