Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
10.1 Volcanoes and Plate Tectonics Lecture Outline Origin of Magma Magma forms in the crust and upper mantle when solid rock particles melt. The Formation of magma depends on several factors. 1. Heat 2. Pressure 3. Water content Heat At a depth of 100 kilometers, the temp of the mantle ranges between 1400 0 C and 16000C. At these temperatures, the solid rock of the lower crust and upper mantle is near, but not at the melting point. The additional heat needed to produce magma comes from: 1. Friction generates as huge slabs of lithosphere slide past each other in the subduction zone 2. Mantle itself heats these subducting slabs 3. Hot mantle rock can rise and intrude into the cooler lithosphere, heating it. Pressure Increasing pressure raises the melting point of rock deep beneath the surface. Decreasing pressure lowers rock’s melting point. When pressure drops enough, decompression melting occurs. This process takes place as hot yet solid mantle rock rises because it is less dense than the surrounding rock. As the rock rises, pressure on the rock decreases. This decrease lowers the rock’s melting point. Water Content The water content of rock also lowers the rock’s melting point. “Wet” rocks deep beneath the surface melts as a much lower temperature than “dry” rocks. Volcanoes and Plate Boundaries Most volcanoes form along divergent and convergent plate boundaries. Some volcanoes form far from plate boundaries above “hot spots” in the crust. Divergent Boundary Volcanism At divergent boundaries, volcanic activity occurs where the plates pull apart. 1. Mantle rock rises to fill the gap between the plates 2. As the rock rises, decompression melting occurs. 3. This forms magma, which erupts along the axis of the spreading center. Spreading centers are located along mid-ocean ridges, some are not 1. The Great Rift Valley in East Africa is where continental crust is being pulled apart along a divergent boundary. 2. Mount Kilimanjaro in Tanzania is a volcano that formed near the rift valley. Convergent Boundary Volcanism Volcanoes form at convergent plate boundaries where slabs of oceanic crust are pushed down into the mantle. As a slab sinks deeper into the mantle, the increased in temperature and pressure drives water from the oceanic crust. Once the sinking slab reaches a depth of about 100 to 150 kilometer, this water reduces the melting point of hot mantle rock low enough for melting to begin. The magma formed slowly migrates upward, forming volcanoes. Volcanoes from as convergent boundaries where two oceanic plates meet and where the oceanic lithosphere is subducted beneath another oceanic plate. This process results in the formation of a chain of volcanic mountains on the ocean floor. Eventually this volcanic mountain grow large enough to rise above the ocean surface. Called Volcanic Island Arcs. Ring of Fire is the long belt of volcanoes that circles the Pacific Ocean. Volcanism may also occur at convergent plate boundaries where the continental plate meets an oceanic plate. Result the oceanic plate is subducted under the continental lithosphere. This forms a Continental Volcanic Arc. Intraplate Volcanism Volcanic activity that occurs within a plate. Example : Yellowstone National Park. Most Intraplate volcanisms occurs where hotter - than – normal mantle material, called a mantle plume, rises toward the surface. This may create a small volcanic region called a hot spot. The more recently formed island of Hawaii Island has two active volcanoes-Mauna Loa and Kilauea.