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Answer Key for Effects of Plate Tectonics Note-taking Guide What are Tectonic Plates? alters, solid, asthenosphere, tectonic, surface The most powerful force that alters the Earth’s surface is the movement of tectonic plates. Tectonic plates are large, solid pieces of the Earth’s lithosphere that move in different directions on top of a hotter flowing layer called the asthenosphere. There are 7 major tectonic plates and some minor ones that make up the surface of the Earth. Plate Tectonics Theory interior, sea floor, earthquakes, ocean floor, Pangaea, new, age, mid-ocean, moved, away, continental drift, plate tectonic, geological, convection, older In the early 1900’s, Alfred Wegener, put forward the theory that the continents were once part of one large land mass called Pangaea. This theory is called the continental drift theory, and explained why identical animal fossils, like that of the Mesosaurus, were found on coastlines separated by vast oceans. It also explained why similar geological features were found across continents like the North American Appalachian Mountains lining up with mountains in Scotland and Scandinavia. © 2010–2015 Edusmart 1 But this theory didn’t explain “why” the continents moved. It was only with the ocean floor explorations of the early 1960s that the continental drift theory was verified. These explorations led to the discovery of magma oozing up in the middle of underwater mountain chains, known as mid-ocean ridges, to create new ocean floor. This process was called sea floor spreading. Sea floor spreading occurs when tectonic plates move away from each other and create a gap in the Earth’s crust. Magma rises up and fills this gap, creating new oceanic crust. The age of the ocean floor is an important piece of evidence that supports sea floor spreading. New crust is found at the mid ocean ridges and gets progressively older as it moves away from both sides of the ridge. The continental drift theory and sea floor spreading, in addition to the discovery that earthquakes and volcanoes occur most frequently at mid-ocean ridges and specific areas around the world, led to the development of a new unifying theory called the plate tectonic theory. The plate tectonic theory explains how earth works. Heat from Earth’s interior, creates convection currents which move the lithospheric plates around in different directions, at different speeds. Divergent Plate Boundaries continental, rift, divergent, fault block, fault, away, new A divergent boundary is the place where two tectonic plates move away, or spread apart, from one another. The majority of divergent boundaries are located at mid-ocean ridges where sea floor spreading takes place and new crust is formed. © 2010–2015 Edusmart 2 A divergent boundary is also formed when two continental plates move apart; a fault develops on the Earth’s crust. Some of the landforms on Earth are created by the movement of divergent plates that cause rift valleys and fault block mountains. Convergent Plate Boundaries asthenosphere, subduction, lighter, convergent, folded, denser, increases, magma, towards, Himalayas, oceanic, volcanoes, trench A convergent boundary is a place where two tectonic plates move towards, or push against each other. When two continental plates converge, they can squeeze together and force the crust upwards. This leads to the formation of folded mountains. This is how the highest mountain range, the Himalayas, were formed. When an oceanic plate converges with a continental plate, the denser oceanic plate slides beneath the lighter continental plate, into the asthenosphere. This region is called a subduction zone and the tectonic forces are so strong that a trench or crack can form in the ocean floor. As the oceanic crust sinks downward toward the asthenosphere, the temperature increases and the edge of the oceanic plate melts to form magma. The magma can eventually erupt through Earth’s crust to form volcanoes. Transform Plate Boundaries California, © 2010–2015 Edusmart earthquakes, slide past, tectonics, fault 3 At transform boundaries, instead of moving towards or away from each other, tectonic plates slide past one another. This kind of boundary results in a large fault. An example of this in the San Andreas Fault in California. Because tectonic plates are not smooth, a lot of jolting and gnashing occurs while the plates slide past one another, creating earthquakes. Plate tectonics has led to a sweeping change in our understanding of Earth and the forces that shape it. © 2010–2015 Edusmart 4