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The Theory of Plate Tectonics

Section 5 of Chapter 1

Christopher Landry

on 9 December 2013

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Transcript of The Theory of Plate Tectonics

The Theory of
Plate Tectonics Inside Earth Objectives J. Tuzo Wilson A Canadian scientist, J. Tuzo Wilson, observed that there are cracks in the continents similar to those on the ocean floor. In 1965, Wilson proposed a new way of looking at these cracks. According to Wilson, the lithosphere is broken into separate sections called plates. The plates fit closely together along cracks in the crust. They carry the continents, or parts of the ocean floor, or both. Wilson combined what geologists knew about sea-floor spreading, Earth's plates, and continental drift into a single theory. How Plates Move Plate tectonics is the geological theory that states that pieces of Earth's lithosphere are in constant, slow motion, driven by convection currents in the mantle. The theory of plate tectonics explains the formation, movement, and subduction of Earth's plates. The plates float on top of the asthenosphere. Convection currents rise in the asthenosphere and spread out beneath the lithosphere, causing the movement of Earth's plates. As the plates move, they produce changes in Earth's surface, including volcanoes, mountain ranges, and deep-ocean trenches. What is the theory of plate tectonics? Plate Boundaries The edges of different pieces of the lithosphere meet at lines called plates boundaries. Faults – breaks in the Earth's crust where rocks have slipped past each other – form along these boundaries. Scientists have been using satellites to measure plate motion very precisely. The plates move at amazingly slow rates, from about 1 to 24 centimeters per year. They have been moving for tens of millions of years. There are three types of plate boundaries: Transform boundaries, divergent boundaries, and convergent boundaries. Divergent Boundaries Convergent Boundaries Transform Boundaries The place where two plates move apart, or diverge, is called a divergent boundary. Most divergent boundaries occur at a mid-ocean ridge. When a divergent boundary develops on land, two slabs of Earth's crust slide apart. A deep valley called a rift valley forms along the divergent boundary. The place where two plates come together, or converge, is a convergent boundary. When two plates converge, the result is called a collision. When two plates collide, the density of the plates determines which one comes out on top. When two plates carrying oceanic crust meet at a trench, the plate that is less dense dives under the other plate and returns to the mantle. This is the process of subduction. When a plate carrying oceanic crust collides with a plate carrying continental crust, the more dense oceanic plate plunges beneath the continental plate through the process of subduction. Oceanic crust is more dense than continental crust. When two plates carrying continental crust collide, subduction does not take place because both plates are mostly low-density granite rock. Instead, the plates crash head-on. The collision squeezes the crust into mighty mountain ranges. A transform boundary is a place where two plates slip past each other, moving in opposite directions. Earthquakes occur frequently along these boundaries, but crust is neither created nor destroyed. What are three types of plate boundaries? Plate Motion
Over Time The movement of Earth's plates has greatly changed Earth's surface. Geologists have evidence that, before Pangaea existed, other supercontinents formed and split apart over billions of years. About 260 million years ago, the continents were joined together in the supercontinent Pangaea. About 225 million years ago, Pangaea began to break apart. Since then, the continents have moved to their present locations. A scientific theory is a well tested concept that explains a wide range of observations.
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