SAC Geography Form 2 Chapter 3: Plate Tectonics Topic 3: Plate Movement

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1 What causes an earthquake? Plate movement causes pressure to build up along faults, or breaks, in the earth's crust. When the rocks cannot take any more pressure, the rock layers shift and an earthquake occurs. Millions of times each year, an average of once every thirty seconds, somewhere around the world the ground shakes and sways. This is called an earthquake. Most are either too small to do any damage, they occur in remote areas, or under the sea. An earthquake occurs when the elastic limit or strength of the rocks are reached. Rocks can bend and stretch to a point when stress is applied. When the rocks cannot take the pressure any longer they shift quickly and break releasing energy. This energy produces vibrations causing the earth to shake and tremble! Different forms of stress can build up at the three different plate boundaries: Divergent-plates moving apart, Convergent-plate moving towards each other, and Transform-plates sliding past each other. First you must understand that there are two types of plates: continental plate and oceanic plate. Oceanic plate refers to those plates that consist mostly of ocean. On the other hand continental plate refers to those plates that consist mainly of a continent or part of it. See table below: Type of Crust Average Thickness Average Age Major Component Continental Crust kilometers 3 billion years Granite Oceanic Crust 10 kilometers Generally 70 to 100 million years old Basalt The earth s plates move because of the intense heat in the Earth s core that causes molten rock in the mantle layer to move. It moves in a pattern called convection currents that form when warm material (magma) rises, cools, and eventually sinks down. As the cooled material (magma) sinks down, it is warmed and rises again. Very similar to what happens in a lava lamp! [see this link: ] 1 P a g e

2 Convergent boundaries Places where plates crash or crunch together are called convergent boundaries. Plates only move a few centimeters each year, so collisions are very slow and last millions of years. There are three types of convergent boundaries: 1. Continental-Oceanic convergence 2. Oceanic-Oceanic convergence 3. Continental-Continental convergence 1. Continental-Oceanic convergence This image is copyright [Tasa Graphic Arts, Inc. 2009] The edge of the continental plate in the diagram has folded into a huge volcanic mountain range, while the edge of the oceanic plate has bent downward and dug deep into the Earth (subducted). A trench has formed at the bend. All that folding and bending makes rock in both plates break and slip, causing earthquakes and volcanoes. 2 P a g e

3 2. Oceanic-Oceanic convergence These plates converge at the ocean floor. The edge of the weakest oceanic plate in the diagram has bent downward and dug deep into the Earth (subducted). A trench has formed at the bend. Magma has been pushed upwards and on the edge of the other oceanic plate submarine volcanoes have formed. These submarine volcanoes can grow to form volcanic islands. 3. Continental-Continental convergence The edge of the weakest continental plate in the diagram has bent downward and dug deep into the Earth (subducted). The Lithosphere of the other continental plate has been pushed upwards forming a mountain range. For a good explanation of the Lithosphere and Asthenosphere see glossary and check out this video: 3 P a g e

4 Divergent boundaries Places where plates are coming apart are called divergent boundaries. This image is copyright [Tasa Graphic Arts, Inc. 2009] As shown in the drawing above, when Earth's brittle surface layer (the lithosphere) is pulled apart, it typically breaks along parallel faults that tilt slightly outward from each other. As the plates separate along the boundary, the rocks between the faults drop down into the Asthenosphere (mantle). The sinking of these rock forms a central valley called a rift or ridge. Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface. The diagram above is better known as sea-floor spreading or mid-oceanic ridge but plates can divert on land as well. The same thing happens but on land. See diagrams below: 4 P a g e

5 Transform boundaries Places where plates slide past each other are called transform boundaries. Since the plates on either side of a transform boundary in the diagram are merely sliding past each other and not tearing or crunching each other, transform boundaries lack the spectacular features found at convergent and divergent boundaries. In fact this type of plate movement causes earthquakes only. Transform boundaries are marked in some places by linear valleys along the boundary where rock has been ground up by the sliding. Perhaps the most famous transform boundary in the world is the San Andreas Fault, shown in the drawing above. The slice of California to the west of the fault is slowly moving north relative to the rest of California. Since motion along the fault is sideways and not vertical, Los Angeles will not crack off and fall into the ocean as popularly thought, but it will simply creep towards San Francisco at about 6 centimeters per year. In about ten million years, the two cities will be side by side! See diagram on your right. An aerial photograph of San Andreas Fault. 5 P a g e