Plate Tectonics – The Moving Crust Name: ______________________________________ Date: ______________________ The surface of the earth (the crust) is not one solid continuous covering (like an egg shell). Instead, it is broken into many smaller areas known as PLATES. It is more like a hard boiled egg that has been dropped and the shell have cracked in many places. The map below shows the major plates of the earth's surface. The name of the plate where you live is called "The __________________________ Plate". All of these plates are constantly moving very slowly over the surface of the earth. The movement of the plates causes some plates to be coming toward each other, some to be moving away from each other and others to be sliding past each other. These different types of motion cause different effects on the earth: mountain building, volcano formation, new land formation, and earthquakes. The movement of the plates is caused by the flowing of the heated magma in the mantle. How? As the magma closer to the core becomes hot, it rises toward the crust and the cooler magma closer to the surface of the earth falls toward the core. This up and down, circular motion is called a convection current. Convection currents force the plates to be pushed along the surface in various directions. What happens where the plates meet? Types of Plate Boundaries Name: ______________________________________ Date: ______________________ The plates of the earth are always moving, very slowly, but moving none-the-less. The edges of a plate where two or more plates meet are called boundaries. What happens at the boundaries depends on whether the plates are coming toward each other, moving away from one another or sliding along each other. There are 3 main types: 1) Divergent boundaries - Plates moving apart 2) Convergent boundaries - Plates moving toward each other 3) Transform boundaries - Plates grinding along, past each other - First proposed by J. Tuzo Wilson (Canadian) Divergent Boundaries Diverging Boundaries are where plates are moving away from each other. New crust is forming when molten magma from deep down in the mantle oozes out of the cracks where the plates are moving apart. There are 2 main types of divergent boundaries: 1) Two Oceanic Crusts Moving Apart This happens on the ocean floor where plates are diverging (moving apart). If enough lava bubbles up, underwater volcanoes and ridges of land can be formed. One of the best examples of a divergent boundary is the mid-Atlantic ridge. It is spreading at a rate of 2.5 cm/year (or 25 km in a million year). This explains why the island country of Iceland, which sits on top of the mid-Atlantic ridge, is growing larger every year. 2) Two Continental Crusts Moving Apart This happens where a plate boundary exists on land. The changes seen on land happen in a number of steps: a valley-like rift develops first. This rift valley is a dropped zone where the plates are pulling apart. As the dropped zone widens and thins, valleys and volcanoes form. Early in the rift formation, streams and rivers flow into the low valleys and long, narrow lakes can be created Eventually, the crust may become thin enough that a piece of the continent breaks off, forming a new and separate land mass. At this point, water from the ocean will rush in, forming a new sea or ocean basin in the rift zone. An excellent example of this is the Great Rift Valley of Central Africa. Convergent Boundaries Converging Boundaries are where plates are moving toward each other. When these slow moving plates push toward each other, we feel the pressure in the form of earthquakes. As the plates are squeezed together, one plate (the older, denser plate) gets pushed down under the other and the other plate crumples and gets shoved up into the air – forming a mountain or a volcano. There are three main types of converging boundaries: 1) Ocean-Continent Oceanic crust tends to be denser and thinner than continental crust. The oceanic crust gets pushed under the continental crust. This is called “subduction”. This forms a subduction zone. The sinking crust creates a deep oceanic trench, or valley, at the edge of the continent. The crust continues to be forced deeper into the earth, where high heat and pressure melt the crust & begin to push it up through the continental crust, forming a volcano. When this magma finds its way to the surface through a vent in the crust, the volcano erupts. An example of this is the band of active volcanoes that encircle the Pacific Ocean (like the Rocky Mountains), often referred to as the Ring of Fire. 2) Oceanic-Ocean Plates Collide When two oceanic plates collide, the older plate is forced below the younger plate. This creates another subduction zones. Instead on forming volcanoes on the land, it creates a chain of volcanic islands known as island arcs. The deepest trenches on the planet also occur this Examples include the Mariana Islands in the western Pacific Ocean and the Aleutian Islands, off the coast of Alaska. 3) Continent-Continent When two continental crusts bump into each other, neither is more dense than the other so no subduction occurs. The edges simply crumple and push high into the air forming very high MOUNTAINS. This is how the Himalayan mountains in Asia were formed. Its best known peaks, Mount Everest and K2, are among several mountains that measure over 8,000 meters high at their summits. Since the Indian Plate is continuing in its northward movement into Asia, the Himalayas continue to grow higher each year by small amounts (5 to 20 mm or 1 inch per year). Colliding Plates – Tsunami Makers The collision and subduction of oceanic plates creates large, powerful earthquakes. Earthquakes generated in a subduction zone can also give rise to tsunamis. A tsunami is a huge ocean wave caused by a sudden shift on the ocean floor. If the wave reaches land, it can cause incredible destruction, like the Asian Tsunami, which killed more than 200,000 people in 11 countries across the Indian Ocean region on December 26, 2004. Transform Boundaries Transform Boundaries are where plates are moving along beside each other in opposite directions. This does not occur in a smooth, gliding motion, though. Instead, the rocks of the two plates grind against each other and rip and tear. This produces shaking motions in the crust known as EARTHQUAKES. This is the type of motion we see along the west coast of California in the US. The North American plate is moving in a southerly direction while the Pacific plate is sliding up toward the north. This explains why there are so many earthquakes in the San Francisco area.