Label rift zones. Draw arrows to show that the two plates are moving apart. Mark each rift zone with this symbol:

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1 Zones Rift Zone What is a rift zone? A rift zone is a region of the Earth s crust where plates are spreading apart, or diverging. Diverging plates move away from each other very slowly. As they do, magma rises from the mantle. It pushes up the crust and eventually breaks through the crust. This process can take place under the ocean or on land. The magma that quietly bubbles out at rift zones is generally runny and dense. Each time the magma erupts new crust is added, creating a ridge of mountains. Most eruptions happen on the ocean floor and go undetected, quietly constructing new crust. The thin magma cools quickly and forms layers of new crust on the sea floor. This is a diagram of the movement that happens along the Mid-Atlantic Ridge, a rift zone. The convection currents in the mantle pull the crust in opposite directions at rift zones. This means that the volcanic debris does not have much time to build up before the plates move the crust away, taking old volcanic debris with them. When volcanic materials rise out of a rift faster than the plates are moving apart, volcanic mountains are built. Sometimes these mountains surface and become islands. The island of Surtsey in the North Atlantic was formed by this process. When a rift zone occurs on land, the same process happens. However, on land we can observe the rising magma and volcano formation. In contrast to the rift zones underwater, the center of the ridge usually sinks to form a valley between the two mountain ranges (giving us the term rift valley ). The sinking valley collapses under the two plates that are moving apart. This sinking valley causes shallow earthquakes and volcanoes at the edge of the rift as well as in the main rift. Eventually this valley sinks below sea level and then fills with sea water, forming a new ocean. To illustrate a rift zone: Use your two hands to represent two plates next to each other. Have another student push his or her hand up in between them, representing the rising magma as the plates move apart this is rifting. Label rift zones Mark each rift zone with this symbol: Draw arrows to show that the two plates are moving apart. 7

2 What does the data at a rift zone look like? Earthquakes are shallow and small. The data pattern forms a narrow band or line of earthquakes. If volcanoes are underwater, there are a few scattered volcanoes near the line of earthquakes. If volcanoes are on land, they appear as a narrow band. There is a narrow ridge of volcanic mountains or a narrow ridge of mountains with a valley in between. 8

3 What is a subduction zone? A subduction zone is a region of the Earth s crust where two plates are coming together or converging. Converging plates are crashing into each other very slowly. At a subduction zone crash, one plate dives under the other plate. The plate that is moving under another is being destroyed. Where the two plates meet a trench is formed. Trenches are long, steep-sided depressions that can be as deep as 30,000 feet below sea level. Subduction Zone This is a diagram explaining the movement that happens at a subduction zone like the Aleutian Islands. At a subduction zone, convection currents in the mantle pull the two plates together, and crust is destroyed. These subduction zones are often found where the thin, dense oceanic crust meets the thick, less dense continental crust. The less dense continental crust floats over the top of the oceanic crust. Sometimes two plates of oceanic crust collide, and one subsides under the other for similar reasons. One oceanic crust may be less dense than the other, so that plate floats over the other. When the cool, denser crust dives under the other plate, the crust crumbles and melts in the mantle. This can cause earthquakes to happen as far as 700 km below the surface of the Earth. When the rock of the subsiding plate melts into the mantle, volcanoes are created on the floating plate. They form a steep, volcanic mountain range right next to the deep ocean trenches. These volcanoes often erupt explosively, ejecting ash, slow-moving lava, and hot rock debris. This magma material is often less dense than the magma at rift zones or hot spots. To illustrate a subduction zone: Use your two hands to represent two plates next to each other, and slide the edge of one hand under the other this is subduction. The edge on the top hand gets pushed up as the bottom hand pushes under. Label subduction zones Mark each subduction zone with this symbol: Draw arrows to show that the two plates are moving together: 9

4 What does the data at a subduction zone look like? Earthquakes happen in a narrow band. Volcanoes happen in a narrow line right next to the line of earthquakes. Earthquakes generally happen deep below the surface as one plate dives under the other. A deep trench is next to a line of steep volcanic mountains. Trench TRENCH 10

5 Buckling Zone What is a buckling zone? A buckling zone is a region of the Earth s crust where two plates are coming together or converging. Converging plates are crashing into each other very slowly. At the buckling zone crash, the two plates push up together to form a large, folded mountain range. Convection currents in the mantle carry these two plates into each other. They are usually thick continental plates. Some of the largest and highest mountain ranges in the world were created in this way. Have you ever pushed two graham crackers together? They are thick and crumbly, much like the continental crust, and both graham crackers have the same density. When they are pushed together, their edges crack and push up into a mountain range. This is a diagram explaining the movement at a buckling zone. When the two plates crash together and fold, the continental crust melts. This melting can cause volcanoes, but often the magma does not escape from the thick crust. The few volcanoes that are created there often eject hot rock, ash, and slow-moving lava. This magma comes from the crumbling continental plates, so it has a different consistency from the magma at rift zones and hot spots. To illustrate a buckling zone: Use your two hands to represent two plates next to each other. Push your two hands against each other, with the touching edges rising up together like a mountain this is buckling. Label buckling zones Mark each buckling zone with this symbol: Draw arrows to show that the two plates are moving together: 11

6 What does the data at a buckling zone look like? Earthquakes happen in a scattered pattern. Volcanoes are few and scattered. Earthquakes generally happen near the surface. A large, folded mountain range sits on continental crust. 12

7 Transform Zone What is a transform zone? A transform zone is an area of the Earth s crust where two plates moving in opposite directions are sliding past each other. How can this happen? Sometimes one side of a plate is subducting (going underneath the plate next to it) while the other side of the plate is rifting. This plate can then slide past one of its neighboring plates. The San Andreas Fault Line in California is a good example of a transform zone: it is a place where the plates are sliding past each other. The Great San Francisco Earthquake of 1906 happened because two plates were sliding past each other. A transform boundary is where two plates move past each other. This movement can displace creek beds and roads and creates bends and folds in the crust. San Andreas fault. Two plates sliding past each other. aults/slideshow/pages/slide2.htm When two plates slide past each other, the rock in the crust gets bent, folded, and cracked. After the plates shift this way, rivers, roads, and fences may no longer line up. Transform Zone How do the plates move past each other? The convection currents within the mantle pull the plates towards or away from each other. Sometimes these convection currents cause the two plates to move past each other. To illustrate a transform zone: Use your two hands to represent two plates next to each other. Keeping the edges of your hands touching, move both hands, sliding one past the other. Label transform zones Mark each transform zone with this symbol: Draw arrows to show that the two plates are sliding past each other: 13

8 What does the data at a transform zone look like? Transform zones are difficult to identify from earthquake and volcano data. You can identify all other plate boundary predictions (rift, subduction, and buckling) and then try to predict which remaining boundaries must be transform boundaries. Lots of earthquakes happen near the surface Volcanoes occur in a variety of patterns 14