Why Does the Mantle Move the Way it Does?
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- Leonard Campbell
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1 Why Does the Mantle Move the Way it Does? In the demonstration, you observed warm water rising through cool water. You also observed cool water sinking to replace the warm water. The movement of a fluid material caused by density differences is called convection. Convection carries heat energy from one location to another location within a fluid. Often, convection is driven by either heating from below or cooling from above the fluid. The warm water you observed carried heat energy as it moved from an area of higher temperature to an area of cooler temperature. You saw this happen in the demonstration as warm water from the cup moved upward through the cooler water until it reached the top. cooling rising sinking heating convection: the movement of a fluid material caused by density differences. Often, convection is driven by either heating from below or cooling from above the fluid. Convection occurs in many places. Think of what happens when a pot of water is heated on a stove. When the water reaches a full boil, the liquid will bubble and roll. However, long before the bubbles start forming, the heated water is already moving due to convection, even though it is hard to see. That is because the heat from the stove adds energy to the water molecules at the bottom of the pot. Molecules are always moving and bumping into one another. In the pot, the heat energy from the stove will cause the water molecules to move faster and hit one another harder. This hitting causes them to spread apart. EE 159
2 Learning Set 4 What Causes Earth s Plates to Move? convection current: the cycle of movement when warm, less-dense fluids rise and cool, and moredense fluids sink due to differences in temperature. That spreading means that when water is hot, the same amount of space will have fewer molecules in it than when the water is cool. Where there are fewer water molecules in a space, the water is less dense. Like any liquid, water that is less dense than the surrounding liquid will rise. The warmer water on the bottom of the pot rises because it is less dense than the water around it. The floating of the heated, less dense water is not all that happens in a pot of water on a hot stove. As the water molecules reach the top of the pot, they release some of their heat energy to material in the surroundings, such as air. As it releases its heat energy into other materials, the water cools. It becomes more dense, and it begins to sink. The heating and cooling of the water causes it to float and then sink in a process that can repeat many times. That process is what is known as convection. When a substance rises and sinks due to differences in heat energy, the movement is called a convection current. A convection current is a flowing pattern in which warmer, less-dense quantities of a substance rise and are replaced by cooler quantities that sink because they are more dense. plate mantle outer core inner core The heating and cooling of mantle material changes the density of the material. This sets convection currents in motion within Earth. The same process by which water in a heated pot moves in a convection current happens in Earth s mantle, but on a much larger scale and over a longer period of time. Earth s core is very hot, so it can provide a source Project-Based Inquiry Science EE 160
3 of heat that causes convection in the mantle. Scientists have calculated that temperatures at Earth s core reach as much as 5000 C (over 9000 F). The mantle material that is farthest down is heated by the core. Remember that the mantle extends a long way from its upper to lower parts (its thickness is about 2700 km, or 1800 mi). Because of its thickness, temperatures in the upper mantle, nearest to the crust, are less hot (just under 900 C, about 1600 F), than the temperatures in the lower mantle, nearest the core (more like 2200 C, just under 4000 F). This temperature difference is thought by scientists to cause convection in Earth s mantle. In Earth s mantle, convection occurs when the core heats material in the lower mantle. The molecules making up the mantle material begin to move more quickly and spread apart. As they spread farther apart, the mantle material nearest the core becomes less dense than the material above it, and it rises. This heated, less dense mantle material eventually plume reaches the upper region of the mantle. As it rises up to just below the crust, the mantle material spreads outwards. As it spreads under the crust, the material loses heat energy. With less heat energy, the molecules move less quickly, and begin to get closer together. This loss of heat causes the material in the upper mantle to become more dense than the material below it, so it sinks slowly back down toward Earth s core. This rising and sinking due to differences in heat in the mantle material is a convection current. Some evidence indicates that convection currents in the Earth cause material that is hotter than the material around it to rise as a column of upward flowing magma in the mantle. This kind of hot, rising column of magma is called a plume. Scientists are studying the possibility that a plume may contribute to several kinds of geologic events. plume: a hot, rising column of mantle material. EE 161
4 Learning Set 4 What Causes Earth s Plates to Move? You have thought about three models that help you understand how Earth s mantle can move. The first was the model you built in Section 4.2 with the cup of colored water. The second was in the video. The third was when you thought about heating water in a pot as a model of convection in the mantle. All of these can help you understand how convection might occur in Earth s mantle. Like any model in science, though, there are some parts of the models you thought about that do not represent the real thing very well. For example, both models used water to represent Earth s mantle material. However, Earth s mantle material is not a thin liquid like water. In fact, the mantle is much more like a solid than like a liquid, so the convection currents occur very slowly, over millions of years. It can take millions of years for material from deep in the mantle to reach the upper part of the mantle. Also, other than its temperature, the water in each model does not differ from the bottom to the top. However, evidence from seismic waves and studies of the composition of lava indicate that the mantle might have layers, each made of different materials. It may be that convection currents in the lower part of the mantle, very close to the core, are different from convection currents in the upper part of the mantle. As well, it may be that Earth s core is not the only thing that heats mantle material and causes convection currents. Some of the material in the mantle may be radioactive, giving off its own heat and affecting convection currents. Scientists are still studying evidence to learn more about the composition and structure of Earth s mantle. hotspot on magma from the core magma plume crust upper mantle lower mantle crust upper mantle lower mantle hotspot on magma from the core magma plume convection cell core core convection cell Scientists are still studying evidence to understand whether Earth s mantle moves continuously from bottom to top, or if the movement occurs in layers. Project-Based Inquiry Science EE 162
5 Stop and Think 1. How do changes in heat energy affect the density of Earth s mantle material? 2. Write a short story detailing the journey of a cubic centimeter (cm 3 ) of mantle material through a convection current beginning at the core. Be sure to include information about what happens to its molecules. Support your story with sketches. 3. Draw a cross section of Earth, showing the core, mantle, and crust. Draw arrows to represent a convection current in Earth s mantle. Sometimes called the Blue Planet, Earth s blue waters and land areas look very different from other planets in our solar system. However, beneath its cool and inviting crust is a very different scene where temperatures are high enough to melt rock. EE 163
6 Learning Set 4 What Causes Earth s Plates to Move? mid-ocean ridge: a continuous mountain range through the oceans, which is subject to earthquakes. lava: fluid rock that comes from a volcano; also, the same material that becomes solid by cooling. ocean-floor spreading: a process by which new ocean floor is created in the area where two plates are moving apart. What Else Drives Plate Motion? Scientists generally agree that circular motion created by convection in the mantle is the strongest force driving plate movement. But there are other forces, too, that cause Earth s plates to move. When Earth s plates move apart from each other, mantle material rises to Earth s surface. This causes two more forces that drive plate movement: ridge push and slab pull. There are places where two plates are moving away from each other and mantle material reaches Earth s surface. One such place is within a large system of underwater mountains known as mid-ocean ridges. A deep crack runs through their center. Mantle material rises up from this crack, filling in the gap between the two plates. As this lava cools and hardens, it forms new crust on the ocean floor. This process is called ocean-floor spreading. As the mantle material fills in gaps between two plates at a mid-ocean ridge, new crust is formed on the ocean floor. Project-Based Inquiry Science EE 164
7 In places where new ocean floor is being added, a depressed valley fills the gap between the plates. The valley is surrounded by the shoulders of the plates that are moving apart. The gap is filled in, but the plates continue to move. As the plates move farther apart, new gaps are created, and the same thing happens again. As new gaps form and fill again and again, a pattern forms on the seabed. At the mid-ocean ridge, where the plates meet, the crust is composed of young rocks. On either side, the rocks become older as you move away from the plate boundary. Successive bands of rock form, with the youngest rock near the current plate boundary and older rock on both sides of it. This evidence first led scientists to recognize that the sea floor was spreading apart at the mid-ocean ridges. The push of older crust away from the plate boundary, called ridge push, is a force that drives plate motion. ridge push: the push of older crust away from a plate boundary. mid-ocean ridge younger crust older crust As new crust is created on the ocean floor along the mid-ocean ridge, older crust is pushed outward. This means that rocks making up the crust nearest to the ridge are the youngest. Moving away from the ridge, the rocks making up the crust are increasingly older. Earth s crust stays the same size because the older crust that is pushed away melts back into the mantle. As the older crust moves away from the midocean ridges, it cools and becomes more and more dense. Eventually, it sinks deep down into Earth. The weight of the plunging crust pulls the rest of the plate along with it. This is called slab pull, and it also drives plate slab pull: the movement of older crust down toward the center of Earth. EE 165
8 Learning Set 4 What Causes Earth s Plates to Move? movement. When the rocks making up this crust move deep enough, they are melted by the heat of the Earth s core and, once more, their material becomes part of the mantle. Eventually, this material may rise once again to the surface in a mid-ocean ridge. The Mid-Atlantic Ridge, the part of the mid-ocean ridges beneath the Atlantic Ocean, rises above the surface of the ocean in Iceland. This allows scientists to observe phenomena that more commonly occur deep under water. Reflect 1. Look at your sketch of the convection current you drew earlier. Add arrows and notes to show how convection currents in the mantle cause the plates to move. 2. Think back to the example of the pot of water. If you take the water off the heat, the bubbling stops after a few seconds. Why does this happen? Why is this not happening on Earth? 3. Describe three factors that drive plate motion and discuss how they work together to move Earth s plates. 4. At the beginning of this Learning Set, you evaluated several ideas about the forces that cause Earth s plate to move. Reread and evaluate these ideas again. Which ideas do you now have evidence to support? Describe the evidence and how each force moves Earth s plates. Project-Based Inquiry Science EE 166
9 What s the Point? Scientists have evidence that motion in Earth s mantle is caused by convection currents. Convection currents are formed when material is heated while other material around it is cooler. The heating causes molecules in the material to move faster and spread apart, making the material less dense. The less-dense material rises, while cooler material sinks. This rising and sinking due to temperature differences is a convection current. In Earth s mantle, material is heated by Earth s very hot core. This heating causes mantle material to rise, possibly in the form of plumes of heated magma. While convection currents in water move quickly, convection currents within Earth s mantle move very slowly, taking many millions of years to move material from the lower mantle up to Earth s crust. Hot magma can escape through cracks in Earth s crust. As the magma reaches the surface, it cools and forms new crust. This happens in the ocean at mid-ocean ridges. As new crust forms at a mid-ocean ridge, older crust is pushed aside exerting a force called ridge push. The formation of new crust, along with the force of ridge push, means that usually the crust material that is farther away from a mid-ocean ridge is older than the material nearer to the center of the mid-ocean ridge. Eventually, that slab of crust material is pushed to a place where it sinks deep into Earth. The slab of crust material exerts a force as it sinks called slab pull, which helps pull more crust material back down into Earth. As slabs of crust material reach Earth s mantle, the heat there melts them. It may be that as the old crust material melts, it is recycled, becoming magma that can eventually contribute to the formation of new crust. EE 167
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