T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 1 Name: Section: Plate Tectonics Unit III: A Few More Details (2 pts) How do we know what the interior of the Earth is like? Like everyone else, scientists believe that the interior of the Earth is hot, because lava comes out of volcanoes. We cannot drill all the way through the lithosphere to reach the mantle (the lithosphere is about 60 miles thick!), so we must use other methods to learn about the inside of the Earth. Every earthquake that occurs ripples outward, giving us clues about the Earth s interior. By measuring how long it takes each earthquake to reach each monitoring station of our worldwide network, scientists have built up a picture of the interior of the Earth. Earthquake waves travel at different speeds depending upon the kind of rock and the rock s temperature and pressure, something that can be measured in laboratory experiments. By combining travel times from many different earthquakes and earthquake speeds for different rocks under different conditions, scientists can work out the thickness of the layers of the Earth, their composition, temperature, pressure, and more. The same principles are used to visualize a baby in the womb using ultrasound waves. The earth scientists figured out the technique first, so you can see your child in the womb thanks to earthquake research! The earthquake data suggests that the Earth s iron core has two parts, a solid inner core and a fluid outer core. The motion of the fluid part of the core is presumably responsible for the Earth s magnetic field, since iron is a magnetic substance. It makes sense that the core is iron, since iron is the densest substance made by stars in large quantities before they explode in a supernova. The early Earth formed from more and more asteroids being pulled together by gravity, and the collisions produced vast amounts of heat, making the early Earth a molten ball of rock. The higher-density elements like iron tended to sink towards the center, while the lower-density substances at the surface cooled and became solid by radiating their heat into space. The cool, outer later then served as an insulator, trapping the heat inside except where it leaks out, mainly at places like volcanoes. High density radioactive materials tended to sink towards the center, and their decay continues to warm the Earth s core to this day. The earthquakes detection network was built to monitor nuclear test-ban treaties. Nuclear explosions set off vibrations that can be detected by the network, so the network is used to make sure that nations live up to their agreements. (In other words, the network is used to detect nuclear cheaters. ). Nuclear explosions are different from natural earthquakes, so the network can easily tell them apart. Nuclear explosions create very small earthquakes, so the network has no difficulty detecting and measuring even small natural earthquakes. 1. What do scientists measure to determine what the interior of the Earth is like? In other words, how did scientists determine the characteristics of the layers of the Earth?
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 2 Why do the plates move? What drives plate tectonics? The ultimate cause of motion inside the Earth is the heat of the Earth s core. The Earth s core is warm, because of heat produced during the initial collisions between the rocky asteroids that produced the earth and the decay of dense, radioactive materials that sank down into the Earth s interior early in its history. The core heats the bottom of the mantle, lowering its density and causing it to rise. Cooler mantle material slides in to replace it, and the process repeats in giant, slowmotion convection cells. The mantle rock rubs against the bottom of the plates of the lithosphere, pushing them and thus making them move. There are other mechanisms that can cause plates to move. For example, the end of a subducting ( diving ) plate can be more dense than the mantle, because it has cooled down and has thick, heavy layers of sediments piled on top of it. It dives down owing to its higher density, and can pull the rest of the plate with it. (This is called slab pull. ) The diving plate also drags the nearby mantle rock with it, helping to push the convection cell in the mantle. (This is called slab suction. ) Note: There is a debate within the geophysics community about whether convection cells reach all the way down to the core. Some scientists think that there 2 layers within the mantle, each with its own convections cells. There is some data which support this view, but if this is true, then it is not clear why large deep earthquakes occur: stay tuned! 2. What causes plates to move? 3. True or false? Since oceanic lithosphere gets more and more dense over time, one edge of a plate can become dense enough to begin to sink down into the mantle on its own.
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 3 Plate Speed and Measuring Plate Motion with Modern Technology Countless studies over the last century have verified the motion and the boundaries of the plates suggested by plate tectonics. Scientists currently use the GPS (global positioning system) to monitor plate motion, but it is simple enough to do without advanced technology: measure the distance between two objects (houses, trees, fences, etc.) on two different plates, and then do so again after each earthquake. Most plates move at least few centimeters per year (an inch or a bit more per year) and no faster than about 15 centimeters per year (about 6 inches per year). 4. About how fast do plates moves, on average: less than an inch per year, a few inches per year, a few feet per year, tens of feet per year, hundreds of feet per year, thousands of feet per year, or miles per year?
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 4 A single plate often contains both continental lithosphere and oceanic lithosphere Notice that a plate can contain both continental and oceanic lithosphere. For example, the North American plate includes both North America (continental lithosphere) and the bottom of the western Atlantic Ocean (oceanic lithosphere). Lava coming out of the mid-ocean ridge cools and solidifies onto whatever plate is next to it and becomes part of that plate, whether it is continental or oceanic lithosphere. 5. True or false? Plates are either made of continents or ocean floor, either continental lithosphere or oceanic lithosphere, but not both. In other words, a single plate cannot have one part made of continental lithosphere and another part made of oceanic lithosphere. 6. Some plates contain some continental lithosphere and some oceanic lithosphere. What is the relationship between them: the continental lithosphere is on top of the oceanic lithosphere, the oceanic lithosphere is on top of the continental lithosphere, or they are attached side-by-side?
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 5 Earthquakes Earthquakes occur when the Earth moves suddenly. The magma of the mantle flows in convection cells which steadily rub against the bottom of the plates, pushing them slowly away from mid-ocean ridges and towards trenches. However, the plates run into one another, so they resist being moved. Instead of moving, the plates are squeezed and twisted where they collide. Eventually, they cannot deform any more. The pressure to resist builds up until it overcomes the resistance: then the plates move (slip) all at once, and snap back into their original shape (or as close as they can get to it). This is an earthquake. Most earthquakes occur near the edges of plates, because most of the resistance to motion is exerted at the edges, so this is where most of the deformation (bending, squeezing) takes place and the tension builds up the most. Plates can resist moving for several reasons. A plate can rub against the side or bottom of another plate, and the friction between them keeps them from moving. The rock in front of a moving plate (another plate or the semi-solid asthenosphere) can block it, possibly forcing it to bend. In short: The plates move slowly and gently with the convection cells over time, slowly bending and twisting where they rub or bash into other plates. The plates give (deform) until they cannot give any more, and then suddenly move a lot all at once, snapping back into their original shape. There are other mechanisms that can cause earthquakes. For example, as a plate dives into the Earth, some parts get warmer than others, so they get larger and the plate cracks where warm rock meets cold rock. This is why glass trays can break when you take them out of the oven or glass pitchers break when you pour hot water into them: one part cools more quickly than another part. 7. True or false? Most earthquakes occur near the edges of plates because this is where plates are bent or deformed due to the plates bumping into one another or running into one another.
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 6 Volcanoes near Trenches At trenches, a plate containing oceanic lithosphere subducts (dives down into the Earth), and is destroyed by melting into and becoming part of the mantle. It is not only the diving plate that melts. The mantle rock above the subducting (diving) ocean crust undergoes what is called partial melting. The diving oceanic lithosphere is heated by the surrounding mantle (friction between the plate and mantle rock is an important factor) and the increased pressure. Water and sediments are also carried down by the plate. As the water and carbon dioxide (from the breakdown of calcium carbonate sediments) mix with mantle material, they lower its melting point and its density, allowing the resulting magma to rise upwards until it hits the nonsubducting plate above. The magma begins melting the bottom of the plate and mixing with the plate s material, which alters the composition of the magma. Wherever a crack in the plate permits, the magma works its way toward the surface of the Earth until it comes out as lava. The magma itself can force its way up: (1) it opens cracks by expanding remember: heating an object makes it larger and (2) it melts the surrounding material. 8. True or false? At trenches, two plates made of oceanic lithosphere are moving apart. 9. True or false? The source of the lava for the volcanoes near a trench is the melting of the plate that is subducting (diving down into the Earth).
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 7 10. True or false? Friction with the mantle rock helps melt the subducting (diving) plate. 11. True or false? Water and sediments in the subducting (diving) plate help melt the plate and the nearby mantle. 12. Does the magma rising up to create the volcanoes near trenches (a) melt through the subducting plate, (b) melt through the plate which does not subduct, or (c) come up into the gap created by the trench?
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 8 Hotspots and Volcanic Islands Molten rock rises at certain, special spots beneath the lithosphere called hotspots, not just at the mid-ocean ridge or next to trenches. The magma heats the bottom of the lithosphere. The lithosphere expands more in some places than others, which opens up cracks through which magma flows to the surface of the lithosphere. Lava breaks through again and again, slowly building an underwater volcano. The volcano is a seamount, an underwater mountain, until it reaches the surface and at which point it becomes an island. The volcano grows on the lithosphere of a plate. Since plates move and the island is on the plate, the island is slowly carried off the hotspot by the motion of the plate, causing the volcano to become extinct (it loses its source of lava). As time passes, the island (and the lithosphere it rides on) cool down more and more, causing them to contract (shrink). This makes the island smaller, so it sinks, and this can carry it beneath the waves, making it into a seamount. In addition, as the lithosphere and volcano cool, they become more dense, so they sink down a bit into the mantle. Oceanographers often find flat-topped seamounts called tablemounts or guyots (gee-ohs). They were once islands at the surface and had their top eroded away by waves before sinking deeper into the ocean. The existence of coral atolls also supports the idea that islands sink. Coral atolls are coral reef islands that are not close to land. However, coral reefs can only grow close to land, because corals live on the ocean floor and need to grow in shallow water so that the algae living inside them can get sunlight. The algae living inside the coral use the sunlight for photosynthesis, and give some of the food that they make to the corals. We think that the original coral reef did grow close to land, and as the island sank, the corals grew upwards to stay close the Sun by adding layers of white calcium carbonate beneath them.
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 9 13. What is an underwater volcano called? 14. What do we call a volcano that reaches the surface of the ocean? 15. What do we call an underwater mountain with a flat top? 16. How do underwater mountains with flat tops form? 17. How do corals survive if the island that they are living on sinks?
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 10 Hotspot Island-Seamount Chains Volcanoes leave a hotspot because the volcanoes are on top of a plate and plates move. After one volcano leaves the hotspot, another volcano grows on the hotspot from the magma rising at the hotspot. This volcano also leaves as the plate continues to move, and another volcano grows due to the magma rising upwards beneath the plate. This happens again and again, creating an island-seamount chain: a long line of islands and seamounts. The classic example is the Hawaiian Islands which are one end of the much larger Hawaii Islands-Emperor Seamount Chain. Currently the southeast corner of the big island is above hotspot; this is where you can see active volcanoes at Hawaiian Volcanoes National Park. (The big island of Hawaii is actually 2 smaller islands linked together by the lava that flowed from the volcanoes.) Notice how the Hawaiian Islands extend northwest, away from the hotspot and thus away from the mid-ocean ridge in the Pacific Ocean, and towards the trenches along the edge of the Pacific Ocean. The New England Seamounts along the Great Meteor hot-spot track near the east coast of the United States also formed in this manner. 18. How or why do hotspots create chains of islands and seamounts (a line of islands and seamounts)?
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 11 19. Examine the picture above. Do plates move towards or away from the mid-ocean ridge? Do plates move towards or away from trenches? 20. Examine the picture above. Does an island-seamount chain extend towards or away from the mid-ocean ridge? Does an island-seamount chain extend towards or away from trenches?
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 12 Hotspot Island-Seamount Chains as Evidence for Sea-Floor Spreading Sea-floor spreading is the idea that the ocean floor is moving away from the mid-ocean ridge. The motion of the sea floor is necessary to produce a chain of islands and seamounts. If the sea floor did not move, then at a hotspot we would find one volcanic island or seamount; there would not be a chain of extinct volcanic islands and seamounts extending away from the hotspot and mid-ocean ridge. In addition, coral atolls in hotspot island-seamount chains are found in places where the water is too cold for corals to grow well. Corals do not grow well in cold water, because their reefs are made of calcium carbonate which dissolves in cold water. Nonetheless, some coral reefs are found in cold water, because they live on volcanic islands which grew on top of plates and the plates have slowly moved from the tropics towards the Poles. Thus, the islands move when the sea floor they are on moves from the tropics to the Poles. 21. If the sea-floor did not move, how many islands would be created by a stationary (nonmoving) hotspot? 22. What substance are coral reefs made of? 23. Do coral reefs dissolve in warm water or cold water? Do corals grow best in warm water or cold water? 24. True or false? Some coral reefs on islands in hotspot island-seamount chains are found outside the tropics.
T. James Noyes, El Camino College Plate Tectonics Unit III: A Few More Details (Topic 11A-3) page 13 Hotspot Locations Hotspots can exist near the mid-ocean ridge (e.g., Iceland), and cause even more volcanism than normal. Hotspots can also be located under a continent. For example, the geysers of Yellowstone National Park are caused by the heat of a hotspot. These hotspots can also cause volcanoes. Hotspots can and do move, but observations suggest they typically move slower than the plates move above them, so it is primarily the motion of the plates that produce hotspot island chains. Recent research has shown that there is not one big magma chamber directly under Hawaii. Its head may have cooled and been dragged away by the plate, or there may be lots of little fingers of magma instead of one big chamber. If this is the case, lookout Maui, because this would make it harder to predict where volcanoes will occur! Stay tuned: there are new things to be discovered! 25. Which of the following statements about hotspots are true? Which are false? a. Hotspots can occur on continents, not just on the ocean floor. b. Hotspots can occur at the mid-ocean ridge. c. Hotspots do not move over time; they always remain in one spot.
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