Lessons From the Sea Page Grade 5 Unit 4
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- Navigate this page:
- friction
- Divergent boundaries
- Mid-Atlantic ridge
- Reading Review Questions: Answers
- Activity Questions: Answers Plate Tectonics Activity Directions
- Move the Nazca/Cocos plate in the direction of its arrows, and the North/South American plate in the direction of its arrows.
- Move the Australia/India plate in the direction of its arrows, and the Eurasia plate in the direction of its arrows.
- Use your plate models as needed to figure out the following
Discussion Question: Why do you think the rock under the oceans would be thinner and denser (heavier) than the rock of landmasses? Would the weight of the water have anything to do with it? Would the relative age or composition of the two types of crusts have anything to do with it? Yes, probably. No absolute answer, just a thought provoker. As the leading edge of the ocean plate sinks beneath the continental crust, the friction of the two great rock masses pushing past each other creates so much heat that it melts the rock into a thick liquid called magma. Some of this magma shoots up through cracks in the rock as lava, forming volcanoes. That is the reason why anywhere there are convergent plates, there are lots of volcanoes. Where this sinking area — or subduction zone — occurs, deep trenches are formed beneath the sea. Some of these deep ocean trenches, like the Mariana trench — where the Pacific plate dives beneath the Eurasia plate — are the deepest places on Earth. At over 11 kilometers (over 6.8 miles deep) the Mariana trench is 1 mile deeper than the tallest mountain, almost 7 times as deep as the Grand Canyon in Arizona, and 10 times deeper than Waimea Canyon in Kaua‘i, which Mark Twain called the “Grand Canyon of the Pacific.” In some places, the oceanic crust is particularly thin, or the cracks in it particularly large, so that magma has a direct route to the surface. These areas are called hot spots. Most hot spots are found near the plate boundaries, but some, like the hot spot that created the Hawaiian Islands, are in the middle of a plate. These hot spots may be associated with a peculiarity in the asthenosphere of an unusually hot bubble of magma in that area. Discussion Question: What would cause one area of magma in the asthenosphere to get and stay hotter than the rest of the magma around it? Compare/Contrast: Why is our sun so hot when space around it is so cold? Could there be a similar reason? No real answer, just another thought provoker for discussion and guesses. Frequently, the plates stick against each other for awhile as they try to move past, and then suddenly break free. This is what causes earthquakes to happen. Great chunks of rock are pushed up to the surface by these earthquakes, gradually growing into tall mountain ranges. One convergent plate, the Indian plate, carries upon it a huge piece of continental crust that used to be separate from the rest of Asia. Millions of years ago, the Indian plate was like Australia, a big island in the middle of the ocean. Then it began to drift northward. When the Indian continent atop finally reached the continent of Asia, it began to push up against it. Since it was made of light continental crust, like that of the Asian continent, it did not sink beneath the land, but just kept pushing into it, so that both sides bent upward to eventually form the highest mountain range in the world, the Himalayas. Discussion Questions: Who can name the most famous mountain in the Himalayas? Mount Everest. Explorers have found fossils of sea shells more than five miles above sea level on Mount Everest. How did they get there? Sea shells carried up with the land as it rose. If convergent plates form mountains and volcanoes, what do you think happens at the boundaries of plates that move away from each other? Form valleys? Rift valleys, actually. But also low mountains from the magma rising up through the rift or crack in the crust between the plates. Where do you think plates that move away from each other would be found? All guesses are valid, but actually most divergent zones are in the middle of the ocean, although one divergent zone goes through Africa and is called the Great Rift Valley. Divergent boundaries occur when two plates move away from each other. Interestingly, almost all these boundaries are under the sea. One of the few places where there is a divergent boundary between two parts of a continent is in Africa. This boundary has created the Great Rift Valley, which runs north/south through Africa, splitting apart the continent and forming the Red Sea. When the rift valley becomes deep enough, the sea will invade and divide Africa in two. Where divergent boundaries occur under the sea, magma flows up from the cracks that are created in the Earth’s crust, forming new seafloor and widening the ocean. They may form underwater mountain ranges, like the Mid-Atlantic ridge, and deep-sea hydrothermal vents, such as those found off the Nazca plate near the Galapagos Islands. One of the fastest spreading boundaries is between the Pacific and Antarctic plates, widening both oceans as well as the ocean between Antarctica and the other continents. Discussion Question: Look at the world map. If the Antarctic plate is moving away from all the other plates, what must be happening to the size of the continent of Antarctica? How can it move toward itself on all sides? What kinds of boundaries must exist along the edge of the continent? The Antarctica plate is getting bigger or staying the same, and the oceans are getting smaller. It is not moving; ocean crusts are. Subduction zones. Reading Review Questions: Answers What are tectonic plates? Rigid pieces of the earth’s lithosphere When two tectonic plates move toward each other, what is the boundary where they meet called? Convergent Plate Boundary. What are three kinds of geologic formations that occur at these boundaries? Volcanoes, mountains, and deep ocean trenches. What is a hot spot, and where is it found? A hot spot is an area where superheated magma comes up through the crust to form volcanoes. Hot spots are found at plate boundaries and sometime in the middle of tectonic plates. What causes most earthquakes? Movements of Earth’s crust from slippage at fault lines, or volcanic action. What are divergent boundaries, and where are most divergent boundaries found? Areas where two plates are moving away from each other; most are found under the ocean. Activity Questions: Answers Plate Tectonics Activity Directions: You will move a plate per the directions provided below, answering the questions in writing each time you move one plate. Move the North and South American plate away from the Nubia and Eurasia plate in the directions indicated by the arrows. 1. What color appears in the middle after you move the plates apart? Red. 2. What does this color represent, and why does it appear? Magma, because plates separate and magma is underneath. 3. When you move the North/South American plate to the left, what happens to the Pacific plate? The Pacific plate gets smaller. 4. When you move the Eurasia plate to the right, what happens to the Pacific plate? The Pacific plate gets even smaller. Move the Nazca/Cocos plate in the direction of its arrows, and the North/South American plate in the direction of its arrows. What happens at the boundaries of these two plates? The Nazca Cocos plate goes under the South American plate. What do you see at the boundaries of the Nazca/Cocos and the Pacific plates, and why? Divergence zone, because the plates are moving away from each other. Move the Australia/India plate in the direction of its arrows, and the Eurasia plate in the direction of its arrows. What happens at the two plate boundaries when they meet? They collide. Six of the eleven largest hot spots in the world are located near what kind of plate boundaries? Divergent plate boundaries. What three major hot spots are not located by any plate boundaries? Hawaiian, Yellowstone–North American, and South Pacific hot spots. What kind of plate boundaries are the other two hot spots found on? Transform plate boundaries. On which boundaries would most earthquakes happen, and why? Convergent plate boundaries, from stresses of plate subduction. On which boundaries would most volcanoes happen, and why? Convergent plate boundaries, from stresses of plate subduction. (Same as item 11.). d. Use your plate models as needed to figure out the following: In a million years, will the Atlantic Ocean be bigger or smaller? The Atlantic Ocean will be bigger. In a million years, will the North Pacific Ocean be bigger or smaller? The North Pacific Ocean will be smaller. In a million years, where will Australia be relative to Asia, and what may happen to the island nations between the two? The two continents will be closer; Asia may disappear or become part of Australia. In a million years, what will happen to Alaska and Siberia (Russia)? Alaska and Siberia may be joined. Lesson 1 Convergent and Divergent Boundaries 
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