Earth s Tectonic Plates

Transcription

1 Earth s Tectonic Plates Lesson 3 The Earth is known as the blue planet because two-thirds of its surface lies beneath the ocean. Before the 19 th century most people thought the ocean floors were flat and featureless. Early sailors used hand lines and later piano wire attached to a cannon ball to measure the depths of the oceans. They found the oceans varied considerably in depth. In the middle of the 20 th century, scientists began to study the ocean floor using sonar. Today we know that the oceans have gigantic mountain chains running through all the major oceans of the world like the seams on a baseball. These mountains formed at mid-ocean ridges located between crustal plates that are separating. Tectonic plate movement Diagram by USGS Spreading centers at mid-ocean ridges create new ocean floor as lava wells up in cracks between two crustal plates. As new seafloor forms on one side of an oceanic plate the leading edge of the plate, which is on the opposite side, subducts beneath another plate. As the oceanic plate moves downward it pulls the rest of the plate away from the spreading center allowing new lava to flow out of the mid-ocean ridge. Our planet is not increasing in size because as one edge of a plate is increasing in size at a spreading center another edge of a plate is subducting and getting smaller. Other plate edges bump into each other. The areas where two plates meet are convergent margins. When two plates meet, one plate must sink forming a subduction zone, or collide producing a collision zones. The Aleutian Islands in the North Pacific is an example of a subduction zone where the Pacific Plate is subducting beneath the North American Plate. The Himalayan Mountains are rising between India and Tibet because the Indian and Eurasian Plates are colliding.

2 Scientists named the largest tectonic plates (crustal plates) for the continents and oceans they contain. The seven largest plates are the North American Plate, Eurasian Plate, African Plate, South American Plate, Australian Plate, Indian Plate, and the Pacific Plate. The rest of the crustal plates are important but smaller. Scientists do not agree on the number of these small plates. For instance, off the coast of Northern California and Oregon you will find information listing the Blanco and the Juan de Fuca Plates. Other authorities list the entire landmass as one plate and call it the Juan de Fuca Plate. On the map below are Earth's largest crustal plates. Earth s Tectonic Plates Map by USGS Plates move as individual units and they are moving in different directions around the Earth. Today using laser beams bounced off satellites, scientists are able to measure the distance between two points on Earth with accuracy of about one centimeter. By taking measurements several times a year scientists are able to measure the velocities of the plates. Scientists have taken measurements between two points, one on the Pacific Plate and the other on the Cocos Plate. They found that the two plates are separating at 11.7 cm per year. Two points on the Eurasian Plate and the Pacific Plate in the North Pacific are coming closer together at a rate of 7.9 cm per year. Christopher Columbus crossed the Atlantic Ocean on his first voyage in The Mid-Atlantic ridge is separating at a rate of 2.5 cm per year. How much farther would Columbus have had to sail before he reached land today? Lesson summary Early sailors used hand lines and later piano wire attached to a cannon ball to measure the depths of the oceans. The oceans have gigantic mountain chains running through all the major oceans of the world like the seams on a baseball. Our planet is not increasing in size because as one edge of a plate is increasing in size another edge of a plate is subducting and getting smaller. Scientists use laser beams bounced off satellites to measure the velocity of tectonic plates.

3 Name Date Quiz 3 Earth s Tectonic Plates Fill in the blanks using words from the Word Bank 1. When two plates collide they form mountain ranges. 2. Mid-ocean ridges run through the entire major oceans of the world like the seams on a. 3. Today using bounced off satellites, scientists are able to measure the speed that tectonic plates are moving around the Earth. 4. When two plates meet one plate must sink forming a zone, or they collide and produce a collision zone. 5. Spreading ridges in the oceans around the world form large ranges called mid ocean ridges. 6. Early sailors used hand lines and later piano wire attached to a cannon ball to measure the of the oceans. 7. When a continental plate meets an oceanic plate, a subduction zone is created at a plate margin. 8. The Islands were created because the Pacific Plate is subducting beneath the North American Plate. 9. The Mountains are forming as the Eurasian Plate collides with the Indian Plate. 10. The largest tectonic plates are named for the continents or they contain. Word Bank subduction laser beams mountain depths Aleutian baseball Himalayan convergent continental oceans

4 Mantle Rocks Activity 3 Introduction Earth s tectonic plates ride on rocks in the upper mantle. Scientists believe that the rocks deform under heat and pressure in the upper mantle so that sometimes the rocks act more like a plastic than a solid. In this activity you will be doing some experiments using a recipe that is similar to Silly Putty. Materials 2 tbsp of Elmer s Glue 1 tbsp of liquid starch Food coloring (optional) Fork Bowl Plate Silly Putty breaking apart Directions 1. Pour 2 tbsp of Elmer s Glue into a small bowl. 2. Pour 1 tbsp of liquid starch on top of the glue. 3. Stir the glue and starch together with a fork. 4. At first the silly putty will be very sticky. 5. When the silly putty starts to form a ball pick it up and squeeze the mixture between your hands. 6. If the mixture sticks to your hands and does not form a ball put it back into the bowl and add a small amount of starch. 7. Mix the starch into the mixture until it holds a shape like a round ball.

5 8. If the mixture forms little individual globs but will not form a ball mix in a little more glue to your silly putty. 9. After you have formed a ball with your silly putty it is time to do some experiments. 10. First hold the silly putty in your hand above the plate and let it ooze down toward the plate. It should flow very slowly toward the plate if your mixture has been correctly made. 11. Next, hold the silly putty between your fingers of both hands and pull the mixture apart. It should break apart instead of just pulling apart. 12. If your mixture is not the right consistency try it again until it does. 13. This activity demonstrates how rocks in the upper Silly Putty flowing downward mantle can flow very slowly like a liquid. 14. After you have formed a ball with your silly putty it is time to do some experiments. 15. First, hold the silly putty in your hand above the plate and let it ooze down toward the plate. It should flow very slowly toward the plate if your mixture has been made correctly. 16. Next, hold the silly putty between your fingers of both hands and pull the mixture apart. It should break apart instead of just pulling apart. 17. If your mixture is not the right consistency try it again until it does. 18. This activity demonstrates how rocks in the upper mantle can flow very slowly like a liquid. 19. The experiment also demonstrates how the rocks can break apart during an earthquake. Extending the activity Place a 1 x 4 inch board over your silly putty and see if you can force it along the surface of the mixture similar to the way tectonic plates move over the surface of the upper mantle.