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Transcript
Grade 9 Geography of Canada Unit 3 Lesson 1 Continental Drift Question How could continents move (Continental Drift)? Facts Earth’s crust consists of about 20 plates Plates move (Wegener’s Continental Drift theory) Plates “float” on top of the Mantle. Part of the Mantle is the asthenosphere. It is extremely hot, liquid and moving slowly Wegener found many similarities between continents (e.g., fossils, landforms) Movement of liquid within the Mantle occurs by convection Other Information Ideas and Hypotheses As gas and liquid is heated, it loses density (e.g., hot air rises because molecules in the air become further apart and, thus, less dense). As the heated substance rises, it cools, becomes more dense, and begins to fall. Ocean currents push the continents through the Earth’s crust (Wegener’s idea. Why did ploughing not change shape of continents? This is one reason why few people supported Wegener’s thesis.) Tidal forces caused by gravitational pull of the moon. Centrifugal forces as the Earth rotates Pressures created by rising and falling within Mantle caused by convection movement Grade 9 Geography of Canada Unit 3 Lesson 1 Theory of Plate Tetonics and Continental Drift Alfred Wegener At the University of Marberg in 1911, Wegener read a scientific paper that listed fossils of identical plants and animals found on opposite sides of the Atlantic. He soon found more cases of similar organisms separated by great oceans. Science of the period suggested land bridges, now sunken, had once connected far-flung continents. Wegener noticed the close fit between the coastlines of Africa and South America. He hypothesized that similarities among organisms were due, not to land bridges, but to the continents having been joined together at one time. Such an insight, to be accepted, would require large amounts of supporting evidence. Wegener found that large-scale geological features on separated continents often matched very closely when the continents were brought together. For example, the Appalachian Mountains of eastern North America matched with the Scottish Highlands, and the distinctive rock strata of the Karroo system of South Africa were identical to those of the Santa Catarina system in Brazil. Wegener also found that the fossils found in a certain place often indicated a climate utterly different from the climate of today. For example, fossils of tropical plants such as ferns and cycads are found today on the Arctic island of Spitsbergen. All of these facts supported Wegener's theory of Continental Drift. About 300 million years ago, the continents had formed a single mass called Pangaea (from the Greek for "all the Earth"). Pangaea had split and its pieces had been moving away from each other ever since. Why was Wegener’s Theory Not Readily Accepted? Wegener could not explain how the continents might move. He thought the continents were moving through the earth's crust, like icebreakers plowing through ice sheets, and that centrifugal and tidal forces were responsible for moving the continents. Wegener overestimated the rate of continental movement. He suggested that North America and Europe were moving apart at over 250 cm per year or about 100X faster than observed today. Today, Plate Tetonics is widely accepted. Note, Wegener's theory was wrong in one major point. Continents do not plow through the ocean floor. Instead, both continents and ocean floor form solid plates which "float" on the asthenosphere, the underlying rock that is under such tremendous heat and pressure that it behaves as an extremely viscous liquid. (Incidentally, this is why the older term "continental drift" is not quite accurate -- both continents and oceanic crust move.) The main features of plate tectonics are: The Earth's surface is covered by a series of crustal plates. The ocean floors are continually, moving, spreading from the center, sinking at the edges, and being regenerated. Convection currents beneath the plates move the crustal plates in different directions. The source of heat driving the convection currents is deep in the Earth’s mantle. In 1929, Arthur Holmes elaborated on one of Wegener's hypotheses: the mantle undergoes thermal convection. As a substance is heated, its density decreases and rises to the surface until it is cooled and sinks again. This repeated heating and cooling results in a current which may be enough to cause continents to move. Holmes suggested that this thermal convection was like a conveyor belt and that the upwelling pressure could break apart a continent and then force the broken continent in opposite directions carried by the convection currents. In 1960, Holmes’ theory was modified and called the "Sea-floor Spreading". The supporting evidence was seafloor features: mid-oceanic ridges, deep sea trenches, island arcs, geomagnetic patterns and fault patterns. Mid-Oceanic Ridges The mid-oceanic ridges rise 3000 meters from the ocean floor and are more than 2000 kilometers wide. These huge underwater mountain ranges have a deep trench which bisects the length of the ridges and in places is more than 2000 meters deep. Heat flow research revealed that the greatest heat flow was centered at the crests of these mid-oceanic ridges. Seismic studies show that the mid-oceanic ridges experience an elevated number of earthquakes. All these observations indicate intense geological activity at the mid-oceanic ridges. Geomagnetic Anomalies Periodically, the Earth's magnetic field reverses. New rock formed from magma records the orientation of Earth's magnetic field at the time the magma cools. Study of the sea floor with magnometers revealed "stripes" of alternating magnetization parallel to the mid-oceanic ridges. This is evidence for continuous formation of new rock at the ridges. As more rock forms, older rock is pushed farther away from the ridge, producing symmetrical stripes to either side of the ridge. In the diagram to the right, the dark stripes represent ocean floor generated during "reversed" polar orientation and the lighter stripes represent the polar orientation we have today. Notice that the patterns on either side of the line representing the mid-oceanic ridge are mirror images of one another. The shaded stripes also represent older and older rock as they move away from the mid-oceanic ridge. Geologists have determined that rocks found in different parts of the planet with similar ages have the same magnetic characteristics. Deep Sea Trenches The deepest waters are found in oceanic trenches, which plunge as deep as 35,000 feet below the ocean surface. These trenches are usually long and narrow, and run parallel to and near the ocean margins. They are often associated with and parallel to large continental mountain ranges. There is also an observed parallel association of trenches and island arcs. Like the midoceanic ridges, the trenches are seismically active, but unlike the ridges they have low levels of heat flow. Scientists also began to realize that the youngest regions of the ocean floor were along the mid-oceanic ridges, and that the age of the ocean floor increased as the distance from the ridges increased. In addition, it has been determined that the oldest seafloor often ends in the deep-sea trenches. Island Arcs Chains of islands are found throughout the oceans and especially in the western Pacific margins: the Aleutians, Japan, Philippines, and Indonesia. These "Island arcs" are usually situated along deep-sea trenches and are situated on the continental side of the trench. These observations, along with many other studies of our planet, support the theory that underneath the Earth's crust (the lithosphere: a solid array of plates) is a malleable layer of heated rock known as the asthenosphere which is heated by radioactive decay of elements such as Uranium, Thorium, and Potassium. Because the radioactive source of heat is deep within the mantle, the fluid asthenosphere circulates as convection currents underneath the solid lithosphere. This heated layer is the source of lava we see in volcanoes, the source of heat that drives hot springs and geysers, and the source of raw material which pushes up the mid-oceanic ridges and forms new ocean floor. Magma continuously wells upwards at the mid-oceanic ridges (arrows) producing currents of magma flowing in opposite directions and thus generating the forces that pull the sea floor apart at the mid-oceanic ridges. As the ocean floor is spread apart cracks appear in the middle of the ridges allowing molten magma to surface through the cracks to form the newest ocean floor. As the ocean floor moves away from the mid-oceanic ridge it will eventually come into contact with a continental plate and will be subducted underneath the continent. Finally, the lithosphere will be driven back into the asthenosphere where it returns to a heated state. Grade 9 Geography of Canada - Unit 3 Lesson 1 Our Dynamic Earth Video – National Geographic – Teacher Resource Centre – Call 551.D96 KIT Questions 1. What does plate tectonics theory attempt to explain? Plate tectonic theory primarily attempts to explain movements of the Earth’s crust (lithosphere). 2. What was the name of the single supercontinent that existed 200 million years ago? The name of the supercontinent was the Pangaea. 3. What created the Himalayas? The HImalayas were created by the Indian Plate crashing into the Eurasian Plate. Crashing is the common wording. Since plates only move a few centimetres per year, it can hardly be called crashing. 4. What do the plates of the lithosphere (crust and upper mantle) float on? The plates float on the asthenosphere within the mantle. 5. Name three major plates. 6. Describe the major forms of plate margin behaviour? Divergent (spreading) – occurs when plates separate. Subduction – occurs when converging plates collide and the lighter plate plunges under the other. Lateral or transform – occurs when plates slide past each other. Continent-continent convergence – occurs when two continental plates collide. 7. What type of plate activity is occurring off the west coast of South America and what is the result? Subduction is taking place as the Nazca Plate subducts under the South American Plate to form the Andes Mountain. Volcanoes and earthquakes also are produced during this process. 8. What is occurring alon gthe San Andreas Fault and what can be the result? Lateral motion is occurring as a result of the Pacific Plate sliding past the North American Plate. As stress is released along the plate boundaries, large earthquakes are produced. 9. Describe the appearance of lava flows along the ocean floor They look like pillows. Hence, they are called Pillow Lavas. Our Dynamic Earth…Page 2 10. Describe the underwater flow of lava in Hawaii. Lava quickly changes form as it is cooled in the seawater. It also creates tubes through which molten lava travels, re-emerges at the end of the tube, and then, flows into the sea. When seawater comes in contact with lava, the lava is quickly chilled and breaks down into small black particles forming black-sand beaches. 11. Describe how life can flourish at great depths where light cannot penetrate. Seawater, which mixes with the magma beneath the seafloor, is heated and chemically altered. It then rises to the seafloor, emerging as thermal vents and “black smokers.” Chemosynthetic bacteria are able to metabolize sulfur chemicals in this hot water, forming the basis of the entire food web, including crabs, tube worms and clams. 12. What enabled people to discover the deep-sea vents? Deep-diving submarine enabled researchers to discover deep-sea vents and study life at extreme depths. 13. What is the promise of plate tectonics? Knowledge of plate tectonics may allow researchers to predict future volcanic and seismic activity, and to discover how to live in harmony with our dynamic Earth. 14. How might additional understanding of plate dynamics improve our lives? Additional knowledge of the movement of plates and the dynamics of Earth processes could lead to better prediction of earthquakes and volcanoes, and improve our ability to harness the Earth’s energy. 15. What evidence is there that the Earth is a dynamic planet? Volcanoes and earthquakes are strong indicators that the Earth is in motion. Less obvious, but just as strong, evidence is provided by the gradual movement by the plates.