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1-2 Notes: Continental Drift Continents Join Together and Split Apart In the ___________’s, mapmakers noticed that the coasts of Africa and South America looked like fit together like puzzle pieces. In ___________, Alfred Wegener proposed a hypothesis called continental drift. His hypothesis stated that Earth’s landmasses were once joined together in a single continent and have gradually drifted _________________. This idea was not accepted until the mid-1900’s. Evidence for Continental Drift Extraordinary claims require extraordinary evidence! What evidence is there for continental drift? There are ____ main kinds of evidence for continental drift: fossils, climate, and geology. Fossils Mesosaurus is an ancient reptile whose fossils are found ONLY in South America and Western _____________________. There are many other fossils found around the world that support the theory of continental drift. Climate Greenland is an island near the Arctic circle that is covered in ice. BUT…Fossils of ____________________ plants can be found on the shores of Greenland. How??? Greenland was probably closer to the _______________________ in the past. South Africa has a warm climate, but its rocks have deep scratches clearly made by _____________________. South Africa once existed much closer to the _________________________________. Geology The types of rock found in Brazil perfectly match rock found in western Africa. Limestone layers in the eastern U.S. match limestone layers found in Scotland. Pangaea and Continental Drift The continents were once joined in a huge supercontinent called Pangaea. The word Pangaea comes from the Greek for “_________ lands.” When Wegener developed his hypothesis, he could not explain __________ the continents moved. Because of this, people disregarded his idea at first. The theory of plate ___________________ built on Wegener’s ideas but also explained HOW plates and their continents move. Evidence from the Sea Floor When ______________________ technology was invented, scientists began mapping the dips and bumps in the ocean floor in the 1950’s. They expected it to be mostly smooth and level, like a flat underwater ____________________. What scientists found instead were huge underwater mountain ranges called mid-ocean ridges. Sea-Floor Spreading Mid-ocean ridges form along _______________________ in the crust. _____________________ rises through these cracks, cools, and forms new crust. Age of the Sea Floor We can verify that sea floor spreading is occurring and creating new crust by testing the _____________ of rocks on the ocean floor. The youngest rocks are close to spreading centers, while older rocks are found _____________________ away. p. 113 The oldest rocks on the sea floor are ____________________ million years old. ___________________________ crust can be upwards of 4 billion years old! Why do you think continental crust is so much older than oceanic crust? The ocean floor is constantly being created AND ______________________________. Ocean Trenches If the sea floor is always spreading and creating new crust, why is Earth not constantly getting bigger? Crust is being destroyed at the same _________________ it is being created. Causes of Plate Movement Remember-tectonic plates float on the _____________________________-a layer of hot, soft rock. ____________________________: the transfer of energy (heat) by the movement of a material. Convection is what happens when you boil a pot of water. The water at the bottom of the bottom heats up, becomes less dense and _____________________. At the surface, it cools, becomes more _______________________, and sinks. Rock in the asthenosphere acts the same way as a boiling pot of water-it moves by ________________________. When this rising and sinking pattern repeats, it forms a circular motion called a convection current. The movement of the mantle is much slower than a pot of boiling water-only a few __________________ per year! The movement in the asthenosphere is like heavy boxes on rollers-but moving very, very slowly. Slab Pull and Ridge Push Two other possible causes of plate movement are slab pull and ridge push. Slab Pull: when ______________________________ pulls the edge of a cool plate into the asthenosphere. Ridge Push: when material from a mid-ocean ridge slides ____________________________ from the ridge. Putting it All Together The theory of plate tectonics puts together scientists’ knowledge of Earth’s plates, the ocean floor, and the asthenosphere. There are ____ major tectonic plates on Earth. One plate cannot shift without affecting the others nearby. Review ____1. The existence of fossilized tropical plants in Antarctica indicates that millions of years ago the continent had _____. A. a warm, rainy climate. C. a cold, dry climate B. been part of Africa. D. been farther from the equator ____2. The driving forces of tectonic plates are related to convection currents in Earth’s A. crust C. inner core B. mantle D. outer core ____3. Which of these did Wegener use to support his theory of continental drift? A. Similar rocks and similar fossils on different continents B. Sea-floor spreading and similar rocks on different continents C. Convection currents in the asthenosphere and sea-floor spreading D. Fossils from ancient organisms and convection currents in the asthenosphere. ____4. Studies of the ocean floor support plate tectonics because oceanic crust A. is younger at a ridge and older near a trench C. is younger at a trench and older near a ridge B. is much older than continental crust D. is about the same age as Pangaea _____5. Scientists think that the main driving force of plate tectonics is A. ridge push C. slab pull B. convection in the lithosphere D. convection in the asthenosphere