1 Wegener earthobservatory.nasa.gov Alfred Wegener first proposed the theory of continental drift. 2 Continental drift was Wegeners theory that all continents had once been joined together in a single landmass and have drifted apart since. Wegener named this supercontinent Pangaea. Wegeners theory was rejected by scientists because he could not explain what force pushes or pulls continents. 3 4 Fit of Continents Across the Atlantic Mountain ranges in South America line up exactly with those in Africa! 5 Notice how fossils lined up across continents!6 Amazing Facts: Did you know......that India was once in the Southern Hemisphere connected to Antarctica?...that North America was once surrounded by warm, tropical seas?...that Africa was once covered by glaciers, which were kilometers in thickness?...that the Sahara desert was once a tropical rain forest? 7 As North America has drifted to different latitudes, changing climates were experienced. This shows the changes in the state of New Mexico. An example of the proof for changing climate: At one time, the area that is New Mexico was below the equator! 8 He could not find the force that was causing the continents to drift. Because of this, he could not convince anyone that continents could move. He died in Greenland on an expedition. At the time of his death, no one believed his hypothesis! Technology developed during the 1940s changed all that! 9 Discovery of Sea-Floor Spreading 10 Sea-Floor Spreading m.y. means million years ago Notice this compass. It is important for the next slide 11 As the sea floor spreads, the lava cools according to the magnetic poles at the time. The rocks on the ocean floor have proved that the earths magnetic field sometimes reverses. The inner core flips and so the north pole moves to the southern hemisphere! The earthh itself does not flip.12 What causes sea floor spreading? Convection currents! 13 Click here for a hyperlink to an animation of convection Be sure to click the play button! 14 Animation of sea floor spreading Can you explain this diagram!? 15 The place where two plates move apart or diverge is called a divergent boundary. 16 This is a model of sea floor spreading at a divergent boundary called a mid ocean ridge. 17 Did you know that the Earths longest mountain range is underwater and is called the mid-ocean ridge? :The Mid-Ocean Ridge system, shown above snaking its way between the continents, is more than 56,000 kilometers (35,000 mi) long. It circles the earth like the stitching on a baseball! 18 A convergent boundary is where two plates come together, or converge. The result of the plates hitting together is called a collision. 19 A transform boundary is a place where two plates slip past each other, moving in opposite directions. 20 Continental crust to continental Continental crust to oceanic Oceanic crust to oceanic 21 from:Continental crust to continental crust collision India-Asia (Himalayas) Before collision After collision 22 Also the way the Appalachian Mountains formed! 23 Because one plate gets pushed under another, it is called subduction. This is where volcanoes form! Oceanic crust colliding with oceanic crust Oceanic crust colliding with continental crust all from:24 The process by which the ocean floor sinks beneath a deep- ocean trench and back into the mantle is called 25 What causes an Earthquake? An earthquake is the movement of Earths crust resulting from the release of built-up potential energy between two stuck lithospheric plates. The lithosphere is cold and brittle and can fracture during an earthquake. The lithosphere is divided into pieces called tectonic plates. They are like broken ice fragments that float on the surface of water. Any sudden shifting of the tectonic plates results in earthquakes. The tectonic plates may slide past each other, push apart, or collide together. The interactions at the plate boundaries are important to consider. (Notice the relationship in the previous slides between the plate boundaries and the location earthquakes 26 Worldwide Earthquake patterns help us know where the edges of plates are located 27 The Earths Plates 28 Here you see the two slides together. What do you see? 29 Plate Boundary Map 30 Earthquakes and Plate Boundaries Earthquakes commonly occur at the boundaries of lithospheric plates. This is because plate boundaries tend to be zones of (earthquake) seismic activity 31 Seismic data around Japan ( ) 32 Seismic Data at the Pacific Rim A region of seismic volatility runs along the Pacific rim called the Ring of Fire. It marks the boundary between a number of huge tectonic plates in the earths crust. The pressure which builds up when one plate pushes against the other can lead to violent volcanic activity and powerful earthquakes. 33 World Volcanoes Map 34 How do Volcanoes form? Deep inside Earth, between the molten iron core and the thin crust at the surface (lithosphere), there is a solid body of rock called the mantle. When one tectonic plate is subducted by another, rock from the subducted plate melts, moves to the surface through the crust, and releases pent- up gases, volcanoes erupt. Extremely high temperature and pressure cause the mantle rock to melt and become liquid rock or magma. When a large body of magma has formed, it rises through the denser rock layers toward Earth's surface. Magma that has reached the surface is called lava. Extremely high temperature and pressure cause the rock to melt and become liquid rock or magma. When a large body of magma has formed, it rises thorugh the denser rock layers toward Earth's surface. Magma that has reached the surface is called lava. 35 36