Presentation on theme: "Chapter 19 Plate Tectonics. Alfred Wegener l A man with an idea. l In 1912 l Noticed similar shapes l similar fossils l Similar rock structures l Similar."— Presentation transcript:
Chapter 19 Plate Tectonics
Alfred Wegener l A man with an idea. l In 1912 l Noticed similar shapes l similar fossils l Similar rock structures l Similar evidence from glaciers l on continents
Problems With the Theory l Though winds or ocean currents could explain the fossils l Thought the poles moved, so that you could get the glaciers l Didnt explain how it happened. l Though that it was due to the Earth spinning on its axis l Scientists in the southern hemisphere accepted it first l They believed the rocks
Solution l Arthur Holmes l Convection currents move the plates.
Ocean-Floor Spreading l In the 1950s and 1960s mapped the ocean floor l Examined underwater mountain ranges l Run for 37,500 miles around the earth. l Found a rift valley in the center l Harry Hess explained – new ocean floor is made at the mid- ocean ridge – Is pushed under other crust at the trenches
Evidence trapped in rocks l Earth is a magnet l When lava cools it keeps track of which way the magnetic pole points. l Every few hundred thousand years the magnetic field switches direction. l Has made stripes of alternating directions on the ocean floor.
Other Evidence l By drilling they have found that rocks near the ridge are youngest, and they get older as you get further away.
Where does it go? l At the edges of the plates one goes under the other l called subduction l It is melted and may make volcanoes. l Evidence comes from earthquakes. l The further you get from the edge, the deeper earthquakes are.
Earthquakes and Volcanoes l Occur in zones around the world
Lithospheric Plates l There are 7 major ones,and several others
Parts of the lithosphere
Convergent Boundaries l Continental and oceanic- ocean subducted, volcanoes on shore l Oceanic and Oceanic- one is subducted, forms an island arc l Continental and continental- folded mountains and a plateau
How Fast? l Arctic Ridge 2.5 cm per year l East Pacific rise, off the coast of Chile 15 cm/year l Current movement tracked by satellites