Plate Tectonics

1. Plate Tectonics By: Sierra Boyle Intended for sixth through eighth grade

2. Historical Background Continental Drift Theory: • Super-Continent Pangea • Theory proposed by Alfred Wegner in 1912 • Evidence: • fitting of continents • distribution of fossils • similar rock sequences • ancient climates • Splitting of Pangea

3. Sea-Floor Spreading • Henry Hess proposed the theory of sea-floor spreading. The theory was the mechanism for Alfred Wagner’s continental drift. • Discovered by studies of earth’s magnetic fields • Earth experiences periods of reversed polarity • These are recorded in rocks and symmetrically about mid-ocean ridges • Magma acts as a “recorder” taping the earths magnetic field reversals • Ocean floor near the edge of continents would be the oldest, and floor near the mid-ocean ridge would be newly formed Together Continental Drift and Sea-Floor Spreading led to the Plate Tectonic Theory

4. Understanding:Make-up of Earth • The Earth is divided into three chemical layers: the core, the mantle and the crust. • The outermost layers of the Earth can be divided by their physical properties into lithosphere and asthenosphere. • Lithosphere-rigid outermost layer made of crust and uppermost mantle. The lithosphere is the "plate" of the plate tectonic theory. • Asthenosphere-mantle that flows, a characteristic called plastic behavior. Part of mantle convection, which plays an important role in moving lithospheric plates.

5. How Continents Drift Apart:Sea Floor Spreading • New sea floor is produced at the rifts of mid-oceans ridges( divergent boundary). The ocean floor, and the rock beneath it, are produced by magma that rises from deeper levels filling the gap to form new oceanic crust. This creates continental drift, thus continents drifting apart.. • Old sea floor is carried away from the ridge. The lithosphere arrives at the edge of a continent, where it is subducted or sinks into the asthenosphere. • The dip accounts for earthquakes, and magma generated along the top of the sinking slab rises to the surface to form stratovolcanoes.

6. Distribution of Plates Can you find the connection between these two pictures? • The term plate was coined for the broken pieces of the Earth's lithosphere . • Earth's surface consists of 12 rigid plates that move relative to each other. • Where plates interact, along their margins, important geological processes take place, such as the formation of mountain belts, seafloor spreading, earthquakes, and volcanoes.

7. Plate Movement Plates move in different ways dependant upon location and relative motion Explanation of Plate Movement

8. Plates Relation to Volcanic Activity Mount St. Helens • Most of the world's active above-sea volcanoes are located near convergent plate boundaries where subduction is occurring. • Convergent boundary volcanoes are called stratovolcanoes. They typically erupt with explosive force, because the magma is too stiff to allow easy escape of volcanic gases. Before Eruption After Eruption

9. Plates Relation to Earthquake Activity • An earthquake is the shaking of the ground caused by an abrupt shift of rock along a fracture in the Earth, called a fault. • A fault is generally located along a transform plate • An earthquake releases stress that has slowly accumulated within the rock. Effects of Earthquakes

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