Plate Tectonics

Earth’s lithosphere is NOT one unbroken layer It is like a cracked egg shell Broken into pieces separated by jagged cracks

Tectonic or Crustal plates are segments of the Earth's crust move relative to other segments are characterized by volcanic and seismic activity around its margins. The lithosphere is broken into separate sections called plates move over the asthenosphere carrying pieces of continental and oceanic crust.

How Plates Move The theory of plate tectonics states: pieces of Earth’s lithosphere are in slow, constant motion, driven by currents in the mantle This explains the formation, movement and subduction of Earth’s plates

So, I got a question for ya… How can Earth’s plates move? What force is great enough to move the heavy continents?

By Geologists…I think you’ve got it! The movement of currents in the mantle is the force that causes movement of the plates As plates moves, they collide, pull apart, or grind past each other This creates changes in Earth’s surface (volcanoes, mountain ranges, trenches)

Plate Boundaries Edge of Earth’s plates meet at plate boundaries Extend deep into lithosphere Faults – breaks in Earth’s crust where rocks have slipped past each other from along these boundaries

Plate Boundaries… 3 kinds of plate boundaries Divergent boundaries Convergent boundaries Transform boundaries A different type of movement occurs along each type of boundary Plates move SLOW; from 1-24 cm per year (about as fast as your fingernails grow)

Divergent Boundaries Place where two plates move apart (diverge) Most occur along mid-ocean ridge where sea-floor spreading occurs Also occur on land If this occurs, two of Earth’s plates slide apart creating a deep valley called a rift valley

Divergent Boundary along Ocean Ridge

Divergent Boundary along Land

Remember Density (Information only) A. Density 1. The measure of how much mass is in a given volume 2. Density = Mass/Volume

Convergent Boundaries Place where two plates come together (converge) Result is a collision The density of the plates determines which one comes out on top If plate carrying oceanic crust (more dense) collides with plate carrying continental crust (less dense), the oceanic crust will sink (subduct) beneath continental crust

Convergent Boundaries If two plates with continental crust collide, neither is dense enough to sink so the crust squeezes into mountain ranges If two oceanic plates collide the older plate will sink since the older plate is more dense.

Convergent Boundaries (Oceanic vs. Continental)

Convergent Boundaries (Continental vs. Continental)

Convergent Boundaries (Oceanic vs. Oceanic) older plate will sink (subduct) Older crust is more dense

Transform Boundaries Place where two plates slip past each other, moving in opposite directions Where earthquakes often occur Crust neither created nor destroyed

Review Questions Explain the theory of plate tectonics. What type of plate boundary is it where two plates collide? What do diverging plates form? What is likely to occur at a plate boundary where oceanic crust collides with continental crust? There are 2 islands on opposite sides of the mid-ocean ridge in the Atlantic Ocean. During the last 8 million years, the distance between the islands has increased by 200 kilometers. Calculate the rate at which the two plates are diverging. (in terms of centimeters per year)