Plate Tectonics

Oceanic and Continental Crust The top layer of the Earth’s surface is called the crust. Oceanic crust (the thin crust under the oceans) is thinner and denser than continental crust. Type of Crust Average Thickness Average Age Major Component Continental Crust 20 - 80 kilometers 3 billion years Granite Oceanic crust 10 kilometers Generally 70 - 100 million years old Basalt

The Lithosphere and Asthenosphere Under the crust is the rocky mantle, which is composed of silicon, oxygen, magnetism, iron, aluminum and calcium. The upper mantle is rigid and is part of the lithosphere (together with the crust). The lower mantle flows slowly, at a rate of few centimeters per year. The asthenosphere is a part of the upper mantle that exhibits plastic properties.

Earth’s Major Plates Have you ever dropped a hard boiled egg? If so, you may have noticed that the eggshell cracked in an irregular pattern of broken pieces. The Earth’s lithosphere (the solid outer shell) is like the cracked eggshell. It is broken into pieces separated by jagged cracks. These pieces are called plates.

T. Juzo Wilson The plates fit closely together along cracks in the lithosphere. The plates carry the continents or parts of the ocean floor or both. In, 1965 a Canadian scientist, J. Tuzo Wilson, combined what geologists knew about sea floor spreading, Earth’s plates and continental drift into one theory – the theory of plate tectonics.

Theory of Plate Tectonics Plate tectonics (meaning “plate structure”) is the geological theory that states the pieces of Earth’s lithosphere are in constant, slow motion, driven by the convection currents in the mantle. Plate tectonics explains the formation, movement and subduction of the Earth’s plates. Most of the Earth’s seismic activity (volcanoes and earthquakes) occurs at the plate boundaries as they interact.

Earth’s Major Plates The current continental and oceanic plates include the: Eurasian, Australian-Indian, Philippine, Pacific, Juan de Fuca, Nazca, Cocos, North American, Caribbean, South American, African, Arabian, Antarctic and the Scotia plates

Theory of Plate Motion The plates of the lithosphere float on top of the asthenosphere. Convection currents rise in the asthenosphere and spread out beneath the lithosphere. Most geologists think that the flow of these currents causes the movement of Earth’s plates. No plate can budge without affecting the other plates surrounding it. As the plates move, they collide, pull apart or grind past each other producing spectacular changes in Earth’s surface, including volcanoes, mountain ranges and deep sea trenches.

Convergent boundaries Plate Boundaries The edges of different pieces of the lithosphere meet at lines called plate boundaries. Plate boundaries extend deep into the lithosphere. Faults (breaks in the Earth's crust where rocks have slipped past each other) form along these boundaries. There are three kinds of boundaries: Divergent boundaries Convergent boundaries . Transform boundaries

Divergent Plate Boundaries The place where two plates move apart, or diverge, is called a divergent boundary. Most divergent boundaries occur at the mid-ocean ridge. Oceanic crust is formed along these boundaries.

Divergent Plate Boundaries Divergent boundaries are not confined to the ocean. They can also occur on land. When a divergent boundary develops on land, two of Earth’s plates slide apart, forming a deep valley called a rift valley. For example, The Great Rift Valley in Africa marks a deep crack in the African continent that runs for about 3,000 km. Someday the rift may split the eastern part of Africa from the rest of the continent.

Divergent Plate Boundary: Sea Floor Spreading

Convergent Plate Boundaries The place where two plates come together, or converge, is called a convergent boundary. When two plates converge, the result is a collision. Collisions may bring together oceanic crust and oceanic crust, oceanic crust and continental crust or continental crust and continental crust .

Convergent Plate Boundaries Crust is destroyed at convergent boundaries and it is at these boundaries where we find some of the most violent geological activity. The resulting destruction depends on the types of plates involved. When the two plates collide, the density of the plates involved determines which one comes out on top.

Convergent Plate Boundaries: Two Oceanic Plates When two oceanic plates collide, the younger of the two plates, because it is less dense, will ride over the edge of the older plate. The older, heavier plate bends and plunges steeply through the asthenosphere, and descending into the earth, it forms a trench that can be as much as 70 miles wide. Where there are deep ocean trenches, subduction takes place. Subduction is the process by which the ocean floor sinks beneath a deep ocean trench and back into the mantle.

Convergent Plate Boundaries: Two Oceanic Plates The result is magma being pushed toward the surface and the formation of volcanoes which eventually form curving chains of islands (the Hawaiian islands).

Convergent Plate Boundaries: Two Continental Plates When two continental plates collide, subduction does not take place. Neither plate is dense enough to sink to the mantle. Instead the plates crash head on and mountain ranges are created as the colliding crust is compressed and pushed upwards.

Convergent Plate Boundaries: An Oceanic and a Continental Plate The less dense continental crust cannot sink under the more dense oceanic crust. Instead, the oceanic plate begins to sink and plunges beneath the continental plate. When an oceanic plate pushes into and subducts under a continental plate, the overriding continental plate is lifted up and a mountain range is created.

Convergent Plate Boundaries: An Oceanic and a Continental Plate Even though the oceanic plate as a whole sinks smoothly and continuously into the subduction trench, the deepest part of the subducting plate breaks into smaller pieces. These smaller pieces become locked in place for long periods of time before moving suddenly and generating large earthquakes. Such earthquakes are often accompanied by uplift of the land by as much as a few meters.

Subduction (When an oceanic and continental plate converge)

Transform Plate Boundaries The place where two plates moving in opposite directions slide past one another is called a transform boundary. Crust is neither created nor destroyed at these boundaries. There is a tremendous amount of friction which makes the movement jerky. The plates slip, then stick as the friction and pressure build up to incredible levels. When the pressure is released suddenly, and the plates suddenly jerk apart, this is an earthquake.

Overview of the Plate Tectonic Process

Plate Tectonics and the Rock Cycle The change sin the rock cycle are closely related to plate tectonics. Plate movements drive the rock cycle by pushing rocks back into the mantle, where they melt and become magma again. Plate movements also cause the folding, faulting and uplift of the crust that move rocks though the rock cycle.

Plate Tectonics and the Rock Cycle At least two types of plate movement advance the rock cycle. One type is a collision between subducting oceanic plates. The other type is a collision between continental plates.

Subducting Oceanic Plates and the Rock Cycle Consider what would happen to the sand grains that once were part of Stone Mountain. The sand may become sandstone attached to oceanic crust. On this pathway through the rock cycle, the oceanic crust carrying the sandstone drifts toward a deep ocean trench. At the trench, subduction returns some of the sandstone to the mantle. There it melts and forms magma, which eventually becomes igneous rock.

Colliding Continental Plates and the Rock Cycle Collisions between continental plates can also change a rock’s path through the rock cycle. Such a collision can squeeze some sandstone from the ocean floor. As a result, the sandstone will change to quartzite. Eventually, the collision could form a mountain range or plateau. Then, as the mountains or plateaus containing quartzite are worn away, the rock cycle continues.