Published on March 18, 2014
The Theory of Plate Tectonics Chapter 14 Lesson 3 p510-519
Vocabulary • Plate tectonics (511) – Earth’s surface is made of rigid slabs of rock, or plates, that move with respect to each other • Lithosphere (512) – the cold and rigid outermost rock layer • Divergent plate boundary (513) – forms where two plates separate • Transform plate boundary (513) – Forms where two plates slide past each other • Convergent Plate Boundary (513) – form where two plates collide • Subduction (513) – A process in which the denser plate sinks below the more buoyant plate • Convection (516) – the circulation of material caused by differences in temperature • Ridge Push (517) – the forces that causes rising mantle material at mid- ocean ridges that creates the potential for plates to move away from the ridges • Slab Pull (517) – As a slab sinks, it pulls on the rest of the plate with this force
The Plate Tectonic Theory • Earth’s crust is constantly being created and destroyed • The theory of plate tectonics states that Earth’s surface is made of rigid slabs of rock, or plates, that move with respect to each other • Each plate moves over Earth’s hot and semi- plastic mantle – The term tectonic describes the forces that shape Earth’s surface and the resulting rock structures – Plate tectonics is used to explain earthquakes and volcanic eruptions
Tectonic Plates • The Pacific Plate is the largest plate • The Juan de Fuca is one of the smallest • The boundaries that run through oceans mark the positions of the mid-ocean ridges • Earth’s outermost layers are cold and rigid compared to the layers in Earth’s interior. It is called the lithosphere. – It is made up of the crust and the solid, uppermost mantle – It is thin below ocean ridges and thick below continents – The tectonic plates are just large pieces of the lithosphere • Just below the lithosphere is called the asthenosphere. – This layer is so hot that it behaves like a plastic material – This enables to lithosphere to move
Plate Boundaries Convergent Convergent
Divergent Plate Boundaries • Mid-ocean ridges are located along divergent plate boundaries. • A divergent plate boundary forms where two plates separate • When the seafloor spreads at a mid-ocean ridge, lava erupts, cools, and forms new oceanic crust. • Divergent plate boundaries can also exist in the middle of a continent – They pull continents apart and form rift valleys – The East African Rift is an example.
Transform Plate Boundaries • A transform plate boundary forms where two plates slide past each other. – The San Andrews Fault in California is a well known example. – As the plates move past each other, they can get stuck and stop moving, – Stress builds up where the plates are stuck. – Eventually the stress is too great and the rocks break – The resulting rapid energy release is an earthquake.
San Andreas Fault
Convergent Plant Boundaries • Convergent plate boundaries form where two plates collide • The denser plate sinks below the more buoyant plate in a process called subduction – The area where a denser plate descends into Earth is called a subduction zone.
• When an oceanic plate and continental plate collide the denser oceanic plate subducts under the edge of the continent • This creates a deep ocean trench • A line of volcanoes forms above the subducting plate on the edge of the continent • This process can also happen with two oceanic plates. – Typically the older, denser plate will subduct beneath the younger plate. – This creates a deep ocean trench and a line of volcanoes called an island arc
• When two continental plates collide, neither plate is subducted. • Instead rock is uplifted and create mountains like the Himalayas
Evidence for Plate Tectonics • Continents move apart or come together at speeds of a few centimeters per years • Today scientists use a network of satellites called the Global Positioning System (GPS) to measure the movement of the plates • The theory of plate tectonics explains why earthquakes and volcanoes occur in certain places
• All types of plate boundaries can result in earthquakes due to the rapid release of energy • Diverging and converging plates result in volcanoes • Mountains form when two continental plates converge
Plate Motion • Convection Currents – Convection is the circulation of material caused by differences in temperature and density • For example, the upstairs floors of most houses often warmer than the lower floor • This is because warm air rises while denser, cold air sinks.
• Tectonic plate activity is related to convection in the mantle. • Radio active elements heat Earth’s interior. • When materials such as solid rock are heated, they expand and become less dense • Hot mantle material rises upward and comes in contact with Earth’s crust • Thermal energy is transferred to the surface • As the mantle cools, it becomes denser and then sinks, forming a convection current • These currents in the asthenosphere act like a conveyor belt moving the lithosphere
FORCES CAUSING PLATE MOTION Scientists are still uncertain about which force has the greatest influence
Forces Causing Plate Motion • Basal Drag – This is when convection currents in the asthenosphere move or drag the lithosphere much like how walking sidewalks at the airport move people.
Ridge Push • Recall that mid-ocean ridges have greater elevation than the surrounding seafloor. • Because they are higher, gravity pulls the surrounding rock down and away from the ridge • Rising mantle material at mid-ocean ridges creates the potential for plates to move away from the ridge with a force called ridge push. – This moves the lithosphere away from the mid-ocean ridge.
Slab Pull • When plates converge, the denser plate will sink into the mantel along a subduction zone • This plate is called a slab. It is usually old and cold which makes it denser. • As a slab sinks, it pulls on the rest of the plate with a force called slab pull.
A Theory in Progress • Plate tectonics has become the unifying theory of geology • It explains the connection between continental drift and the formation and destruction of crust along plate boundaries. • It also helps to explain the occurrence of earthquakes, volcanoes, and mountains
Unanswered Questions • Several questions remain unanswered: – Why is Earth the only planet in the solar system that has plate tectonic activity? – Why do some earthquakes and volcanoes occur far away from plate boundaries? • Part of the answer has to do with plate thickness. The other part is the scientists are still learning how active the mantle really is. – What forces dominate plate motion?
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