The Theory of Plate Tectonics

1. The Theory of Plate Tectonics

3. Plate Tectonics • Plate tectonics is the theory that explains why and how continents move. • By the 1960s evidence supported continental drift which led to the development of plate tectonics.

5. How Continents Move • Oceanic crust is a dense crust made of rock that is rich in iron and magnesium. • Continental crust is low-density crust made of rock that is rich in silica. • Tectonic plates are blocks of earth’s shell that ride on a deformable layer of the mantle. • The lithosphere is the solid outer layer of earth, that consists of the crust and the grid upper part of the mantle.

7. More About How Continents Move • Asthenosphere is the solid, plastic layer of the mantle beneath the lithosphere. • Plastic rock is a solid rock under great pressure that flows slowly, like putty. • The boundaries of the tectonic plates are not always easy to identify because the continents and oceans are carried on tectonic plates in the same way that passengers are carried by a bus.

10. Tectonic Plates • Scientists identified 15 major tectonics plates. • The boundaries of the tectonic plates are not always easy to identify because, they don’t always match the familiar outlines of continents and oceans. • Scientist identify plate boundaries by studying data from earthquakes. • A sudden movement along the boundary of a tectonic plate is an earthquake. • Frequent earthquakes in a given zone is evidence that two or more plates may meet in the area. • Volcanoes help identify the locations of plate boundaries because some volcanoes form when plate motions generate magma that erupts on earth’s surface.

12. More About Tectonic Plates • A zone of active volcanoes that encircles the Pacific ocean is known as the Pacific Ring of Fire. • Earthquakes also occur frequently in the Pacific Ring of Fire. • The characteristics of the Pacific Ring of Fire indicates that the Pacific Ocean is surrounded by plate boundaries.

14. Types of Plate Boundaries • Divergent is the boundary between tectonic plates that are moving away from each other. • Convergent is the boundary between tectonic plates that are colliding. • Transform is the boundary between tectonic plates that are sliding past each other horizontally. • Mid-ocean ridge is an undersea mountain rage. • Subduction zone is a region where one plate moves under another. • Fracture zones are short segments of a mid-ocean ridge that are connected by transform boundaries.

18. Plate Boundaries • The three areas where plate boundaries may be located are in the middle of the ocean floor, around the edges of continents, and within continents. • The magma at divergent boundaries rises to the surface as plates move apart. • Warm and light rock forms when magma cools to form new oceanic lithosphere. • A narrow area that forms where the plates at a divergent boundary separate is called a rift valley. • Most divergent boundaries located on the ocean floor. • An example of a rift valley is the Red sea, between the African and Arabian plates.

19. And More About Plate Boundaries • When oceanic lithosphere collides with continental lithosphere, the oceanic lithosphere is less dense than the continental lithosphere, so it sinks or subducts. • Deep-ocean trenches form at subduction zones. • As the oceanic plate subducts, it release fluids into the mantle, causing magma to form and rise to the surface, forming volcanic mountains. • When two plates made of continental lithosphere collide, the colliding edges crumple and thicken, forming large mountains ranges. • The Himalalya mountains is an example of a large mountain range formed when two plates made of continental lithosphere collided. • When two plates made of oceanic lithosphere collide, one plate subducts under the other, forming a deep-ocean trench. Fluids from the subducted plate cause mantle rock to melt and form magma.

20. And it goes on, and on, and on • A chain of volcanic islands called an island arc is produced from magma formed from melted mantle rock. • An example of a feature that formed when two plates made of oceanic lithosphere collided is Japan. • As plates slide past each other horizontally, they scrape against each other causing earthquakes at transform boundaries. • The transform boundaries are different from other types of boundaries, because they do not produce magma. • An example of a transform boundary is the San Andreas Fault. • The San Andreas Fault is located between North American and Pacific plates. • Transform boundaries that connect short segments of a mid-ocean ridge are called fracture zones.

21. I throw my hands up in the air some times.. • An example of a convergent boundary is the Chilean trench along the west coast of South America. • An example of a divergent boundary in the mid-Atlantic is the boundary of the North American and Eurasian Plates.

22. Causes of Plate Motion • The movements of heated material due to differences in density is called convection. • The cycle in which the cooler, denser water sinks and the warmer water rises to the surface to create a cycle is called a convection cell. • Earth’s mantle is heated by core energy and radioactivity. • Tectonic plate movement is caused by the mantle drags overlying tectonics along.

23. More Causes of Plate Motion • As Newer, warmer rock cools at a mid-ocean ridge, it sinks into the mantle and pulls away from the ridge. • The force on the rest of the plate from the asthenosphere below cooling, sinking rock is called ridge push. • As a result of ridge push, it pushes the rest of the plates away form the mid-ocean ridge. • Scientists studied convergent boundaries for clues to forces that drive plate motion. • Magma rises to the surface in places where plates pull away from each other at mid-ocean ridges. • The force exerted by a sinking plate caused by the subduction of lithosphere into the asthenosphere is called slab pull.

24. Saying A-Oh • Compared to speed of plates that are not subducting, plates that are subducting move faster. • The three forces that work together to cause plate motions are: drag on the bottoms of tectonic plates, ridge push, and slab pull.