5 Plate Tectonics Theory The lithosphere is divided into a number of large and small plates and the plates are floating on the mantleLithosphere = the Earth’s crust plus the upper portion of the mantle layer
6 How Plates MoveA Canadian scientist, J. Tuzo Wilson, claimed the lithosphere is broken into separate sections called plates. These plates fit together along cracks in the lithosphere.Scientists realized that the continental drift idea could be explained by sea floor spreading. Wilson took what these scientists knew and combined it with his idea about Earth’s plates into a single theory.A scientific theory is a well-tested concept that explains a wide range of observations.
7 How Plates Move (Cont.)Sea floor spreading provides the driving mechanism for movementHowever, it is not the continents that are moving, but the “plates” of lithosphere “floating” in effect on the asthenosphereThe lithosphere is made up of about 20 plates which move relative to each other in several waysThe theory of plate tectonics states that pieces of Earth’s lithosphere are in slow, constant motion, driven by convection currents in the mantle. The theory of plate tectonics explains the formation, movement, and subduction of Earth’s plates.
11 “Plates” of lithosphere are moved around by the underlying hot mantle convection cells
12 ConvectionConvection is heat transfer by the movement of currents within a fluid. During convection, heated particles of fluid begin to flow. This flow transfers heat from one part of the fluid to another.Heat from the core and the mantle itself causes convection currents in the mantle.
15 Convection CurrentsMany geologists think that plumes of mantle rock rise slowly from the bottom of the mantle toward the top. The hot rock eventually cools and sinks back through the mantle. Over and over, the cycle of rising and sinking takes place.There are also convection currents in the outer core. These convection currents cause Earth’s magnetic field.
16 As the Earth’s plates move, they collide, pull apart, or grind past each other, producing spectacular changes in Earth’s surface. These changes include volcanoes, mountain ranges, and deep-ocean trenches.
17 Convergent boundaries form where two plates move together. Plate BoundariesTypes of Plate Boundaries Divergent boundaries (also called spreading centers) are the place where two plates move apart. Convergent boundaries form where two plates move together. Transform fault boundaries are margins where two plates grind past each other without the production or destruction of the lithosphere.
21 Oceanic Ridges and Seafloor Spreading Divergent boundaries:• Oceanic ridges are continuous elevated zones on the floor of all major ocean basins. The rifts at the crest of ridges represent divergent plate boundaries.• Rift valleys are deep faulted structures found along the axes of divergent plate boundaries. They can develop on the seafloor or on land.• Seafloor spreading produces new oceanic lithosphere.
23 Oceanic-Continental Convergent Boundaries A subduction zone occurs when one oceanic plate is forced down into the mantle beneath a second plate. Oceanic-Continental• Denser oceanic slab sinks into the asthenosphere.• Pockets of magma develop and rise.• Continental volcanic arcs form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent.• Examples include the Andes, Cascades, and the Sierra Nevadas.
25 Convergent Boundaries (Cont.) Oceanic-Oceanic• Two oceanic slabs converge and one descends beneath the other.• This kind of boundary often forms volcanoes on the ocean floor.• Volcanic island arcs form as volcanoes emerge from the sea.• Examples include the Aleutian, Mariana, and Tonga islands.
27 Convergent Boundaries (Cont.) Continental-Continental• When subducting plates contain continental material, two continents collide.• This kind of boundary can produce new mountain ranges, such as the Himalayas.