Plate Tectonics

What is Plate Tectonics? According to the plate tectonics theory, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer. This layer is known as the lithosphere A plate is one of numerous rigid sections of the lithosphere that move as a unit over the material of the asthenosphere

Types 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.

Plate Boundaries

Divergent Boundaries Oceanic Ridges and Seafloor Spreading 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.

Spreading Center

Actions at Divergent Boundaries CONTINENTAL RIFTS When spreading centers develop within a continent, the landmass may split into two or more smaller segments, forming a rift.

East African Rift Valley

Actions at 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.

Oceanic-continental Plate Boundary

Actions at Convergent Plate Boundaries 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.

Oceanic-Oceanic Convergence

Actions at Convergent Boundaries CONTINENTAL-CONTINENTAL When subducting plates contain continental material, two continents collide. This kind of boundary can produce new mountain ranges, such as the Himalayas.

Continental-Continental Convergence

Collision of India and Asia

Actions at Transform Boundaries  At a transform fault boundary, plates grind past each other without destroying the lithosphere. TRANSFORM FAULTS Most join two segments of a mid-ocean ridge. At the time of formation, they roughly parallel the direction of plate movement. They aid the movement of oceanic crustal material.

Transform Faulting at a Divergent Boundary

Evidence for Plate Tectonics Magnetic Pole Reversals The mid-ocean ridge is a mountain range at the bottom of the ocean that is composed mainly of volcanoes and lava flows. See map p. 5 ESRT  The mid-ocean ridge is a mountain range at the bottom of the ocean that is composed mainly of volcanoes and lava flows. See map p. 5 ESRT The rocks created at the mid-ocean ridge have magnetic minerals that are aligned with Earth’s magnetic field. Earth’s magnetic field reverses polarity on a cycle of thousands of years.

Evidence of Plate Tectonics  The discovery of strips of alternating polarity, which lie as mirror images across the ocean ridges, is among the strongest evidence of seafloor spreading.

Polarity of Ocean Crust

Evidence of Plate Motion Seafloor Spreading The data on the ages of seafloor sediment confirmed what the seafloor spreading hypothesis predicted. The youngest oceanic crust is at the ridge, and the oldest oceanic crust is at the continental margins (away from the ridge).

Evidence of Plate Tectonics HOT SPOTS A hot spot is a concentration of heat in the mantle capable of producing magma, which rises to Earth’s surface; The Pacific plate moves over a hot spot, producing the Hawaiian Islands. Hot spot evidence supports that the plates move over the Earth’s surface.

Ring of Fire

Passive Continental Margin 5 main features 1.Coast – edge of continent 2.Continental Shelf – wide shallow extension of continent 3.Continental Slope – steepest gradient drop toward ocean floor 4.Continental Rise – collection of sediment from turbidity currents 5.Abyssal plain – ocean floor, flattest places on Earth

Passive Continental Margin – simple Trailing Edge of Continent

Active Continental Margin Subduction Zone Subduction is the sinking of the denser crust out of the two plates that are colliding. See map p. 5 ESRT Ocean trenches are the deepest parts of the oceans. created at subduction boundaries by bending down of the subducting plate and warping of the crust created at subduction boundaries by bending down of the subducting plate and warping of the crust

Active Continental Margin Subduction Zone Continental Volcanic Arcs form on the edge of the continental plate due a combination of factors: 1.Friction between plates Subducting slab Overriding slab 2.Increased temperature at depth 3.Lower density magma rises through denser solid rock

Active Continental Margin

Volcanoes created at subduction boundaries due to the melting of the subducting plate at great depth the rock becomes magma and rises because it is now less dense than the surrounding rock the rock becomes magma and rises because it is now less dense than the surrounding rock continental volcanic arcs form on the edge of a continent that the plate is subducting beneath continental volcanic arcs form on the edge of a continent that the plate is subducting beneath Examples: Andes in South America, Cascades in United States Examples: Andes in South America, Cascades in United States volcanic island arcs form on a plate boundary where an oceanic plate is colliding with another oceanic plate and the denser (older) plate is subducting below the less dense (younger) plate volcanic island arcs form on a plate boundary where an oceanic plate is colliding with another oceanic plate and the denser (older) plate is subducting below the less dense (younger) plate Example: Mariana, Aleutian, and Tonga Islands Example: Mariana, Aleutian, and Tonga Islands

Typical Volcano

Hot Spots are areas where volcanism occurs…NOT at plate boundaries where a mass of hotter than normal mantle material called a mantle plume rises toward the surface where a mass of hotter than normal mantle material called a mantle plume rises toward the surface Examples: Hawaiian Islands, Yellowstone, Iceland, Canary Islands See Tectonic Plates p. 5 ESRT

Hot Spot

Causes of Plate Motion CONVECTION CURRENTS WITHIN THE EARTH Scientists generally agree that convection occurring in the mantle is the basic driving force for plate movement. Convective flow is the motion of matter resulting from changes in temperature

Mantle Convection Mantle plumes are masses of hotter-than- normal mantle material that ascend toward the surface, where they may lead to igneous activity. The unequal distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion

Mantle Convection