Plate Tectonics

Transform Faults

Although oceanic spreading ridges appear to be curved on Earth's surface, the ridge are composed of a series of straight-line segments, offset at intervals by faults perpendicular to the ridge.

Asthenosphere

Hot and weak, mostly solid- some molten rock, like silly puddy. Deforms easily, deeper, hotter under more pressure.

Ridge-push/Slab-pull

(a) plates that are attached to subducting slabs (e.g., Pacific, Australian, and Nazca Plates) move the fastest, and plates that are not (e.g., North American, South American, Eurasian, and African Plates) move significantly slower; (b) in order for the traction model to apply, the mantle would have to be moving about five times faster than the plates are moving (because the coupling between the partially liquid asthenosphere and the plates is not strong), and such high rates of convection are not supported by geophysical models; and (c) although large plates have potential for much higher convection traction, plate velocity is not related to plate area. In the ridge-push/slab-pull model, which is the one that has been adopted by most geologists working on plate-tectonic problems, the lithosphere is the upper surface of the convection cells, as is illustrated in Figure 10.29.

Continent-Continent Convergent Boundary

A continent-continent collision occurs when a continent or large island that has been moved along with subducting oceanic crust collides with another continent (Figure 10.23). The colliding continental material will not be subducted because it is too light (i.e., because it is composed largely of light continental rocks [SIAL]), but the root of the oceanic plate will eventually break off and sink into the mantle. There is tremendous deformation of the pre-existing continental rocks, and creation of mountains from that rock, from any sediments that had accumulated along the shores (i.e., within geosynclines) of both continental masses, and commonly also from some ocean crust and upper mantle material.

Oceanic Crust

A lot thinner than continental, more like lava rock, basaltic (mafic). Denser than continental, will sink faster.

Mantle Plum/Hotspot

A place where hot mantle material rises in a stationary and semi-permanent plume and affects the overlying crust.

Rules of tectonic Plates

Boundaries either terminate against other boundaries or encircle the entire Earth, they do not simply end. Motion of a plate is generally consistent throughout, motions can always be described as rotations about axes that pass through the center of spherical Earth, Plates move as rigid blocks. Rate of crustal consumption (at convergent boundaries) must equal rate of crustal creation (at divergent boundaries), or the Earth would expand. Transform boundaries link different boundary types and are only active where opposite sides of the transform are moving relative to each other. Everything is in motion, plates and boundaries- moving to a fixed hotspot reference frame.

Convection of the Mantle

Critical to plate tectonics. Without convection there would be no ridges to push from because upward convection brings hot buoyant rock to surface.

Lithosphere

Crust+Mantle, it is the plate.

Important physical features of the ocean floor

Extensive linear ridges (commonly in the central parts of the oceans) with water depths in the order of 2,000 to 3,000 m Fracture zones perpendicular to the ridges Deep-ocean plains at depths of 5,000 to 6,000 m Relatively flat and shallow continental shelves with depths under 500 m Deep trenches (up to 11,000 m deep), most near the continents Seamounts and chains of seamounts

Plate Tectonics

Idea that the crust can be divided into a series of rigid plates.

Plates

Made up of crust and the lithospheric (upper) part of the mantle. Even though they are moving all the time there is never a significant amount of space between them. Thought to move along the lithosphere-asthenosphere boundary, as the asthenosphere is the zone of partial melting. Assumed that the relative lack of strength of the partial melting zone facilitates the sliding of the lithospheric plates. Plates include both crustal material and lithospheric mantle material which makes it possible for a single plate to be made up of both continental crust and oceanic.

Processes taking place at divergent boundaries

Magma from the mantle pushing up to fill the voids left by divergence of the two plates. Pillow lavas forming where magma is pushed out into seawater. Vertical sheeted dykes intruding into cracks resulting from the spreading. Magma cooling more slowly in the lower part of the new crust and forming gabbro bodies.

Hess' Idea

New sea floor was generated from mantle material at the ocean ridges, and that old sea floor was dragged down at the ocean trenches and re-incorporated into the mantle. That the process was driven by mantle convection currents, rising at the ridges and descending at the trenches. That the less-dense continental crust did not descend with oceanic crust into trenches, but that colliding land masses were thrust up to form mountains. Formed the basis for our ideas on sea-floor spreading and continental drift, but it did not deal with the concept that the crust is made up of specific plates.

Ocean-Ocean convergent Boundary

One of the plates (oceanic crust and lithosphere) is pushed, or subducted under the other. Usually it is the older and colder plate that is denser that subducts. There is commonly an ocean trench along the boundary. The significant volume of water within the subducting material is released as the subducting crust is heated. This water is mostly derived from alteration of pyroxene and olivine to serpentine near the spreading ridge shortly after the rock's formation. It mixes with the overlying mantle, and the addition of water to the hot mantle lowers the crust's melting point and leads to the formation of magma (flux melting). The magma, which is lighter than the surrounding mantle material, rises through the mantle and the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands known as an island arc. Earthquakes take place at the boundary between the subducting crust and overriding crust. Largest ones occur near the surface where the subducting plate is still cold and strong. Density determines which will subduct. The colder plate will be denser than the hotter plate.

Divergent Boundaries

Spreading boundaries where new oceanic crust is created from magma derived from partial melting of the mantle caused by decompression as hot mantle rock from depth is moved toward the surface. Most boundaries are located at the oceanic ridges (although some are on land) and the crustal material created at a spreading boundary is always oceanic, mafic igneous rock. Mountains at divergent boundaries because the crust at the boundary is less dense.

Rift Valley Formation

Spreading is thought to start within a continental area with up-warping related to an underlying mantle plume. When a series of mantle plumes exists beneath a large continent, the resulting rifts may align and lead to the formation of a rift valley, which eventually develops into a linear sea and finally into an ocean.

Continental Crust

Tends to be thicker, similar to granite composition.

Ocean-Continent Convergent Boundary

The oceanic plate is pushed under the continental plate in the same manner as at an ocean-ocean boundary. Sediment that has accumulated on the continental slope is thrust up into an accretionary wedge, and compression leads to thrusting within the continental plate. The mafic magma produced adjacent to the subduction zone rises to the base of the continental crust and leads to partial melting of the crustal rock. The resulting magma ascends through the crust, producing a mountain chain with many volcanoes. No subduction.

Important ideas about Plate Tectonics

The outermost layer of the Earth is broken up into tectonic plates. Tectonic plates move relative to each other. The boundary between two tectonic plates can be convergent, divergent or transform. The movement and interaction of tectonic plates helps to explain many of Earth's features.

Transform Boundaries

Transform boundaries exist where one plate slides past another without production or destruction of crustal material. As explained above, most transform faults connect segments of mid-ocean ridges and are thus ocean-ocean plate boundaries (Figure 10.15). Some transform faults connect continental parts of plates. Transform faults do not just connect divergent boundaries.

Convergent Boundaries

Where two plates are moving toward each other. Three types: ocean-ocean, ocean-continent, continent-continent.