Lecture #28: Plate Tectonics
Plate Tectonics - is the paradigm that the Earth's lithosphere is divided into a dozen major plates which slide over the asthenosphere in various directions. Because of the motion of the plates they can collide, pull apart, or scrap against each other. Each type of interaction causes a characteristic set of earth structures or "Tectonic" features. The word, tectonic, refers to the deformation of the Lithosphere as a consequence of plate interaction. The dozen major plates are as follows:
North American -------- South American
Pacific ------------------- Cocos
Nazca -------------------- Antarctic
Caribbean --------------- African
Eurasian ----------------- Australian-Indian
Philippine Sea ---------- Arabian
Plate Interaction - The plates interact with each other along three types of plate boundaries:
Convergent Plate Boundaries - these boundaries are found where two plates run into each other just like two cars hitting head on in an accident. The type of tectonic features which form depend on the types of lithosphere involved in the collision. There are three major types of convergence zones.
Ocean-Ocean Convergence - Oceanic lithosphere is relatively dense and can easily sink into the Earth's mantle. When two oceanic lithospheres collide, one runs over the other which causes the latter to sink into the mantle along a zone called a subduction zone. The subducting lithosphere is bent downward to form a very deep depression in the ocean floor called a trench. The deepest ocean in the world is found along trenches. Often andesitic magma forms within the heated lithosphere down in the subduction zone. This magma makes it way to the surface to erupt in a chain of volcanoes called an island arc which forms in the over riding plate. The inner wall of the trench toward the arc, has a very complex structure called the subduction complex where underthrusting adds many slices of ocean floor rocks to the inner wall of the trench. Adding slices to the bottom of the subduction causes the complex to lift upward and a forearc basin forms between the complex and the volcanic arc.
Ocean-Continent Convergence - Continental lithosphere is less dense than oceanic lithosphere so the latter always subducts beneath the former. Andesitic magma rises from the subduction zone to form a magmatic arc on the continental crust. For example, the Andes mountains is a magmatic arc on the South American continental crust which formed as the Nazca plate is subducted below South America. Hot magma rising upward from the subduction zone thickens the continental crust and probably makes it somewhat weaker and more mobile than cold crust. It is within this hot mobile zone that regional metamorphism takes place. On the landward side of the arc (the back arc) thrusting of the hot mobile core takes place over cold lithosphere of the continental interior.
Continent-Continent Convergence - Continental lithosphere does not easily subduct so that when two continents collide the two continents weld together in a suture zone. The continent crust is often thickened because one continent slides a little way under the other. The result is a mountain range in the interior of a continent. The Himalaya Mountains formed in this manner.
Backarc spreading - If an island arc forms a short distance away from a continent, it may eventually move farther away from the continent by a process called backarc spreading. The Sea of Japan apparently formed by backarc spreading where it is believed that a large blob of hot mantle rose and spread to widen the backarc region. Backarc spreading may occur in a continent behind a magmatic arc. Such spreading will thin a continental crust and create a region of high heat flow as is the case for the Basin and Range in Nevada.
Divergent Plate Boundaries - these boundaries form where plates are breaking apart to move away from each other. The breakup of lithosphere starts in the continental portion of lithospheric plates, a process now taking place along the east Africa Rift Zone.
Continental Rifts - When a supercontinent such as Pangaea breaks up, a diverging boundary can be found in the middle of a continent. In the early stages of continental breakup, the center of a continent is elevated which stretches the crust, making it thinner above the uplift. Tension produces normal faults on either side of a rift valley which is a central graben. Central Africa has a rift valley which is marked by high heat flow and basaltic volcanism. As divergence continues, the continental crust on the upper part of the lithospheric plate is infilled with basalt to form oceanic crust.
Mid-ocean Ridge - As divergence continues the rift valley becomes part of the oceanic crust. The Red Sea, between Africa and Arabia is a narrow sea of newly formed oceanic crust. Basalt continues to erupt where heat flow is the highest between two lithospheric plates. As sea floor spreading continues, the rift valley is lifted to become the mid-ocean ridge.
Transform Boundaries - these boundaries are found where plates are sliding past each other along one fault or a group of faults. The motion of the two plates is parallel but in opposite directions so that the boundary is a shear zone but neither plate is gaining or loosing area. The San Andreas fault in California is a transform boundary between the Pacific and North American Plates.