Topic 3: PLATE TECTONICS: The Unifying Earth Model Plate Tectonics is a model that unifies many explanations of earth processes, from sedimentation patterns to mountain ranges to mineral resources. The model is introduced early in Geosc 71, so that it may be used as a framework for subsequent lectures. You should become familiar with the basic concepts and terms as introduced below, but need not be concerned at this point if some details remain incompletely understood. Your understanding will improve as more info is given in lectures that follow. You might want to return to this section from time to time, as you learn more about related topics. What is Plate Tectonics? Plate Tectonics is the "paradigm" (Thomas Kuhn's word for a revolutionary concept that changes the course of future work in a particular field) that the Earth's lithosphere is divided into a dozen major plates which slide over the asthenosphere in various, but non-random, directions. What is lithosphere?: the crust, and strong upper part of the mantle, roughly 100 km thick. What is asthenosphere?: the weak layer of mantle underlying the lithosphere. What is crust, mantle, and core? Check your textbook; you must know these terms. To continue the story of the shifting plates: because of the relative motion of these plates, they can collide, pull apart, or scrape against each other. Each type of interaction causes a characteristic type of deformation, resulting in the formation of specific "structures". The word "tectonic" (with the same root as "architecture") refers to the deformations 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 Australiahn-Indian Philippine Sea Arabian Try to locate these on a map. Check text p.17; your lab manual may have a better map with more details. --------------- More terms to know (maybe more than you want to know!): Plate Interaction - The plates "interact" with each other along three types of plate boundaries, discussed below: Convergent Plate Boundaries - where two plates run into each other, like a halfback meeting a linebacker head-on. Something has to give. The type of tectonic features that form depend on the types of lithosphere involved in the collision. There are three major types: Ocean-Ocean Convergence - Oceanic lithosphere is relatively dense and can sink into the Earth's mantle. When two oceanic lithosphere plates collide, one overrides the other, deflecting the lower plate downward and causing it to sink into the mantle. The resulting contact zone is called a subduction zone. The subducting lithosphere plate is bent downward to form a very deep depression in the ocean floor, called an oceanic trench. The deepest parts of the world's oceans are found along these trenches. Often, magma is generated within lithosphere near the subduction zone. This magma, commonly andesitic in composition, rises toward the surface to erupt in a chain of volcanoes, some submarine; over time an arcuate chain of islands (an island-arc) is formed on top of the over-riding plate. The "inner" wall of the trench (toward the island arc), can form a very complex structure called the "subduction complex", where "underthrusting" (see glossary) plasters many slices of ocean floor rocks to the inner wall of the trench. Adding slices to the bottom of this upper plate in the subduction zone causes the "subduction complex" to lift upward. Thus, a "forearc basin" is formed between the two "high" areas -- the subduction complex, and the volcanic island arc. The above explains why island areas such as Indonesia, or the Caribbean, have two sets of islands, one with volcanoes, and the other, outer ones, with complex thrust faults. Ocean-Continent Convergence - Continental lithosphere is less dense than oceanic lithosphere, so the oceanic plate always subducts beneath the continent when the two plates collide. Magma, usually andesitic, rises from deep in the subduction zone (under the continental plate) to form a string of volcanoes (a magmatic arc) on the continental crust. Thus, the Andes mountain range is a magmatic arc on the South American continental crust, which formed as the Nazca oceanic plate was (and is) subducted below South America. Hot magma rises upward from near the subduction zone. The new material thickens the continental crust and makes it somewhat weaker and more plastic than cold crust. It is within this hot, mobile zone that regional metamorphism takes place. On the landward side of the magmatic arc (the back arc), compression and thrusting of the hot mobile core takes place, pushing plastic core material over colder lithosphere of the continental interior. Continent-Continent Convergence - Low-density continental lithosphere does not easily subduct, so that when two continents collide the two continents may more or less "weld" together in a suture zone. The continent crust may be thickened, because one continent plate can slide (grudgingly) a little way under the other. The result is a mountain range or high plateau in the interior of an enlarged (accreted) continent. The above explains why some of these mountains are so high. The Himalaya Ranges apparently formed in this manner. Backarc spreading - If an island arc forms a short distance away from a continent (by ocean-ocean convergence), it may eventually be moved even farther away from the continent by a process called "backarc spreading". The Sea of Japan is one example. It is believed that a large blob of hot mantle rose and gradually spread (by the sea-floor spreading mechanism), to widen the backarc region. Backarc spreading may also 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 province in Nevada and adjacent states. --------------- Divergent Plate Boundaries - these boundaries form where plates are breaking apart to move away (diverge) from each other. The breakup of lithosphere plates typically starts in the continental portion of lithospheric plates, a process apparently now taking place along the East Africa Rift Zone. Relic rift features similarly formed near Harrisburg in early-Mesozoic time, just before the Atlantic Ocean was formed by splitting a supercontinent (North America plus Africa, together). Continental Rifts - When a "supercontinent" (a collection of attached continents) such as Pangaea begins to break up, a "diverging boundary" can be found in its interior. In the early stages of continental breakup, the center of a continent becomes elevated; this elevation stretches the crust, making it thinner above the uplift. Tension produces normal faults (see topic discussion on structural geology), on both sides of a rift valley; the down-dropped block is called a graben. Central Africa has a rift valley which is marked by high heat flow and basaltic volcanism. If the lithospheric plates continue to move apart, the intervening space may ultimately be infilled with basalt to form new oceanic crust. This process is now occurring under the Red Sea. Mid-Ocean Ridge - As divergence continues the rift valley becomes part of the oceanic crust. As mentioned above, 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 may become elevated (relative to the adjacent (sunken) ocean floor to become the mid-ocean ridge (MOR). ------------------- Transform Boundaries - boundaries found where lithosphere plates are sliding past each other, along one fault or a system of faults. The relative motion of the two plates is parallel but in opposite directions. Thus the boundary is a shear zone, but neither plate is gaining or losing area. The San Andreas fault system in California is a transform boundary between the Pacific and North American Plates. Similar faults are found elsewhere, in New Zealand, Nicaragua, etc. Transforms also offset parts of the MOR.