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Chapter 4 – The theory of plate tectonics Geologists surmise that the earth’s crust is made up of ten or more rigid pieces of lithosphere called plates • These crustal plates or slabs move with respect to one another as they glide on the hot, plastic asthenosphere • Remember that the athenosphere is that part of the mantle that is solid, but also somewhat plastic so it flows very slowly. • Plates move around the earth’s surface and sometimes collide with or slide past each other. Plates can also break or tear apart. • There are three types of plate boundaries: • convergent boundaries (plates move towards one another) • divergent boundaries (plates move away from each other) • transform boundaries (plates slide past one another). • Nothing on earth is fixed or static; every part of the earth is moving although normally at a rate so slow as to go unnoticed by humans. • Not only are the plates moving, but plate boundaries also move or migrate • Ridge crests (divergent boundaries) can migrate across the earth’s surface • Convergent boundaries can also migrate (subduction stops in one place and starts in another) • Transform boundaries (for example: San Andreas fault where two plates are sliding past one another) can also change positions over geologic time. The fault has been in its present position for only about 5 M yrs. Before that, the fault was located further west. The 1992 Landers earthquake in California took place on a new fault located further west in the Mojave Desert. This suggests the fault is trying to migrate further inland. If this is so, more of California may be attaching itself to the Pacific plate instead of the NA plate and will continue to slide northwestward relative to the rest of NA. The image below shows what geologists believe are the boundaries of the major lithospheric plates. Some plates consist of ocean floor and some contain both oceans and continents. For example, the South American plate contains both ocean and continental lithosphere and it’s eastern boundary lies far out in the ocean. Active continental margins define a plate boundary. Passive continental margins are not plate boundaries. • Plates can change in size, growing larger or smaller, such as when new sea floor is being created at a ridge zone. Plates can grow smaller when crust is subducted or when a plate splits apart to become two at a divergent zone. • The leading edge of the NA plate (western edge of NA) is not being subducted because it is made up of lightweight continental rock. More NA plate material is being created at the mid-Atlantic ridge; therefore, the NA plate is growing larger. • Continental lithosphere averages about 150 km (range 125 to 250 km) thick. • Old oceanic lithosphere at continental edges may be 100 km thick • Young oceanic lithosphere at mid-ocean ridges may be only 10 km thick. •Ocean basins exist because continental lithosphere is made of lighter density rock and therefore “floats” higher than oceanic lithosphere, which is made of denser rock. Passive margins accumulate sediments washed down from the adjacent continent and these sediments can, over time, form a continental shelf. Earthquake belts (red markings in map below) help define the position of plate boundaries. Divergent plate boundaries and sea-floor spreading. • A divergent boundary is usually a mid-ocean ridge and is a place where new sea floor is created by rising of the hot asthenosphere. As the asthenosphere rises it melts to form magma, which, because it is more buoyant that the surrounding rock, continues to rise and push apart existing rocks as it fills the magma chamber located in the crust below the ridge axis. •Magma that solidifies along the chamber sides makes gabbro, a coarse-grained igneous, intrusive mafic rock that is the chemical twin to basalt, a fine-grained extrusive rock that solidifies at or above the surface. •Basalt is an extrusive igneous rock that cools very quickly upon reaching the surface and hitting the ice cold sea water. There are a peculiar kind of basalt formation that is found on the sea floor called “pillow basalt”. As it is created, the new oceanic crust moves away from the ridge on both sides to allow room for still more crust to form. As the oceanic crust and upper mantle move away from the ridges, it cools, becomes denser, sinks and becomes part of the lithosphere (cool rock is denser than warm rock). Thus, the sea floor grows older (green) with increasing distance from the ridge axis with the oldest sea floor found near the continental margins and the youngest sea floor (dark red) found directly over the ridge axes. The concept of subduction. Two plates converge and one plate is forced to bend and subduct beneath the overriding plate. At convergent plate boundaries, the downgoing plate grinds along the base of the overriding plate, generating earthquakes. In a continental-oceanic convergence, the downgoing plate is always the denser, heavier, oceanic slab, never the continental lithosphere because the continental slab is too buoyant to subduct, being made of lighter, silica-rich rocks. Because oceanic lithosphere is constantly being returned to the mantle via subduction zones, no oceanic lithosphere is greater than 200 million years old. Continental lithosphere, however, can be several billion years old. A chain of volcanoes can develop behind the accretionary prism (wedge). If an oceanic plate is being subducted beneath a continental plate, this arc is called a continental volcanic arc (the Andes in S.A. and the Cascade Mtns in N.A.). If the subduction occurs between two oceanic plates, it is called a volcanic island arc. Example of a volcanic island arc. The Aleutian Islands, where two oceanic plates are involved in a subduction process. Transform faults are the actively slipping segment of a fracture zone between two ridge segments. At this kind of boundary, one plate slides sideways past another but no new plate is formed or consumed. A famous example of a continental transform fault is the San Andreas fault in California which defines part of the plate boundary between the North American Plate and the Pacific Plate. LA. is part of the Pacific Plate while east of the fault is part of the N.A. Plate. In 100 million years, LA. will be a suburb of Anchorage, Alaska. Transform faults can generate huge earthquakes as in the San Francisco Bay area in 1906 and 1989. A triple junction is where three plate boundaries intersect a point. Figure A shows a ridgeridge-ridge triple junction. Figure B shows a trench-transform-transform triple junction. • Hot spot volcanoes develop from rising mantle plumes. A mantle plume is a narrow • • • • column of hot mantle rock that rises through the mantle. A mantle plumes are thought to have spherical or mushroom shaped heads rising above a narrow tail Plumes form hot spots of active volcanism on the earth’s surface When the head of the plume nears the surface, it can cause uplift and the eruption of vast amounts of flood basalts As the head widens beneath the crust the flood basalt area widens and the crust is stretched The narrow tail that follows produces a narrow spot of volcanic activity Some scientists theorize that the outward flow of the expanding head of the plume may be strong enough to start plates moving Some geologists feel plumes located at the mid-Atlantic ridge are forceful enough to drive the NA plate westward Plumes are essentially stationary with respect to moving plates and to each other If plumes are located beneath a continent, they may eventually cause that continent to split apart • Yellowstone National Park in northwestern Wyoming is a place where a mantle plume may exist. • Evidence for this includes past volcanism in the area, high elevation of the region (uplift occurs when a plume is beneath), high heat flow, and hot spring and geyser activity • Radial flow of mantle rock beneath Yellowstone may also be causing earthquakes in the area • Eventually, the NA continent may split apart at this site, and a new ocean will form • Plumes may also rise beneath oceanic plates • The plume under Hawaii rises in the center of the Pacific plate • As the plate moves along the plume, a line of volcanoes form, creating an aseismic ridge • Eventually, the volcano is carried off the hot spot and becomes extinct (no new material is added to it). • Without the continuous addition of new rock by volcanic activity, weathering and/or subsidence will eventually cause inactive volcanic islands to disappear beneath the sea (seamounts and guyots) Hot spot volcanoes develop from rising mantle plumes underneath that portion of the plate. As the plate moves, the volcano is carried with it and off the hot spot to eventually becomes extinct. Over eons of geologic time, the process causes a chain of extinct volcanoes to develop. The oldest volcano is furthest from the hot spot while the youngest, still active volcanic island is directly over the hot spot. The extinct volcanoes gradually sink below sea level to become seamounts. • Divergent plate boundaries can occur in the middle of the ocean or in the middle of a continent • The result of divergence, ultimately, is the creation of new ocean basins • The breakup of the ancient continent of Pangaea was caused by divergent boundary activity • The divergent boundary is marked by rifting, basaltic volcanism, and uplift • During rifting, the continental crust is stretched and thinned, a process than can produce earthquakes and normal faulting • Some geologists feels rifting causes uplift • Others feel uplift is caused by the mantle plume first, then rifting happens Continental rifting. This occurs when the continental lithosphere thins and pulls apart to eventually separate and form a new continent. Continental rifting is the East African Rift zone. Oceanic plate – continental plate convergence • When an oceanic plate is subducted under the continental lithosphere, an accretionary prism (wedge) and forearc basin form an active continental margin between the trench and the continent • The Wadati-Benioff zone (named after the scientists who discovered it) is an earthquake zone caused by the subducting plate dipping under the edge of the continental plate • A belt of igneous activity called a magmatic arc (similar to an island volcanic arc if in the ocean) is created • An example of this on the NA continent would be the Cascade mountain range in the Pacific NW • The hot rising magma causes uplift and a mountain range is created by the thickened crust which rises isostatically • A second contributor towards mountain building is stacking up of thrust sheets on the continental side of the magmatic arc Continent – continent convergence In this case, two continental plates approach each other and eventually collide. A good example of this is the collision between the Indian plate and the Asian plate. • This happens when the sea floor that lies between them is subducted, and the ocean becomes narrower and narrower until it is all gone and the continents, which as we’ve learned cannot subduct, collide with each other. • One continent may slide a short distance beneath another, but it will not go down into a subduction zone because they are both too buoyant to be subducted into the mantle. • The two continents become welded together along a suture zone that marks the old site of subduction and quite large mountain ranges can form • An example of a mountain range formed in this kind of collision would be the Himalayan Mountains in central Asia. Continent-continent convergence. Collision causes mountain ranges to arise Crust beneath a mountain range is about 60-70 km thick, twice the thickness of normal crust. Plates move in response to ridge-push force and to slab-pull force. Ridge-push force develops because mid-ocean ridges are at a higher elevation than the adjacent abyssal plain. Gravity causes the elevated lithosphere at the axis to push on the abyssal plain lithosphere. Slab-pull force results from the more dense, older and cooler lithosphere being subducted into a less dense asthenosphere. Once a plate starts to sink, it pulls the rest of the plate along behind it. The velocity of plate motion varies from 1 to 15 cm per year. Geologists using GPS can now track plate motion in rates as small as millimeters per year. End of Chapter 4.