Plate Tectonics 2
Plate Structure
•The various plate margins leave their mark on the plates they define. This helps geologists piece together both the present and past geology of the regions.
•Plates bordering a subduction zone have specific features depending on whether both plates are oceanic or one is a continental plate. Features in the first case include a trench, a melange of piled sediments (a zone of high-pressure, low-temperature metamorphism), a volcanic island arc (Japan is one of these), and a back-arc basin (the Sea of Japan). In the second case, the back-arc basin is absent and the island arc is replaced by a volcanic mountain range (such as the Cascades).
Ocean-ocean plate convergent boundary.
Structure of a continent-ocean convergent boundary.
Continent-continent collision.
•Plates bordering a mature zone of extension (a divergent boundary) are commonly oceanic plates. The structure includes the mid-ocean ridge, which is a line of mountains with a rift valley in the middle where magma extrudes to the surface. The crust slopes away from the ridge, decreasing in height and increasing in age with distance from the ridge. Along the ridge are transform faults. These are the most predictable (and likely least dangerous) volcanoes in the world.
Mid-ocean ridge divergent boundary showing transform faults.
•Plates at a new zone of extension can also be continental plates. Upwelling of magma underneath a continental plate first causes a bulge, then a rift zone with fault blocks. Once the ridge sinks below sea level, the zone is mature and a mid-ocean ridge results.
Two stages of an opening rift. The top image shows the second stage of the rift zone, while the bottom image shows the next stage, in which the rift has sunk below sea level.
•Plates defining a transform fault can be of any type. Typically the features on either plate consist of a main fault and a network of lesser faults.
•In some areas plate motions can raft small fragments of crust tremendous distances. Eventually, any fragment that is not consumed by subduction will be added to a larger continental mass (accreted). These fragments are called terranes, and they are geologically transient. Their ultimate fate is to be accreted to a larger continental mass.
Portions of accreted terrane on the Pacific plate.
•Examples of this type of tectonics include the islands of Taiwan and the Philippines. Other fragments form when they are sliced off the margin of a large continent; this is the case with the western side of the San Andreas Fault.
What Drives Plate Tectonics?
•Although the driving motion of plate tectonics is not known for certain, scientists believe the mechanism is related to thermal convection in the Earth's mantle.
•Thermal convection is a process in which hot material within the mantle rises toward the surface due to buoyancy (it is hotter and therefore less dense). As it rises, it cools and spreads. When it cools enough, it begins to sink.
•This convection forms thermal cells that act as conveyor belts, moving the plates along. Divergent plates form at the rising portions of these cells, while subduction zones occur where the cells sink.
•Cells may be confined to the aesthenosphere, may involve the entire mantle, or may be more complex, with thermal plumes rising from the core-mantle boundary.
Upper mantle convection cells.
Full-mantle convection cells.
•Another force that might be at work is called slab-pull or ridge-push.
•In slab-pull, subducting slabs are pulled down because of their greater density. They take the rest of the plate along with it. In ridge-push, the upwelling magma pushes the extending crustal plates out of the way as it extrudes.
•It appears that all three of these forces are at work in plate motion, but the contribution of each is currently unclear.
Turning Back the Clock: Past Plate Motions
Determining past plate positions
•Past plate positions must be reconstructed from the evidence that remains from those time periods.
•Magnetic anomalies along the mid-ocean ridges (discussed earlier) can be reconstructed —essentially pushing the plates back together — to trace past plate motion. However, this method only works so far, as the plates are not only created but destroyed (at subduction zones), thus erasing previous magnetic stripes.
•Continental fit can also be used to piece together previous continental (and thus plate) positions. Regions where the continents do not fit together perfectly can often be explained through subsequent erosion or other activity (the formation of deltas and deep-sea fans, creation of volcanic rocks at hot spots and so on).
•Portions of poor fit can also be the result of erosion of regions that were deposited from other plates (sedimentary basins and such).
•Paleomagnetism also works in volcanic regions away from the mid-ocean ridge, and can be very useful when occurring on the lighter, more long-lived continental crust.
Putting it together
•Evidence suggests that the mechanism or mechanisms that drive plate tectonics have been operating for at least 2 billion years and maybe longer.
•The cycle of supercontinent formation and breakup is known as the Wilson Cycle.
•1 billion years ago — The supercontinent Rodinia broke up along western and eastern N. America.
•450 million years ago — N. America collided with Europe.
•250 million years ago — Pangaea formed when N. America and Europe collided with Gondwanaland (S. America, Africa, India, Antarctica, Australia).
The Earth as scientists have reconstructed it 250 million years ago.
•180 million years ago — The beginning of the current state of the continents. A complicated series of events occurred, in which N. America and Africa separated, and Africa and India began their collision with Eurasia.
•20 million years ago — the remains of the Tethys Sea (small gaps between Eurasia and Africa) closed.
•6 million years ago — Mediterranean Sea was cut off from the Atlantic.
•What's next — Assuming, as we must, that the plates continue their current speed and direction of movement, the next 50 million years will see Australia crush the islands of Indonesia in its move towards Asia, the disappearance of the Mediterranean and the opening of a new ocean at the East African Rift Zone. California will continue its move towards the Aleutian Islands.
Australia moves north in this projection of the state of the continents 110 million years from now.
Resources
•Subduction zones are regions where hot fluids are percolating through igneous and metamorphic rock. These are ideal conditions for the deposition of heavy precious metals such as gold and silver.
•Sedimentary resources like gypsum and salt are most likely deposited in geologically quiet regions such as the continental interiors.
Parental Moment:
Gordon-Michael Scallion has predicted that three large earthquakes, each one larger than the one that preceded it would strike the LA area. The third of these great quakes, which Scallion says will measure about 8.3 on the Richter scale (+/- .5), will initiate the breaking up of America. According to Scallion, the first two of these predicted LA quakes have already taken place: A 6+ on April 22, 1992 and the 7.5 Landers Earthquake which took place on June 28, 1992. Scallion is predicting the third one will occur before
December of 1995. When the third earthquake occurs, a three stage land fracture and break-up of the entire western portion of America will begin.
According to Scallion, the cause of this massive break-up is a very large magma bubble that is pushing up beneath the United States. This magma bubble is caused by a
build up of ice at the poles. The ice build-up at the poles is causing an instability in the Earth's rotation which, in turn, is creating instability in the earth's magma and core.
A third fracture will occur. This time vast sections of Utah, Nevada, Arizona, and Colorado will be swallowed up by the ocean. Phoenix, now a coastal city, will eventually emerge as a major seaport city. A very large seaport city will also be established in the Nebraska region. Denver and Sedona will become coastal regions.