The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which “float” on the fluid-like asthenosphere. Due to convective currents in the asthenosphere, the tectonic plates move in different directions. The point where one plate meets another is known as a plate boundary; these areas are commonly associated with geological events and features such as earthquakes, mountains, volcanoes, and oceanic trenches. Plate boundaries are home to most of the world’s active volcanoes.
Tectonic plates are broadly divisible into two groups of plates: continental and oceanic. The distinction is based on the density of their constituent materials; oceanic plates are denser than continental plates due to their greater silicate mineral content. As a result, the oceanic plates are generally below sea level, the continental plates above.
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The left or right lateral motion of one plate against another along transform or strike-slip faults can cause highly visible surface effects. Because of friction, the plates can’t simply glide past each other. Instead, stress builds up in both plates, and when it reaches a level that exceeds the slipping-point of rocks on either side of the transform-faults, the accumulated potential energy is released as motion along the fault. The massive amount of energy released is the cause of earthquakes, a common occurrence along transform boundaries(see figure 3). (Tilling, 1985)
At divergent boundaries(see figure 2), two plates move apart from each other. The space that this creates is filled with new crustal material from the molten magma that forms below. The driving force that moves the plates apart is not fully understood. Two theories are the ridge-push and slab-pull hypotheses. In the ridge-push hypothesis, upwelling convective currents in the mantle bring hot material close to the Earth’s surface. As the material reaches shallow levels it starts to melt and is expelled at the divergent boundary, thus forcing the plates apart. The slab-pull hypothesis suggests that if one end of a plate is being subducted at a convergent boundary, the downward slab of material will release stress on the other end, pulling it away. (Mantovani, E. et all, 2001)
The birth of divergent boundaries is sometimes thought to be associated with phenomena known as hotspots. Giant convective cells bring large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be enough to break apart the lithosphere. It is believed that there is a hot spot located in the Mid-Atlantic Ridge system, which currently is under Iceland and widening at a rate of a few centimetres per year.
Divergent boundaries are shown in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge, and in the continental lithosphere by rift valleys such as the famous East-African Rift. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not parallel, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are fracture zones, a major source of submarine earthquakes. (Tilling, 1985)
Scientists found one of the most important pieces of evidence at the mid-ocearn ridges, forcing acceptance of the sea-floor-spreading hypothesis. Geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental, as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals, and the link was made. The magnetic banding directly corresponded with the Earth’s polar reversals. This was confirmed by measuring the ages of the rocks within each band. (http://www.nasca.org.uk/Strange_relics_/reversal/reversal.html, 2001)
The nature of a convergent boundary(see figure1) depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically pushed underneath, forming a subduction zone. At the surface, the topographic expression is commonly an oceanic trench on the ocean side and a mountain range on the continental side. Where two continental plates collide, the effect is for the plates to crumple and compress, creating extensive mountain ranges, such as the Himalayan range. When two oceanic plates converge they form an island arc as one oceanic plate is subducted below the other. Japan is a good example of this. (Noson, Qamar, and Thorsen, 1988)
The Continental Drift was first proposed in 1912 by Alfred Wegener, who noticed the similarity in the shape of the coasts of Africa and South America. His controversial and radical ideas were not taken seriously by geologists of the time, who pointed out that there was no visible or possible mechanism for continental drift.
This changed drastically in the 1960s, when Wegener’s theory was verified by a number of discoveries, most notably the Mid-Atlantic ridge. With plate tectonics evidence quickly falling into place, the answer became clear. Collisions of converging plates had the force to lift the seafloor into thin atmospheres. The cause of marine trenches strangely placed just off-island arcs or continents and their associated volcanoes, became clear when the processes of subduction at converging plates were understood. Within a matter of only a few years, geophysics and geology were revolutionized.
Figure 1 Subduction Boundaries (www.tahomascience.com/geology/ notes.contdrift.html)
Figure 2 Divergent Boundaries (www.tahomascience.com/geology/ notes.contdrift.html)
Figure 3 Transform Boundary (cronopio.geo.lsa.umich.edu/ Lec3/lec3.html)
Continental Drift, 1/20/04, http://www.zephryus.demon.co.uk/geography/resources/earth/tect.html
Earth Floor: Plate Tectonics, ETE Team. 1/20/04, http://www.cotf.edu/ete/modules/msese/earthsysflr/plates1.html
Kious, W.Jacquelyne and Robert I. Tilling, This Dynamic Earth: the story of plate tectonics, USGS, 1/20/04, http://pubs.usgs.gov/publications/text/dynamic.html
Soper, Davison E, Plate Tectonics, 1/20/04, http://zebu.uoregon.edu/~soper/Earth/tectonics.html
Noson, Qamar, and Thorsen, 1988, Washington State Earthquake Hazards: Washington State Department of Natural Resources, Washington Division of Geology and Earth Resources Information Circular 85
This, “Ring of Fire”, Plate Tectonics, Sea-Floor Spreading, Subduction Zones, “Hot Spots” USGS,1/20/04, http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/description_plate_tectonics.html
Mantovani, E. et all, Back Arc Extension: Which Driving Mechanism?, 2001, http://www.virtualexplorer.com.au/2001/Volume3review/Mantovani/paper2.html
The Continental Slide, 1998, http://www.pbs.org/wgbh/aso/tryit/tectonics/convergent.html
Polar reversal, 2001, http://www.nasca.org.uk/Strange_relics_/reversal/reversal.html
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