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Plate Tectonics

Plate tectonics – now a household name in Earth sciences – is a global model devised in the 1960s and 70s that encompasses most geological phenomena observed on the planet. It explains earthquakes and eruptions, mountain building and even the arrangement of ocean ridges and continents around the globe. In the model of plate tectonics, the solid crust of the Earth can tear apart. It grows thinner in places, in response to horizontal stretching, and hot mantle rises to fill the void. Vertical sheets of magma are injected into the overlying crust.

Hence, the Earth’s crust grows like a deck of cards that is constantly replenished with new cards up the middle of the deck. This process takes place in the middle of ocean basins, basins that grow wider and shove the continents apart on either side – the process of continental drift (Frankel, 2005). Before World War II, the knowledge of the ocean basins was extremely sketchy. Geological concepts were dominated by what scientists saw and studied on the continents, and thus represented only one-third of the Earth’s surface.

However, the accelerated exploration of the oceans after the war added to the data on the topography, the structure, the chronology, the composition and the magnetic characteristics of the basins. By the 1960s, this information had been combined with old ideas about continental drift into a new global theory of plate tectonics. This unifying concept correlated and explained many seemingly unrelated geological processes and phenomena. In brief, the theory proposed that the apparently rigid outer layer of the Earth, the lithosphere, was actually subdivided into a series of segmented plates that moved laterally relative to one another.

The plates were created at midocean ridges and rises where basaltic material welled up from the interior. And in a continuous process known as sea-floor spreading, the recently formed material divided and spread laterally as newer material was intruded along the axis of the ridge or rise. A decrease in elevations occurred away from the ridge as the new plate cooled. Thus, ridges and rises represent divergent lithospheric-plate boundaries. But if Earth maintains a constant radius, how do we account for these rapidly forming and spreading lithospheric plates?

The answer lies at convergent plate boundaries. Where two oceanic plates come together, one must be subducted, or slide under, the other. At the convergent zones, one of the relatively rigid lithospheric plates flexes and descends under the other to be remelted and reincorporated into the mantle. Continental regions fit into the picture in several ways. At present, continents exist in the upper part of the lithospheric over one-third of the Earth’s surface. Continents have formed over geologic time from the accretion of low-density crustal material in a poorly understood process.

Because of their low density, they stand topographically high and in general, they ride passively on the laterally moving lithospheric plates. Geologists recognize there major types of plate boundaries; divergent, convergent and transform. Along these boundaries new plates are formed, are consumed or slide laterally past one another. In interaction of pates at their boundaries accounts for most of Earth’s seismic and volcanic activity and origin of mountain systems. Divergent plate boundaries or spreading ridges occur where plates are separating and new oceanic lithosphere is forming.

Divergent boundaries are places where the crust is extended, thinned and fractured as magma, derived form the partial melting of the mantle rises to the surface. Divergent boundaries most commonly occur along the crests of oceanic ridges – for example, the Mid-Atlantic Ridge. Oceanic ridges are thus characterized by rugged topography with high relief resulting from displacement of rocks along large fractures, shallow-focus earthquakes, high heat flow and basaltic flows or pillow lavas (Monroe and Wicander, 2005)

Tectonic activity or earthquake is not as dramatic within the laterally moving plates as at their boundaries because of their relatively rigid nature. However, intraplate activity includes loading and downwarping of the lithosphere by ice and sediments. Localized sublithospheric plates moves over the hot spot, volcanoes are produced and then removed, creating a linear array of volcanoes such as the Hawaiian-Emperor seamount chain (Cornell and Gorenstein, 1984). Not all of Earth’s plates are moving apart or crashing into one another.

Some plates slide horizontally against each other at transform boundaries. No crust is created or destroyed at these boundaries, which is why they are sometimes called “conservative” plate boundaries. This means that there are no volcanoes. However, the movement of these giant plates releases enormous amounts of energy, as the plates tend to stick, then slip violently, rather than sliding smoothly against each other. The movement at transform boundaries can cause fractures in Earth’s crust, called faults. Fold mountains and earthquakes are common features of faults (Edwards, 2005).

As a summary, the plate tectonics is explains the movement about the Earth’s crust. With regards to the plate tectonics, there are plate boundaries that characterize how these movements are relative to one another, which can be observed in figure 1. These plate boundaries are transform boundaries, divergent boundaries and convergent boundaries. The plates slide towards each other which creates the mountains in the convergent boundaries while plates slide apart from each other resulting to the formation of ridges and earthquakes in the divergent boundaries.On the other hand, the plates only slide each other in transform boundaries forming faults. Figure 1. Tectonic Plates Boundaries

References

Cornell, J. , & Gorenstein, P. (1984). Astronomy from Space: Sputnik to Space Telescope. USA: MIT Press. Edwards, J. (2005). Plate Tectonics and Continental Drift. London: Evans Brothers Limited. Frankel, C. (2005). Worlds on Fire: Volcanoes on the Earth, the Moon, Mars, Venus and New York: Cambridge University Press. Monroe, J. S. , & Wicander, R. (2005). Physical Geology: Exploring the Earth. New York: Thomson Brooks/Cole.

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