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Geological Time

| Geologic Time Scale | Plate Tectonics | Radiometric Dating | Deep Time | Geological History of New Zealand |

Plate Tectonics

Scientists theorise that one of the great driving mechanisms of evolution is the movement of plates and the forces that act on them. This concept is commonly known as the theory of plate tectonics. This theory describes how the Earth's lithosphere is broken up into about a dozen plates or so which slide by, collide with or move away from each other as new ocean floor is created. The continents that are embedded in the lithosphere drift with the moving plates. This theory explains the concept of continental drift which describes the large scale movement of the continents over the globe.

The theory of plate tectonics indicates that the rigid plates slide over a partially molten, weak asthenosphere with the continents drifting passively along. Continental drift is therefore a consequence of the movement of the plates. Plate tectonics allows the plates to move with relatively little buckling or breaking except at plate boundaries. There are four types of plate boundaries:

  • Divergent boundaries -- where new crust is generated as the plates pull away from each other.
  • Convergent boundaries -- where crust is destroyed as one plate dives under another.
  • Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other.
  • Plate boundary zones -- broad belts in which boundaries are not well defined and the effects of plate interaction are unclear.

Continental Drift

The hypothesis of continental drift was largely developed by Alfred L. Wegener, a German lecturer in astronomy and meteorology, who suggested that the Earth's continents had at one time been joined. In 1912, Wegener proposed that all the continents were previously one large continent, but then broke apart, and had drifted through the ocean floor to where they are now located. Wegener studied the distribution of animals and fossil land plants to help him in his interpretations. He found that the plant Glossopteris had left behind leaf remains which were relatively common in the Southern Hemisphere continents. This supported his hypothesis, as Wegener reasoned that in order for Glossopteris leaves to be found in the widely spaced continents of the Southern Hemisphere, the continents must once have been joined. Using this idea, he reconstructed all the continents which contained evidence for commonality into a supercontinent which he named Pangea. Interestingly this was all the southern continents, along with India. Wegener also studied the distribution of major rocks and mineral deposits. He found that when he fitted Africa and South America together along their continental shelves, large blocks of ancient rock called cratons formed continuous patterns across the dividing line. The mountains that run from east to west across South Africa seemed to link with the range near Buenos Aires in Argentina.

Wegeners's findings were published in 1915 in his book Die Entstehung der Kontinente und Ozeane (The origin of the continents and oceans). His ideas were not widely accepted as critics thought that the evidence was not strong enough, that the underlying cause of the drift was not explained and that the drift was impossible. He was attacked by many critics and this eventually took its toll on his career. Despite his undisputed talents as a teacher and the continuing loyalty of his close associates, he was unable to obtain a professorship in a German university and eventually left Germany for the University of Graz in Austria.

Through the discovery of palaeomagnetism after the second world war and the development of oceanography it was possible to support Wegener's theories and convert the critics.

Paleomagnetism is based on the principle that magnetic particles will align themselves with the Earth's magnetic field in molten igneous rocks, or unlithified sediments. This magnetic record is stored within the rocks when they cool and within the sediments when they become lithified. The deviations in the alignment of these paleomagnetic particles from the current direction of the Earth's magnetic filed shows that the continents have moved. Through the development of a sensitive device called the astatic magnetometer by a British physicist Patrick Blackett, it was possible for the first time to detect the orientation of extremely weak magnetic fields.

Two English scientists, Drummond Matthews and Fred Vine discovered during the 1960's a series of linear magnetic anomalies on either side of the Mid-Atlantic Ridge. They observed that strips of ocean crust had alternating magnetic orientations. They explained these observations through a sea floor spreading model that shows how new oceanic crust forms along mid-ocean ridges as the two halves of an ocean move apart.

Causes for continental drift

In 1928, Arthur Holmes, proposed that the mechanisms of thermal convection in the mantel were the driving forces of continental drift. He proposed that subcrustal convection currents will pull two halves of continent apart causing mountain building in the front, where currents are descending and new formation of ocean floor where the continent is pushed apart and currents are ascending.

The mapping of the Mid-Atlantic Ridge and the discovery of deep valleys or rifts running down its centre provided convincing evidence. Although the question of the causes for continental drift has yet to be fully resolved there are four main hypotheses to explain the movement of the plates.

  1. The hypothesis of convection currents suggests that flow in the mantle is induced by currents which drag and move the lithospheric plates above the asthenosphere. Convection currents rise and spread below divergent plate boundaries and converge and descend along convergent. The convection currents are sought to be produced by three sources: first the cooling of the Earth's core, second the radioactivity within the mantle and crust and third through the cooling of the mantle.
  2. The hypothesis of magma injection postulates that the injection of magma at a spreading centre pushes plates apart and thereby causes plate movement.
  3. The hypothesis of continental drift through gravity says that the oceanic lithosphere thickens as it moves away from a spreading centre and cools. The cooler and heavier plates might tend to slide under the force of gravity, from a divergent margin towards a convergent margin.
  4. This hypothesis of descending plates suggests that a cold dense plate descending into the mantle at a subduction zone may pull the rest of the plate with it and thus cause plate motion.

Reference Websites  Reference Websites

A map of the tectonic plates can be found at: http://geology.er.usgs.gov/eastern/plates.html

Go on this website straight to the plate tectonics activity, a great animation: http://www.pbs.org/wgbh/aso/tryit/tectonics/#

Great animations of continental drift due to plate tectonics on: http://www.ucmp.berkeley.edu/geology/tectonics.html.

View a map with the average movement of the plates per year: http://www.uwsp.edu/geo/faculty/ozsvath/images/plate%20rates.htm

More about the Mid-Atlantic Ridge on: http://en2.wikipedia.org/wiki/Mid-Atlantic_Ridge.

New Zealand Example  New Zealand Example

Look up this site for New Zealand's paleogeographic history: http://www.gns.cri.nz/store/download/index.html#Pali

Reference Books  Reference Books

Hamblin W. K., and Christiansen E.H., (1998). Earth's dynamic systems. New Jersey: Prentice Hall.

Press F., and Siever R., (1994). Understanding Earth, New York: Freeman and Company.

Selby M.J., (1985). Earth's changing surface. Oxford: Clarendon Press.


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