| Geologic Time Scale | Plate
Tectonics | Radiometric
Dating | Deep Time | Geological
History of New Zealand |
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.
- 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.
- The hypothesis of magma injection postulates that the injection of magma at a spreading centre pushes plates apart and thereby causes plate movement.
- 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.
- 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
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
Look up this site for New Zealand's paleogeographic history: http://www.gns.cri.nz/store/download/index.html#Pali
 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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