Plate tectonics (from the Greek word for "one who
constructs", τεκτων (tekton)) is a theory of
geology developed to explain the phenomenon of continental drift. In the theory of plate tectonics the outermost part of the Earth's interior is made up of two layers, the outer lithosphere and the inner asthenosphere.
The lithosphere essentially "floats" on the asthenosphere and is broken-up into seven major plates: African, Antarctic,
Australian, Eurasian, North American, South American, and the Pacific. These plates (and the more numerous minor plates) move in relation to one another at one of three
types of plate boundaries; Convergent (two plates push against one another), Divergent (two plates move away from each other),
and transform (two plates slide past one another). Earthquakes, volcanic activity, mountain-building, and
oceanic trench formation occur along plate boundaries (most notably
around the so-called "Pacific Ring of Fire").
Plate tectonic theory arose out of two separate geological observations: seafloor spreading and continental
drift. The theory itself was developed during the late 1960s and has since almost
universally been accepted by scientists and has revolutionized the Earth
sciences (akin to the development of the periodic table for
chemistry, the discovery of the genetic code for genetics, or evolution in biology).
Key principles
The division of the Earth's interior into lithospheric and asthenospheric components is based on their mechanical differences. The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and
mechanically weaker. This division should not be confused with the chemical subdivision of the Earth into (from innermost
to outermost) core, mantle, and crust. The key principle of
plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which "float" on the fluid-like asthenosphere. The relative fluidity of the
asthenosphere allows the tectonic plates to undergo motion in different directions.
One plate meets another along a plate boundary, and plate boundaries are commonly associated with geological events
such as earthquakes and the creation of topographic features like mountains, volcanoes and oceanic trenches. The majority of the world's active volcanoes occur along
plate boundaries, with the Pacific Plate's Ring of Fire
being most active and famous. These boundaries are discussed in further detail below.
Tectonic plates are comprised of two types of lithosphere: continental and oceanic lithospheres; for
example, the African Plate includes the continent and parts of the floor
of the Atlantic and Indian Oceans. The distinction is based on the density of constituent materials; oceanic lithospheres are
denser than continental ones due to their greater mafic mineral content. As a result, the
oceanic lithospheres generally lie below sea level (for example the entire Pacific Plate, which carries no continent), while the continental ones project above sea level (see isostasy for explanation of this principle, which is essentially a large-scale version of
Archimedes' Bath).
Types of plate boundaries
There are three types of plate boundaries, characterised by the way the plates move relative to each other. They are
associated with different types of surface phenomena. The different types of plate boundaries are:
- Transform boundaries occur where plates slide, or
perhaps more accurately grind, past each other along transform-faults. The relative motion of the two plates is therefore either sinistral or dextral.
- Divergent boundaries occur where two plates slide
apart from each other.
- Convergent boundaries (or active margins)
occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or an orogenic
belt (if the two simply collide and compress).
Plate boundary zones occur in more complex situations where three or more plates meet and exhibit a mixture of the above three
boundary types.
Transform (conservative) boundaries
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 cannot simply glide past each other. Rather, 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 strain, or motion along the fault. The massive amounts of
energy that are released are the cause of earthquakes, a common phenomenon
along transform boundaries.
A good example of this type of plate boundary is the San Andreas
Fault complex, which is found in the western coast of North America
and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move
relative to each other such that the Pacific plate is moving north with respect to North
America.
Divergent (constructive) boundaries
At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal
material sourced from molten magma that forms below. The genesis of divergent boundaries
is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric
material near the surface and the kinetic energy is thought to be
sufficient to break apart the lithosphere. The hot spot believed to have created the Mid-Atlantic Ridge system currently
underlies Iceland which is widening at a rate of a few centimetres per century. Such
hot spots can be very productive of geothermal power and Iceland is
actively developing this resource and is expected to be the world's first hydrogen economy within twenty years.
Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge, and in the continentental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive
fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks
are different massive transform faults occur. These are the fracture
zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange
pattern of blocky structures that are separated by linear features (http://pubs.usgs.gov/publications/text/baseball.html) perpendicular to the ridge axis. If one
views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the
spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be
older and deeper (due to thermal contraction and subsidence).
It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was
found. Airborne geomagnetic surveys showed a strange
pattern of symmetrical magnetic reversals on opposite
sides of ridge centres. 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 corresponds with the
Earth's polar reversals. This
was confirmed by measuring the ages of the rocks within each band. In reality the banding furnishes a map in time and space of
both spreading rate and polar reversals.
Convergent (destructive) boundaries
The nature of a convergent boundary 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 thrust 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. An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate. As organic material from the ocean bottom is transformed and heated by friction
a liquid magma with a great amount of dissolved gasses will be created. This can erupt to the surface, forming long chains of
volcanos inland from the continental shelf and parallel to it. The continental spine
of South America is dense with this type of volcanos. In North America the Cascade mountain range, extending north from California's
Sierra Nevada, is also of this type. Such volcanos are characterized by alternating periods of quiet and episodic eruptions that
start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy cinders, followed by a rebuilding phase
with hot magma. The entire Pacific ocean boundary is surrounded by long stretches of volcanos and is known collectively as The
Ring of Fire.
Where two continental plates collide the plates either crumple and compress or one plate burrows under or (potentially)
overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern
margins of the Indian subcontinental plate is being thrust under a portion of the Eurasian plate, lifting it and creating the
Himalaya.
When two oceanic plates converge they form an island arc as one oceanic
plate is subducted below the other. A good example of this type of plate
convergence would be Japan.
Sources of plate motion
As noted above, the plates are able to move because of the relative weakness of the asthenosphere. Dissipation of heat from
the mantle is acknowledged to be the source of energy driving plate tectonics. Somehow, this energy must be converted into force in order for the plates to move. There are essentially two forces that could be
driving plate motion: friction and gravity. These are further subdivided below.
Friction
- Mantle drag
- Convection currents in the mantle are transmitted through the
asthenosphere; motion is driven by friction between the asthenosphere and the lithosphere.
- Trench suction
- Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches.
Gravity
- Ridge-push
- Plate motion is driven by the higher elevation of plates at mid-ocean ridges. Essentially stuff slides downhill. The
higher elevation is caused by the relatively low density of hot material upwelling in the mantle. The real motion producing force
is the upwelling and the energy source that runs it. This is a mis-nomer as nothing is pushing and tensional features are
dominant along ridges. Also, it is difficult to explain continental break-up with this.
- Slab-pull
- Plate motion is driven by the weight of cold, dense plates sinking into the mantle at trenches.
There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at
mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding
on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere is not
strong enough to directly cause motion by friction. Slab pull is widely believed to be the strongest force directly operating on
plates. Recent models indicate that trench suction plays an important role as well. The over-all driving force and its energy
source are still debatable subjects of on-going research.
Major plates
The main plates are
Notable minor plates include the Indian Plate and the Arabian Plate.
The movement of plates has caused the formation and breakup of continents over time, including occasional formation of a
supercontinent that contains most or all of the continents. The supercontinent Rodinia is thought to have formed about 1000 million years ago and to have embodied most or all of Earth's
continents, and broken up into eight continents around 600 million years ago. The eight continents later re-assembled into
another supercontinent called Pangaea; Pangea eventually broke up into Laurasia (which became North America and Europe-Asia) and Gondwana (which became the rest).
- Related article
History and impact
Continental drift was one of many ideas about tectonics proposed in the late 19th century. By 1915 Alfred Wegener was making
serious arguments for the idea with the first edition of The Origin of Continents and Oceans. In that book he noted how
the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener wasn't the first to note this (Francis Bacon and Snider-Pellegrini preceded him), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation.
However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for
continental drift (see continental drift for more on this
controversy). Specifically they did not see how continental rock could plow through the much denser rock that makes up oceanic
crust. This changed radically in the 1960s, and was prompted by a number of discoveries,
most notably the Mid-Atlantic ridge. The most notable was the
1962 publication of a paper by American geologist Harry Hess. Hess suggested that instead of continents moving through oceanic crust (as was suggested by
continental drift) that an ocean basin and its adjoining continent moved together on the same crustal unit, or plate.
The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) can be
compared to the Copernican revolution in astronomy (see Nicolaus Copernicus). Within a matter of only several years geophysics and geology in particular were revolutionized. The parallel is striking:
just as pre-Copernican astronomy was highly descriptive but still able to make predictions, pre-tectonic plate geological
theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question "How?".
Before acceptance of plate tectonics, geology in particular was trapped in a 'pre-Copernican' box.
However, by comparison to astronomy the geological revolution was much more sudden. What had been rejected for decades by any
respectable scientific journal was eagerly accepted within a few short years in the 1960s
and 1970s. Any geological description before this had been highly descriptive. All the
rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The
descriptions are still valid. The reasons, however, today sound much like pre-Copernican astronomy.
One simply has to read the pre-plate descriptions of why the Alps or Himalaya exist to see the difference. In an attempt to answer "how" questions like "How can
rocks that are clearly marine in origin exist thousands of meters above sea-level in the Dolomites?", or "How did the convex and concave margins of the Alpine chain form?", any true insight was hidden
by complexity that boiled down to technical jargon without much fundamental insight as to the underlying mechanics.
With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had
the force to lift sea floor into thin atmospheres. The cause of marine trenches oddly placed just off island arcs or continents
and their associated volcanoes became clear when the processes of subduction at converging plates were understood.
Mysteries were no longer mysteries. Forests of complex and obtuse answers were swept away. Why were there striking parallels
in the geology of parts of Africa and South America? Why did Africa and South America look strangely like two pieces that should
fit to anyone having done a jigsaw puzzle? Look at some pre-tectonics explanations for complexity. For simplicity and one that
explained a great deal more look at plate tectonics. A great rift, (similar to the Great Rift Valley in northeastern Africa) had split apart a
single continent, eventually forming the Atlantic Ocean, and the forces were still at work in the Mid-Atlantic Ridge.
We have inherited some of the old terminology, but the underlying concept is as radical and simple as "The Earth moves" was in
astronomy.
Plate tectonics on Mars
As a result of 1999 observations of the magnetic fields on Mars by the Mars Global
Surveyor spacecraft, it has been proposed that the mechanisms of plate tectonics may once have been active on the planet -
see Geology of Mars.
References
- Earth System History, Steven M. Stanley, (W.H. Freeman and Company; 1999) pages 211-228 ISBN 0-7167-2882-6
- Geographica: The complete illustrated Atlas of the world, Editors of James Mills-Hicks (Barnes and Noble Books; New
York; 2004) ISBN 0-7607-5974-X
External links
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