Thursday, January 12, 2012



Plate tectonics is the theory that the outer rigid layer of the earth (the lithosphere) is divided into a couple of dozen "plates" that move around across the earth's surface relative to each other, like slabs of ice on a lake.
The drawing above is a cross section of the earth showing the components that lie within plate tectonic theory. The cross section should really be curved to correspond to the earth's curvature, but it has been straightened out here. 

Note the continental craton (stable continent) in the middle of the drawing. Note the line under the craton; that is the lower boundary of the plate. Everything above that line is the plate. All similar lines in the cross section mark the bottom of the plates. Technically, everything above that line is lithosphere, the rigid, brittle shell of the earth. Everything below is asthenosphere, the hot, plastic interior of the earth.

Within the asthenosphere are convection cells, slowly turning over hot, plastic rock. The convection cells bring heat from the earth's interior out to the surface, but slowly. Movement is about 10 centimeters a year. When the convection cells reach the base of the lithosphere they release heat to the surface at the divergent plate boundary to escape to space. The cooled plastic rock then turns sideways and moves parallel to the earth's surface before descending back into the earth at subduction zones to become reheated. It is this turning over of the convection cells the drives the plate movements. 

Simplistically, the earth consists of the plates, and plate boundaries, those zones where the plates contact and interact. Observe that 7 different plates are labeled in the cross section. Plates are combinations of two units, continents and ocean basins. A plate may be an ocean basin alone, or a continent alone, or a combination of ocean basin+continent (common).

It is possible a plate could be a continent alone, but for this to occur all edges of the continent would have to be a plate boundary (very rare, perhaps not practically possible). Note that in the cross section several different ocean basin/continent combinations are present, but that it is difficult to get a continent with all plate boundaries.

The three kinds of plate boundaries are also illustrated in the cross section, divergent, convergent, and transform. Plate interact at these boundaries.

Two divergent margins (plate boundaries) are present in the cross section, one labeled as such to the right of the continental craton, and the other on the left side. The left side divergent margin is labeledBack Arc (Marginal) Basin. Back arc basins are formed by minor convection cells above subduction zones. Divergent plate boundaries always create new ocean floor (that is, new oceanic lithosphere, called the ophiolite suite).

Three convergent boundaries are present, all of them one way or another involving a subduction zone. In the continent-continent collision the subduction zone is now extinct but can be seen below the surface. Subduction zones generate lots of igneous magma that rises to the surface to form volcanic mountains (volcanic arcs; also island arcs). The igneous batholiths that feed the volcanoes are the beginning of generation of new continental crust. Continents are created above subduction zones as small proto- and microcontinents. They enlarge by colliding and fusing together, or suturing onto a larger continent, at a convergent plate boundary. 

At convergent boundaries oceanic lithosphere is always destroyed by descending into a subduction zone. This is because oceanic rock is heavy, compared to the continents, and sinks easily. Because oceanic lithosphere is created and destroyed so easily ocean basins are young; the oldest we have is only about 200 million years old. Continents, on the other hand, composed of light weight rock never subducts. Thus, continental rock once formed is more or less permanent; the oldest continental fragment is 3.9 billion years old, virtually as old as the earth itself. 

Only one transform boundary is present, on the left side of the drawing. At transform boundaries two plates just slide past one another horizontally, and quietly compared to convergent and divergent plate boundaries. Most of these are found in the ocean basins, but the San Andreas fault in California and Mexico is an example coming on land.

The essence of plate tectonic theory is that the plates (ocean basins plus or minus continents) slide around over the earth surface, interacting as they do at the plate boundaries. Thus, any time there is a divergent plate boundary where two plates are separating, there must be a convergent plate boundary (subduction zone) where the earth comes together again. And convergent boundaries always, eventually, lead to collisions between continents, or continents and terranes (island arcs plus or minus microcontinents). 

Observe the subduction zones in the cross section. Next to each one is a remnant ocean basin (ROB). An ROB is one that is disappearing down a subduction zone; it is a remnant of its former self. But all subduction zones must eventually disappear completely and when they do the floating blocks on either side will collide, and create a mountain range. The continent-continent collision in the cross section is a case where the collision has already occurred. 

But in the larger picture, it is common for a divergent plate boundary to come into existence and create a new ocean basin, and then for that ocean basin to close again along a convergent plate boundary until two continents collide. This opening and closing of ocean basins is the Wilson Cycle, and is the simplest model we have of how the earth operates historically.

  • The Earth's surface is made up of a series of large plates (like pieces of a giant jigsaw puzzle).
  • These plates are in constant motion travelling at a few centimetres per year.
  • The ocean floors are continually moving, spreading from the centre and sinking at the edges.
  • Convection currents beneath the plates move the plates in different directions.
  • The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth.

Major Tectonic Plates of the World

plate_colour_diagram2.gif (21726 bytes)

  1. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building.
  2. Plate tectonics is a relatively new theory and it wasn't until the 1960's that Geologists, with the help of ocean surveys, began to understand what goes on beneath our feet.
  1. The continents seem to fit together like a giant jigsaw puzzle: If you look at a map, Africa seems to snuggle nicely into the east coast of South America and the Caribbean sea. In 1912 a German Scientist called Alfred Wegener proposed that these two continents were once joined together then somehow drifted apart. He proposed that all the continents were once stuck together as one big land mass called Pangea. He believed that Pangea was intact until about 200 million years ago
  2. Continental Drift. When Wegener first put forward the idea in 1912 people thought he was nuts. His big problem was that he knew the continents had drifted but he couldn't explain how they drifted. The old (AND VERY WRONG!!) theory before this time was the "Contraction theory" which suggested that the planet was once a molten ball and in the process of cooling the surface cracked and folded up on itself. The big problem with this idea was that all mountain ranges should be approximately the same age, and this was known not to be true. Wegener's explanation was that as the continents moved, the leading edge of the continent would encounter resistance and thus compress and fold upwards forming mountains near the leading edges of the drifting continents. Wegener also suggested that India drifted northward into the Asia forming the Himalayas and of course Mount Everest.
  3. Sea floor spreading. It is hard to imagine that these great big solid slabs of rock could wander around the globe. Scientists needed a clue as to how the continents drifted. The discovery of the chain of mountains that lie under the oceans was the clue that they were waiting for.

  • PLATES ARE CREATED: In the diagram below you can see that the continental crust is beginning to separate creating a diverging plate boundary. When a divergence occurs within a continent it is called rifting. A plume of hot magma rises from deep within the mantle pushing up the crust and causing pressure forcing the continent to break and separate. Lava flows and earthquakes would be seen. In the diagram below you can see that the continental crust is beginning to separate creating a diverging plate boundary. When a divergence occurs within a continent it is called rifting. A plume of hot magma rises from deep within the mantle pushing up the crust and causing pressure forcing the continent to break and separate. Lava flows and earthquakes would be seen.

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This is an example of a divergent plate boundary (where the plates move away from each other). The Atlantic Ocean was created by this process. The mid-Atlantic Ridge is an area where new sea floor is being created.

diverge plates with opening rift valley animation.gif (18569 bytes)

As the rift valley expands two continental plates have been constructed from the original one. The molten rock continues to push the crust apart creating new crust as it does

diverge3 animation  widening ocean.gif (17635 bytes)

As the rift valley expands, water collects forming a sea. The Mid-Atlantic Ridge is now 2,000 metres above the adjacent sea floor, which is at a depth of about 6,000 metres below sea level.

diverge4 animated sequence.gif (25011 bytes) 

The sea floor continues to spread and the plates get bigger and bigger. This process can be seen all over the world and produces about 17 square kilometres of new plate every year.


island arcs.gif (39562 bytes) 

This is a convergent plate boundary, the plates move towards each other. The amount of crust on the surface of the earth remains relatively constant. Therefore, when plates diverge (separate) and form new crust in one area, the plates must converge (come together) in another area and be destroyed. An example of this is the Nazca plate being subducted under the South American plate to form the Andes Mountain Chain. 

Here we can see the oceanic plate moving from left to right. The top layer of the mantle and the crust (all called the lithosphere) sinks beneath the continent. A deep ocean trench is formed. Water gets carried down with the oceanic crust and the rocks begin to heat up as they travel slowly into the earth. Water is then driven off triggering the formation of pools of molten rock which slowly rises. The plate moves downwards at a rate of a few centimetres per year. The molten rock can take tens of thousands of years to then either:
  • Solidify slowly underground as intrusive igneous rock such as granite. 

  • Reach the surface and erupt as lava flows. Cooling rapidly to form extrusive igneous rock such as basalt.

The floor of the Easter Pacific is moving towards South America at a rate of 9 centimetres per year. It might not seem much but over the past 10 million years the Pacific crust has been subducted under South America and has sunk nearly 1000 kilometres into the Earth's interior.

Types of Convergent Boundaries
The example above showed what happened when the dense oceanic plate subducts under a lighter continental plate (ie, oceanic - continental). Two other types of subduction can take place:

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When two oceanic plate meet each other (oceanic-oceanic) this often results in the formation of an island arc system. As the subducting oceanic crust melts as it goes deeper into the Earth, the newly-created magma rises to the surface and forms volcanoes. If the activity continues, the volcano may grow tall enough to breech the surface of the ocean creating an island.

Himalayas collision.gif (37280 bytes)

Millions of years ago India and an ancient ocean called the Tethys Ocean were sat on a tectonic plate. This plate was moving northwards towards Asia at a rate of 10 centimetres per year. The Tethys oceanic crust was being subducted under the Asian Continent. The ocean got progressively smaller until about 55 milion years ago when India 'hit' Asia. There was no more ocean left to lubricate the subduction and so the plates welled up to form the High Plateau of Tibet and the Himalayan Mountains. The continental crust under Tibet is over 70 kilometres thick. North of Katmandu, the capital of Nepal, is a deep gorge in the Himalayas. the rock here is made of schist and granite with contorted and folded layers of marine sediments which were deposited by the Tethys ocean over 60 million years ago.

The fourth type of plate movement involves plates sliding past one another without the construction or destruction of crust. This boundary is called a conservation zone because plate is neither created nor destroyed An example of such a boundary is the San Andreas fault in California. The force needed to move billions of tonnes of rock is unimaginable. When plates move some of the energy is released as earthquake



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