Here is an explanation on Plate Tectonic Theory:
|Configuration of todays Plate Tectonics
Plate Tectonic Theory is a scientific theory that basically says that the surface of the Earth can be described as large chunks
or "plates" that drift on the surface of the Upper Mantle. The plates can be differentiated into two different,
distinct types. These are Oceanic Plates and Continental Plates. However, its a little more complicated then that. As a general
rule, most plates consist of some portion of both. For example, the North American Plate consists of the North American Continent
and Central America, but also consists of a portion of the Atlantic Ocean. Therefore, the North American Plate consists of
a mixture of Continental and Oceanic Plate composition. Keep this in mind for later.
The basic processes involved in Plate Tectonics is:
1) Rifting: The processes of two plates spreading apart.
2) Subduction: The process of two plates coming together, and the more dense plate going under the lighter plate.
3) Collision: The process of two plates coming together, however there is no density change. They collide.
The plate tectonics are described to be the rigid and brittle part of the uppermost portion of the crust which overrides
the Upper Mantle. The Upper Mantle is a chemically different composition and is ductile and plastic in nature. The lithosphere
is the portion of Crust and Cold Upper Mantle which show brittle deformation. The asthenosphere is a portion of the Upper
Mantle which shows semi-molten and plastic behavior. This is the only portion of the Mantle that is semi-molten. The rest
is solid rock, however thermal energy and pressure cause it to act like a plastic. The plates move through the forces applied
onto them by the spreading and subduction of the plates, all caused by Mantle Convection. Mantle Convection is the movement
of hot solid Mantle rocks moving in convective circles within the mantle.
Continental Crust is simply thought as what ever makes up the continent. Fair enough. It is mostly composed of granite,
which is an intrusive felsic igneous rock. It has certain properties that make it important. Namely, its density. It has an
average density of 2.75g/cm^3 (grams per cubic centimeter). Continental crust also composes of a small portion of sedimentary
rocks on the surface, and gneissic metamorphic rocks further down, but its mostly granite. The average thickness of continental
crust is approximately 30km deep.
Oceanic Crust however is chemically different then continental crust. Oceanic crust is composed of mostly an intrusive/extrusive
mafic igneous rock gabbro/basalt, respectively. Gabbro and basalt are chemically the same, they just have different crystal
sizes, reflecting the variance in cooling time. Larger crystals result from longer cooling times. However, the density of
basalt is 3.3g/cm^3. That means that its denser then granite, and therefore denser then continental crust. This is why science
believes that oceanic crusts can subduct under continental crust. The average thickness of Oceanic Crust is 9km deep.
Oceanic Crust is also more complicated to understand then Continental Crust. Geophysical analysis has determined that
the thickness of oceanic crust is related to its age. Newly formed Oceanic Crust is very thin and undersaterated with respect
to water. However, as we age the Oceanic Crust, and we head away from the spreading ridge, the Oceanic Crust gets saturated
with seawater, making it heavier, and it also thickens as described by the equation:
Oceanic Crust Younger then 70Ma can be simply calculated as:
L = 11*sqrt(T)
L = thickness of Oceanic Crust
T = Age (Ma)
11 comes come the combination of several constants
Therefore, a older, more water saturated and thicker oceanic crust would be relatively heavier then a younger one. Thats
why an older oceanic crust will subduct under a younger one.
|Model of Plate Tectonics
Plate Tectonic Theory has been confirmed by the scientific community as it has been witnessed by several scientists first
hand. Seismic tomography imaging has seen past oceanic plates subducted under current continental plates, and we witness seafloor
spreading as well as subduction in ocean trenches.
For a more indepth explanation of Plate Tectonic Theory, consult the link below or a credible Geology or Geophysics Textbook.