Unlike the other planets , Earth's crust is divided into several sepearde solid plates which float around indpendeently on the top of the hot mantle below.
The theory which describes this is known as Plate Tectonics which we will talk about it here.
Plate Tectonics is characterized by two major processes : spreading and subduction.
Occurs when two plates move away from each other and new crust is created by upwelling magma from below.
Occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle.
There is also transvers motion at some plate boundaries ( i.e the San Andreas Fault in California ) and collisons between continental paltes ( i.e India/Eurasia).There are (at present) eight major plates :
North America Plate-North America , Western North Atlantic and Greenland
South American Plate-South America and Westeren South Atlantic
Antarctic Plate-Antarctica and the " Southern Ocean "
Eurasian Plate-eastern North Atlantic , Europe and Asia except for India
African Plate-Africa , eastern South Atlantic and western Indian Ocean
Indian-Australian Plate-india , Australia , New zealand and most of Indian Ocean
Nazca Plate-eastern Pacific Ocean adjacent to South America
Pacific Plate-most of the Pacific Ocean ( and the southern coast of California )
It is not all Plates in Earth There are also Plates but small plates about 20 as the Arabian , Cocos , and Philippine Plates.
What you have to know that the continents have changed because they sit on blocks of the lithosphere that are in horizontal motion with respect to each other , and they continue to change because the hrizontal motion continues.
The following figure illustrates the present continents seprated from two supercontients called laurasia and gondwanaland through this process of plate tectonics.
The two supercontients may have once been united in a single supercontinent called Pangaea , though this is less certain.
Indications of Tectonic Activity
Among the calsses of evidence for continentale drift and the underlying tectonics we may list
1-The shapes of many continents are such that they look like very seperated pieces of a puzzle.For example , look in the adjacent map at the shape of the east coast of North and South Americal relative to the shape of the west coast of Africa and Europe.
2-Many fossil comparisons along the edges of continents that look like they fit together suggest species similarities that would only make sense if the two continents were joined at some point in the past.
3-There is a large amounts of seismic , volcanic , and geothermal activity along the conjectured plate boundaries.This is shown clearly below in figure where the epicenters of earthquakes above Richter magnitude 5.0 are plotted for a 10-year period .The concentration is strikin , and indeed this plot serves to define the plate boundaries extremely well.
4-There are ridges , such as the Mid-Atlantic Ridge where plates are separating that are produced by lava welling up from between the plates as pushing against each other (e.g,the Himalayas , which are still growing)
Plate Tectonic motion , which may be only centimeter per century , is now being studied by careful laser ranging techniques that are capable of detecting such small motions.
Age of the Sea Floor
If the crustal plates are pulling apart at boundaries like the Mid-Atlantic Ridge (see the line of earthquake epicenters down the center of the Atlantic in the precceding figure) , the sea floor near these ridges these ridges should be very young geologically , since it is formed of material upwelling from the interior.This is indeed the case , as the following figure shows.
This figure displays the estimated age of sea floor crustal plates with red the youngest and blue the oldest.
Consequences of Plate Tectonics
We will divide the Consequences of Plate Tectonics into two title:
Some Past and Present Consequences
Plate tectonics has been responsible for many of the features that we find on the surface of the Earth today . A few examples include
The Appalachian Mountains were formed from wrinkling of the Earth's surface produced by the collison of the North American and African plates .
The seismic and volcanic activity of the West Coast of the United states (for example , the San Andreas Fault) is produced by the grinding of the Pacific and North American Plates against each other.indeed , the entire " ring of fire " around the Pacific , corresponding to regions of high volcanic and seismic activity , is caused primarly by the motion of the Pacific Plate.
The Dead Sea in Israel is part of a rift system produced by plates that are pulling apart in that region.
The Himalayan Mountains were formed (indeed are still growing) as a result of the Indian subplate burrowing under the Eurasian plate and raising its edge.
Some Future Consequences pf Plate Tectonics
Plate tectonic is still an active process , and will drastically reshape the face of the Earth over the next 50 million years or so . A few consequences of plate tectonics based on projections of present motion include :
Portions of California will seperate from the rest of North America.
The Italian " boot " will disappear.
Ausrtralia will become linked to Asia.
Africa will seperate from the Near East.
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Last Modified : 5 Sep. 2001
Created By#C0115361 Team