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Plate Borders
by Shannon Lee Bruce
Physical Geology
Spring 2009
                  

  

Types of Tectonic Borders

The earth’s surface is formed out of several large plates called tectonic plates.  These plates are floating on top of magma.  The tectonic plates move a few centimeters each year.  There are three different borders that occur when the edge of tectonic plates meet. Divergent boundaries  occur when the outer edges of the tectonic plates move apart allowing magma to flow to the surface of the earth.  The mid-Atlantic ridge is an example of this.  Transform boundaries are created when the two edges of the tectonic plates slide past one another such as the San Andreas Fault in California.  The third and final borders are convergent boundaries which occur when the edge of one tectonic plate is forced under the other tectonic plate.  This type of border creates magma underground that comes to the surface through volcanoes.    The picture below shows what the three different types of borders look like.
 

 

Picture copyright Wikipedia, the free encyclopedia

Picture copyright Wikipedia, the free encyclopedia

          This picture above shows where the three different types of Tectonic plate borders occur on the earth.

When divergent boundaries occur under the ocean surface, formations like the mid-Atlantic ridge are formed.  The two edges of the tectonic plates are pulled apart and hot magma flows to the surface where it meets the water and cools forming new sections of the earth’s crust which is also known as seafloor spreading.  When they occur on land you get land like the picture below.

Picture copyright Wikipedia, the free encyclopedia
 

           Transform boundaries like the San Andreas Fault in California is a good example of strike- slip faults.  Strike- slip faults occur when two fault blocks move horizontally past each other in opposite directions along the direction of the strike line. This is a likely setup for disasters such as earth quakes. An earth quake is a sudden release of energy underground usually associated with a fault.  Another type of fault that we know of is a thrust fault, which is a low angle reverse fault where the headwall block moves up and dips over the younger rock.  Below is an illustration showing an example of a thrust fault.
 

Photo Copyright Roger Weller/Cochise College
 

Convergent boundaries are created when the edge of one tectonic plate is pushed under the other one.  There are three different types of convergent boundaries that are possible and these are the oceanic to continental, the continental to continental, and the oceanic to oceanic. 

When you have a oceanic to continental convergent boundary the thinner and less denser tectonic plate is pushed under the thicker and denser tectonic plate.  With this type of boundary the oceanic crustal slab is pushed beneath the continental crust at about a 30 degree angle.  This type of boundary is the cause for the majority of volcanoes such as the ones that are found in the Ring of Fire around the Pacific Ocean.  Below is an diagram of oceanic to continental boundary.

 

Oceanic to continental

Picture copyright Wikipedia, the free encyclopedia

 

          When you have a continental to continental divergent boundary which is when two crustal plates at a boundary are moving away from each other, a rift zone will form above the boundary.  If it is a convergent boundary, a folded mountain range will be pushed up.  An example of this would be the Appalachians or the Ural mountains. The diagram below show how this takes place.

 

Continental to continental

Picture copyright Wikipedia, the free encyclopedia

 

          When an oceanic to oceanic type of boundary occurs, if it is a divergent plate boundary, an oceanic ridge will form at the boundary.  On the other hand, if it is a convergent boundary, the oceanic plate on one side of the boundary will be shoved under the other at a steep angle and can cause islands to form in the ocean.  An example of this is the Aleutian Islands in Alaska. The diagram below further illustrates this type of boundary.
 

oceanic to oceanic

Picture copyright Wikipedia, the free encyclopedia
 

References 

http://www.sismo.unr.edu/ftp/pub/louie/class/100/plate-tectonics.html
http://www.moorlandschool.co.uk/earth/tectonic.htm
http://skywalker.cochise.edu/wellerr/aawellerweb.htm
http://en.wikipedia.org/wiki/Plate_tectonics
http://pubs.usgs.gov/gip/dynamic/understanding.html