Plate Boundaries


 
~Plate Boundaries and Interplate Relationships~
 
 
The drawing above represents a cross section of the earth showing the components that make up the plate tectonic theory (click for Larger Version). 
     
Note the continental Craton (stable continent) in the middle of the drawing. The line under the Craton is the lower boundary of the lithosphere, the rigid, brittle shell of the earth. Everything above that line is the lithosphere. Everything below the line is asthenosphere, the hot, plastic interior of the earth with convection cells bringing heat to the surface from the earth's hot core.
    
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. The process is very slow; moving approximately 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 in the form of volcanoes. The cooled plastic rock then moves parallel to the earth's surface before descending back into the earth at subduction zones to become reheated, similar to a conveyor belt. It is this turning over of the convection cells the drives plate movement. 

~The Plates~
 
Simplistically, the earth consists of plates, and plate boundaries, those zones where the plates contact and interact. Everything between plate boundaries are part of some plate. 
     
Observe that 7 different plates are labeled in the cross section. Plates are combinations of two units, Continents and Ocean Basins. Plates are composed either of a fragment of ocean basin, or (more commonly) a fragment of ocean Basin with an attached continent.
     
It might be possible for a plate to 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 surrounded on all edges by plate boundaries

~Plate boundaries~
 
The three kinds of plate boundaries are also illustrated in the cross section, divergent, convergent, and transform. Plates 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 divergent margin is labeled Back 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 mafic oceanic lithosphere, called the Ophiolite Suite) when magma oozes into the crack as plates separate. The implication is, ocean basins get larger at divergent plate boundaries, but since the earth cannot grow larger, older crust is destroyed at a convergent boundary.

Three Convergent Boundaries are present.  Oceanic lithosphere descends into the earth's mantle at subduction zones and is destroyed because mafic oceanic rock is heavy compared to felsic continents, and sinks easily. Since oceanic lithosphere is created and destroyed so easily, ocean basins are young; the oldest is only about 200 million years old. Continents, on the other hand, composed of light weight rock, never subduct. Thus, continental rock is more or less permanent; the oldest continental fragment is 3.96 billion years old, virtually as old as the earth itself.
 
In the continent-continent collision (see figure above) the subduction zone is extinct but can be seen below the surface.  Subduction zones generate igneous magma that rises to the surface to form volcanic mountains (volcanic arcs; also island arcs, such as the islands of Japan, or Sumatra, or the Aleutian islands off Alaska). 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 grown larger by colliding and fusing together, or suturing onto a larger continent, at a convergent plate boundary. 

At transform boundaries, two plates 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.

~The Six Lithospheric Tectonic Regimes~
 
There are six tectonic regimes that make up the lithosphere. A tectonic regime is a portion of the earth that is more or less uniform in its structure, rock composition, and processes. The six tectonic regimes are the individual components that interact in plate tectonic theory. 

1. Continents (or, Cratons)
2. Ocean Basins
3. Divergent boundaries
4. Convergent boundaries
6. Hot Spots 

Each of these tectonic regimes is described in the Glossary

~Plate Collisions~  
  
The essence of plate tectonic theory is that the plates (ocean basins plus or minus continents) slide around over the earth's surface, interacting 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 two (or more) places come together again. Convergent boundaries always, eventually, lead to collisions between continents, or continents and terranes (island arcs plus or minus microcontinents).
     
The drawings below show the major types of collisions within plate tectonic theory;


Observe the subduction zones in the cross section at the top of the page (or Larger Version). 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. 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. 
     
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 Supercontinent Cycle, but it is also the Wilson cycle, which is the simplest model we have of how the earth operates historically.

Contributed by Lynn Fichter 

Wednesday, November 19, 2014
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