The Theory of Plate Tectonics

The earth's surface layers are divided up into large segments called crustal plates. These plates are always on the move due to the slow movement of semi-molten rock within the earth’s interior (convection currents). Years ago, scholars commented on how the continents fit together like a jigsaw puzzle. They discovered that if the continents could be moved and put together they will form on large land mass. This idea was given the name continental drift and was developed by Alfred Wegener. We call this general movement of the earth’s crust and the landforms which result from those movements Plate Tectonics. The plates are a rigid shell of the earth’s crust and are approximately 100km. thick. Each plate measures several thousand kilometers across. Some of these plates have actual continents on them such as the South American Plate while others do not such as the Caribbean Plate.

Figure 1 shows the largest plates and a number of smaller plates that makes up the earth’s crust.                      

Largest Plates	Smaller Plates
Antarctic Plate	Philippine Plate
North American Plate	Caribbean Plate
South American Plate	Cocos Plate
Pacific Plate	Arabian Plate
African Plate
Indo-Australian Plate
Eurasian Plate
Nazca Plate

                      Figure 1: Crustal plates that makes up the Earth's crust. 

                    

The main crustal plates and their margins                                                                                            

When plates move they do so over the face of the earth in a circular path around its pole of rotation. In doing so, they move away from each other, towards another and alongside each other. These movements results in three types of plate boundaries or Plate Margins (Figure 2). Where the two plates meet forms a weakness in the crust and earthquakes and volcanoes are usually associated in these area.

                                                             Figure 2: Crustal plate margins or boundaries.  

There are three types of plate margins:
Divergent Margin:

Where two plates are slowly pushed apart, there is a divergent margin. Hot molten rock comes up from the mantle between the cracks and hardens to form a strip of new crust. In some places the magma flows very slowly, whereas, in others, there are volcanic eruptions. Divergent plate margins are also called constructive plate margins because new crust is formed when there is movement. These types of boundaries or margins can be found beneath the world’s ocean. 

                                                                Figure 3: Divegent plate margins

Convergent Margin:

Remember plates are being created and are spreading, there must be a place where they stop moving, this area is called a convergent plate margin. The plates in these areas are pushed together. Convergent margins are usually formed close to the edge of the oceans, however there other types of convergent margins (See figure 4B). When the plates collide, the heavier, denser plate will normally be deflected or pushed beneath the less dense plate. It will be destroyed and absorbed into the mantle.  This area is called the subduction zone. Convergent plate margins are known as destructive plate margin because plates are destroyed in this region. Volcanoes are formed at convergent plate margins, where a plume of magma rises towards the surface. When this occurs on the oceanic crust, a volcanic island arch is formed. 

                                                                              Figure 4: Convergent plate margins

Figure B4: Convergent Margins (See more at: http://www.passmyexams.co.uk/GCSE/physics/convergent-boundaries.html) 

Transform Margin:

The third type of plate margin is a transform margin. We also refer to this margin as a neutral (passive) or a conservative margin. At this type of margin the plates slide pass each other without converging or diverging. Earthquakes are present at all margins. However, there is little or no volcanic activity at transform plate margins. Friction prevents the plates from moving smoothly. They jam to each other and jerk and snap causing earthquakes. The plates move in parallel but in opposite directions they pass each other without causing destruction of the plates or adding new material to the ocean floor. The northern boundary of the Caribbean plate is a transform margin. It slides passes the North American plate roughly though Hispaniola and Jamaica. This explains why earthquakes are common along this line. This type of margin can also be found at the San Andreas Fault, California, USA.   

Figure 5: Transform plate margins

                                                                         
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How Plates Move

The plates are always moving like a raft or boat on the top of the semi-molten mantle below. However, this movement is very slow and ranges from less than 2.5cm a year to over 15cm. a year. The plates move due to convection currents which occur in the semi-molten mantle. These currents are created from the heat generated from the inner most part of the earth, the core. 

As the semi-molten rock in the mantle is heated it becomes less dense and begins to rise (heat rises). As it reaches the crust above, it spreads out and moves the plates with it. The semi-molten rocks then cools and gradually sinks back down to be re-heated. This process occurs constantly resulting in the plates moving. 

                                                                                 Figure 6: Convection Currents in the mantle. 

Plate Margin Summary 

Read More>>> http://www.learner.org/interactives/dynamicearth/

Sources: 

The Caribbean Environment for CSEC Geography. Fourth Edition (2012) Mark Wilson 

New Caribbean Geography with map reading and CXC questions. Fourth Edition. Vohn A.M. Rahil

GeoBytesGCSE. http://geobytesgcse.blogspot.com/2007/01/plates-and-convection-currents.html. 

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