The Earth’s surface is not a continuous layer. The lithosphere is in fact broken up into several large pieces called crustal plates or tectonic plates. These large segments of the crust and upper mantle float on the denser semi- molten rock beneath them and move very slowly across the surface of the planet. Each plate moves only a few centimeters in a year but their movement affects us all. Plate tectonics can be described as the study of crustal plates and how their movement affects the Earth’s surface.
Crustal Plates
Most of the territories which make up the Caribbean are located on one plate which is called the Caribbean Plate. To the north of the Caribbean Plate lies the North American Plate and to the south of it lies the South American Plate. To the south west lie the Nazca Plate and Cocos Plate. These are just a few of the Earth’s crustal plates. There are seven primary crustal plates and several smaller ones. The primary crustal plates are the North American, South American, African, Antarctic, Indo-Australian, Eurasian and Pacific plates. The numerous smaller ones include the Nazca Plate, Cocos Plate, Madagascar Plate and Okinawa Plate. For a better idea of the Earth’s crustal plates and the directions in which they are moving click the link below. You can click on the map to zoom in.
Earth’s tectonic plates (by Eric Gaba)
The Earth’s crustal plates move slowly over the planet’s surface, interacting in various ways. The area where the edges of two plates meet is called a plate boundary or a plate margin. The way that these plates are moving relative to each other determines the type of plate boundary. There are three types of plate boundary; convergent, divergent and transcurrent boundaries.
Convergent Boundaries
In some areas the plates move toward each other. This is called convergence. At convergent plate boundaries (also called destructive plate boundaries), crustal plates collide in super slow motion causing earthquakes, folding and volcanic activity. It is estimated that about three quarters of all earthquakes occur along convergent plate boundaries.
As the massive plates are pushed together, the rocks along the plate boundary are placed under tremendous pressure. This pressure often causes them to buckle and fold. In some areas, rocks are pushed downward while in others rocks are pushed upward. Rock may be pushed thousands of feet upward forming mountain ranges. Most of the very high mountains in the world were formed in this way.
Many convergent boundaries occur along the edges of our oceans. In these regions, oceanic crust collides with continental crust. The denser oceanic crust is pushed under the less dense continental crust. This process is known as subduction and the area where it occurs is known as a subduction zone. As the oceanic crust is pushed below the continental crust massive forces are generated. This causes the edges of the oceanic and continental plates to bend forming a V-shaped depression called an ocean trench. The oceanic crust melts as it is pushed down into the mantle. This molten material may rise through weaknesses in the crust forming volcanoes near the plate boundary.
There are instances where oceanic crust collides with oceanic crust. In this case one of the oceanic plates will be pushed under the other forming a subduction zone and creating an ocean trench. As the plate is pushed down into the mantle it melts. Molten material may rise through weaknesses in the crust forming underwater volcanoes. These underwater volcanoes may eventually grow tall enough to rise above the surface, forming volcanic islands. A chain of islands known as an island arc may form in this way. The Lesser Antilles is a good example of an island arc. This arc formed as the crust beneath the western edge of the Atlantic ocean was forced below the eastern edge of the Caribbean plate. There are still active or dormant volcanoes on many of the islands in this arc. An ocean trench called the Lesser Antilles Trench lies to the east of the Lesser Antilles. The Puerto Rico Trench to the north of Puerto Rico and the Cayman Trench between Jamaica and the Cayman Islands were formed on the boundary between the Caribbean and North American plates.
Divergent Boundaries
In some areas, crustal plates are moving away from each other. This is called divergence. At divergent plate boundaries the slow separation of the plates creates a small space through which magma rises and cools, creating or constructing new crust. For this reason, these boundaries are also called constructive plate boundaries. Earthquakes sometimes occur at divergent plate boundaries. Also the magma rising through weaknesses in the crust near divergent plate boundaries causes volcanic activity. Divergent plate margins often occur beneath our oceans. On the ocean floor in the middle of the Atlantic lies a divergent plate boundary called the Mid-Atlantic Ridge which runs almost the entire length of the Atlantic Ocean. Because the plates there are moving apart, the Atlantic Ocean becomes a few centimeters wider each year.
Transcurrent Boundaries
In some areas plates are not moving toward or away from each other. Instead, they simply slide past each other. This is called transcurrence. At transcurrent (or transform) plate boundaries, earthquakes sometimes occur. The famous San Andreas Fault zone is an example of a transcurrent or transform plate boundary.
Watch the video below which explains the theory of plate tectonics, then take the Plate Tectonics quiz!