Although we think of the land on Earth as being fixed and stable, it turns out that it is constantly moving. This movement is way too slow for us to notice, however, because it only moves between one to 6 inches per year. It takes millions of years for the land to move a significant amount.
The Lithosphere
The part of the land that is moving is the Earth's surface called the lithosphere. The lithosphere is made up of the Earth's crust and a part of the upper mantle. The lithosphere moves in big chunks of land called tectonic plates. Some of these plates are huge and cover entire continents.
Major and Minor Tectonic Plates
Most of the Earth is covered by seven major plates and another eight or so minor plates. The seven major plates include the African, Antarctic, Eurasian, North American, South American, India-Australian, and the Pacific plates. Some of the minor plates include the Arabian, Caribbean, Nazca, and Scotia plates.
Here is a picture showing the major tectonic plates of the world.
Click on the picture to see a larger view
Continents and Oceans
Tectonic plates are around 62 miles thick. There are two main types of tectonic plates: oceanic and continental.
Oceanic - Oceanic plates consist of an oceanic crust called "sima". Sima is made up primarily of silicon and magnesium (which is where it gets its name).
Continental - Continental plates consist of a continental crust called "sial". Sial is made up primarily of silicon and aluminum.
Plate Boundaries
The movement of tectonic plates is most evident at the boundaries between the plates. There are three main types of boundaries:
Convergent Boundaries - A convergent boundary is where two tectonic plates push together. Sometimes one plate will move under the other. This is called subduction. Although the movement is slow, convergent boundaries can be areas of geological activity such as the forming of mountains and volcanoes. They can also be areas of high earthquake activity.
Tectonic plate convergence
Divergent Boundaries - A divergent boundary is one where two plates are getting pushed apart. The area on land where the boundary occurs is called a rift. New land is formed by magma pushing up from the mantle and cooling as it reaches the surface.
Transform Boundaries - A transform boundary is one where two plates slide past each other. These places are often called faults and can be areas where earthquakes often occur.
Interesting Facts about Plate Tectonics
One famous transform boundary is the San Andreas Fault in California. It is the boundary between the North American Plate and the Pacific Plate. It is the cause of so many earthquakes in California.
The Mariana Trench is the deepest part of the ocean. It is formed by a convergent boundary between the Pacific Plate and the Mariana Plate. The Pacific Plate is being subducted under the Mariana Plate.
Scientists are now able to track the movement of tectonic plates using GPS.
The Himalayan Mountains, including Mount Everest, were formed by the convergent boundary of the Indian Plate and the Eurasian Plate.
