How many?

Following the devastating earthquake in Japan, many people were made aware of the importance of tectonic plates and the role that they play in causing earthquakes. A good earth’s tectonic plates map will outline the seven primary tectonic plates, but it may not show the entire picture. This is because plate tectonics encompasses a more detailed study than just the major plates. There are seven primary tectonic plates in the world, but there are also seven secondary plates and sixty tertiary plates, for a grand total of more than seventy tectonic plates.

Each tectonic plate is approximate 60 miles thick and composed primarily of either continental crust or oceanic crust. These plates are part of the Earth’s lithosphere which is comprised of the upper mantle and the crust. The boundaries between plates are where major earth activity occurs, such as volcanoes, earthquakes and the formation of mountains and ocean trenches. There are three different types of plate boundaries: conservative transform boundaries, convergent boundaries (also known as collisional boundaries), and divergent boundaries which are often referred to as spreading centers.

Because plate tectonics is still a relatively new science (having replaced the theory of continental drift in the late 1950s and early 1960s), there is some dispute about the actual categorization of plates into the three levels. Most experts do agree that there are seven primary plates that make up the majority of the earth’s surface and the Pacific Ocean. These plates are the African Plate, the Antarctic Plate, the Eurasian Plate, the Indo-Australian Plate, the North and South American Plates, and the Pacific Plate.