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Plate tectonics is the study of how the earth’s crust is shaped by geological forces. The crust is divided into plates that move, causing geological events like earthquakes, mountains, and volcanoes. There are 14 major plates, and they move at a rate of 1-3 inches per year. Subduction zones and divergent zones are marked by deep trenches and create new crust. When two continental plates converge, they create mountain ranges. The “Ring of Fire” is a series of active volcanoes located along the perimeter of the Pacific Ocean. Continental drift was a forerunner of plate tectonics, and the theory of Pangea explains how the continents moved.
Plate tectonics is the study of how the earth’s crust is shaped by geological forces. It is based on the understanding that the crust is divided into large chunks, or plates, that sit on molten magma beneath the surface. The currents within cause the plates to move, causing many different geological events, including earthquakes and the formation of mountains and volcanoes. Understanding how plates move and interact is the main purpose of plate tectonics.
The earth’s crust
While it may appear that the earth’s crust is a solid shell, plate tectonics states that it is cracked into several large pieces. These pieces are called tectonic plates and are, on average, about 50 miles (80 km) thick. Underneath the plates is the partially molten layer of the Earth’s core, called the mantle. The mantle is in a constant state of motion, propelled by heat from the Earth’s inner core; it acts like a conveyor belt that slowly moves the slabs floating above.
According to plate tectonics, there are 14 major plates:
Pacific Plate
Piatto Juan de Fuca
North American dish
South American dish
Caribbean dish
Cocos Plate
Nazca plate
Scotia Plate
Antarctica plate
African dish
Arabic dish
Eurasian Piastra
Indian-Australian dish
Filipino dish
The plates move at a rate of about 1-3 inches (2.5-7.5 cm) per year. As they move, pressures build up at their boundaries, creating various types of geological events: crust is created, destroyed, or broken apart; earthquakes occur; mountain ranges arise; and the continents shrink and grow.
Subduction zones and divergent zones
When a thin oceanic plate converges with – or is pushed into – a thicker continental plate, the oceanic plate will be pushed down under the continental plate. This is called the subduction zone and is usually marked by a deep trench. As the edge of the oceanic plate slides into the soft molten mantle, it drags the rest of the plate along. This process is referred to as slab mining.
As the crust is consumed in subduction zones, it is created in divergent zones. In these areas, the plates move away from each other. The best example is the Mid-Atlantic Ridge, which lies midway between the East Coast of the United States and Africa, and marks the plate boundaries of the North American and African plates. Volcanic material is constantly bubbling up from the seafloor at the site of spreading plates, creating new marine crust as old crust moves outward.
Mountains, earthquakes and volcanoes
When two continental plates converge, they create mountain ranges. This occurs when the plates compress and push the crust upwards, somewhat like the folds in a blanket. The highest mountain range on Earth, the Himalayas, were formed when the Indo-Australian plate collided with the Eurasian plate. Indeed, the Indo-Australian Plate continues to move northward and the mountains continue to grow.
Instead of colliding, some plates rub against each other. Because the rocks on plate boundaries cannot slide smoothly past each other, the very slow motion causes friction to gradually build up until the plates “slip,” causing an earthquake. The San Andreas Fault in California is a prime example of this slip; the Pacific and North American plates slide past each other near this area, causing the famous California earthquakes. The strength and length of these earthquakes are related to how the fault zone is deformed by plate movement.
The “Ring of Fire” is a series of active volcanoes, including Mt. St. Helena, Mt. Fuji, Mt. Pinatubo and others — located along the perimeter of the Pacific Ocean. As the Pacific Plate moves in a northwesterly direction, it rubs against the surrounding plates. This rubbing causes molten magma to be pushed along all the outer edges of the plate, causing many of the volcanoes in this area.
continental drift
A forerunner of plate tectonics was the theory of continental drift, put forward in 1912 by the German scientist Alfred Lothar Wegener. Wegner observed that the coasts of Africa and South America were eerily similar, as if they could fit together. He also found paleontological records that revealed shared coastal fossils. This and other data led Wegener to hypothesize that all the continents were once united into a supercontinent that he called Pangea, which is Greek for “all lands.”
According to Wegener’s theories, Pangea began to slowly break apart 200 million years ago, first into two huge landmasses, which he called Gondwanaland and Laurasia, and then into the continents seen today. This explained conflicting geological records, such as glacial deposits in lands that are now deserts or tropical plant remains found in Antarctica. However, only when a theory was developed about how continents might move did plate tectonics become a viable science.
