The Earth’s surface is fragmented into enormous, rigid sections known as tectonic plates. These plates, which form the planet’s outermost layer, are constantly in motion, moving at speeds comparable to the growth rate of a human fingernail. This slow but relentless movement reshapes continents, builds mountains, and drives geological events like earthquakes and volcanic eruptions. The shifting of these segments dramatically alters the planet’s geography, a process known as plate tectonics.
Defining the Earth’s Tectonic Plates
Tectonic plates are composed of the lithosphere, the planet’s solid, approximately 100-kilometer-thick outer layer. The lithosphere includes the crust and the rigid, uppermost portion of the mantle. Below this layer lies the asthenosphere, a layer of the mantle made of rock that is so hot and under high pressure that it is plastic and semi-molten.
The plates move horizontally because the lithosphere rests upon the weaker asthenosphere. This motion is driven by mantle convection, a process fueled by heat escaping from the planet’s interior. Hotter, less dense material rises from the mantle, while cooler, denser material near the surface sinks, creating slow-moving currents that drag the overlying plates. These movements cause plates to converge, diverge, or slide past one another at their boundaries.
Identifying the Seven Major Plates
Scientists agree upon seven large continental- and ocean-sized plates that cover the majority of the Earth’s surface. These major plates are named for the continents or oceans they encompass. The seven principal plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American plates.
The Pacific Plate is the largest and is almost entirely oceanic, lying beneath the Pacific Ocean basin. It is highly active, especially along its margins where it subducts beneath surrounding plates. The North American Plate contains North America, Greenland, and a portion of the Atlantic Ocean floor, carrying both continental and oceanic crust.
The Eurasian Plate encompasses most of Europe, Russia, and Asia, extending into the surrounding oceanic crust. It is bordered on the west by a divergent boundary running through Iceland. The African Plate includes the entire African continent and is surrounded by oceanic crust.
The Antarctic Plate holds Antarctica and the surrounding ocean floor. This plate is bordered almost entirely by divergent boundaries where new crust is created. The Indo-Australian Plate comprises the Indian subcontinent, Australia, and the oceanic crust of the Indian Ocean; it is often studied as two separate plates (Indian and Australian). The South American Plate includes the South American continent and the adjacent western Atlantic Ocean floor.
Key Smaller Tectonic Plates
In addition to the seven major plates, numerous smaller plates play a significant role in localized geology, often at the boundaries of the larger segments. These minor plates are frequently sites of intense seismic and volcanic activity. The Nazca Plate, an oceanic plate in the eastern Pacific, is subducting beneath the South American Plate. This interaction is responsible for the uplift of the Andes Mountains and the high frequency of earthquakes along South America’s western coast.
The Cocos Plate is situated beneath the Pacific Ocean off the coast of Central America. Its movement relative to the North American and Caribbean plates drives significant seismic events in that region. The Juan de Fuca Plate, off the coast of the Pacific Northwest, is subducting beneath the North American Plate. This process fuels the volcanic activity in the Cascade Range.
The Arabian Plate, composed mostly of the Arabian Peninsula, is colliding with the Eurasian Plate. This collision is closing the Persian Gulf and contributing to mountain building in the Middle East. The Philippine Sea Plate is situated between the Pacific and Eurasian plates. These smaller, highly active segments demonstrate the complex, fractured nature of the Earth’s outer shell.