The Earth’s Tectonic Plates

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In the Earth’s lithosphere, gigantic and irregularly-shaped rocks, known as tectonic plates cover the surface (crust and mantle). These tectonic plates can range in size from hundreds of kilometers wide to thousands of kilometers across.

The Earth’s crust and the upper part of the mantle form the lithosphere. This lithosphere is divided into tectonic plates.

Ocean Crust and Continental Crust

Most tectonic plates are composed of both oceanic and continental crust. The Pacific Plate, however, is made up of mostly oceanic crust.

Oceanic crust, which is basaltic, is thinner than continental crust. Continental crust, which granitic, is more buoyant than oceanic crust.

Volcanic and earthquake activity tends to be more concentrated at the margins of these plates.

What is Plate Tectonics?

Plate tectonics is the study of how the plates’ movement and behavior shapes the landform of the Earth.

What are the Earth’s Major Plates?

There are seven major tectonic plates. Major tectonic plates tend to be at least 20 million square kilometers in size.

The Earth’s major tectonic plates are:

  • African Plate
  • Antarctic Plate
  • Eurasian Plate
  • Australian Plate
  • North American Plate 
  • Pacific Plate
  • South American Plate 

What are the Earth’s Minor Plates and Microplates?

Minor tectonic plates are plates that are less than 20 million square kilometers in area but are larger than one million square kilometers. The Indian, Nazca, and Juan de Fuca plates are examples of minor tectonic plates.

Microplates are tectonic plates that are smaller than one million square kilometers. The Bismark, Mariana, Easter, and Juan Fernandez are examples of microplates.

Map of the Earth’s Major and Minor Tectonic Plates

Not all minor tectonic plates are shown on large scale tectonic plate maps. This global map created by the USGS shows all the major and some of the minor tectonic plates.

A colored map showing major and minor tectonic plates on Earth.
Map showing the location of major and minor tectonic plates on Earth. Map: USGS, public domain.

Tectonic Plates are in Motion

Tectonic plates are constantly in motion. Tectonic plates can move in different directions and at different rates of speed.

The movement of the Earth’s tectonic plates is responsible for the break up of the Earth’s supercontinent Pangea and for the subsequent formation of the Atlantic Ocean as the Americas and Africa shifted. More: Continental Drift Theory

Types of Plate Tectonic Boundaries

There are three different types of plate tectonic boundaries. These plate tectonic boundaries are: divergent, convergent, and transform plate.

Divergent Plate Boundaries

Two tectonic plates that are moving away from each other is known as divergent. New crust is formed when the plates move way from each other.

Earthquakes are common near divergent plate boundaries. Rift valleys like the East African rift valley form where tectonic plates diverge.

Mid-ocean ridges are found along divergent plate boundaries.

The Mid-Atlantic Ridge which runs down the middle of the Atlantic Ocean, is one such example. The Mid-Atlantic Ridge is about 65,000 kilometers (40,390 miles) long and forms part of the longest chain of mountains on Earth. The Mid-Atlantic Ridge extends from the Arctic Ocean in the north to past the south end of the continent of Africa to the south.

Map showing areas in red where seafloor gravity is stronger on Earth.
This gravity map created by NASA highlights the Mid-Atlantic Ridge (red arrow). Seafloor gravity tends to be stronger near tectonic plate boundaries. Map: NASA, public domain.

Ninety percent of those mountains in the Mid-Atlantic Ridge are found in the deep ocean.

Part of the Mid-Atlantic Ridge is a deep rift valley that is growing at the rate of 2 to 5 centimeters (0.8 to 2 inches) per year. The rift valley is about as deep and wide as the Grand Canyon.

At divergent plate boundaries new ocean floor is formed as the tectonic plates move apart.

Convergent Plate Boundaries

When two tectonic plates come together, these are known as convergent plate boundaries.

At convergent boundaries, mountains can be formed as one or both continental plates are pushed up. The Himalayan Mountain range and the Tibetan Plateau formed when the Indian and Eurasian Plates collided. Millions of years of convergence of these plates resulted in the Himalayan Mountain range as the highest continental mountains in the world. Mount Everest, part of the Himalayas, is the world’s highest mountain.

A subduction zone is where one plate is pushed (or subducted) beneath the other plate.

When the plates are pushed down, underwater trenches can be formed. Trenches form the deepest part of the ocean. The deepest point in the ocean is Challenger’s Deep in the Mariana Trench. The Mariana Trench is the boundary where the fast-moving Pacific Plate converges against the slower moving Philippine Plate.

Diagram showing how a subduction zone is created by one plate being pushed under the second plate.
Subduction Fault Zone Diagram. Image: USGS, public domain.

Volcanic and seismic activity is common along convergent plate boundaries. The Pacific Ring of Fire is a horseshoe line of more than 450 volcanoes that runs 40,250 kilometers (25,000 miles) in the Pacific Ocean.

Transform Plate Boundaries

When two plates slide past each other, these are called transform plate boundaries. The San Andreas Fault occurs at a transform plate boundary where two plates meet in western California. The Pacific Plate on the west moves northwestward relative to the North American Plate on the east.

View of the San Andreas Fault looking southeast along the surface trace in the Carrizo Plain, north of Wallace Creek. Elkhorn Rd. meets the fault near the top of the photo. Photo: Scott Haefner, U.S. Geological Survey. Public domain.
View of the San Andreas Fault looking southeast along the surface trace in the Carrizo Plain, north of Wallace Creek. Elkhorn Rd. meets the fault near the top of the photo. Photo: Scott Haefner, U.S. Geological Survey. Public domain.

Earthquakes are common along transform plate boundaries.

References

NWS JetStream Max – World’s major tectonic plates. (n.d.). National Weather Service. https://www.weather.gov/jetstream/plates_max

What are the different types of plate tectonic boundaries? (n.d.). NOAA Office of Ocean Exploration and Research. https://oceanexplorer.noaa.gov/facts/plate-boundaries.html

What is the mid-ocean ridge? (n.d.). NOAA Office of Ocean Exploration and Research. https://oceanexplorer.noaa.gov/facts/mid-ocean-ridge.html

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