What is a Continent? - Definition & Explanation Video

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  • 0:00 What Is a Continent?
  • 1:04 Continental Crust:…
  • 3:09 Building a Continent…
  • 4:27 Changing Shapes
  • 5:04 The Supercontinent Cycle
  • 6:01 Lesson Summary
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Lesson Transcript
Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

At some point in your elementary school career, you were likely taught that the earth has seven continents. But, geologically, the situation is a bit more complicated. Learn how our continents formed, and why, in this lesson.

What Is a Continent?

The Earth is known to have seven continents, but that can vary according to the physical layout of these land masses.
continents

When you hear the word continent, what image pops into your head? North America? South America? Africa? Most likely it is one of the seven large land areas you learned about in school, the other four being Europe, Asia, Australia, and Antarctica. But those names were created based as much on cultural characteristics as on physical ones.

A continent is commonly defined as a large land mass separated from other land masses by oceans. But, if you think about it, that definition is a little imprecise. For example, Asia, Europe, and Africa aren't separated by oceans. Neither are North America and South America. In fact, if you combine those set of land masses, you get only four continents. It seems that a continent can be defined in different ways.

From the geologic perspective, a continent is not defined by its size, location, who discovered it, or whether it is surrounded by oceans. Rather, it is defined by the rocks it's made of and how it came to be that matter.

Continental Crust: Definition and Origin

Geologists look at continents within the framework of plate tectonics. Continents are the part of the earth's crust, the outermost layer of the planet composed of igneous, sedimentary, and metamorphic rocks that have lower densities than the rocks of the adjacent seafloor crust and underlying mantle. In fact, the term continental crust is used specifically to describe these landmasses. Since continental crust is less dense, it remains at higher elevation and is not recycled back into the mantle at plate subduction zones, like the seafloor crust.

Continental crust is thicker than seafloor crust due to how it is formed and its lower density, which prevents it from recycling into the mantle.
thickness

The original crust of our planet formed from crystallization of magma and was composed entirely of igneous rocks that probably had a fairly uniform chemical composition and density, since there was little to distinguish one area of crust from the other.

It was probably the operation of plate tectonics that created continental crust. The process began early in Earth's history, because the oldest continental crust rocks we've found are about four billion years old. Melting of the original crust at plate subduction zones and above mantle hot spots, combined with widespread volcanic eruptions, produced magma with different chemical compositions. That magma crystallized into igneous and volcanic rocks that were slightly less dense and more buoyant than the original rocks.

The earliest continents were widely scattered islands, perhaps similar in size and shape to Indonesia, the Philippines, or Japan. Over time, those island chains, moved about by plate tectonics, collided, and merged into larger landmasses, sometimes called proto-continents.

Additional continental crust was added as weathering and erosion of the igneous rocks produced sediment that became sedimentary rocks. Although none of those early sedimentary rocks remain, some of them were altered to metamorphic rocks during plate collisions and mountain-building episodes. Many of those changed rocks survive as part of the modern continental crust.

Building a Continent Piece by Piece

Each of the modern continents contains a core of the ancient igneous and metamorphic rocks. That part of the continent is called the craton. They are the oldest rocks we find. For example, in North America, the craton in Canada contains rocks that are around four billion years old.

Cratons (shown here in orange, pink, and dark blue) make up the cores of each continent and are the oldest rocks of the crust.
provinces

The area where the craton is eroded and exposed at the surface is called the continental shield. The places where the craton is buried beneath younger sedimentary rocks is known as the continental platform. In North America, the shield extends from central Canada, south into Minnesota and the northern Great Lakes region. The platform is the area between the Appalachian and Rocky Mountains.

Around the edges of the craton are areas where the continent has been subjected to plate collisions and mountain building. They are known as mobile belts, in reference to the plate collisions, or orogenic belts. The Rocky Mountains and the Appalachians are examples found in North America. The eroded rocks along the coastal margins of the mobile belts are covered by young sediment deposits that form the coastal plains and the continental shelves, part of the continent that lie below sea level.

Changing Shapes

During the last ice age, large areas of continental shelf were exposed when sea level fell. The outlines of the continents were much different as a result.
shelf

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