Continental mass meeting up with a large body

Continental margins are areas that separate the continental crust and oceanic crust. Discover the differences between the tectonically active continental margins and the relatively quiet passive continental margins.

Earth

When you are active, you are always on the go and have a lot of energy. When you’re passive, you tend to lie around and things happen slowly.

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Planet Earth behaves in a similar way. It has active areas where earthquakes rattle the ground and volcanoes erupt, and it has passive areas where sediment slowly deposits and few earth-shaking occurrences happen. In this lesson, you will learn about these active and passive areas of the world, as you take a look at active and passive continental margins.

Continental and Oceanic Crust

To understand active and passive continental margins, it’s important to gain a basic understanding of the way our planet is put together.

The earth is not a solid clump of rock. Instead, it has different layers comprised of different materials. You can relate the main layers of the earth to a hard-boiled egg.

The yellow inner yolk of the egg relates to the earth’s core, which is a very hot place.The white part of the egg relates to the earth’s mantle, which is the largest layer and also pretty hot. The mantle is mostly solid, but the rock and material within the mantle can melt and form magma, which is the hot, molten rock that we associate with volcanoes. The shell covering the egg relates to the earth’s crust.Like the eggshell, the earth’s crust is relatively thin and can break into sections of crust or plates. The continental crust is the part of the earth’s crust that makes up the continents.

The oceanic crust is the part of the earth’s crust that underlies the oceans.

Continental Margin

The continental crust is less dense than the material that makes up the underlying mantle, so it tends to float on top of the mantle and move around. Of course, even though we live on the continental crust, we do not feel this movement because it’s very slow, in the range of about 1-10 centimeters during the course of a year.

The continental crust is also less dense than the oceanic crust, so the oceanic crust does not move as much. But, because the continental crust is moving, it can knock into the oceanic crust at a place called the continental margin. We can define the continental margin as the zone that separates the continental crust from the oceanic crust.

Active Continental Margin

So, as you can imagine, just about anywhere there is a large land mass meeting up with a large body of water, we would find a continental margin.

But, not all continental margins are created equal. Some are active and some are passive. Active continental margins are continental margins that are tectonically active. When we look at the term ‘tectonics‘ in geology, we see that it refers to the study of the deformation of the earth’s rocky crust and the forces that cause this deformation.So, an area of the world that is tectonically active would be an area where the rocky crust folds or deforms, as seen in mountain building. It would also be the site of earthquakes and volcanoes. The West Coast of the United States is a good example of an active continental margin.

Earthquakes have been known to shake California and other west coast states, and the Aleutian Islands of Alaska have many highly active volcanoes.Active continental margins tend to have a narrow continental shelf, which is the submerged border of the continent. So, the continent drops off quickly along an active margin and meets up with the more dense oceanic crust.

Because the oceanic crust is heavier, it gets forced underneath the continental crust, carrying lots of rock toward the hotter mantle of the earth, where it can melt. This area where plates converge and one plate is driven below the other is called a subduction zone. This contributes to the earthquakes and volcanic activity that we see associated with active continental margins.

Passive Continental Margin

Passive continental margins are continental margins that are not tectonically active. So, instead of seeing mountainous areas prone to earthquakes and volcanoes, with passive continental margins, we see gently rolling flat land and a wide continental shelf that slopes slowly into the ocean. The East Coast of the United States is a good example of a passive continental margin.The East Coast has long beaches from New Jersey down to Florida where the continental shelf is so wide in some areas that visitors to the beach can wade far out into the ocean and still touch bottom. Because subduction zones are not common, tectonic activity is low and we see passive processes, such as coastal erosion, and the passive depositing of sand and sediment dominate these landscapes.

Lesson Summary

Let’s review. The outer layer of the earth is called the crust.

The continental crust is the part of the earth’s crust that makes up the continents. The oceanic crust is the part of the earth’s crust that underlies the oceans. The continental margin is the zone that separates the continental crust from the oceanic crust.

Active continental margins are continental margins that are tectonically active. Tectonics is the study of the deformation of the earth’s rocky crust and the forces that cause this deformation. So, an area of the world that is tectonically active would be a mountainous area with earthquake and volcanic activity.

The West Coast of the United States is a good example of an active continental margin. Active continental margins tend to have narrow continental shelves.Passive continental margins are continental margins that are not tectonically active. These areas have flat lands and have a wide continental shelf, which is the submerged border of the continent.

The East Coast of the United States is a good example of a passive continental margin.A subduction zone is the area where plates converge and one plate is driven below the other. This contributes to the earthquakes and volcanic activity that we see associated with active continental margins. These zones are not common in passive continental margins, so coastal erosion and the passive depositing of sand and sediment dominate these areas.

Learning Outcomes

You’ll have the ability to do the following after this lesson:

  • Define continental crust, oceanic crust, and continental margin
  • Describe what causes active and passive continental margins
  • Identify examples of both active and passive continental margins
  • Explain what a subduction zone is
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