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Sea Floor Spreading: Definition, Theory & Facts

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  • 0:03 What Is Sea Floor Spreading?
  • 0:42 Spreading: How and Where?
  • 2:01 How Did People Learn This?
  • 2:58 Lesson Summary
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Lesson Transcript
Instructor: Terry Dunn

Terry has a master's degree in environmental communications and has taught in a variety of settings.

The continents are on the move and sea floor spreading helps to explain how. Here you will learn what sea floor spreading is, how it works, where it is happening, and what led to its discovery.

What Is Sea Floor Spreading?

Have you ever watched a cake baking in the oven? You know that moment when the top starts to cook and split when you can see that that gooey batter in the crack? It eventually cooks, too. Well, it turns out that the surface of the earth is also a little like that. The crusty surface of the cake is similar to the earth's tectonic plates, which are gigantic, slowly-moving pieces of the earth's crust. The uncooked 'batter' between the plates would be the magma. When the magma cools, it becomes part of the earth's crust. Where the plates are moving away from each other, they are called divergent plates. The shifting divergent plates under the ocean are the reason for sea floor spreading.

Spreading: How and Where?

Just like the cracks on a baking cake, long ridges called mid-ocean ridges form at the point of sea floor spreading. Think of them as underwater mountain chains. One of the more well-known mid-ocean ridges is the Mid-Atlantic Ridge. It splits the North American and Eurasian plates apart in the North, and the South American and African plates in the South. But there is a global mid-ocean ridge system which looks like a series of global zippers that the Mid-Atlantic Ridge is part of.

Wherever the sea floor is spreading, the edges move apart while the magma in the middle hardens. That means the rock in the center is the youngest. As the continents move apart, whole new oceans can form. Scientists believe that's how the Red Sea came into existence. 

So you may be wondering: if the sea floor is spreading is some places, wouldn't the planet be getting bigger? It seems logical, but the answer is that other parts the planet are shrinking at the same time! There are also convergent plates, where one plate moves underneath another plate. At those points in the ocean, called oceanic trenches, the earth is literally losing ground. Oceanic trenches tend to be along the margins of the continents, and most are on the margins of the Pacific Ocean. It's these areas that are most prone to earthquakes.

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