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Oceanic Crust: Definition, Composition & Facts

Oceanic Crust: Definition, Composition & Facts
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  • 0:01 What Is Oceanic Crust?
  • 1:00 Composition of Oceanic Crust
  • 2:26 Where Does It Come From?
  • 3:10 Subduction and Volcanoes
  • 4:13 Magnetic Anomalies
  • 4:54 Lesson Summary
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Lesson Transcript
Instructor: Suzanne Rebert

Suzanne has taught college economics, geography, and statistics, and has master's degrees in agricultural economics and marine affairs (marine resource management).

A thin layer of rock separates the Earth's oceans from the hot mantle beneath them. In this lesson, you'll read about the composition and life cycle of this oceanic crust. Then you can take a quiz to test your understanding.

What Is Oceanic Crust?

All over the Earth, just a few miles below our feet, the powerful tectonic forces that create and destroy the Earth's crust are at work 24 hours a day. We're usually unaware of them until a violent earthquake, volcanic eruption, or tsunami threatens lives and property, but this cycle is essential to all life. It creates the crust of the Earth we walk on, and the crust that lies at the ocean floor.

Oceanic crust is the part of the Earth's crust that makes up the seafloor. It's thinner, denser, and simpler in structure than the continental crust. Oceanic crust is also younger, on average; from its birth at a mid-ocean ridge to its end at a subduction zone is no more than 250 million years.

Fragments of the oceanic crust sometimes become stranded high and dry on top of a continent due to uplift. It is partly thanks to these ancient ophiolites that we understand as much as we do about the composition of this layer of the Earth's lithosphere.

Composition of Oceanic Crust

The oceanic crust, which, on average, is only about six kilometers thick, is primarily made up of the igneous rock basalt. Basalt tends to come from lava that flows smoothly and quietly from a volcanic vent, unlike the viscous lava typical of the violent eruptions of many continental volcanoes.

A term that is sometimes used for oceanic crust rocks is sima, which is short for magnesium silicate, a common component of these rocks. Another term for these undersea igneous rocks is mafic, which comes from the fact that they are high in magnesium and iron. Basalt and gabbro are examples of mafic rocks.

Not all 'gentle' eruptions happen under water - Hawaiian volcanoes are famous exceptions - but due in part to the incredible pressure exerted by thousands of feet of water, ocean floor eruptions are usually the source of smooth 'pillows' or 'sheets' of dark, dense basalt. In fact, these pillows and sheets make up the top 500 meters of the seabed, not counting the thin layer of sediment and detritus over that.

Farther down lies a 1-kilometer layer of basalt dikes, the remnants of channels that once brought molten rock up from deeper in the lithosphere. Finally, two layers of gabbro, totaling about four and a half kilometers in thickness, represent the old magma chamber from which all the overlying basalt originally flowed.

Where Does It Come From?

But where does the oceanic crust come from? Mid-ocean ridges are underwater mountain ranges that form where plates of crust are pulling away from each other. Even though oceanic crust is very heavy, the powerful forces of convection from the hot mantle beneath the crust are strong enough to push this dense mafic material up at the ridges.

There are a few places on land where you can see and even stand on one of these ridges. Iceland is one. There, the European and North American plates are pulling away from each other at about the same rate that your fingernails grow. The mid-ocean ridges actually are one long ridge that continues through every ocean in the world, and at a total of 49,700 miles, it's the longest mountain range on Earth.

Subduction and Volcanoes

Because oceanic crust is denser than continental crust, it sinks very slowly under the tectonic plates of the continents as it is pushed away from the mid-ocean ridges. This is the process known as subduction.

At the plate boundaries where this occurs, which are known as subduction zones, the melting oceanic crust combines with seawater that gets dragged down with it to form explosive volcanoes, the likes of which include Krakatoa and Mount St. Helens.

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