What is the Oceanic Crust Made of

Earth crust is made up of continental crust as well as oceanic crust, therefore, one should know what is the oceanic crust made of, to fully understand the composition of earth’s crust. Crust is the thin layer of rocks that make up the surface of the earth. Most of us are aware of the rocks that make up the surface of the earth and the earth’s crust, but few know about the oceanic crust. This is because of the fact that oceans remained covered with water while the rocks can be easily seen on the land surface. It is the floor of the oceans where oceanic crust lies. This article takes a close look at the crust of the earth that is covered by the oceans.

What is the Oceanic Crust made of – Facts

Oceanic crust is thin and very dense

Oceanic crust makes up just 0.01% of the mass of the earth. It has a thickness of almost 7 kilometers and it is basically made of basaltic rocks. The floor of the oceans is made up of rocks containing mainly silica and magnesium. This is why it is referred to as Sima. Oceanic crust is not only thinner and denser than continental crust, and it is also much younger than it and has a different chemical composition. When magma from the mantle of the earth finds a way up, it comes into contact with water that cools it down quickly. It is forced to take the shape of pillows.

What is the Oceanic Crust made of

Magma cools down quickly to form oceanic crust

Unlike continental crust that remains at its place and is quite old, oceanic crust is continuously being made by oceanic ridges where tectonic plates slide and rub with each other. This friction allows the magma from below to move upwards. It cools down quickly and gets converted into oceanic crust. These oceanic ridges make up a very large network of volcanoes that spreads to about 40000 kilometers. This system generates new oceanic crust that covers the floor of the ocean with basaltic rocks.

Subduction process is responsible for the creation of new oceanic crust

However, the network of volcanoes creates subduction zones. Heavier oceanic crust sinks below the lighter rocks to reach these subduction zones where it starts to melt again. This molten crust once again finds its way up to form the ocean crust. This cycle continues again and again and this is why oceans get a new crust after passage of nearly 200 million years. Subduction process does not allow the oceanic crust to become older than 200 million years as a new crust is formed. On the other hand, continental crust remains where it is for billions of years and it is much older than oceanic crust.

Composition of oceanic crust

Oceanic crust is made up of several layers with the topmost layer being just 500 meters thick. This is made up of basaltic rocks in the shape of pillows and sheets. The lower layer of oceanic crust is made up of two sub-layers that cover a thickness of 4.5 kilometers. These layers are made up of gabbros that are basically basalt mixed coarse grains of minerals. These gabbros contain pockets or chambers of lava that finds its way up to the floor of the ocean.


Images Courtesy:

  1. Subduction by Eround1 (CC BY-SA 3.0)

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