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2.2. Structure and composition of the crust

Structure of the earth's crust

The Earth's crust is the outermost and thinnest layer of the Earth, and is in contact with the atmosphere and the hydrosphere.

The earth's crust is made up of thin materials, such as light silicates, carbonates, and oxides.

Geologically, the earth's crust is a very active layer, since two types of geological processes take place:

There are two types of earth's crust:

Continental crust

The continental crust has the following characteristics:

  • It is made up of rocks of varied composition, although sedimentary rocks tend to predominate in the most superficial part, metamorphic rocks  in the deepest areas, and there are also igneous rock outcrops.
  • Its thickness is greater than that of the oceanic crust, having an average thickness of about 33 km, although it is greater in areas with orogens, and can reach up to the 75 km thickness that it has in the Himalayas.
  • The continental crust can be as old as 3.8 billion years.
  • The continental crust is the one found on the continents (in their mountains, valleys, plateaus, etc.) and on the continental shelf (an area near the coast of not too deep, covered by sea ​​water).

Ocean crust

The main characteristics of the oceanic crust are:

  • The oceanic crust is thinner than the continental crust. It varies between 6 and 12 km, with an average thickness of 7 km.
  • The structure of the oceanic crust is divided into three levels:
    • Lower level: Formed by gabbros, some basic plutonic rocks.
    • Intermediate level: Formed by basalts, some basic volcanic rocks.
    • Upper level: Formed by marine and continental sediments (near the coast).
  • Although most of the oceanic crust is under the sea, there are exceptions like Iceland, which are above sea level in the mid-ocean ridges.
  • The oceanic lithosphere, formed by the oceanic crust and part of the upper mantle, is generated in the oceanic ridges and disappears, descending towards the mantle, in the oceanic trenches. That's why the oldest ocean rocks are only 180 million years old.
  • The oceanic crust is characterized by its great abyssal plains of the seabed in which the oceanic ridgesguyots (seamounts) and oceanic trenches stand out.

What is the lithosphere?

The lithosphere is made up of the crust and the upper part of the mantle.

The lithosphere is divided into lithospheric or tectonic plates, in whose limits volcanoes and earthquakes usually occur. It is at these plate edges that the lithosphere is created and destroyed.

Chemical composition of the earth's crust

Although any chemical element can be found in the earth's crust, as the layers of the geosphere were formed by the different density of terrestrial materials, the chemical elements that are part of the less dense materials predominate.

The most abundant chemical elements in the earth's crust are the following:

Oxygen
46.6%
Silicon
27.7%
Aluminum
8.1%
Iron
5.0%
Calcium
3.6%
Sodium
2.8%
Potassium
2.6%
Magnesium
2.1%

These chemical elements combine to form the minerals found in the earth's crust, among which silicates stand out, such as quartz, feldspars, micas, clays, etc.

The Earth's crust in other courses

The topic of the earth's crust is also covered in: