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Biology 2nd Baccalaureate

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6.2.1. Geochemical model


The crust is the outermost and thinnest layer of the Earth, with an average thickness of about 35 km. It occupies from the surface to the Mohorovicic discontinuity. Both tectonic movements and external geological processes (erosion, transport and sedimentation) can be seen in the crust. Two types of bark are distinguished:

  • Oceanic crust.
    • Its thickness is 8-10 km.
    • It is made up of rocks such as basalt and gabbro.
    • In the oceanic crust these forms are distinguished:
      • Ridges, which cross the oceans and have intense volcanic activity.
      • Abyssal plain, a flat and extensive bottom.
      • Pits, deep depressions of up to 11,000 meters and elongated.
    • The oceanic crust is relatively young, with maximum ages of 180 million years , being more modern in the areas near the ridges and older near the continents.
  • Continental crust.
    • It is thicker than the oceanic crust, and can reach up to 70 km deep in mountainous areas.
    • It is made up of highly varied sedimentarymetamorphic and igneous rocks. Pimples abound.
    • It is much older than the oceanic crust, being able to have rocks of more than 3.800 million years, although it varies according to the zones:
      • Cratons or continental shields: The oldest areas usually form the central areas of the continents, and are geologically stable, without volcanoes or earthquakes.
      • Orogens: they are the most recent areas of the continental crust. They are usually found on the periphery of the continents and with volcanic and seismic geological activity.

Interactive Activity: Continental or Oceanic Crust?.


The mantle is the layer of the Earth that lies between the crust and the core, from the Mohorovicic discontinuity (about 35 km) to the Gutenberg discontinuity (2900 km). It makes up 84% of the planet's volume. It is formed by rocks rich in iron and magnesium silicates, such as peridotite, composed mainly of the mineral olivine.

The physical conditions of the rocks of the mantle vary due to the increase in pressure and temperature, so three zones are distinguished in the mantle:

  • Upper mantle. It goes up to 650 km deep. As the pressure is lower, the rocks are less compacted.
  • Lower mantle. From 650 to 2900 km deep, approximately. The rocks are more compacted and their density is higher.


The core is the innermost part of the Earth, made up of iron and a little nickel, sulfur, and oxygen. Due to the behavior of seismic waves (s waves do not cross it), two layers are distinguished:

  • Outer core: fluid, from 2,900 km to 5,100 km.
  • Inner core: solid, from 5100 km to 6370 km.

In the outer core convection currents are produced that are the cause of the generation of the Earth's magnetic field.


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.