Discovery Science: Structure Of The Earth’s Crust

Earth Science: Structure Of The Earth’s Crust

The Earth’s crust is the thin and rigid outer shell of the Earth. The Earth has a continental and an oceanic crust, differing in thickness, density, and type of rock content, as well as the age and origin of the rock material.

The thin outer shell of the Earth is on average only about 22 miles (35 km) thick. It may reach 19 to 44 miles (30 to 70 km) of thickness where the continents are situated. The crust is thickest where the roots of mountain ranges reach deep into the Earth’s mantle. The crust is thinnest underneath the oceans: only 3 to 5.6 miles (5 to 9 km) thick. The Mohorovicic discontinuity, which is also referred to as “Moho,” separates the Earth’s crust from the mantle.

The temperatures in the crust and the outer mantle are not hot enough to maintain the rock material in a liquid state. These brittle rock layers combined are called the lithosphere, moving according to plate tectonics and the underlying movement of the lower layer, called the asthenosphere.

Oceanic and continental crust

One third of the Earth’s crust is continental crust and two-thirds are oceanic crust. In between these two types of crust, there is a slightly broken-up seismic layer of discontinuity referred to as the Conrad discontinuity. At no point is the oceanic crust older than 200 million years as it continuously regenerates at the edges of the large continental plates in the areas of the mid-oceanic ridges. In contrast, the continental crust contains the oldest rocks ever found.

They are about four billion years old. Nevertheless the continental crust changes shape all the time due to tectonics, volcanic activity, erosion, and sedimentation. The comparatively young oceanic crust mainly consists of heavy rock material such as basalt or gabbro with a high density of 1.6 to 1.7 ounces per cubic inches (2.9-3.1 g/cm3). Due to its prevalent silicon magnesium compounds, it is also called sima rock.

Continental crust material is slightly lighter by 1.5oz/in 3 (2.7 g/cm3) and consists of granite with a high silicon and aluminum content. Therefore, it is often referred to as sial rock.


Andrija Mohorovicic (1857-1936), former director of the Institute for Meteorology and Geodynamics in Zagreb, started studying seismology in 1900. He produced seismic diagrams following an earthquake near the capital of Croatia in 1909 and found that seismic waves were dispersing at different speeds.

From these findings, he concluded that the density must be changing within the Earth’s crust and defined the limiting value between the Earth’s crust and the mantle.



     Crust          Weight
     element      proportion

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