earth, in geology and astronomy: The Earth's Crust and the Moho

The outer shell, or crust, varies from 5 to 25 mi (8 to 40 km) in thickness, and consists of the continents and ocean basins at the surface. The continents are composed of rock types collectively called sial, a classification based on their densities and composition. Beneath the ocean basins and the sial of continents lie denser rock types called sima. The sial and sima together form the crust, beneath which lies a shell called the mantle. The boundary between the crust and the mantle is marked by a sharp alteration in the velocity of earthquake waves passing through that region. This boundary layer is called the Mohorovičić discontinuity, or Moho.

Sections in this article:

• Introduction
• The Origin of the Earth
• The Change in Seasons
• Rotation and Revolution
• The Astronomical Earth
• The Earth's Core
• The Earth's Mantle
• The Earth's Crust and the Moho
• The Geologic Earth
• Bibliography

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