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Geologic Map
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Generalized Geologic Map of the Conterminous United States
North America Shaded Relief


  The North American Tapestry of Time and Terrain

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Legend and Rock Ages
Rock Types
Political Boundaries

  Legend and Rock Ages

Archean Rocks - 4 to 2.5 billion years ago

map of Archean rocks


The oldest rocks on earth are Archean-aged rocks. During the Archean, the cratons--the masses of rock that make up the basic, initial structure of continents--formed. The crustal bodies that formed were smaller than today's continents and are referred to as protocontinents.

During the Archean the earth was pummeled with meteors. Scientists believe that a huge impact tore away a large chunk of the earth and trapped it in orbit, forming the moon. Seawater and an atmosphere are believed to have developed during the Archean, and it was during this time that the oceans became populated with algae, bacteria, and colonies of microorganisms known as stromatolites.

Today, Archean-aged rocks are found in areas such as the Superior Upland and the Rocky Mountains.

We have divided the Archean into the following sections (youngest to oldest):
Late Archean
Middle Archean
Early Archean