Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Archean
Eon Overview
Early Continents and Oceans
The First Life on Earth
Changes in the Atmosphere
Acasta gneiss, one of the oldest known rocks on Earth.
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Banded Iron Formation
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Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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Early Continents and Oceans
Analyses of the approximately 4 billion-year-old Acasta Gneiss suggests that the first continents and oceans developed before the Archean. The Archean, however, is the period during which the present continents took shape. Most present continents have shields at their cores that formed between 3 to 2.5 billion years ago during the early Archean. Evidence of ancient oceanic crust is often found in today's greenstone belts. Continental crust gradually grew from selective melting of dark-colored basaltic igneous rocks within the oceanic crust. Through time, these melts became increasingly rich in silica, as geologic processes melted more of the lower-temperature, lighter-density minerals. Much as a helium balloon rises through air, these silica-rich melts rose from deep within Earth and formed granitic plutons (intrusive bodies) nearer the surface. Early in the Archean the granitic crust of continents had begun to form from basaltic crust of the ocean floor. Continental landmasses began forming about 3.7 billion years ago from the horizontal accretion of smaller micro-continents. The Kenoran orogeny was one such event, which formed what is now the Great Lakes region of North America during the Late Archean.

Direct evidence for running water on Earth comes from the Isukasia region of Greenland. There, 3.8-billion-year-old rocks preserve original sedimentary layering that formed when sediments weathered from exposed rocks were deposited by water. This is evidence that the hydrologic cycle was in place by the Early Archean, with water evaporating from the seas, condensing and falling as rain, and forming oceans. Weathering and erosion of early continents had built widespread continental shelves by 2.7–3.0 billion years ago.

Eon Overview | Early Continents and Oceans | The First Life on Earth
Changes in the Atmosphere

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