Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Proterozoic
Contents
Eon Overview
Earth's Crust as a Platform for Prokaryotic Life
Eukaryotes and the First Multicellular Life Forms
Changes in the Earth's Atmosphere
Proterozoic Mountains and Glaciers
Evidence
Rangea from the Ediacaran Fauna, Namibia.
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sample
First eukaryotes
Ediacaran Fauna 1
Ediacaran Fauna 2
Ediacaran Fauna 3
Ediacaran Fauna 4
Banded Iron Formations
Stromatolites
references and links
Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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Proterozoic Mountains and Glaciers
In the Proterozoic Eon, four major mountain-building episodes occurred, each of which was followed by an interval of continental erosion. Mountain-building was caused by converging plates, just as occurs in present-day plate tectonics. It was accompanied by intrusions of molten granite that welded an additional belt of younger, igneous rock around the edges of the original microcontinents. In North America, the Proterozoic episodes of mountain-building greatly expanded the size of the continent.

Widespread continental glaciations evidently occurred at least twice in the Proterozoic, once near its beginning and again near its end. Several of these glaciations extended almost to the equator, much farther south than any recent cooling events. This unusual situation has led a few geologists to propose that the Earth was almost entirely covered by glaciers for perhaps several million years during the Proterozoic. During this “Snowball Earth” phase, life would have been relegated to hydrothermal vents and other such refuges until the build-up of carbon dioxide released from volcanoes warmed Earth from its deep-freeze. The final Marinoan Ice Age marked the transition to the Cambrian, the first period of the Paleozoic Era.



Eon Overview | Earth's Crust as a Platform for Prokaryotic Life | Eukaryotes and the First Multicellular Life Forms | Changes in the Atmosphere | Proterozoic Mountains and Glaciers



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