Geology Reference
In-Depth Information
OUTLINE
Introduction
What Happened During the Eoarchean?
Continental Foundations—Shields, Platforms, and Cratons
Archean Earth History
Proterozoic Earth History
Origin and Evolution of the Atmosphere and Hydrosphere
GEO-INSIGHT: North America's Precambrian Rocks
Life—Its Origin and Early History
Resources in Precambrian Rocks
GEO-FOCUS: Banded Iron Formation—From Mine to Steel Mill
Geo-Recap
OBJECTIVES
At the end of this chapter, you will have learned that
The Precambrian encompasses more than 88% of all geologic
time, yet we know less about its history than we do about more
recent intervals of geologic time.
During the earliest part of the Precambrian, called the
Eoarchean Era, Earth differentiated and crust began to form.
The Archean Eon was a time during which several small
continental nuclei began to evolve, and the most common
rocks were granite-gneiss complexes and greenstone belts.
During the Proterozoic Eon, an essentially modern style of plate
tectonics developed, and crust that formed during the Archean
amalgamated into a large craton we now call Laurentia.
Proterozoic rocks consist of sandstone-carbonate-shale
assemblages, red beds, banded iron formations, glacial
deposits, and a variety of others.
The Precambrian atmosphere evolved from one that lacked
free oxygen and an ozone layer, but even at the end of the
Precambrian, the atmosphere had much less free oxygen than
it does now.
These rocks of the Belt Supergroup in Glacier National Park in Montana con-
sist of Mesoproterozoic to Neoproterozoic sedimentary rocks. They have been
slightly metamorphosed, but they retain features of the original rock. St. Mary
Lake is in the foreground.
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