Further Reading

Powell, John W. The Exploration of the Colorado River.

New York: Doubleday and Company, 1961. -. "The Laws of Hydraulic Degradation." Science 16

-. "Remarks on the Structural Geology of the Valley of the Colorado of the West." Bulletin—Philosophical Society of Washington 1 (1874): 48-51. Stegner, Wallace. Beyond the Hundredth Meridian: John Wesley Powell and the Second Opening of the West. Omaha: University of Nebraska Press, 1954.

Precambrian Comprising nearly 90 percent of geologic time, the Precambrian eon includes the time interval in which all rocks older than 544 million years formed. The Precambrian is preceded by the Hadean Eon, representing the time interval during which the Earth and other planets were accreting and from which no rocks are preserved, and is succeeded by the Cambrian, the dawn of advanced life on Earth. The Precambrian consists of two eras: the Archean, ranging in age from the oldest known rocks at about 4.0 billion years old to 2.5 billion years ago, and the Proterozoic, ranging from 2.5 billion years ago until 544 million years ago. The Archean is further divided into the Early (4.0 Ga-3.0 Ga) and Late (3.0-2.5 Ga), and the Proterozoic is divided into the Early or Paleoproterozoic (2.5 Ga-1.6 Ga), Middle or Mesoproterozoic (1.6 Ga-1.3 Ga), and Late or Neoproterozoic (1.3 Ga-0.54 Ga).

Most Precambrian rocks are found in cratons, areas of generally thick crust that have been stable since the Precambrian and that exhibit low heat flow, subdued topography, and few earthquakes. Many also preserve a thick lithospheric keel known as the tectosphere. Exposed parts of Precambrian cratons are known as shields. Many of the rocks in cratons are preserved in granite-greenstone terrains, fewer are preserved as linear high-grade gneiss complexes, and still fewer form relatively undeformed sedimentary and volcanic sequences deposited in shallow water or platform basins, resting on older Precambrian rocks. Platformal sequences form a thin veneer over many older Precambrian terrains, so geologic maps of cratons and continents show many essentially flat-lying platformal units, but these are volumetrically less significant than the underlying sections of the crust. Many other areas of Precambrian rocks are found as linear tectonic blocks within younger orogenic belts. These probably represent fragments of older cratons that have been rifted, dispersed, and accreted to younger orogens by plate tectonic processes, some traveling huge distances from where they initially formed in their primary tectonic settings.

The Precambrian is the most dramatic of all geological eons, as it marks the transition from the accretion of the Earth to a planet that has plate tectonics, a stable atmosphere-ocean system, and a temperature range all delicately balanced in such a

Accretion Plate Tectonics

Map of the world, showing the distribution of Precambrian rocks in Archean cratons and other undifferentiated areas of Precambrian crust way as to allow advanced life to develop and persist on the planet. The planet has been cooling steadily since accretion and was producing more heat by radioactive decay in the Precambrian than it has been since. However, scientists do not know if this greater amount of heat significantly heated the mantle and crust, or if this additional heat was simply lost faster than it is by the present style of plate tectonics. It is likely that more rapid seafloor spreading or a greater total length of oceanic ridges with active volcanism was able to accommodate this higher heat flow, keeping mantle and crustal temperatures close to what they have been in the Phanerozoic.

understanding of the development of life in the Precambrian has been undergoing rapid advancement, and the close links between life, atmospheric chemistry, plate tectonics, and global heat loss are only recently being explored. Many mysteries remain about the events that led to the initial creation of life, its evolution to more complex forms, and the eventual development of multi-celled complex organisms at the end of the Precambrian.

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