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HISTORY
  Big Bang
  Galaxies etc.
  The Earth
  Continents
  Ice Epochs
  Life
  Precambrian
  Paleozoic
  Mesozoic
  Cenozoic
  First People
  Europeans

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The Earth

© 1985 by Andrew J. Morris

By about four and a half billion years ago the planet Earth had condensed sufficiently to generate great heat and pressure at its center, and as the planet continued to condense due to its own gravitational attraction, enough heat was produced to melt the entire planet. While in this molten state the heavier elements tended to sink inwards while lighter elements rose to the surface. Today, the Earth has a core rich in Iron, giving the planet its magnetic field, while the crust consists largely of lighter oxygen and silicon compounds.

The Earth stabilized at approximately its current dimensions, and as heat was radiated into space the surface cooled and hardened into a thin crust, around four billion years ago. Volcanic activity spewed forth magma and gasses with great intensity for the first half billion years or so. The gasses thus released were primarily carbon dioxide, water vapor and nitrogen compounds. As the water vapor cooled it condensed and fell to the surface as rain, eventually giving rise to the oceans. These oceans and the clouds of water vapor in the atmosphere helped cool the planet further. The clouds reflected more sunlight back into space than had the crusty rocks of the Earth's surface, so the planet absorbed less solar energy. The seas helped by dissolving some of the carbon dioxide out of the atmosphere, thus lessening the "greenhouse effect," a property of atmosphere that tends to make it act as a heat trap.

The crust of the Earth consisted of two general types, a thin crust of relatively dense materials some two or three miles thick, and a lighter thicker crust up to ten miles thick. The thin crust was largely covered by the oceans, while the thicker crust stuck up above the waters to form one or more continents. The crust does not form one solid covering for the Earth, but consists of a number of separate sections, or "plates," floating on the magma of the Earth's interior. These plates each move separately, driven by the forces of currents and flows in the underlying magma. Where two plates moved toward one another one will often sink beneath the other to be absorbed into the underlying magma, as when the heavier, thin, crust of the ocean floor slips beneath a lighter, thicker, continental plate. In other places, where two plates move away from one another, new material rises from below to form new crust, as is happening today on the ocean's floor.

This process is really much more complex than this brief explanation would indicate, but it is mentioned here because it provides the mechanism for an important phenomena, continental drift. Because they ride upon these shifting plates, the continents are not the fixed immutable masses they seem, but instead are a series of floating islands drifting constantly about the Earth's surface. This has had important consequences, affecting the rate of evolution of life on the planet, and possibly helping to trigger ice ages and other global climatic changes.

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