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					                       Evolution of Planet Earth
Formation:
 Planet Earth condensed out of the solar nebula, a cloud of gas and dust,
  which collapsed under its own gravity about 4.6 billion years ago.
 Earth formed from accretion of millions of small boulder to mountain to
  continent-sized objects, under the influence of gravity.
 Earth initially collected quantities of the light gases, hydrogen & helium, but as
  the Sun turned on, these gases were lost due to solar heating. Earth formed
  from heavier elements, those that could condense at the Earth’s location from
  the Sun.
 The formation process lasted about 100 million years, as the growing planets
  “swept up” most of the matter in the inner solar system
 The Moon formed during this period, most likely due to a violent collision with
  a Mars-sized object.

The First 400 Million Years:
 The Earth was largely molten due to: (1) the residual heat of formation, (2)
  continuing impacts from asteroids, & (3) internal radioactivity, which cooked
  Earth from within.
 Differentiation occurred as the heavier elements sank towards the center,
  producing the core/mantle/crust structure.
 A secondary atmosphere formed from the release of gases trapped within the
  Earth: carbon dioxide, nitrogen, water, but no free oxygen.
 Eventually, the rate of impacts declined, permitting the crust to cool and
  solidify. Water in the atmosphere began to fill ocean basins.
 Continental land masses formed from lighter rock, which “floated” on top of
  more dense material.
 Plate tectonics initiated, driven by heat escaping from the interior.

The Next 4 Billion Years:
 Continents circulated around the globe, a result of plate motions, and
  continued volcanic activity.
 Life originated in the early oceans.
 The geologic cycle was established: erosion by water, wind, and ice, new
  crust formed by volcanic activity, mountain-building where plates collided.
 Earth’s climate did not vary sufficiently to cause permanent freezing of the
  oceans, or a runaway greenhouse effect.
 Over time living organisms altered the atmosphere by releasing free oxygen.
 About 400 million years ago, life moves onto land.
 During the last 400 million years, biology has added to the alterations of the
  land surface, by locally affecting the hydrological cycle, and the ability of
  water and wind to erode the surface.

				
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posted:5/3/2009
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