The Cryogenian Period: The Equator Under IceLast Updated on 2010-12-18 00:00:00
About 0.73 Ga, Earth underwent severe glaciations that eliminated much of life. The Cryogenian (Greek; ice + birth) period was initiated by several conditions.  The breakup of the supercontinent Rodinia exposed organic matter to anaerobic respiration, creating an atmosphere in which the major greenhouse gas was methane (CH4). Atmospheric CH4, in contrast with CO2, reacts with O2 and is not buffered by a massive ocean reservoir; therefore, atmospheric CH4 levels decreased rapidly once continental movements diminished and less organic matter was exposed. As CH4 declined, Earth cooled. Cooler temperatures inhibited biological production of CH4, accelerating the cooling. Ice appeared on the land masses near the equator, increasing the reflectance of solar energy (a phenomenon called albedo) and decreasing the solar energy retained at Earth’s surface. Temperatures sank until even... More »
Ancient Earth: The First Three Billion YearsLast Updated on 2010-11-08 00:00:00
The Big Bang, the single point in space and time from which all matter and energy in the universe supposedly emanated, is thought to have occurred sometime around 13.7 Ga (gigayears ago; 1 Ga means 1000 million years ago). Our solar system began to coalesce about 4.5 Ga. When the surface of primitive Earth cooled to a temperature below the boiling point of water, sometime around 4.1 Ga, the atmosphere consisted primarily of gases released from volcanoes.  These gases included high concentrations of carbon dioxide (CO2), carbon monoxide (CO), water vapor (H2O), dinitrogen (N2), hydrogen chloride (HCl), and, perhaps, small amounts of methane (CH4). Within a relatively short time, perhaps as early as 3.8 Ga, photosynthetic bacteria (cyanobacteria) were present. 
Cyanobacteria proliferated widely during the next few billion years. Their photosynthesis depleted the CO2... More »
ProterozoicLast Updated on 2010-07-26 00:00:00
The Proterozoic eon is a time in the Earth's history that occurred from 2.5 billion to 543 million years ago. Many important events in the geologic history of the Earth occurred during this eon. For example, stable continental masses first appear and began to significantly grow in size through the accretion of sedimentary deposits. Also occurring during this time is the fossilization of our planet's earliest organisms, mainly bacteria and archaeans. By about 1.8 billion years ago, eukaryotic organisms begin to appear in the fossil record as well. Finally, multicellular eukaryotic life-forms become common near the end of the Proterozoic.
During the middle of Proterozoic oxygen concentration in the atmosphere increases significantly. This global change reduced the abundance of many anerobic bacterial groups, but made possible for the domination of... More »
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