ABSTRACT
The geologic time scale refers to sequential periods of time as defined by the fossil succession found in rocks and described by European geologists in the 1700s to 1800s. The major divisions represent phases in development of the earth. By the Archean, 4 billion years ago, Earth's crust had solidified and was cooling. There is an ocean derived from condensed atmospheric water vapor. Gases released by volcanoes created a primitive atmosphere rich in water vapor, carbon dioxide (CO2), sulfur dioxide, and ammonia, with little or no free oxygen. Some surface water is thought to have existed, despite the high temperatures, owing to the high atmospheric pressure of the CO2-rich atmosphere. Ozone in the atmosphere protects surface life from the harmful effects of solar ultraviolet radiation. The primary driver for the cold snap is controversial but probably has something to do with variations in solar irradiance, the amount of energy Earth gets from the sun.
