I need to buy (or borrow, our bookcases are full) a modern book on geo-history. When I was a kid snowball earth was still a mystery ...
... Several [Caltech] graduate students, along with supervising professor Joe Kirschvink, have released a paper presenting their explanation of what caused "Snowball Earth," a periodic deep freeze of Earth's atmosphere that has been theorized for years. The Caltech team argues that 2.3 billion years ago, cyanobacteria, or blue-green algae, gained the ability to break down water, which in turn released a flood of oxygen into the atmosphere.
That oxygen reacted with the atmospheric methane, which insulated the Earth at the time, and broke it down. While the oxygen-methane reaction created the greenhouse gas carbon dioxide, the protective nature of the barrier cracked.
Temperatures plunged to minus 50 degrees Celsius, and ice at the equator grew to 1 mile thick. Although this process took several million years, substantial damage to the methane layer could have occurred in the first 100,000 years.