Ice Ages

In the first 2.7 billion years of Earth, its chemistry changed as impacts occurred and geological layers churned.
Around 1.8 billion years ago:
- the Solar System moved between spiral arms of the Milky Way,
- so there were fewer stars forming nearby,
- and fewer cosmic hitting the atmosphere,
- and fewer cloud formations,
- so the crust remained hot,
- preventing the tectonic plates from sticking to the magma below,
- so the surface of the Earth changed very slowly,
- so the chemical composition of the Earth remained relatively constant for a billion years or so.
Eventually, the crust cooled enough to bind to the mantle, tectonic plates moved more quickly, the chemical composition of the biome changed, and the Earth became a snowball for about 55 million years.
After 12 million warm years, Earth became a snowball again for about 15 million years.
After 56 million warm years, Earth became a snowball again for about 340,000 years.

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