Scientists now believe that at between two to four times between 850 and 580 million years ago, the planet was covered completely in ice and early life forms were nearly wiped out: a “Snowball Earth.” There are a couple of theories as to why this occurred: one being that the Sun was about six percent cooler than it is today; the other is that the continents had all traveled—due to plate tectonics—mostly south; ocean currents traveled easily around the planet without interruption, and volcanic activity dwindled to a minimum. The result was that significantly less carbon dioxide was being generated and expelled into the atmosphere. In addition, the Earth’s axis was significantly more tilted at the time (by 54 degrees versus today’s 23.5 degrees), causing more drastic seasonal extremes.

Once a period of cooling began and ice shelves formed, more of the Sun’s light was reflected, thus causing the cooling cycle to accelerate more and more until everything was frozen. Early theories about this, proposed by such scientists as George Williams at the University of Adelaide and Joe Kirschvink at the California Institute of Technology, include the effects of plate tectonics as causing massive volcanic eruptions worldwide. This would lead to an extended winter of unprecedented scale that turned Earth into one big frozen lump in space that would last for millions of years. Nearly all life was wiped out. In fact, one matter of debate concerning this theory was that critics felt such a Snowball Earth would have extinguished all life. This notion was laid to rest in the 1990s when life was found to thrive near geothermal vents deep under the ocean.

The Snowball Earth cycle was only broken because volcanic action and plate tectonics would cause carbon dioxide levels to build under the ice, especially since there would be no liquid water to dissolve minerals or aid in the dissipation of CO2 levels. The result would be an inevitable, sudden massive release of CO2 that would lead to a comparatively brief period of extreme heat with temperatures averaging 120°F (50°C). However, the thaw retreated to another snowball period a couple more times over millions of years before the continents moved into positions that created a more stable geophysical state of the planet. Volcanic activity moderated, as did carbon dioxide levels. Another Snowball Earth could happen again, however, as the continents, hundreds of millions of years from now, drift back together to form a new supercontinent.