Life on Earth is only here by chance, 99% of all species on Earth that ever lived has been wiped out by one form of catastrophe or another. These disaster have changed the course of evolution making it possible for our presents on Earth today. The Earth turned in to a frozen glacier snowball 715 million years ago and once again 635 million years ago. These periods of time occurred in what is known as the cryogenian "time of ice" which lasted approximately 120 million years. The first snowball earth episode known as the Sturtian Glaciation, started 716 million years ago and lasted about 36 million years. The last snowball earth Known as the Marinoan Glaciation, last from about 650 - 635 million years ago. During this time the sun was also about 7% dimmer than it is today which was only one contributing fact that kept snowball earth in a global deepfreeze. Estimates vary but some scientist think that at the north and south poles the temperature could have been -130 degrees Celsius and at the equator the temperature would have been below 0 degrees with sheets of ice thousands of meters thick that covered both land and sea.
The evidence for snowball earth is wide spread and found on every continent today in the form of drop stones that have been carried by glaciers for thousands of miles. Every rock has a unique magnetic signature, this allows scientist to determine the point on the earth where the rock was formed and rocks carried by glaciers have been found thousands of miles from where they where formed. Scientists have determined that the whole globe was covered in ice as at the time of snowball earth Australia was at the equator and drop stones carried by glaciers for thousands of miles have been found there in abundance.
In addition to drop stones, more evidence has been found in the form of carbonate rock. Carbon dioxide mixes with water vapor to form acid rain. The acid reacts with the rocks as it falls on them and forms carbonate. As the rocks get striped away with acid rain that takes carbon dioxide out of the atmosphere which is locked in the carbonate rock and carried out to the oceans. Studies of ocean sediments around the world have reveled that during the cryogenian period the carbonate rocks disappear because when the earth was covered in ice almost no weathering took place on land and carbonate became vary rare. When the ice began to melt, weathering resumed and huge deposits of carbonate began form again. Most scientist believe that the absence of carbonate rocks in a sure sign that earth was completely covered in ice.
The most popular theory for theses massive ice glaciers is that earth's natural thermostat of the carbon cycle failed. The carbon cycle is the swapping back and forth of carbon between the atmosphere and the Earth's crust. This failed process started with volcanoes, which over the course of thousands to millions of years gradually emit Co2 into the atmosphere where it helped to keep the earth warm. Co2 gas gets stored in carbonate rocks from the process of weathering which in turn keeps the earth cool. Volcanic emissions and rock weathering are the 2 main processes that keeps the earth's temperature not to hot and not to cold.
In the Cryogenian period the super continent known as Rodinia, changed this balanced Co2 cycle by breaking up. The breaking up of Rodinia release a lot of basalt from volcanic rock which is excellent at absorbing Co2 in the process of weathering. At this time, Rodinia was located at the equator where it was warmer and wetter which weathered the rock faster and the rock absorbed Co2 much quicker. Scientist believe that this altered the Co2 cycle as carbonate rock absorbed the Co2 faster than volcanoes released it changing the balance of the green house gases that keeps earth warm. With the contributing factor of the sun being 7% dimmer than it is today, this temperature change was enough for glaciers to start to form.
In the last few years scientist have discovered another contributing occurrence that helped create snowball earth. A massive volcano eruption, known as the Franklin Large Igneous Province, took place 18 million years before the glacier period started. The volcanic rock that came from this eruption was full of sulfur, which another planet cooling gas. When sulfur is injected into the atmosphere it cools the air by blocking sunlight. Normally sulfur dose not block the sunlight for long periods of time as it interacts with the moisture in the atmosphere and comes down as acid rain typically leaving the atmosphere in a few years. Theses eruptions where massive fire fountains that took years ejecting huge flumes of sulfur gases up to 12km into the atmosphere. At that height above the Earth's surface into the stratosphere, it would take much longer to break down and rain out as acid rain. With lower Co2, as the planets rocks absorbed the Co2 and a dimmer sun, the atmosphere started to cool down and ice began to form.
Ice reflects much more sunlight than water and this added to the cooling down of the earth known as the Runaway Icehouse Effect as the ice became unstoppable. Even though the supercontinent Rodinia was covered in ice, it continued to separate volcanoes kept forming and releasing Co2 into the atmosphere. As the earth was covered in ice and all the rocks where locked below ice sheets they were not able to absorb the Co2 gas than began to build up in the atmosphere. It took almost 50 million year for enough Co2 to build up the green house effect to melt the first glacier period, and about 10-15 million years to melt the second glacier period. The Earth's natural carbon cycle broke down twice as the supercontinent continued to break up near the equator. After the second glacier period the supercontinent was breaking up largely in the southern hemisphere so the Earth's natural thermostat could balance itself. Earth began to evolve to what we know it as today.