Forming Fossils

Time Periods

Why do scientists believe that several ice ages occurred during the late Pre-Cambrian era?

Chemical and isotopic analysis of rocks found in Africa show that Earth may have gone through at least four ice ages between 750 and 570 million years ago. These were very deep ice ages, essentially turning Earth into a “snowball planet.” From the evidence to date, some scientists think the oceans were covered with ice almost 300 feet (91 meters) deep, and the land was completely dry and barren of life.

Some scientists believe the Pre-Cambrian ice ages may have been caused by Earth’s tilt toward the Sun. The planet may have been tilted at a much larger angle—upwards of 55 degrees—than today’s angle of 23.5 degrees. This large degree of tilt meant that the polar areas received most of the Sun’s warmth, keeping them ice-free. But the areas around the equator would have been colder, allowing glaciers to form. If this was true, the buildup and melting of the glaciers around the equator during the Pre-Cambrian era may have created enough force to move the planet’s axis to its modern position. Some scientists have equated this process to repeatedly pushing on a swing at just the right moment in its movement, adding energy to make it go higher. The influence of the alternating advance and retreat of the glaciers could have caused the axis to straighten to its present angle.

Some scientists believe the “heroes” that thawed the snowball planet and paved the way for an explosion of life were none other than volcanoes. As these surface blisters erupted toward the end of the Pre-Cambrian era, they sent massive amounts of carbon dioxide into the atmosphere, an increase of approximately 350 times its present concentration. This increase trapped re-radiating solar energy, warming the planet as it created a super greenhouse effect. The temperatures rose enough to melt the ice-covered oceans and end the ice age.


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