The glaciations were probably triggered by the Great Oxygenation Event (GOE), which removed atmospheric methane (a greenhouse gas), and eventually supplied free oxygen to the atmosphere. The alternate warm and ice age periods was probably caused by a repeating cycle. Cyanobacteria flourished in the warm periods, producing huge amounts of oxygen. The oxygen removed the free methane, and used up carbon dioxide. This caused the temperature to crash. This slowed down the bacteria. So the temperature rose again.