Was Earth a solid snowball 700 million years ago? Some geologists have been looking for an answer in an ancient, well-preserved glacial formation in the mountains of northern Oman. Their conclusion–that Earth experienced intermittent ice ages like those in Earth’s more recent history–weakens the snowball theory.
The snowball scenario has Earth freezing several times between 750 and 580 million years ago, during the Neoproterozoic period (SN: 8/29/98, p. 137: https://www.sciencenews.org/sn_arc98/8_29_98/bob1.htm). Each time, so the theory goes, ice covered all of Earth for about 10 million years, until carbon dioxide from volcanoes accumulated and created a greenhouse effect that thawed the planet.