The Snowball Earth Hypothesis proposes that during an ice age 900 mya, the entire land and oceanic surface the Earth was covered by ice. This animation shows four proposed stages to the formation and destruction of Snowball Earth conditions:

• During "normal" climate periods there are ice caps at the poles;  sea level rises and falls.
• During "metastable" climate times ice sheets expand and contract dramatically.
• "Runaway snowball" conditions develop and ice nearly envelopes the Earth; atmospheric carbon dioxide is not absorbed by the frozen ocean.
• The rising concentration of unabsorbed carbon dioxide gas leads to a "runaway greenhouse effect"; Earth warms and the ice shell rapidly vanishes.

by Declan DePaor

>>see: p.720