IN THE EARLY 1960s, Brian Harland, a geologist at Cambridge University, observed that rocks on several continents, dating from the Neoproterozoic era (approximately 800–680 million years ago), contain glacial debris. Some of the glacial debris included carbonate rocks, which are known to form in the tropics (e.g., in the present-day Bahama Banks). This conclusion later gained additional support from paleo-magnetic data. One potential explanation is that the [Page 904]Earth entire Earth was covered by ice and snow during the Neoproterozoic. This has come to be known as the “Snowball Earth” hypothesis.

The Snowball Earth hypothesis proposes that the Earth was entirely covered by ice in part of the Cryogenian period.

One early problem was understanding how a global ice age could have commenced. During the 1960s, the ...