Between roughly 650 and 635 million years ago, most – if not all – of the Earth was covered by ice and snow. Dating of rock units formed immediately after the glaciation ceased suggests that the termination of this Marinoan Snowball Earth event was synchronous around the world.
Clive Calver of Mineral Resources Tasmania and colleagues measured the uranium and lead contents of volcanic zircons from the Cottons Breccia unit, located on Kings Island, Tasmania. The unit was formed at low latitudes, and consists of glacial and marine rocks that were deposited during and immediately following the Marinoan glaciation. The team was able to identify volcanic ash within the younger layers, which gave a U-Pb age of about 636 million years.
This timing is consistent with ages for the end of the Marinoan glaciation obtained in Namibia and South China, indicating that the end of this Snowball Earth occurred synchronously, at least on a million-year time scale. The full article published in Geology magazine can be viewed at:
http://geology.gsapubs.org/content/early/2013/07/30/G34568.1.abstract