<P> Thick glaciers were heavy enough to reach the sea bottom in several important areas, thus blocking the passage of ocean water and thereby affecting ocean currents . In addition to direct effects, this caused feedback effects as ocean currents contribute to global heat transfer . </P> <P> Glaciation has been a rare event in Earth's history, but there is evidence of widespread glaciation during the late Paleozoic Era (200 to 300 Ma) and during late Precambrian (i.e. in the Neoproterozoic Era, 600 to 800 Ma). Before the current ice age, which began 2 to 3 Ma, Earth's climate was typically mild and uniform for long periods of time . This climatic history is implied by the types of fossil plants and animals and by the characteristics of sediments preserved in the stratigraphic record . There are, however, widespread glacial deposits, recording several major periods of ancient glaciation in various parts of the geologic record . Such evidence suggests major periods of glaciation prior to the current Quaternary glaciation . </P> <P> One of the best documented records of pre-Quaternary glaciation, called the Karoo Ice Age, is found in the late Paleozoic rocks in South Africa, India, South America, Antarctica, and Australia . Exposures of ancient glacial deposits are numerous in these areas . Deposits of even older glacial sediment exist on every continent except South America . These indicate that two other periods of widespread glaciation occurred during the late Precambrian, producing the Snowball Earth during the Cryogenian Period . </P> <P> In popular culture, there is often reference to "the next ice age". Technically, because Earth is already in an ice age at present, albeit an interglacial period, this usually refers to the next glacial period . </P>

How far will the plates spread in the next 200 000 years