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Geodaze 2015, Tucson, AZ Dates and dynamics: Snowball Earth comes of age Paul F. Hoffman Dept of Earth & Planetary Sciences, Harvard University, Cambridge, MA School of Earth & Ocean Sciences, University of Victoria, Victoria, BC Abstract Snowball Earth is a theoretically well-defined climate state in which the entire ocean and most continental areas are covered by dynamic glaciers, driven by sublimation in the equatorial zone of the sea glacier. Ice albedo instability is the trigger and deglaciation is self-induced, due to the steady rise of atmospheric CO2 from normal volcanic outgassing in the absence of sinks for carbon on the frozen planetary surface. Snowball Earth is invoked for two Cryogenian glacial epochs when dynamic ice sheets are known from geological evidence to have existed on all continents including low-latitude carbonate platforms. The snowball hypothesis makes three sets of falsifiable predictions: (1) Cryogenian glacial epochs were long-lived (millions to tens of millions of years), and both the onsets and terminations were globally synchronous at low paleolatitudes; (2) deglaciations occurred under extreme (105 ppmv) atmospheric CO2 radiative forcing and consequent ocean acidification, and were followed by very high surface temperatures and intense weathering; and (3) the biosphere was irreversibly impacted. The first two prediction sets and arguably the third are now strongly supported by observations unavailable when the hypothesis was proposed. At the same time, climate models suggest that the snowball atmosphere, cryosphere, ocean and lithosphere were more dynamic and unstable than initially assumed, consistent with geological evidence. Many aspects of the phenomenon are counterintuitive, meaning that insights from modeling are essential to avoid testing false predictions. The ultimate cause of Cryogenian refrigeration appears to have been basalt weathering following emplacement of the 0.72-Ga Franklin large igneous province across paleoequatorial (present Arctic) Laurentia during the breakup of supercontinent Rodinia. Biomarkers tentatively imply a Cryogenian origin for multicellularity in animals and for the dominance of green algae among eukaryotic primary producers in Phanerozoic oceans. Snowball Earth is a good example of why we study old rocks: to discover things we never would if we only studied young rocks. Paul Hoffman is a sedimentary and tectonic geologist with extensive field experience in northern Canada and southern Africa. His best known papers are United plates of America, the birth of a craton (1988), Did the breakout of Laurentia turn Gondwanaland inside-out? (1991) and A Neoproterozoic snowball Earth (1998).