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I know that at least twice in its history Earth was totally frozen. I also know that the last time it happened there were already life on Earth, and it survived. In deep water, under the ice, which was transparent enough for light to penetrate it and support life.

My question is, why didn't all the water freeze? AFAIK Earth was frozen for 10 million years, so I think it was enough time. Something must have kept deep water from freezing, what was it? Geothermal warmth? Underwater volcanoes? Both seem to be too weak sources of energy for keeping deep ocean liquid.

The same about Antarctica. I know there was a lake found deep under thick ice, why didn't it freeze?

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  • $\begingroup$ It seems to me that the question you're actually asking - why did the entire ocean not freeze in this scenario - is not really related to the survival of life, and hence the title. You might find that you attract the right people to answer it better with a clearer title. $\endgroup$
    – Semidiurnal Simon
    Jan 23, 2015 at 14:32
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    $\begingroup$ I would not assume that the "whole earth" was frozen because glacial moraine was found in some apparently equatorial location. The St. Elias range is covered in glaciers, yet Anchorage and Fairbanks have very livable climates. $\endgroup$
    – Tyler Durden
    Jan 24, 2015 at 3:43

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At least four things combined to prevent solid freeze-up during Ice/Slush Ball Earth periods:

  • It takes a lower temperature to freeze water under pressure. Deep sea pressures are enormous.
  • Ice floats. If it sank and new ice kept forming at surface, the seas might fill with ice.
  • The surface freezes first and acts as an approx zero-degree C barrier, slowing the progress of the ice penetration.
  • Thermal heat from the ocean floor at hotspots, island arcs, subduction zones, and mid-oceanic rifts is substantial, especially locally.
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What makes you think Geothermal energy and underwater volcanoes are too weak? The mid-oceanic ridge system alone is 50000 miles long. The mean heat flow at the surface (91.6 mW/m2) has to be accommodated somewhere.

Also the freezing point of brine at a depth of 5000 m is approx -40 °C.

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The water has an uncommon property: at 4°C it's heavier(most dense) that at 0°C. This makes ice floats on surface and ice is a good thermical isolator, preventing the water below freezing.

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