Question

Thermodynamically why does the ocean not freeze at its deepest depths?

Answer 1

For all teh liquids, the volume increases with the increase in temperature. But for water which has peculiar property that the volume increases with temperature but from 0-4 deg C it will decrease by making it more dense.

Now lets consider a stituation where the water at the surface is at 12°C and its cooling down. Now, as the temperature decreases, its volume decreases and density increases. As the surface water reaches about 4°C, it becomes more dense and "sinks" to the bottom allowing the less dense water to surface up. As temperature falls below 4°C, surface water becomes lighter( less dense) compared to the water below and floats until it freezes. Once frozen, the ice insulates the water at the bottom from cooling so its temperature is maintained between 0 and almost 4°C, keeping it from freezing.