Eli5: How does the bottom of the ocean, miles away from sunlight and volcanic action, stay above freezing?

Question

Obviously all of the ocean is above freezing; but reading about the titanic sub today, I still was surprised to read that it’s still above freezing down 2+ miles down. What keeps it relatively warm? Obviously there is some volcanic activity but it’s not widespread over the entire ocean is it?

In: 2

Answer 1

The better question is if something is covered with a lot of stuff above and below how could it get cold to begin with? For temperature to drop there needs to be a way to remove energy, it spontaneously flows from warm to cold not the other way around. So the water on the bottom of the sea can’t get colder than what is above or below it from just heat transfer to the surrounding. That is if you just have heat transfer and not a moment of any matter.

The surface gets cold because it can radiate heat into space, and the incoming radiation from a dark part of space has a black body temperature of 2.7 kelvin.

Earth’s core is very hot, warmer than the surface of the sun, and heat is transported out all the time. You have a temperature drop from the surface to the area just below it but then if you go down there is a temperature increase that is 25–30 °C/km away from tectonic plate boundaries.

The real question is why it is so cold down not why it is not colder. The same distance below down into the solid part of the earth is a lot warmer. At the same dept on land, you have a temperature around 100C

The explanation is water flows around and is the densest at around 4°C. Ice is also less dense than liquid water. so if you for some reason get water that starts to freeze it will not happen at the bottom and the rise will rise to the surface. Pounds on the ground do not freeze from the bottom but from the surface. The temperature at water freezes depends on salinity, for typical sea water it is -1.8°C so you can have water colder than 0°C and is still liquid, This is why we use salt on roads to keep ice away.

So the reason it is cold deep in the sea is not that there is no sun or volcanic activity there, it is because water gets cooled down on the surface and can then drop because density increases and there iw warm surface currents. Water that gets below the freezing point can sink, it is ice that is less dense and floats. So only water above freezing temperature can move down into the deep part of the ocean.