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I have found following information on Wikipedia:

The temperature is $1$ to $4\,°\mathrm C$ and the pressure is about $0.156$ GPa.

The Graph tells us that it's liquid. However, I don't quite understand why the temperature is not below $0\,°\mathrm C$. Light is unable to reach these depths and thus unable to transfer heat.

My reasoning is that eventually particles with higher kinetic energy will have risen due to convection, lowering the temperature below $0\,°\mathrm C$, resulting in freezing the water.

enter image description here

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  • $\begingroup$ The ground could be hot at places due to magma. $\endgroup$ Feb 5, 2020 at 9:54
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    $\begingroup$ @JohnRennie You could start with a block of water at (say) 20 degrees, wait for it to separate, then run a heat engine with the 40 degree water as the hot side and the 0 degree water as the cold side. Repeat until the water gets to 0 degrees. This violates the Second Law of Thermodynamics. $\endgroup$ Feb 5, 2020 at 10:41
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    $\begingroup$ @user253751 It''s called a thermocline. It's also not really "spontaneous". People do run heat engines off it (OTEC). $\endgroup$
    – JMac
    Feb 5, 2020 at 13:39
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    $\begingroup$ The graph in this question pertains to pure water. Ocean water freezes at about -2° C; the exact temperature depends on salinity and pressure. $\endgroup$ Feb 5, 2020 at 14:33
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    $\begingroup$ It might be geothermal energy that prevents it from freezing. $\endgroup$ Feb 5, 2020 at 16:12

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You are basically asking why water does not freeze at the bottom of the trench. It is basically because of two reasons:

  1. it is very important to understand the difference between fresh and salt water. We are talking about salt water here and the freezing point (phase change) is below $0^\circ \,\rm{C}$, at around $-2^\circ \,\rm{C}$. This takes your question as to why water at the bottom of the trench cannot reach $-2^\circ \,\rm{C}$.

  2. you are asking about the bottom of the trench, where the water is in direct contact with the ground. To reach freezing (phase change), the ground would need to have the same (or even colder) $-2^\circ \,\rm{C}$. In reality, because of geothermal energy, the ground is way warmer then that.

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  • $\begingroup$ Why the downvote? $\endgroup$ Feb 9, 2020 at 16:27
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Your intuition is that warm water rises, so eventually the coldest water molecules end up at the bottom and freeze. There are two problems with this:

(1) individual water molecules do not really feel much of buoyancy if they move fast (=are hot). The buoyancy of warm fluids and gases is because warm molecules bounce more vigorously against each other, producing an expansion of the medium that lowers its density and this makes it tend to move upwards if it is surrounded by colder, more dense medium. So there will not be any strong sorting of molecules, just of water parcels.

(2) Water conducts heat, and a cold water parcel surrounded by warmer water or rock will tend to heat up. This is the main reason the deepest ocean is not arbitrarily cold. There is some heat coming from the upper warmer layers even if they do not mix, and there is geothermal heat (around 60 mW/m$^2$) from below.

There are further issues. There are slow currents (the thermohaline circulation) driven by the polar regions dripping cold, salty water into the deep, and upwellings in some regions where the deep water gets mixed with warm surface waters.

Finally, the pressure at the deepest trenches is still somewhat below the level where high-pressure ice would form. Were the trenches deeper we would actually get ice formation down there up until the level where the pressure was insufficient. This is fairly temperature-independent (the upper curve between blue and green is fairly horizontal), so it would mostly depend on depth.

(Note that in the uppermost atmosphere where the density is low enough molecules actually do show some sorting since they do not collide much, and the hottest molecules go on long parabolic arcs.)

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  • $\begingroup$ @DavidHammen - Yes, I see that the mention of the temperature dependency is incorrect. I have removed that statement, which did not affect the main answer. $\endgroup$ Feb 5, 2020 at 20:30
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Frozen water is less dense than the liquid water, so when icicles are formed, they are moving up, so the ice always builds up at the top of a body of water. If the water reservoir is deep, then, because of thermal capacity of a large volume of water it does not get frozen completely from top to bottom. The ocean water does not get frozen to the bottom because of many underwater streams moving warm water from one part of the world to the other. Gulfstream is one example of such streams.

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  • $\begingroup$ This answer seems consistent with the relatively small (for whatever latitude) seasonal variations in temperature that characterize coastal areas and islands, as well as with the well-known "turning over" of pond and lake water in autumn. $\endgroup$
    – Edouard
    Nov 28, 2021 at 5:14
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The water down there does not freeze because of freezing point dpression, plus the effect of the pressure (In order to freeze water needs to expand).
Since it is not frozen it keeps mixing up with the water coming from above.

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  • $\begingroup$ That doesn't mean that water is unable to reach temperatures below $0\,°\mathrm C$ $\endgroup$ Feb 5, 2020 at 14:27
  • $\begingroup$ im not entirely sure anymore if the guy is trolling or serious...the densest water goes to the bottom Alessio. And the densest water is 1°C to 4°C as wikipedia states (according to you) that that is the temperature at the bottom of the trench. $\endgroup$
    – Leviathan
    Feb 5, 2020 at 20:54
  • $\begingroup$ @SomeGuy normal ice (not the one that forms under high pressure) is less dense than water. Is not water density at low temperature the problem, but the transition from liquid to solid state that requires expansion, so, yes pressure could in some circumstances prevent the transition. $\endgroup$
    – FluidCode
    Feb 5, 2020 at 21:01
  • $\begingroup$ @FluidCode From where do you get the notion that ice is less dense than water at this pressure? Because the phase diagram does not show that. No transition from liquid to solid does not necessarily 'take space'. That is a specific characteristic of water, that becomes less dense when freezing! It expands. Same mass, higher volume, thus less density. Liquid metal for example will not show this behaviour. It will decrease in volume upon freezing, thus it will harden under pressure or upon cooling. At high enough pressures, even water will do so. But not at pressures as low as in the trench . $\endgroup$
    – Leviathan
    Feb 5, 2020 at 21:19

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