One of the Episodes of BBC's documentary Frozen Planet describes an upwell phenomenon in the Antarctic Ocean. I am trying to understand the cause of seawater freezing when this upwell occurs.
Liquid sea water can remain liquid below salt water's one-atmosphere freezing temperature when that salt water is maintained under pressure at deep ocean depths. The high pressure conditions deep in the ocean apparently prevent the salt water from freezing.
According to the documentary, shifting ocean currents occasionally bring a deep mass of liquid antarctic sea water from near the bottom of the ocean all the way up to the surface very quickly. And much of that mass of liquid sea water quickly freezes due to the change in pressure. Pressure is reduces when the water moves from the bottom of the Antarctic Ocean and arrives near the surface.
If temperature remains constant but the state changes from liquid to solid, I assume this state change is caused by the pressure allowing the sea water to expand and form a crystalline lattice - ice.
Someone watching the BBC documentary with me believes the ice formed in such an upwell event is caused by the molecular movement of the water molecules slowing down with the reduction in pressure, and she argues the slowing of movement brought on by reduction in pressure causes the ice to form. She asserts the reduced molecular movement is the cause, not the physical expansion of the overall volume of the water as it rises.
My understanding is that the extreme high-pressure water simply doesn't have enough physical space to expand into a volume which will allow the molecules to form a lattice, and this has little or nothing to do with the molecular movement.
What is the correct way to understand this phenomenon?