Frozen saltwater is fundamental different from frozen freshwater. If fresh water freezes it is building a massive block of ice. Ice cubes are a typical examples. They are glassy and in one piece.
But frozen seawater looks more like a “swiss cheese”. The ice is crossed by a web of holes and channels. The channels have a size from as small as millimetre to centimetre. Under the microscope the ice of seawater looks porous and fragile. Why is this?
The reason for this is that seawater is made up by water and salt particles. But only from the water is ice building, because only water has space in ice crystals the salt particles are to big to fit in. This means that icebergs in the ocean are made up from fresh water. A castaway in the arctic will never be thirsty but probably will freeze.
What happens to the salt when the seawater freezes?
Easy, it remains. It concentrates more and more in the rest of the water. As long as one part of the water freezes, the other part will continuously obtain salt. This mixture is called “brine” and is heavier than normal seawater and sinks down. However not fully. Because sometimes the ice is faster.
As the ice builds, the salt solution gets trapped inside the ice. The salt solution infiltrates through the ice and builds maze made up by holes and channels that are all connected.
The salt ensure that the sea ice is porous and brittle. But this does not mean that the ice is not reliable. Even a small ice floe with a thickness of less than 15 centimetre could carry a seal with about 1 ton and sometimes the ice is even as heavy, that icebreaker can not break it.
[Sources:]
Picture 1: Iceberg
Picture 2: Ice cubes
Picture 3: Iceberg cracks
Picture 4: Ice
Book I got most of the information
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