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[w3526602]

Hi,

There was more film of ice falling into the sea, on TV news tonight. I don't know if it was a glacier, or floating ice. 

Can somebody tell me ... if an ice berg melts, does the sea level go up, or down?

Remember, pure water starts to expand as it's temperature drops below 4*, which is what causes engine blocks to split..

Was (or is) there ever such stuff as PIKECRETE?

Is there a way of capturing/recovering the pure water in icebergs? Which raises the questions about the sinking of the SS Britannic in 1912 ... did anyone here see the documentry on UK TV?  Interesting, but unlikely.

602

[w3526602]

Whoops ...

Sorry, it was the SS Olympic, not the Britannic, that sank ... or maybe didn't

602

[A-Ro]

I saw a programme about Pykrete many years ago, it is a mixture of sawdust or other such material and ice which was supposed to be strong enough to make boats out of. I think it was an old Mythbusters programme or something similar and they tried making a boat out of newspaper and ice, as I recall it wasn’t very successful. Google tells me it was invented in WWII by a Geoffrey Pyke as a cheap way of making a seaborne landing strip or aircraft carrier. I can think of stranger inventions that emerged during WWII.

[Craig T]

It was the Titanic that sank in 1912 after bumping into an iceberg..
There were two sister ships, the Olympic was scrapped in 1935 and the Britannic was turned into a hospital ship in the first world war and sank in 1916

Craig.

[Genem]

Hi,

There was more film of ice falling into the sea, on TV news tonight. I don't know if it was a glacier, or floating ice. 

Can somebody tell me ... if an ice berg melts, does the sea level go up, or down?

602

An iceberg, floating, has no impact on sea level when it melts. Feel free to practice with a G&T and a suitable mark on the edge of the glass one evening.  Ice that has previously been sitting on solid ground, melting, increases sea level. Things get complex when it happens in quantity and issues such as decreased salinity and impact on ocean currents start to come into play.  The thought is that in extremis Gulf Stream may be switched off...   

[andrewR]

To be pedantic ... there will be an effect, because seawater is denser than freshwater. So the density of the iceberg is significantly less than the density of the seawater.

The icecube melting in a glass of water description only makes the level stay the same when the glass is full of a liquid and the "cube" is frozen material of the same stuff. This isn't the case with an iceberg, that has a zero or low salt content.

The force exerted up on the berg by the seawater is defined by the displaced mass of seawater:

(Force up on berg) = (Density of seawater) * (Volume of seawater displaced) * G

Meanwhile the (Mass of Berg) = (Density of berg) * (Volume of Berg)
and so (Force to hold berg floating) = (Mass of Berg) * G = (Density of berg) * (Volume of Berg) * G

Now if you assume that there are no air bubbles in the berg so that it is solid water-ice (if there are, it just makes everything harder but comes out the same in the end)
The forces must equal or the berg sinks or flies in the air, so:
(Force up on berg) = (Force to hold berg floating)
so
(Density of seawater) * (Volume of seawater displaced) * G = (Density of berg) * (Volume of Berg) * G
so
(Volume of Berg) = (Volume of seawater displaced) * (Density of seawater)  / (Density of berg)

The density of seawater is about 3% more than freshwater, so this means:
(Volume of Berg when it melts) = (Volume of seawater displaced) * 1.03  / 1.00

So for ever 100 m^3 of seawater displaced by a berg when it is floating, will be replaced by 103 m^3 of freshwater, which adds to the sea volume.

[Clifford Pope]

To be a little bit more pedantic, icebergs are formed mostly from glaciers that have reached the sea and broken up, so are of course composed from fresh water.
Some icebergs are formed from sea water that has frozen, but even these have virtually no salt in them because the salt is expelled as the  water freezes.
Ice is only salty if it has been frozen very quickly, which I don't know whether ever  happens in open water.

Perhaps ice formed from sea spray on the upperworks of arctic convoy vessels is salty?