"Aidan Karley" .group
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In article , Leto2 wrote:
I've got a question about ice caps. Does this fall into geology ?

Close enough to get a perspective, but there are more relevant
groups.
The question is about the dynamics of the rise of sea level.

On a geological time scale, the rate of change of sea levels can be
at
least as fast as the "writing" of signals into our record. Most areas
"see" the
rise (or fall) of sea level as a change between one bed and it's
successor.
Sometimes you have material deposited fast enough to record the
intermediate
stages of a change in sea level, but most sites will just see the low or
high
state.

My understanding is that when floating ice melts, this does not provide
more
water,

Correct.

What raises sea level is when continental ice slides down into the ocean,
either in liquid or solid state.

Strictly, it's when ice that is not free-floating melts. Consider an
iceberg floating in the sea - it's your first case above, no change in net
sea
level. Now consider the case of a grounded iceberg, part of whose weight
is
supported by the seabed. That means that the volume of water displaced by
the
iceberg is going to be less than the volume which the iceberg would
displace
when it's free-floating. When a grounded iceberg is melted, it will
increase
the liquid volume of the system in proportion to the support force of the
seabed on the iceberg.
From this case it is a smooth logical step to considering the case
of a
glacier tongue sliding down into the sea - where the glacier is supported
by
the seabed, then there will be a net increase in liquid volume on melting.
And
obviously it includes the case of a glacier 2 km up in the mountains.

So the dynamics could be much slower than just the melting speed, or on
the
contrary, there could be an avalanche effect,

Well, there would be a delay before the water hit the ocean.
Depending
on distance to travel, amount of wetland to traverse, etc, it could be a
few
hours, it could be a couple of weeks. I don't really see circumstances
where it
could be more than a couple of years though, excepting as a glacier.

I was thinking about big lakes being created by the melting ice, and then
released, as happened in Canada, I think.
On a continent like Antarctic, maybe this could happen.
I read also that this was one of the dangers of warming in the Andes : the
glaciers melt and get filled of water until they break down into an
avalanche. This happened above Huaraz, Peru, maybe 30 years ago. The town
disappeared.

The "avalanche effect" is not as well-known as it should be. In
glaciology it's frequently referred to as a "surge", rather than as an
avalanche, because the ice-flow speeds may increase a hundred-fold, but
still
not make it to walking pace. But it's a well-known phenomenon, if not a
well-understood one. Involves alls sorts of mucky problems like the
friction of
dirty ice against irregular surfaces of dirty rock.

Of course I suspect that at continent-scale, the slope cannot allow an
avalanche to happen. I used the term to mean a threshold effect, between a
phase where the movement can be stopped when temperature cools down (it is a
reversible phase : more temperature = more speed, and reversely, cooler
means slower), and a phase where nothing can be stopped anymore : the slide
cannot be prevented even if temperature cools down, because the dynamics
involved are quicker than the thermal propagation. In an "accelerated motion
movie" this means : you cannot "freeze" the avalanche back.


(I'm not questionning the possible cause for the melting. please no
global
warming threads)

Not a hope in hell.

--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233