|1/ What is pack-ice?
the beginning of the austral winter starting around March,
the loose pack ice that has spent the summer months circling
Antarctica begins to drift northwards.
Pack ice is old sea-ice, frozen sea water
that is a year old or more, it froze and formed elsewhere
and later floated off with the winds and currents.
Pack ice is heavy stuff and when it arrives
somewhere it has the effect of steadying the ocean swell.
The continuous rolling motion of the sea is stopped completely
by a relatively narrow band of pack ice only 100m or so
wide. The result is that where pack ice is present in reasonable
quantity, the sea calms down sufficiently for low temperatures
to freeze it easily - moving water cannot freeze as easily
as static water.
|2/ What is fast-ice?
is sea-ice in the very early stages of formation. Sea-ice
that forms in situ and attached to the coast is called "fast-ice",
it is stuck fast. In this picture the surface of the sea
is beginning to freeze as the temperature is dropping to
-20C and below.
Pack ice has come near to the shore and
so all movement of the sea has been killed completely allowing
low temperatures to freeze the sea water. At this stage
the ice is around an inch or 2.5cm thick but it has a spongy
texture, you could poke a finger or certainly a fist through
it relatively easily.
The patterned effect comes from the rise
and fall of the tides. As the tide rises, so the surface
of the sea enlarges slightly and so the ice cracks apart,
as the tide falls, so the surface of the sea decreases slightly
and so the slabs of ice overlap at the edges.
|3/ Does the tide
change the way the sea freezes?
ice in the process of forming, the shore of the island
in the distance is about 5 miles (8 kilometres) away and
the whole of the sea surface in-between is made of forming
Notice how the slabs of forming ice become
larger further out to sea as there are less undulations
of the coast to push the slabs together as the tide falls.
|4/ What are ice
ice near to the shore here is known as "pancake-ice".
This is formed when slabs of ice that are forming are jostled
by the wind and / or movement of the sea.
The pancakes of ice bash against each other around the edges
and start to curl upwards at the edges
of submerged ice joined with others into great sheets, the
rubbery green ice thickened, an ice foot fastened onto the
shore, binding the sea with the land. Liquid became solid,
solid was buried under crystals."
Annie Proulx - The Shipping News
|5/ Do ice-bergs
move around when the sea freezes?
picture taken of consolidated pack-ice.
The ice that you see is mainly
pack-ice, last years ice that formed elsewhere, broke up
and floated here. As the temperature dropped, then this
ice became stuck together by fast-ice, sea-water frozen
in situ and attached to the coast that acts as a glue for
the loose bits of pack.
The ice-bergs that you see
have been frozen in position and will remain so until they
are freed by the spring break-up of the surrounding sea-ice.
|6/ What is a tide-crack?
fast ice (sea-ice frozen in situ) has become established,
the patterns of the earlier pieces disappears. The tide
still rises and falls however meaning that the sea surface
expands and shrinks slightly as it does so.
Tide cracks result from this (as ice is not
known for its elastic properties!) that are formed when
the ice moves apart, they close again when the tide falls.
A tide crack is often many miles long, in this case stretching
for around 5 miles (8 kilometres), but never more than about
18", 45cm wide between Signy and Coronation Islands
in the South Orkneys group.
Tide cracks are valuable resources for
wild-life as they provide a region where birds such as snow
petrels can fish through for krill and also as a breathing
hole for crabeater and Weddell seals.
|7/ Does pack ice
is pack-ice in the summer months around the Antarctic peninsula.
The ice looks fairly continuous,
but has quite a lot of open water between the pieces and
so can be relatively easily pushed aside by an ice-strengthened
ship, in this case HMS Endurance. Larger pieces such
as this one that are hit by the bow of the ship crack up
into smaller pieces.
Proper Ice breakers have rounded hulls
and rounded bows rather than being sharp and pointed. When
breaking through very thick ice, the front of the ship rides
up over the ice and the weight of the ship breaks through.
Passage is slow though, and heavy on fuel,
but most of all, it takes an experienced and well informed
ice-pilot to be confident in entering such ice so as not
to be locked into the pack should the wind direction change
and consolidate the ice.
|8/ What happens
to fast-ice in the summer?
the end of the winter, rising oceanic swells and increasing
temperatures cause the stable winter sea-ice to break-up
and begin to drift away from where it formed.
This years fast-ice therefore becomes next
years pack-ice with a portion of it melting and disappearing
completely. Here a small inflatable zodiac-like craft is
(not entirely sensibly) negotiating quite close, but relatively
One person drives the boat, while another
sits on the bow pushing the larger pieces of ice out of
the way with his feet.