IN CERTAIN TIMES OF THE YEAR, we can witness tall, stately masses of glacial
ice and snow floating off the northern and eastern coastlines of Newfoundland
as they journey on the currents and waves of a warming Atlantic, and to their
final dissolution. Some have broken away from glaciers in Canada’s north, but most
are from the giant ice sheets of Greenland where the miles deep inland ice flows
inexorably to the coast, calving its wayward
offspring into the sea, just as they have done for thousands of years. Many icebergs
[from Ger. berg— “mountain”] floating past the Newfoundland coast are small—the
size of grand pianos—but larger ones are observed, some as big as a fifteen-story
building and weighing up to 200,000 tonnes. And contrary to popular opinion,
most icebergs are not shaped like “ice cream cones”, with 90% of their mass
below water sharpening to a point. Icebergs are formed by the contest between
the force of gravity and the icy structure’s innate buoyancy.* Gravity pulls
down on the berg while the crystalline structure of the ice pushes it upward. Water
expands when it freezes making it about 10% less dense and therefore lighter,
hence the 10% ‘tip’ above the water’s surface. This is also the reason ice
forms on the water’s surface in winter and doesn’t sink—again, water’s special
properties cause it to expand when it freezes, unlike other liquids (example: liquid
helium, nitrogen, alcohol). Sea water, into which freshwater icebergs calve is denser
because of its high salt content, which adds to the buoyancy of icebergs. However,
another factor determines which part of the berg will be above water, and that
is how balanced it is with respect to its centre of mass. An irregularly
shaped iceberg can sway and flip in wind or currents as gravity pulls most of
its underwater. For example, an “ice cream cone” shaped iceberg sitting
vertically in the water is unstable and prone to overturning by wind and waves,
melting, and sub-surface currents. Most icebergs would ‘overturn’ this
configuration to float on their side, which is a more stable position. It still
would have about 9/10s of its mass below water’s surface.1 The “bummock”, or
underwater mass, tends to be older and denser ice, thus the greater weight of
an iceberg is subsurface. [The above water portion of an iceberg is called the
“hummock”. Ed.]
“An
iceberg, just like anything else that floats, will always displace a volume of
water equal to its weight. Ice is about 89% as dense as seawater, so that’s the
fraction of an iceberg’s volume that must be submerged to displace its weight
in seawater. Only 11% will float above the surface. The depth of an iceberg
will be related to its shape. A thin, flat iceberg will not be very deep because
not much of it will be above the surface. But a taller, rounder iceberg will
have a greater underwater depth in order to keep to the “11% above the surface”
rule.” (Quora)
ICEBERGS THAT FLOAT on their sides are known as “tabular” icebergs. [from
Latin—tabula: “board”, “tablet”] These are large, flat masses of ice
that sheer off a glacial ice shelf as it calves
into the sea. They can be hundreds of feet deep beneath the surface and miles
long. The most famous one is the recently formed “A23a”,
an unfortunate name for a massive, Antarctic iceberg that is almost 1500 square
miles—the size of Greater London. “Non-tabular” icebergs are much smaller and
irregularly shaped, with “dome”, “spire” and “wedge” being the three main categories.)
They are created from “tabular” icebergs calving their own icebergs as well as from
glacial calving directly as the Greenland ice sheet breaks apart into the sea.
White icebergs are made of newer snow and ice deposited on the inland ice
field. Blue or green bergs are composed of glacial ice that has been compressed
under great pressure and may be hundreds of years old. IN THIS CASE, the ice is
not actually blue. Its density allows light to enter the ice deeper and be
refracted to our eyes in a blue wavelength. In newer ice, the air and intact
crystalline structure refracts back light in the white wavelength. Streaks of colours
in icebergs represent rock material (i.e., dust), algal growth and dissolved
organic matter that have blown onto the inland ice shield, impregnating the ice
that forms there. As icebergs melt in the ocean, they release this nutritional
matter and carbon that, in turn, forms micro-habitats for plankton and
microscopic organisms to thrive and become food sources for larger, pelagic
faunae surrounding the bergs as they are carried on the ocean currents.
AND, OF COURSE, there are dramatic scenes of icebergs over-turning as
they adjust to their changing centres of gravity. Most icebergs over-turn in
their ‘youthful’ phase following their calving from the glacial ice sheet. As
they stabilize and adjust their balance, they float with the current, conveying
a sense of solidity and permanence. Which is deceptive. Over-turning events,
when they are observed, surprise and sometimes
frighten as the mass of blue-tinged bottom ice lifts from beneath, and
overturns and rotates the “hummock”, or surface ice, into the water, creating a
new form for the ten percent of the berg that will remain above. This is the life cycle of an iceberg—to grow inland, perhaps with
centuries of snow compacting into ice, then a journey to the coast to break free
of the land, to melt and change form, evolving from water into ice into water
again. They are majestic, imposing, constantly changing, and ephemeral. So it goes.
Cheers, Jake.
_________________________________________
*
The buoyancy of an iceberg is due to the unique properties of water that expands
when it freezes.
“The
density of water is 1 g/mL, or 1 g/cm³. Ice has a density of approximately 0.92
g/cm³. Ice has a crystalline structure, and the molecules are spaced farther
apart from each other than in liquid water. Since there is only a small decrease in density of ice compared to
water, [Approximately 10% less dense] there is a small upward net buoyant
force, and a small amount of the ice remains above the water surface. An object
with very low density will sit much higher on the surface of the water.” (Quora)
Fire and Ice
By Robert Frost
Some say the world will end in
fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
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