An Iceberg is a floating mass of
freshwater ice that has broken from the seaward end of a glacier or a polar ice
sheet. Icebergs are typically found in open seas, especially around Greenland
and Antarctica.
They form mostly during the spring and summer, when warmer
weather increases the rate of calving (separation) of icebergs at the
boundaries of the Greenland and Antarctic ice sheets and smaller outlying
glaciers. In the Northern Hemisphere, for example, about 10,000 icebergs are
produced each year from the West Greenland glaciers, and an average of 375 flow
south of Newfoundland into the North Atlantic shipping lanes, where they are a
hazard to navigation.
Arctic icebergs vary in size from
the size of a large piano, called growlers, to the dimensions of a 10-story
building. Icebergs about the size of a small house are called bergy bits.
Many
icebergs in the Arctic are about 45 meters tall and 180 meters long.
US
Coast Guard C130 airplane flying over a large iceberg
US
Coast Guard International Ice Patrol Image
Icebergs
of the Antarctic not only are far more abundant but are of enormous dimensions
compared with those in the Arctic. Ninety-three percent of the world's mass of
icebergs is found surrounding the Antarctic.
Usually 1/8th of an iceberg is
above the waterline. That part consists of snow, which is not very compact. The
ice in the cold core is very compact (and thus relatively heavy) and keeps
7/8ths of the iceberg under water. The temperature in the core is constant:
between -15 and -20 degr. Centigrade. An iceberg that has tumbled over several
times, has lost is light snow layers and so the iceberg gets relatively heavier
then before (with the snow) and because of the greater compactness, only 1/10th
rises above the surface.
NASA: A Short Tour of the
Cryosphere Video
International Ice Patrol (IIP)
Frequently Asked Questions
Where do North Atlantic icebergs
come from?
The principal origin of those
icebergs that reach the North Atlantic Ocean are the 100 or so major tidewater
glaciers of West Greenland.
Between 10,000 to 15,000 icebergs
are calved each year, primarily from 20 major glaciers between the Jacobshaven
and Humboldt Glaciers. Since icebergs originate from Glaciers, they are
composed of fresh water. As described in the other FAQs, glaciers are formed by
thousands of years of snowfall accumulation which eventually is compressed into
ice. It is estimated that these glaciers account for 85% of the icebergs which
reach the Grand Banks of Newfoundland. Other sources of icebergs are the East
Greenland glaciers, which produce about half the amount of icebergs as the West
Greenland glaciers, but account for only 10% of the icebergs reaching the Grand
Banks. The remaining 5% are thought to come from glaciers and ice shelves of
northern Ellesmere Island.
What
is the life cycle of an iceberg?
The life cycle of a typical
iceberg found in the North Atlantic today might look something like this:
TIME
LINE
ICEBERG
DEVELOPMENT
1,000 B.C.
Snow/Firn
950 B.C.
Ice/Glacier
--
Glacier movement
1998 A.D.
Calving
2001 A.D.
Iceberg melt
Snow falls on the ice cap of Greenland. Then over the course of several months
it changes into firn, which is basically a granular snow. Several decades later
it is compressed into very dense ice by the weight of the firn and snow that
have accumulated on top of it. Therefore, icebergs are composed of fresh water.
Driven by the enormous weight of the ice cap above, the ice begins to flow
seaward through openings in the fringe of the mountains (thinking of it like
water leaking out of a cracked bowl may help). This force moves the rivers of
ice known as glaciers up to sixty five feet a day, eventually pushing the ice to
Greenland's western coast.
At the glacier's terminus or end,
huge slabs of ice are weakened and then broken by the action of the rising and
falling tides. This process is called calving and results in an iceberg's birth.
By the time these mountains
of ice enter Baffin Bay they have seen nearly 3,000 years pass. Once waterborne,
icebergs are driven by strong subsurface currents, the core's of which are
located at a depth of approximately fifty meters (This occurs because 7/8 of an
icebergs mass rests below the waterline). Therefore, deeper currents have
greater surface area to push against compared to winds or wind generated surface
currents. This is why it is not uncommon to see icebergs heading directly into
strong winds. In order for an iceberg to reach the North Atlantic the currents
typically take it from Baffin Bay through the Davis Strait and Labrador
Sea.
This is a long trip and most
icebergs never make it. Most icebergs melt well before entering the Atlantic
Ocean. One estimate is that of the 15,000 to 30,000 icebergs produced annually
by the glaciers of Greenland only one percent (150 to 300) ever make it to the
Atlantic Ocean. When an iceberg does happen to reach the Atlantic its long and
traveled life quickly comes to an end melting rapidly in the warm waters. At
most it will take two months to melt unlike icebergs stuck in parts of Baffin
Bay where it can take upwards of four years for a berg to melt.
How many icebergs last long
enough to reach the Atlantic shipping lanes (south of 48 N)?
The mean number of icebergs passing
south of 48 N is 473 icebergs with a standard deviation of 492 icebergs.
Therefore, yearly totals are highly variable and are subject to highly variable
climatic factors
Where is iceberg alley?
The area we call "Iceberg
Alley" is located about 250 miles east and southeast of the island of
Newfoundland, Canada. Iceberg Alley is usually considered to be that portion of
the Labrador Current, that flows southward from Flemish Pass, along the eastern
edge of the Grand Banks of Newfoundland, to the Tail of the Banks. This area
extends approximately from 48 to 43 degrees North Latitude at 48 degrees West
longitude. Icebergs and sea ice flowing south from Iceberg Alley created the
Titanic disaster of 1912. This is the area of the ocean we patrol and monitor
most carefully.
What
are the shapes and sizes of icebergs?
ICEBERG
SIZE CLASSIFICATION
SIZE
CATEGORY
HEIGHT(FT)
HEIGHT(M)
LENGTH(FT)
LENGTH(M)
Growler
less than 3
less than 1
less than 16
less than 5
Bergy Bit
3-13
1-4
15-46
5-14
Small
14-50
5-15
47-200
15-60
Medium
51-150
16-45
201-400
61-122
Large
151-240
46-75
401-670
123-213
Very Large
Over 240
Over 75
Over 670
Over 213
ICEBERG SHAPE CLASSIFICATION
TABULAR: An iceberg with steep
sides and flat top having a length-to-height ratio greater than 5:1. Many show
horizontal banding.
NON-TABULAR: Describes all icebergs
that are not tabular shaped as described above. This category is further
subdivided to include the specific shapes described below. If no other
description applies, the iceberg is simply referred to as a non-tabular.
Non-Tabular Iceberg Shape
Classifications
DOME: An iceberg with a rounded
top
PINNACLE: An iceberg with one or
more spires
WEDGE: An iceberg having a steep
vertical side on one end and sloping on the other
DRY-DOCK: An iceberg that has
eroded so a slot or channel is formed
BLOCKY: An iceberg with a flat top
and steep vertical sides
What
are the most dangerous icebergs?
All icebergs are dangerous to
shipping but depending on its size, shape and location some icebergs can be
more troublesome than others. Obviously, icebergs nearest the Atlantic shipping
lanes are of greatest concern to mariners. Large icebergs, because of their
great mass, can inflict the most damage on a ship. However, they are usually
easy to detect on a ship's radar and therefore can be avoided. On the other
hand, the smaller an iceberg, the harder it is for ships to detect and avoid.
For example, many growlers or bergy bits are mostly submerged and are about the
size a small vessel. These "hidden" icebergs can cause a significant
amount of damage to a vessel. Lastly, an iceberg's shape is a factor. A
smoothed iceberg can be more difficult to detect.
What
is the typical size of an iceberg in the North Atlantic Ocean?
SIZE
CATEGORY
% OF TOTAL
Growler
5.6%
Small
15.3%
Medium
15.3%
Large
12.5%
Very Large
2.8%
General(Size
Unknown)
48.5%
How much of an iceberg is below
the water?
About 7/8ths of an iceberg is
below the water line. This figure is approximate. Although icebergs are similar,
not all are the same. Varying factors are iceberg density, water density etc.
Keep in mind we are talking about an iceberg's mass. Due to irregular iceberg
shapes, icebergs may have varying heights out of the water, but mass is
relatively consistent. The following provides further background information:
Buoyancy
Buoyancy is the upward force
exerted on an object immersed in a fluid. Of course, water is the most common
fluid, but buoyancy also applies to hot air balloons (where the fluid is the
surrounding air) and many other situations. What's the basic idea?
Archimedes figured out that the
key to buoyancy is how much volume the object displaces compared to its weight.
Archimedes Principle of buoyancy states that the upward force on an object in a
fluid is equal to the weight of the fluid that is displaced. If this buoyant force is less than the weight of the object itself, the object will be left with
a net downward force and will sink. If the object floats, it floats enough that
the buoyant force exactly balances its weight.
For solid, uniform objects like
an iceberg, this boils down to the object's mass density, its mass divided by
its volume, usually represented by the Greek letter . For something like a boat
hull, which is hollow, not uniform, you have to just look at the total weight
and the volume of displaced water.
Example: Icebergs
So let's take the case of the
iceberg. Lets say it has mass Mi and volume Vi. Their ratio is given by the mass
density of ice: M/V = Rhoi ~ 0.90 g/cm³ (iceberg ice is more dense than normal
ice since it has been compressed by thousands of years of pressure - normal ice
is 0.917 g/cm³). Since we already know it floats, lets say that the volume
below the surface of the water is Vw. This is the volume of water displaced, and
the buoyant force is equal to the weight of that displaced water, which has mass
Mw = VwRhow. The mass density of liquid water was originally used to define the
gram, so it has the convenient metric value = 1 g/cm³. Sea water on the other
hand is more dense since it has salts, therefore we shall use Rhow = 1.035 g/cm³
(or 1035 kg/m³).
The weight of an object is given by its mass times the acceleration of gravity,
g = 9.8 m/s²:
W = Mg
The iceberg has weight Wi = Mig and the buoyant force is equal to the weight of
the displaced water, Ww = Mwg. Furthermore, since the iceberg is floating, its
weight exactly balances the buoyant force:
Ww = Wi
Mwg = Mig
VwRhowg = ViRhoig
Vw = Rhoi/Rhow Vi
So, the fraction of ice underwater, Vw/Vi, is given by the ratio of densities
Rhoi/Rhow=0.87. Over 87% of an iceberg's volume (and mass) is underwater. As you
can see, the convenient definition of the gram gives us a quick way to see how
much of a floating substance lies below the surface of fresh water: the fraction
is equal to that substance's mass density in g/cm³.
Summary:
Archimede's Principle of buoyancy
states that the buoyant force on an object is equal to the weight
of the fluid displaced by that object.
The underwater fraction of a
substance floating on water is given by that substance's mass density in
g/cm³.
Iceberg names are derived from the Antarctic quadrant in which they were
originally sighted. The quadrants are divided counter-clockwise in the
following manner:
A = 0-90W (Bellinghausen/Weddell Sea)
B = 90W-180 (Amundsen/Eastern Ross Sea)
C = 180-90E (Western Ross Sea/Wilkesland)
D = 90E-0 (Amery/Eastern Weddell Sea)
When an iceberg is first sighted,
The National Ice Center documents its point of origin. The letter
of the quadrant, along with a sequential number is assigned to the iceberg.
The National Ice Center is a tri-agency operational center represented by the
United States Navy (Department of Defense); the National Oceanic and
Atmospheric Administration Department of Commerce); and the United States Coast
Guard (Department of Transportation). The National Ice Center mission is to
provide world-wide operational ice analyses for the armed forces of the United
States and allied nations, U.S. government agencies, and the private sector.
Iceberg
B-15J Calves Iceberg Named B-15S
MODIS
image of B-15J AND B-15S
February 02, 2007, Washington,
DC-- The National Ice Center (NIC) was informed by the Antarctic
Meteorological Research Center that iceberg B-15J (Figure 1) has calved a new
iceberg that meets criteria for naming and tracking by the NIC. The new
iceberg will be named B-15S. This iceberg marks the 18th calving event of
icebergs belonging to the B-15 series. B-15S is located at 76 03' 11"
South, 168 08' 35" East, near Franklin Island in the Central Ross Sea.
Iceberg B-15S measures 10 nautical miles on its longest axis and 2 nautical
miles on its widest axis.
The National Ice Center is a
tri-agency operational center represented by the United States Navy (Department
of Defense), the National Oceanic and Atmospheric Administration (Department of
Commerce), and the United States Coast Guard (Department of Homeland Security).
The National Ice Center mission is to provide the highest quality strategic and
tactical ice services tailored to meet the operational requirements of U.S.
national interests and to provide specialized meteorological and oceanographic
services to United States government agencies.
Data
compiled from The British Antarctic Study, NASA, Environment Canada,
UNEP, EPA and
other sources as stated and credited Researched by Charles
Welch-Updated dailyThis
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