Large tabular icebergs
such as the one pictured are common in the waters near Antarctica.—Credit:
Ted Scambos, NSIDC
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
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
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
International Ice Patrol (IIP)
Frequently Asked Questions
Where do North Atlantic icebergs
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:
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
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
What are the shapes and sizes of icebergs?
less than 3
less than 1
less than 16
less than 5
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
DOME: An iceberg with a rounded
PINNACLE: An iceberg with one or
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?
% OF TOTAL
How much of an iceberg is below
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 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.
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.
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
Iceberg Collapsing off Battle Harbour, Labrador
Ice Bergs off the New Zealand Coast. First time since 1931. Approx 60 Kilometres
Quick Facts on Icebergs
What is an iceberg?
Icebergs are pieces of
ice that formed on land and float in an ocean or lake. Icebergs come
in all shapes and sizes, from ice-cube-sized chunks to ice islands the
size of a small country. The term “iceberg” refers to chunks of ice
larger than 5 meters (16 feet) across. Smaller icebergs, known as
bergy bits and growlers, can be especially dangerous for ships because
they are harder to spot. The North Atlantic and the cold waters
surrounding Antarctica are home to most of the icebergs on Earth.
How do icebergs form,
and where do they go?
Icebergs form when
chunks of ice calve, or break off, from glaciers, ice shelves, or a
larger iceberg. Icebergs travel with ocean currents, sometimes
smashing up against the shore or getting caught in shallow waters.
When an iceberg reaches warm waters, the new climate attacks it from
all sides. On the iceberg surface, warm air melts snow and ice into
pools called melt ponds that can trickle through the iceberg and widen
cracks. At the same time, warm water laps at the iceberg edges,
melting the ice and causing chunks of ice to break off. On the
underside, warmer waters melt the iceberg from the bottom up.
Why are icebergs important?
Icebergs pose a danger to ships traversing the North Atlantic and the waters
around Antarctica. After the Titanic sank near Newfoundland in 1912, the United
States and twelve other countries formed the International Ice Patrol to warn
ships of icebergs in the North Atlantic. The International Ice Patrol uses
airplanes and radars to track icebergs that float into major shipping lanes. The
U.S. National Ice Center uses satellite data to monitor icebergs near
Antarctica. However, it only tracks icebergs larger than 500 square meters
(5,400 square feet). Icebergs can also serve as tools for scientists, who study
them to learn more about climate and ocean processes.
Why do scientists study icebergs?
Climate scientists study icebergs as they break up for clues to the processes
that cause ice shelf collapse. Scientists have noticed that the way icebergs
break up when they reach warmer waters mirrors the disintegration of Antarctic
ice shelves. By studying the factors that cause icebergs to break up,
researchers hope to better understand the influences that lead to ice shelf
breakup, and to better predict how ice shelves will respond to a warming
climate. Oceanographers follow icebergs because the cold freshwater they
contribute to the sea can influence currents and ocean circulation far away from
their origins. Biologists study icebergs to find out how they influence ocean
life. As icebergs melt, they leak nutrients into the ocean around them. Recent
studies have shown that the water surrounding icebergs teems with plankton,
fish, and other sea life.
Credit: U.S. Coast Guard, The National Snow and Ice Data Center, Royal New
Zealand Air Force
Data compiled from The
British Antarctic Study, NASA, Environment Canada, UNEP, EPA and
other sources as stated and credited Researched by Charles
Welch-Updated daily This Website is a project of the The Ozooe Hole