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Icebergs 

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 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. 

iceberg and ship

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.

 

iceberg coast guard plane

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.

iceberg under water

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. 

globe north pole

Map from Worldatlas.com

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.

iceberg calving

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.

greenland

 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. 

greenland

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.

iceberg alley

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 sizes and classification

ICEBERG SHAPE CLASSIFICATION

iceberg tabulariceberg tabular

TABULAR: An iceberg with steep sides and flat top having a length-to-height ratio greater than 5:1. Many show horizontal banding.

iceberg non-tabulariceberg non-tabular

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

iceberg tabular domeiceberg tabular dome

DOME: An iceberg with a rounded top

iceberg pinnacleiceberg pinnacle

PINNACLE: An iceberg with one or more spires

iceberg wedgeiceberg wedge

WEDGE: An iceberg having a steep vertical side on one end and sloping on the other

iceberg dry dockiceberg dry dock

DRY-DOCK: An iceberg that has eroded so a slot or channel is formed

iceberg blockyiceberg blocky

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

iceberg 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:

archimedes

  • 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 Collapsing off Battle Harbour, Labrador

 

Iceberg Video

Ice Bergs off the New Zealand Coast. First time since 1931. Approx 60 Kilometres offshore. 

 

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.

 

 

http://nsidc.org/index.html

 

http://www.navcen.uscg.gov

 

http://www.natice.noaa.gov

 

 

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 Ozone Hole Inc. a 501(c)(3) Nonprofit Organization http://www.theozonehole.com

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