C.A.M. King has divided ice in the sea into two broad types: icebergs, originating on land, and the sea ice, or pack ice, which forms directly by freezing of sea water.
Icebergs pose great danger to shipping, since they reach fairly low latitudes at times. On the other hand, due to pack ice the harbours are frozen up in winter months.
There are two types of icebergs, which are characteristic of the Northern and Southern Hemispheres. The northern icebergs are produced from calving glaciers, while the flat icebergs of south that are larger in size are derived from the calving of large blocks from the shelf-ice found around some parts of the Antarctic continent.
The Antarctic glaciers move rather very slowly, so the true ice-bergs are smaller in number in that region. But the iceberg formation is most active in Greenland, where there are fast-flowing glaciers. The icebergs in the Northern Hemisphere are not only smaller in size, but they are also irregular in shape.
The northern glaciers are derived from such glaciers which are characterized by a large number of crevasses, and which break into fairly small pieces. On the contrary, the Antarctic icebergs are generally very large and flat-topped.
Since they are less dense, they float higher in the water. These Antarctic icebergs are sometimes 96 km in length, but mostly they are much smaller, being less than about 6 km. Most of the ice bergs are about half a kilometer long, and 30 to 40 m. high above sea surface.
There is a great irregularity in the calving of Antarctic ice bergs. In certain years the number of icebergs is more than in others. It is to be kept in mind that all the icebergs in the Southern Hemisphere are produced around the Antarctic continent. They drift much further north than the pack-ice.
Their northern limits in the Atlantic, the Pacific and the Indian Oceans are usually 35′ S, 50°S, and 45°S respectively. In the South Atlantic Ocean, the cold Falkland Current drives the icebergs along with it to the more northerly limit.
Wherever the icebergs float in the pack-ice, they move at a greater speed through it. They are greatly influenced by the ocean currents, while the movement of the pack-ice is largely controlled by winds.
These icebergs are very dangerous to such ships as are trapped in the pack-ice lying in the icebergs’ path. Remember that since these giant icebergs melt after a long time, they drift to greater distances from the continent than the ice / pack ice.
As an exceptional case, a small piece of floating ice was seen in 26° 30′ S latitude and 25° 40′ W longitude on April 30, 1894.
In the Northern Hemisphere, the main sources of the origin of icebergs are the sea coasts of Greenland, Franz Josef Land, and Novaya Zemlya. Along the north – west and south-east coasts of Greenland the icebergs are most numerous.
From these source regions the icebergs drift south in the cold Labrador and East Greenland currents. Icebergs carried southward by the above- mentioned cold currents merge together east of the Grand Banks between 43°N and 47°N lat.
Remember that it is in this zone that there is dense fog during most of the year as it is the meeting zone of cold Labrador Current and the warm Gulf Stream current. The tragic loss of the Titanic ship occurred in this zone in 1912. However, since then the International Ice-Patrol has been operating here.
There is great variation in the number of icebergs from year to year. Besides, the southerly limit of the icebergs also varies greatly. For example, as many as 1300 icebergs were sighted by the ice-patrol in 1929, whereas only 11 icebergs were seen in 1924.
In those years in which the number of icebergs is abnormally high, some of these may reach as far as 30N lat. The life span of the Greenland icebergs is reported to be less than two years.
The icebergs with a gigantic size of 50 million cu. ft. in the Davit Strait decrease to only 6 to 8 million cu. ft. at the Grand Banks. The large number of icebergs appears in this area from the middle of March to the middle of July with the maximum number arriving in the month of May.
The so-called ice-islands of the Arctic Sea with their smooth surface, and dimensions measuring 17.5 x 7 km originate from the ice-shelves in the vicinity of Greenland and Ellesmere Islands. They drift with the pack-ice and melt from below. However, they build up from above by snow precipitation.
They are always surrounded by the pack-ice. The currents of the Arctic basin make them drift over the Pole. During the course of their long journey the original ice at the bottom disappears by melting away with the result that the ice-island finally consists of only firm.
Sea-ice or pack ice:
Sea-ice, according to its age and place of origin assumes various forms. The characteristics of the sea-ice have already been discussed earlier in this chapter. In the Arctic Sea, ice forming at the sea surface is divided into three categories: pack-ice, polar ice, and fast ice. Pack ice forms around the margin of the Arctic Sea.
It extends through the Bering Strait into the Bering Sea. In the North Atlantic the pack ice can be seen as far south as Newfoundland and Nova Scotia. During the month of May the pack ice reaches its maximum extent.
Its area decreases to the minimum in September. During winter months it attains a maximum thickness of 2 m. It is under the dominant control of winds. However, the ocean currents also modify its structure and break it. The expansion of floes goes a long way in the formation of pack ice.
Polar ice covers the largest area of the Arctic Sea including the polar region. Its maximum thickness exceeds 50 meters. However, during the warmer summer months, it starts melting as a result of which enclosed bodies of water known as polynyas are produced. Remember that the polar ice never disappears completely.
But its average thickness in summer is decreased to about 2 meters. There is a continuous exchange of polar ice, as floes from the pack ice are carried into the polar region in winter, and the floes break out of the polar ice and re-enter the pack ice in summer. About 33% of the pack and polar ice enter the North Atlantic through the East Greenland Current every year.
Fast ice develops during the winter season from the shore to the pack ice. It disappears in summer by melting. It is firmly attached to the shore. In winter its thickness exceeds 2m. Fast ice remains fixed in its position of growth and its only movement is up and down with the tides.
A sheet of fast ice which projects more than 2m above the sea level is known as an ice-shelf. The limit of fast-ice coincides with a water depth of 20 m. In places off Siberia, where water is shallow to a great distance from the shore, it extends seaward over 400 km.
This is particularly so off the mouths of the rivers Yana and Lena, where additional discharge of water causes low salinity and hence conditions favourable for ice-formation.
In the Southern Hemisphere the entire polar region is covered by a continental ice sheet. All the sea ice forming around the continental margin of Antarctica, which is of temporary existence, may be characterized as pack ice. It hardly extends north of 55° S lat.
From October to January it disappears completely except in protected bays which are very calm. Very strong winds do not allow the formation of a greater accumulation of pack ice around the Antarctic continent.
Careful monitoring of the drift ice was conducted by Brennecke during the drift of the Deutschland in the Weddell Sea in 1911-1912. It was found that the direction of the ice drift deviated about 34° from the wind direction and not by 45° as required by the Ekman theory of wind currents.
It may be partly due to the resistance offered by the ice which is packed together in some areas and broken apart in others, while the wind does not blow uniformly over large areas. However, when compared with the ice resistance in the Arctic, it is small in the Antarctic.
It is due to the fact that in the Antarctic the drift of ice is not checked by land masses on all sides. Therefore the Antarctic pack ice comprises larger ice floes than its Arctic counter part. It is less broken and piled up.