Thus a lake can experience ice formation while considerable heat still remains in the deeper parts. This does not apply to sea water. The addition of salt to the water lowers the temperature of maximum density, and once the salinity exceeds Cooling of the ocean surface by a cold atmosphere will therefore always make the surface water more dense and will continue to cause convection right down to the freezing point - which itself is depressed by the addition of salt to about It may seem, then, that the whole water column in an ocean has to be cooled to the freezing point before freezing can begin at the surface, but in fact the Arctic Ocean is composed of layers of water with different properties, and at the base of the surface layer there is a big jump in density known as a pycnocline , so convection only involves the surface layer down to that level about metres.
Even so, it takes some time to cool a heated summer water mass down to the freezing point, and so new sea ice forms on a sea surface later in the autumn than does lake ice in similar climatic conditions. How ice forms in calm water In quiet conditions the first sea ice to form on the surface is a skim of separate crystals which initially are in the form of tiny discs, floating flat on the surface and of diameter less than mm.
Each disc has its c-axis vertical and grows outwards laterally. At a certain point such a disc shape becomes unstable, and the growing isolated crystals take on a hexagonal, stellar form, with long fragile arms stretching out over the surface. These crystals also have their c-axis vertical. The dendritic arms are very fragile, and soon break off, leaving a mixture of discs and arm fragments. With any kind of turbulence in the water, these fragments break up further into random-shaped small crystals which form a suspension of increasing density in the surface water, an ice type called frazil or grease ice.
In quiet conditions the frazil crystals soon freeze together to form a continuous thin sheet of young ice; in its early stages, when it is still transparent, it is called nilas. When only a few centimetres thick this is transparent dark nilas but as the ice grows thicker the nilas takes on a grey and finally a white appearance. Once nilas has formed, a quite different growth process occurs, in which water molecules freeze on to the bottom of the existing ice sheet, a process called congelation growth.
This growth process yields first-year ice, which in a single season in the Arctic reaches a thickness of 1. How ice forms in rough water If the initial ice formation occurs in rough water, for instance at the extreme ice edge in rough seas such as the Greenland or Bering Seas, then the high energy and turbulence in the wave field maintains the new ice as a dense suspension of frazil, rather than forming nilas.
This suspension undergoes cyclic compression because of the particle orbits in the wave field, and during the compression phase the crystals can freeze together to form small coherent cakes of slush which grow larger by accretion from the frazil ice and more solid through continued freezing between the crystals.
This becomes known as pancake ice because collisions between the cakes pump frazil ice suspension onto the edges of the cakes, then the water drains away to leave a raised rim of ice which gives each cake the appearance of a pancake. At the ice edge the pancakes are only a few cm in diameter, but they gradually grow in diameter and thickness with increasing distance from the ice edge, until they may reach m diameter and cm thickness. The surrounding frazil continues to grow and supply material to the growing pancakes.
At greater distances inside the ice edge, where the wave field is calmed, the pancakes may begin to freeze together in groups and eventually coalesce to form first large floes, then finally a continuous sheet of first-year ice known as consolidated pancake ice. Such ice has a different bottom morphology from normal sea ice. The pancakes at the time of consolidation are jumbled together and rafted over one another, and freeze together in this way with the frazil acting as "glue".
The result is a very rough, jagged bottom, with rafted cakes doubling or tripling the normal ice thickness, and with the edges of pancakes protruding upwards to give a surface topography resembling a "stony field".
The rough bottom is an excellent substrate for algal growth and a refuge for krill. The thin ice permits much light to penetrate, and the result is a fertile winter ice ecosystem. In the Arctic, a key area where pancake ice forms the dominant ice type over an entire region is the so-called Odden ice tongue in the Greenland Sea.
Most of the old ice continues south, driven by the wind, so a cold open water surface is exposed on which new ice forms as frazil and pancake in the rough seas. The salt rejected back into the ocean from this ice formation causes the surface water to become more dense and sink, sometimes to great depths m or more , making this one of the few regions of the ocean where winter convection occurs, which helps drive the entire worldwide system of surface and deep currents known as the thermohaline circulation or " Great Ocean Conveyor Belt ".
Growth of the ice Once a continuous sheet of nilas has formed, the individual crystals which are in contact with the ice-water interface grow downwards by freezing of water molecules onto the crystal face. This freezing process is easier for crystals with horizontal c-axes than for those with c-axes vertical. The crystals with c-axis horizontal grow at the expense of the others, and as the ice sheet grows thicker crowd them out in a form of crystalline Darwinism. Thus the crystals near the top of a first-year ice sheet are small and randomly oriented, then there is a transition to a fabric composed of long vertical columnar crystals with horizontal c-axes.
This columnar structure is a key identifier of congelation ice i. The ions of the salts in sea water cannot enter the crystal structure despite its open nature. One might expect all salt to be rejected, therefore, leading to a sea ice cover composed of pure ice. Such is not the case, however. If you suck on a piece of first-year sea ice it will taste distinctly salty. The water from young sea ice may have a salinity of about 10 parts per thousand, dropping to in old ice.
How does this salt get into the ice? The answer lies in the way that the ice sheet grows. The ice-water interface advances in the form of parallel rows of cellular projections called dendrites. Brine rejected from the growing ice sheet accumulates in the grooves between rows of dendrites.
As the dendrites advance, ice bridges develop across the narrow grooves that contain the rejected brine, leaving the brine trapped and isolated.
The walls of the "prison" close in through freezing, until the salt is contained in a very small cell of highly concentrated brine , concentrated enough to lower the freezing point to a level where the surrounding walls can close in no further. The cell then remains, a tiny inclusion. In this way, new continental or ocean crust is formed at the fractures. Climate researchers consider continental drift to be one of the most influential factors in the history of ice formation in the polar regions.
After all, the relative positions of the continents and oceans determine the patterns of air and ocean currents, and thus the distribution of heat on the planet. In the past, parts of both regions have been located in the opposite hemispheres.
Antarctica — an ancient continent In order to understand the origins of the southern polar region, it is necessary to know that the Antarctic continent actually consists of two parts: One is the relatively large, solid landmass of East Antarctica, which is composed of continental crust up to 3. The other is West Antarctica, which comprises four considerably smaller and thinner crustal blocks. These four crustal fragments even today are not firmly connected to one another.
They are constantly drifting. The geographic position and remoteness of Antarctica are relatively recent phenomena from a geological perspective. At least twice, in fact, it has been located far from the South Pole at the centre of a supercontinent.
The first time was around one billion years ago, when all of the continents united as a consequence of worldwide mountain building to form the supercontinent Rodinia. Some reconstructions place it beside Australia or Mexico. Which scenario is correct is still being debated today. Approximately million years later, during the Ordovician geological period, the Antarctic Continental Plate again moved to the centre of a great continent. This time it formed the core area of the giant Gondwana continent.
This time it was bounded by India and Australia to the west and South America to the south. Extra Info A volcanic landscape hidden below the ice Gondwana existed for a period of more than million years. These initiated the movement of the Antarctic Plate toward the south, which became possible as all of the neighbouring continents slowly broke off.
The land masses of India and Madagascar then slowly drifted away toward the north, centimetre by centimetre. Then, between 90 and 80 million years ago, as New Zealand separated from Antarctica, the crustal blocks of West Antarctica were reorganized. Hot magma currents within the Earth began to lift the blocks along their border to East Antarctica.
It is to kilometres wide, more than kilometres long, and is one of the largest continental trench fault systems on the Earth — comparable in size to the East African Rift Valley, which runs through Africa from the Red Sea to Mozambique.
The Antarctic continent could someday break apart along this active fault zone, but currently the trench is only widening by two millimetres per year, which is equal to about one metre every years. The formation of the West Antarctic Rift zone about 80 million years ago was not the last tectonic milestone in the drift history of the Antarctic Continental Plate. One occurred at the plate boundary between South America and the Antarctic Peninsula where the spreading increased significantly 50 million years ago.
Around 41 million years ago the Drake Passage opened here, an oceanic strait that is about kilometres wide today and connects the Pacific and Atlantic Oceans. The second notable spreading process occurred on the other side, in East Antarctica, where Australia was drifting away from the Antarctic Plate. Researchers today find this separation fascinating because it occurred in part, at least from a geological perspective, at breath-taking speed.
It is now believed that the Australian Plate separated from the Antarctic Plate in two steps. This land bridge, however, already bordered on a long, shallow gulf that was formed between the two plates.
A strait was created that opened a pathway for cold oceanic deep water from the Southern Ocean, which could now flow unimpeded between Australia and Antarctica. The ring of water around Antarctica was now complete and the Southern Ocean was born. The continuous band of current today still climatically insulates the southern continent from the rest of the world, and this same situation significantly contributed to the initiation of ice formation in Antarctica 34 million years ago.
The Arctic — an ocean opens The land masses in the present-day Arctic region have undergone a much longer voyage than that experienced by the Antarctic continent. Since then Spitsbergen has drifted 12, kilometres to the north at an average speed of less than two centimetres per year. The climate was hot and humid, and dense rain forests grew on Svalbard.
When the age of dinosaurs began million years ago, the land mass of Svalbard was covered by a sea in which first ichthyosaurs, and a few million years later metre-long plesiosaurs swam and hunted their prey. Researchers have discovered large numbers of the skeletons of both of these marine reptiles. At the same time, rivers that existed then must have transported large amounts of sediment and organic material into the sea.
These sank to the bottom and produced kilometre-thick deposits in large basins. One possible explanation is that the continental plates of North America and Siberia drifted apart rotationally, thereby creating room for the Amerasian Basin. In the Late Jurassic epoch, million years ago, plate-tectonic processes began to act that led to the formation of the Arctic Ocean and the present-day configuration of the continents.
Geologists believe that at that time a small ocean basin formed between North America and Siberia, which was the beginning of a division and the subsequent rotational spreading between the two plates.
Based on present knowledge, the exact motions that occurred in this scenario can only be surmised. Some lava masses also escaped to the surface and formed volcanoes. At this time, Svalbard had reached its position in the high latitudes, but was still a part of the large land mass of Laurasia, which, like all of the areas surrounding the new Arctic Basin, was covered with dense forests of giant redwoods.
These are also indicative of the tropical conditions in the high north. Laurasia began to break apart completely as crustal spreading between Canada and Greenland around 95 million years ago created the Labrador Sea and Baffin Bay.
In the process of this separation, the Eurasian Basin of the Arctic Ocean opened between the continental margin of Eurasia and the Lomonosov Ridge.
In its centre there is an active mid-ocean ridge today, the Gakkel Ridge, named after the Russian oceanographer Yakov Yakovlevich Gakkel. This ridge is a continuation of the North Atlantic Ridge. It extends from the north coast of Greenland to near the Lena River Delta and divides the Eurasian Basin into the northerly Amundsen Basin and the Nansen Basin, which lies to the south and thus nearer to the coast.
As is typical of mid-ocean ridges, the Gakkel Ridge is a tectonic spreading zone. This means that the ocean floor is spreading apart along the kilometre-long ridge. The sea floor is spreading here at a rate of only one centimetre per year. Where plates collide, large zones of deformation and buckling occur.
Mountains fold upwards — for example, on the west coast of Svalbard, in northern Greenland and in the Canadian Arctic. Such fault zones exist today on Banks Island and Ellesmere Island, for instance. Researchers have found that plate movements have shaped the entire continental margin of North America over the long term.
This is supported by the fact that the margin of northern Canada is surprisingly straight from the Mackenzie Delta in the southwest to the northern edge of Greenland.
Here they have set up camp on a remote part of Ellesmere Island. Ocean formation in the Labrador Sea and Baffin Bay ended around 35 million years ago. Greenland, which had existed for some time as a separate continental plate, now became part of the North American plate again.
During this separation, 17 to 15 million years ago, a trench up to metres deep was created between the archipelago and the east coast of Greenland. Despite all of these geological indications, the history of the Arctic Ocean remains a plate-tectonic enigma. Many of the details are still not understood today. For example, geologists do not know the origin of the Alpha-Mendeleev Ridge.
This undersea mountain chain divides the Amerasia Basin into the Makarov Basin in the north and the Canada Basin to the south. Ship expeditions to this vast marine region are extremely rare and expensive because, despite climate change, this part of the Arctic Ocean is covered with sea ice even in summer, making geological drilling particularly costly and risky. For most of the approximately 4. The planet has been predominately ice-free.
Large-scale glaciation in the high latitudes has only occurred during the glacial periods. If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media. Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. The cryosphere contains the frozen parts of the planet.
It includes snow and ice on land, ice caps, glaciers, permafrost, and sea ice. As the world warms due to increasing greenhouse gases being added to the atmosphere by humans, the snow and ice are melting.
At sea, this exposes more of the dark ocean below the ice, and on land, the dark vegetation below. These dark surfaces then absorb the solar radiation causing more melting. This creates a positive feedback loop, which exacerbates the impacts of climate change. Learn more about this vulnerable sphere with this collection of resources. Climate describes the average weather conditions of a particular place over a 30 year period.
All places on earth have their own climates. Different from weather events, which are short-term and temporary phenomenon, climates are usually steady and predictable, and shape how organisms and human civilizations evolve and adapt in any given region.
However, climates are not always permanent, and can change drastically due to human activity. Explore the world's climates and how they affect local regions and the planet with this curated collection of resources. A biome is an area classified according to the species that live in that location. Temperature range, soil type, and the amount of light and water are unique to a particular place and form the niches for specific species allowing scientists to define the biome.
However, scientists disagree on how many biomes exist. Some count six forest, grassland, freshwater, marine, desert, and tundra , others eight separating two types of forests and adding tropical savannah , and still others are more specific and count as many as 11 biomes. Use these resources to teach middle school students about biomes around the world. The northern hemisphere experiences summer during the months of June, July, and August because it is tilted toward the sun and receives the most direct sunlight.
Inversely, summer for the southern hemisphere takes place during the months of December, January, and February because that is when it receives the most direct sunlight. Did you know that the earth is approximately 3.
Learn more about the relationship between the earth and the sun with these resources. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.
Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. The Arctic is the northernmost region of Earth. Most scientists define the Arctic as the area within the Arctic Circle , a line of latitude about Within this circle are the Arctic ocean basin and the northern parts of Scandinavia , Russia, Canada, Greenland, and the U. The Arctic is almost entirely covered by water, much of it frozen. Some frozen features, such as glacier s and iceberg s, are frozen freshwater.
Most of the Arctic, however, is the liquid saltwater of the Arctic ocean basin. This frozen seawater is called sea ice. Often, sea ice is covered with a thick blanket of snow. Sea ice has a very bright surface, or albedo. The Arctic experiences the extremes of solar radiation. The sun rises again during the March equinox, and increases the light and heat reaching the Arctic. By the June solstice , the Arctic experiences hour sunshine.
The Arctic ocean basin is the shallowest of the five ocean basins on Earth. It is also the least salty, due to low evaporation and huge influx es of freshwater from rivers and glaciers. River mouth s, calving glaciers, and constantly moving ocean current s contribute to a vibrant marine ecosystem in the Arctic. The cold, circulating water is rich in nutrient s, as well as the microscopic organisms such as phytoplankton and algae that need them to grow.
Marine animals thrive in the Arctic. Primary consumer s such as jellies and shrimp consume plankton, the basis of the Arctic marine food web. Secondary consumer s include fish, seabirds such as gulls and puffins , and a wide variety of baleen whales, including giant blue whales and bowhead whales. Tertiary consumer s, animals that prey mostly on other carnivore s, include toothed whales and dolphins such as orcas and narwhals and pinniped s such as seals, sea lions, and walruses.
Scavenger s including some sharks and crabs and decomposer s such as marine worms and algae break down dead and decaying materials. Organic nutrients are thus recycled into the marine ecosystem of the Arctic. The varied landscape s of the Arctic provide for a variety of ecosystems.
The Arctic includes the peaks of the Brooks mountain range in western North America, the enormous Greenland ice sheet , the isolate d islands of the Svalbard archipelago , the fjord s of northern Scandinavia, and the grassland plateau s and rich river valley s of northern Siberia.
Although some forest s lie near the Arctic Circle, plant life is mostly limited to grasses, sedge s, and tundra vegetation such as moss es and lichen s. These autotroph s have the ability to survive despite being covered in snow and ice for much of the year.
Insects such as mosquitoes and moths are common, especially as icemelt creates ponds during spring and summer. Insects and insect larvae provide a crucial diet for birds, such as wrens and sandpipers, and freshwater fish.
Primary consumers across the region range from tiny lemmings to enormous muskoxen. One of the most familiar Arctic herbivore s is the caribou, often known as the reindeer in Europe and Asia. Secondary consumers include Arctic foxes, and raptor s such as owls and eagles.
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