Fjord

Geologically, a fjord or fiord (/ˈfjɔːrd/ (listen), /fiˈɔːrd/ (listen)) is a long, narrow inlet with steep sides or cliffs, created by a glacier. There are many fjords on the coasts of Alaska, Antarctica, British Columbia, Chile, Greenland, the Faroe Islands, Iceland, Kamchatka, the Kerguelen Islands, New Zealand, Norway, Novaya Zemlya, Labrador, Nunavut, Newfoundland, Quebec, Scotland, South Georgia Island, and Washington state. Norway's coastline is estimated at 29,000 kilometres (18,000 mi) with nearly 1,200 fjords, but only 2,500 kilometres (1,600 mi) when fjords are excluded. Geologically, a fjord or fiord (/ˈfjɔːrd/ (listen), /fiˈɔːrd/ (listen)) is a long, narrow inlet with steep sides or cliffs, created by a glacier. There are many fjords on the coasts of Alaska, Antarctica, British Columbia, Chile, Greenland, the Faroe Islands, Iceland, Kamchatka, the Kerguelen Islands, New Zealand, Norway, Novaya Zemlya, Labrador, Nunavut, Newfoundland, Quebec, Scotland, South Georgia Island, and Washington state. Norway's coastline is estimated at 29,000 kilometres (18,000 mi) with nearly 1,200 fjords, but only 2,500 kilometres (1,600 mi) when fjords are excluded. A true fjord is formed when a glacier cuts a U-shaped valley by ice segregation and abrasion of the surrounding bedrock. According to the standard model, glaciers formed in pre-glacial valleys with a gently sloping valley floor. The work of the glacier then left an overdeepened U-shaped valley that ends abruptly at a valley or trough end. Such valleys are fjords when flooded by the ocean. Thresholds above sea level create freshwater lakes. Glacial melting is accompanied by the rebounding of Earth's crust as the ice load and eroded sediment is removed (also called isostasy or glacial rebound). In some cases this rebound is faster than sea level rise. Most fjords are deeper than the adjacent sea; Sognefjord, Norway, reaches as much as 1,300 m (4,265 ft) below sea level. Fjords generally have a sill or shoal (bedrock) at their mouth caused by the previous glacier's reduced erosion rate and terminal moraine. In many cases this sill causes extreme currents and large saltwater rapids (see skookumchuck). Saltstraumen in Norway is often described as the world's strongest tidal current. These characteristics distinguish fjords from rias (e.g. the Bay of Kotor), which are drowned valleys flooded by the rising sea. Drammensfjorden is cut almost in two by the Svelvik 'ridge', a sandy moraine that during the ice cover was under sea level but after the post-glacial rebound reaches 60 m (200 ft) above the fjord. Jens Esmark in the 19th century introduced the theory that fjords are or have been created by glaciers and that large parts of Northern Europe had been covered by thick ice in prehistory. Thresholds at the mouths and overdeepening of fjords compared to the ocean are the strongest evidence of glacial origin, and these thresholds are mostly rocky. Thresholds are related to sounds and low land where the ice could spread out and therefore have less erosive force. John Walter Gregory argued that fjords are of tectonic origin and that glaciers had a negligible role in their formation. Gregory's views were rejected by subsequent research and publications. In the case of Hardangerfjord the fractures of the Caledonian fold has guided the erosion by glaciers, while there is no clear relation between the direction of Sognefjord and the fold pattern. This relationship between fractures and direction of fjords is also observed in Lyngen. Preglacial, tertiary rivers presumably eroded the surface and created valleys that later guided the glacial flow and erosion of the bedrock. This may in particular have been the case in Western Norway where the tertiary uplift of the landmass amplified eroding forces of rivers. Confluence of tributatry fjords led to excavation of the deepest fjord basins. Near the very coast the typical West Norwegian glacier spread out (presumably through sounds and low valleys) and lost their concentration and reduced the glaciers' power to erode leaving bedrock thresholds. Bolstadfjorden is 160 m (520 ft) deep with a threshold of only 1.5 m (4 ft 11 in), while the 1,300 m (4,300 ft) deep Sognefjorden has a threshold around 100 to 200 m (330 to 660 ft) deep. Hardangerfjord is made up of several basins separated by thresholds: The deepest basin Samlafjorden between Jonaneset (Jondal) og Ålvik with a distinct threshold at Vikingneset in Kvam. Hanging valleys are common along glaciated fjords and U-shaped valleys. A hanging valley is a tributary valley that is higher than the main valley and were created by tributary glacier flows into a glacier of larger volume. The shallower valley appears to be 'hanging' above the main valley or a fjord. Often, waterfalls form at or near the outlet of the upper valley. Hanging valleys also occur under water in fjord systems. The branches of Sognefjord are for instance much shallower than the main fjord. The mouth of Fjærlandsfjord is about 400 m (1,300 ft) deep while the main fjord is 1,200 m (3,900 ft) nearby. The mouth of Ikjefjord is only 50 meters deep while the main fjord is around 1,300 m (4,300 ft) at the same point. During the winter season there is usually little inflow of freshwater. Surface water and deeper water (down to 100 m or 330 ft or more) are mixed during winter because of the steady cooling of the surface and wind. In the deep fjords there is still fresh water from the summer with less density than the saltier water along the coast. Offshore wind, common in the fjord areas during winter, sets up a current on the surface from the inner to the outer parts. This current on the surface in turn pulls dense salt water from the coast across the fjord threshold and into the deepest parts of the fjord. Bolstadfjorden has a threshold of only 1.5 m (4 ft 11 in) and strong inflow of freshwater from Vosso river creates a brackish surface that blocks circulation of the deep fjord. The deeper, salt layers of Bolstadfjorden are deprived of oxygen and the seabed is covered with organic material. The shallow threshold also creates a strong tidal current. During the summer season there is usually a large inflow of river water in the inner areas. This freshwater gets mixed with saltwater creating a layer of brackish water with a slightly higher surface than the ocean which in turn sets up a current from the river mouths towards the ocean. This current is gradually more salty towards the coast and right under the surface current there is a reverse current of saltier water from the coast. In the deeper parts of the fjord the cold water remaining from winter is still and separated from the atmosphere by the brackish top layer. In fjords with a shallow threshold this deep water is not replaced every year and low oxygen concentration makes the deep water unsuitable for fish and animals. In the most extreme cases there is a constant barrier of freshwater on the surface and the fjord freezes over such that there is no oxygen below the surface. Drammensfjorden is one example. The Gaupnefjorden branch of Sognefjorden is strongly affected by freshwater as a glacial river flows in. Velfjorden has little inflow of freshwater. As late as 2000, some coral reefs were discovered along the bottoms of the Norwegian fjords. These reefs were found in fjords from the north of Norway to the south. The marine life on the reefs is believed to be one of the most important reasons why the Norwegian coastline is such a generous fishing ground. Since this discovery is fairly new, little research has been done. The reefs are host to thousands of lifeforms such as plankton, coral, anemones, fish, several species of shark, and many more. Most are specially adapted to life under the greater pressure of the water column above it, and the total darkness of the deep sea.