Fjord
In physical geography, a fjord is a long, narrow sea inlet with steep sides or cliffs in a valley created by a former glacier, which has since become inundated with water. They are the glacial equivalent of drowned river valleys, known as rias. Fjords exist on the coasts of Antarctica, the Arctic, and surrounding landmasses of the northern and southern hemispheres. Areas with extensive fjords demonstrate an extreme example of the coastline paradox; Norway's coastline is estimated to be long with its nearly 1,200 fjords, but only long when excluding the fjords.
Formation
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. 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 below sea level. Fjords generally have a sill or shoal 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. Saltstraumen in Norway is often described as the world's strongest tidal current. These characteristics distinguish fjords from rias, which are drowned valleys flooded by the rising sea. Drammensfjorden is cut almost in two by the Svelvik "ridge", a sandy moraine that was below sea level when it was covered by ice, but after the post-glacial rebound reaches above the fjord.In the 19th century, Jens Esmark 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 tributary fjords led to excavation of the deepest fjord basins. Near the very coast, the typical West Norwegian glacier spread out and lost their concentration and reduced the glaciers' power to erode leaving bedrock thresholds. Bolstadfjorden is deep with a threshold of only, while the deep Sognefjorden has a threshold around deep. Hardangerfjord is made up of several basins separated by thresholds: The deepest basin Samlafjorden between Jonaneset and Ålvik with a distinct threshold at Vikingneset in Kvam Municipality.
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 was 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. Small waterfalls within these fjords are also used as freshwater resources. Hanging valleys also occur underwater in fjord systems. The branches of Sognefjord are for instance much shallower than the main fjord. The mouth of Fjærlandsfjord is about deep while the main fjord is nearby. The mouth of Ikjefjord is only deep while the main fjord is around at the same point.
Features and variations
Hydrology
During the winter season, there is usually little inflow of water that is fresh. Surface water and deeper water 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 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. This deep water is ventilated by mixing with the upper layer causing it to warm and freshen over the summer. In fjords with a shallow threshold or low levels of mixing 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 mixing in fjords predominantly results from the propagation of an internal tide from the entrance sill or internal seiching.
The Gaupnefjorden branch of Sognefjorden is strongly affected by freshwater as a glacial river flows in. Velfjorden has little inflow of freshwater.
Coral reefs
In 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.New Zealand's fjords are also host to deep-water corals, but a surface layer of dark fresh water allows these corals to grow in much shallower water than usual. An underwater observatory in Milford Sound allows tourists to view them without diving.
Skerries
In some places near the seaward margins of areas with fjords, the ice-scoured channels are so numerous and varied in direction that the rocky coast is divided into thousands of island blocks, some large and mountainous while others are merely rocky points or rock reefs, menacing navigation. These are called skerries. The term skerry is derived from the Old Norse sker, which means a rock in the sea.Skerries most commonly formed at the outlet of fjords where submerged glacially formed valleys perpendicular to the coast join with other cross valleys in a complex array. The island fringe of Norway is such a group of skerries ; many of the cross fjords are so arranged that they parallel the coast and provide a protected channel behind an almost unbroken succession of mountainous islands and skerries. By this channel, one can travel through a protected passage almost the entire route from Stavanger to North Cape, Norway. The Blindleia is a skerry-protected waterway that starts near Kristiansand in southern Norway and continues past Lillesand. The Swedish coast along Bohuslän is likewise skerry guarded. The Inside Passage provides a similar route from Seattle, Washington, and Vancouver, British Columbia, to Skagway, Alaska. Yet another such skerry-protected passage extends from the Straits of Magellan north for.
Phytoplankton
Fjords provide unique environmental conditions for phytoplankton communities. In polar fjords, glacier and ice sheet outflow add cold, fresh meltwater along with transported sediment into the body of water. Nutrients provided by this outflow can significantly enhance phytoplankton growth. For example, in some fjords of the West Antarctic Peninsula, nutrient enrichment from meltwater drives diatom blooms, a highly productive group of phytoplankton that enable such fjords to be valuable feeding grounds for other species. It is possible that as climate change reduces long-term meltwater output, nutrient dynamics within such fjords will shift to favor less productive species, destabilizing the food web ecology of fjord systems.In addition to nutrient flux, sediment carried by flowing glaciers can become suspended in the water column, increasing turbidity and reducing light penetration into greater depths of the fjord. This effect can limit the available light for photosynthesis in deeper areas of the water mass, reducing phytoplankton abundance beneath the surface.
Overall, phytoplankton abundance and species composition within fjords is highly seasonal, varying as a result of seasonal light availability and water properties that depend on glacial melt and the formation of sea ice. The study of phytoplankton communities within fjords is an active area of research, supported by groups such as FjordPhyto, a citizen science initiative to study phytoplankton samples collected by local residents, tourists, and boaters of all backgrounds.