What Is a Fjord? A Glacial Valley the Sea Moved Into
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A fjord is a valley cut by a glacier and later flooded by the sea, which gives it steep walls, enormous depth and a shallow sill at the mouth. That shape controls the water inside it and produces conditions found almost nowhere else.
How they are cut
Glaciers erode differently from rivers, cutting a cross section that is broad and steep-sided rather than narrow and V-shaped, because the ice fills the valley and grinds against the walls as well as the floor. They also erode below sea level, since the ice is driven by its own thickness rather than by gravity flowing downhill to a base level, so a glacier reaching the coast continues excavating well beneath the waterline. Erosion concentrates where the ice is thickest and fastest, which produces an uneven floor with deep basins where the glacier was confined and shallower sections elsewhere. At the mouth the ice thinned and spread on reaching the sea, so erosion weakened and material was deposited, leaving a raised sill. When the ice retreated the sea flooded the excavated valley.
What the shape produces
The distinctive form has direct consequences for the water:
- •Great depth, with several fjords exceeding a thousand metres and far deeper than the sea outside
- •A shallow sill at the mouth, frequently only tens of metres deep, which restricts exchange with the open ocean
- •Stagnant deep water behind the sill, which can become depleted of oxygen since it is not renewed for long periods
- •A layer of fresh water on the surface from rivers and melt, sitting above denser salt water
- •Strong currents at the sill and at narrows, where the restricted cross section accelerates tidal flow
- •Sheltered conditions inside, since the steep walls block wind and the sill limits swell
Living in and on them
The conditions are unusual enough to support distinctive communities. The layered water means organisms adapted to fresh, brackish and fully marine conditions occupy different depths within a few metres of each other. Deep-water species that normally live far offshore occur at accessible depths because the basin is deep and cold, which is why several fjords contain corals and other assemblages that would otherwise require deep ocean work to study. Where deep water stagnates, the low-oxygen conditions exclude most animals and permit sulphur-based microbial communities, which are studied as accessible analogues for conditions in ancient oceans. For people the shelter, the depth and the fresh water made fjords valuable, supporting settlement, fishing, harbours and more recently aquaculture and hydroelectric generation, with the last two producing their own environmental pressures on enclosed water bodies with restricted exchange.
Measuring how deep
The depths involved are worth stating because they are counterintuitive. Several Norwegian fjords exceed a kilometre in depth while the sill at the mouth may be only a hundred metres or less, so a vessel sails over shallow water into a basin far deeper than the sea it came from. The deepest known examples are in Antarctica and Greenland, with figures over a kilometre and a half. That geometry exists because a glacier erodes below sea level where it is thick and confined and stops doing so where it thins and floats, which sets the sill depth at roughly where the ice began to float. The same relationship means that the sill depth of a fjord records where the ice front sat, which is used to reconstruct past ice extent, and that fjord basins act as sediment traps preserving a continuous record of what the glacier and the region were doing.
Where they are
Fjords occur wherever glaciers reached the sea, which restricts them to high latitudes and to mountainous coasts. Norway supplies the name and the classic examples, with an intricate coastline of long narrow inlets. Comparable coasts occur in Greenland, Iceland, Alaska, British Columbia, Chile, New Zealand and parts of Scotland, and the deepest and longest examples are in Greenland and Antarctica. The distribution is a direct map of where ice sheets and valley glaciers extended during recent glaciations. Sea level rise since the ice retreated flooded the valleys, and continuing uplift of land relieved of the weight of ice is raising some of them slowly, which means the shorelines are still changing measurably. Several coasts show submerged fjord systems offshore from periods when sea level was lower still.
The takeaway
Glaciers cut broad steep-sided valleys and erode below sea level, leaving deep basins and a shallow sill where the ice thinned at the coast, and the sea flooded the result. The sill restricts exchange, so deep water stagnates and can lose its oxygen, and fresh water layers on top. Deep-sea species occur at accessible depths, which is why these places are studied intensively.