Fjords are one of the more prominent and spectacular landforms found on Earth, formed in areas that have been exposed to extensive glaciations. Fjords are glacially carved valleys, commonly termed U-shaped valleys or glacial troughs, that have been flooded by the sea during and after the glaciers that formed them retreated. Thus, they demonstrate the erosive powers of outlet glaciers draining continental ice caps and ice sheets. This entry discusses the location of fjords, characteristics of their profiles, and processes of formation.
Fjords are channels of formerly radiating outlet glaciers that drained large continental glaciers during the ice ages, as seen in Greenland and Antarctica today. Fjords are therefore only found along high-latitude coasts on both hemispheres. Among countries well known for their fjords and fjord regions are New Zealand, British Columbia, Chile, Greenland, Alaska, and Norway. Two Norwegian fjords, Geirangerfjord and Nær⊘yfjord, are considered archetypical fjords and are therefore on UNESCO's World Heritage List.
Sognefjorden, in Norway, may serve as an example of the size, characteristic features, and dimensions of a fjord system. Sognefjorden cuts its way more than 200 km (kilometers) from the west coast of Norway into the mountain area of Jotunheimen. The maximum depth (1,308 m [meters]) is found more than 50 km inland, giving rise to an overdeepened longitudinal profile (see Figure 1). Adjacent mountains peaks, rising more than 1,700 m above sea level, give rise to extreme reliefs. Commonly, a fjord system consists of several tributary fjords and valleys that coalesce toward the coast, producing a dendritic pattern.
The rate at which glaciers erode depends on the ice thickness and flow velocity; hence glacier troughs will not form in a flat landscape with a uniform ice thickness. Fjord dimensions are thought to be adjusted to the volume of ice that is discharged during maximum glacial conditions. The result is a wide, steep-sided valley, a shape that efficiently conveys glacier ice by sliding and deformation from the source area to lower altitudes, where there is a net ablation. The typical U-shape can be observed in some deep, narrow fjords, but more often the cross-profile of glacial troughs has a parabolic shape, which is favored by ice flow. However, not only glaciological parameters control the trough cross-profile, but the rock mass strength is also found to be an important controlling factor of cross-profile development.
Glacial troughs are likely to have developed in preglacial valleys, which are deepened and widened, while the adjacent mountain plateaus may show limited signs of glacial erosion. The pattern and size of the fjord system are therefore largely inherited from the preglacial landscape. However, ice divides of continental ice sheets may not coincide with the underlying watershed; thus, fjords may drain ice from substantially larger areas than the preglacial fluvial catchment. It has also become increasingly evident that linear geological structures in the bedrock, for example, faults, strongly control the development of glacial troughs. It is suggested that in Patagonia outlet glaciers from repeated glaciations have widened and deepened preexisting geological structures, with less effect on the intervening landscape.
Seen from space, or in a map view, glacial troughs tend to be more linear than those carved by rivers. This relates to the properties of ice compared with water, preventing it from making the same steep turns that rivers are able to do. Thus, a glacier tends to straighten the fluvial preglacial valley.
Glacial fjords also have characteristic irregular longitudinal profiles, consisting of overdeepened troughs with intervening thresholds. An overdeepened trough is a landform where the slope near the mouth is reversed. In other words, the trough is deepest in the middle and shallower toward the ends. This is clearly seen in many fjords, which can be more than 500 m deep in the central part and less than 100 m deep at the fjord mouth. Although ice has a lower density than water and will float, glaciers may grow thicker than the water depth they enter. When the ice thickness exceeds the water depth by approximately 10%, the ice is grounded and can erode the fjord bed. Fjords with depths of many hundreds of meters thus provide evidence of great ice thicknesses and the fact that fjords conveyed huge volumes of ice during the maximal glacial conditions.
Glacier erosion takes place as glaciers slide over their bed, tearing loose particles from the underlying bedrock and abrading the bed by dragging rock particles along their base. Although glaciers may produce huge quantities of sediment each year, glacier erosion is a slow process taking place over large areas. Glacier erosion is estimated to proceed at rates of the order of millimeters per year. It thus follows that fjords are old landscape elements that have formed during multiple glaciations.
Glaciers: Continental, Glaciers: Mountain, Landforms
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