What is a fjord

Fjords are found in locations where current or past glaciation extended below current sea level. A fjord is formed when a glacier retreats, after carving its typical U-shaped valley, and the sea fills the resulting valley floor. This forms a narrow, steep sided inlet (sometimes deeper than 1300 metres) connected to the sea. The terminal moraine pushed down the valley by the glacier is left underwater at the fjord’s entrance, causing the water at the neck of the fjord to be shallower than the main body of the fjord behind it.

Pictures of some fjords in western Norway

Fjord, from the norse “fjörðr”, means “der man ferder over” (English “where you travel across”) or “å sette over på den andre siden” (english “put across to the other side”).

It has the same origin as the norwegian word “ferd” (english “fare” or “travel”). The verb “fare” (english “travel”) and the substantive “ferje” (english “ferry”) has the same origin. (Source: Wikipedia.no).

 

Visitnorway - Fjords

 

Description of Sognefjord drainage basin (Short version)
A fjord, as the Sognefjord, is per definition a glacially overdeepened valley, usually narrow and steepsided, extended below sea level and naturally filled with seawater. The major part of the Sognefjord relief as we know it today started with glacial erosive process at the beginning of the Quaternary period/Pleistocene epoch, 2, 5 million years before present. Give or take the usual Geological uncertainty and disagreements.




The Pleistocene climate was characterized by repeated glacial cycles, ending 10000 years before now, by the introduction of the mild Holocene sub-epoch. Four major glacial events have been identified through the Pleistocene, as well as many minor intervening events.

As for the Sognefjord, the result was dramatic. From the original (paleic) landscape, at that time only penetrated by a single river system, the glaciers abrased, plucked, gnawed and washed away an amount of rock corresponding to roughly 7600 cubic kilometres, resulting in a valley 204 000m long and a maximum relief of 2850m. From the fjord region of Western Norway alone, a total of 35 000 cubic kilometres of solid rock was removed and dumped on the continental shelf.

See below an illustration of the geologic time table covering Tertiary and Quaternary ages:

Geological Time Table


Description of Sognefjord drainage basin (Long version)
Fjords are some of the most dramatic landscape features on earth, and the origin and processes related to this feature have been discussed for almost a hundred years. This debate has mainly focused on the classic fjords and fjord lakes in Norway and Canada. Most authors agree that there has been a clear glacial-erosive influence on the fjords, but the importance of glacial activity relative to such other processes as tectonism and fluvial erosion has not been clear. To explain how a fjord is formed, we use Sognefjord as an example…

Quaternary erosion in the Sognefjord drainage basin, Western Norway.
By A. Nesje, S.O. Dahl, V. Valen and J. Øvstedal
Department of Geography and Geology, University of Bergen.

In this paper, we have calculated quantitatively the amount of Quarternary bedrock erosion in the Sognefjord drainage basin in western Norway. The volume of eroded bedrock is then used for simple calculations of mean Quaternary glacial erosion rates.



Fig.1: Map of the Sognefjord drainage system, Western Norway. Fig: Atle Nesje
Fig.1

The Sognefjord is the largest fjord system in Norway, penetrating more than 200 km inland from the coast of western Norway (Fig. 1). There is a correspondance between the fracture systems in the bedrock and the trend of the main fjord and its branches. These fracture systems have been important for guiding erosion.

 

 

Fig.2: Longitudal profile of the Sognefjord with maximum summit levels in the drainage basin north and south of the fjord. Assuming no glacial erosion at the threshold, a mature river profile for the preglacial drainage system is indicated. Fig: Atle Nesje
Fig.2

The longitudinal profile of the Sognefjord (Fig. 2) shows one main basin with a relatively flat bottom bounded to the west by a high threshold. The main fjord, starting in the eastern part at Årdal (190 km), becomes abruptly deeper westwards to reach depths of about 800m below the present sea level where it coalesces with the Lustrafjord. The maximum depth (1308m) of the Sognefjord is at Vadheim, further out. The fjord bottom then rises to the Solund area, and the sea bottom extends westwards at depths of 100-150m.

he outer Sognefjord has few tributary or distributary fjords. The inner part, however, has five branches (Fjærlandsfjord, Årdalsfjord, Nærøyfjord, Aurlandsfjord and Lustrafjord) (Fig. 1). These tributary fjords to the Sognefjord all “hang” above the bottom of the main fjord, and some of the branches have minor basins and thresholds.

The mountains along the Sognefjord rise gradually eastward from about 500m in the coastal region to altitudes above 2000 metres in Jotunheimen (Fig. 1). The highest mountain adjacent to the Sognefjord is Bleia (1721m), and the largest relief along the fjord of 2850m is found here. However, the average relief along the fjord is about 2000 metres.

  • Quaternary glacial erosion in the Sognefjord drainage basin
Fig.3: Transverse profile south of Sogndal (see fig.1 for location) illustrating that the Sognefjord isincised into the Paleic valley. At this place, the Sognefjord is 935m deep with 200m of sediments. Fig: Atle Nesje
Fig.3

The Sognefjord is presumed to follow a preglacial (original/ paleic) river system. In many places the paleic surface is preserved more or less unaltered and the paleic surface and the present landscape commonly occur together (Fig. 3). The consistent and gradually rising summit level eastwards along the Sognefjord (Fig. 2) may therefore be regarded as remnants of the paleic surface. However, the preglacial valley floor is difficult to reconstruct accurately along the present fjord (Fig. 2), and this introduces some uncertainties when calculating glacial erosion.

Assuming that consistent summit levels and the wide, high-lying valley represent the paleic, pre-Quaternary land surface, the present relief represents the total Quaternary erosion and denudation. Both glacial and fluvial erosion are selective and follows zones of bedrock weakness, but at different rates. Therefore, the relative amounts of glacial and fluvial erosion must have varied greatly from site to site. The total Quaternary erosion and denudation is quantified by subtracting the present landscape from the reconstructed paleic surface.

Excluding the Quaternary sediments at the bottom of the Sognefjord, the volume difference between the reconstructed paleic surface and present topography is 7610 km3.




  • Erosion Rates

The volume of 7610 km3 of eroded bedrock distributed over the Sognefjord drainage basin (12,518 km2 ), yields an average vertical erosion of 610 m. Supposing that the erosion of the Sognefjord basin started at the beginning of the first significant Quaternary glaciations in Scandinavia 2.57 million years ago (Ma), the average erosion rate was 24 cm/1000 yr (0.024 mm/yr). The volumes of large submarine fans on the mid-Norwegian continental shelf deposited during the last 2.5 million years correspond to a total erosion of 500­1000 m in a zone along the Norwegian coast. In the Barents Sea and on the Norwegian continental shelf these fans represent a sudden increase in the rate of sedimentation after tectonically stable conditions during Oligocene/Miocene, and this is assumed to be associated with a significant uplift of western Scandinavia after 2.5 million years. Some of the increased sedimentation may be the result of fluvial erosion following the uplift, but it is difficult to separate from glacial erosion.

During the Quaternary there have been several glacials and interglacials. The first significant expansion of the Scandinavian ice sheet occurred at 2.57 Ma. Minor glacier expansions, however, are recorded at 5.5 to 5 Ma, 4.5 Ma, between 4 and 3.5 Ma, and at 2.8 Ma. The period between 2.57 to 1.2 Ma was dominated by about constant glacial activity with small amplitudes between glacials and interglacials. After 1.2 Ma the amount of ice ratfed detritus (IRD) increased, indicating larger glacial activity in Scandinavia . The last 0.6 Ma were characterized by short, but warm interglacials between significant glacials with deposition of large amounts of IRD on the Vøring Plateau, indicating that Scandinavian ice sheets frequently reached the edge of the continental shelf. 600,000 years (23%) during the last 2.57 Ma were dominated by ice sheets reaching coastal regions.

Based on this value and assuming only glacial erosion, the average rate of ice erosion in the Sognefjord drainage basin was 102 cm/ 1000 yr (1.02 mm/yr). This is of similar magnitude as erosion rates calculated for modern Norwegian glaciers, which range from 1.6-O.OS mm/yr.

Using the average relief along the fjord basin (2000 m), the mean erosion rate over the last 2.57 Ma was 80 cm/1000 yr (0.8 mm/yr). Averaged over 600,000 years and assuming only minor fluvial erosion, the rate of glacial erosion was 330 cm/1000 yr (3.3 mm/yr).

The highest rate of glacial erosion may be estimated from the maximum relief along the Sognefjord (2850 m). From this, the maximum average erosion rate over the last 2.57 Ma was 110 cm/ 1000 yr (1.1 mm /yr). Averaged over 600,000 years and assuming only minor fluvial erosion, the mean glacial erosion rate was 475 cm/1000 yr (4.75 mm/yr).

Taking into account the selective nature of ice streams, annual erosionrates for ice streams in the Sognefjord drainage basin probably varied between 1.02 and 3.3 mm/yr. An annual erosion rate of about 2 ± 0.5 mm/yr seems therefore most reasonable.

  • Overdeepening of the Sognefjord

The degree to which overdeepening can take place depends on the relationship between the thickness of the ice and its velocity. These variables critically affect the effectiveness of abrasion and other erosional processes. When the ice is thick, threshold values between increasing and decreasing abrasion show that the ice must have a high velocity to erode effectively. Within a narrow critical zone, the combination of great thickness and high velocity thus provides the optimum conditions for a high abrasion rate. As overdeepening has taken place in the Sognefjord, and the ice is suggested to have been more than 2000m thick along inner parts of the fjord, ice velocities must have been very high along the Sognefjord drainage channel. Similar channelized ice flows occur at present in Antarctica and Greenland with measured annual ice velocities of about 500m. However, these fast moving ice streams are connected with rapid subglacial deformation of water-saturated unconsolidated till.

  • Quaternary glacial erosion in fjord region, western Norway

Limited to the east by the main watershed, the fjord region of western Norway is about 58,000 km2. If the average estimate of 610m from the Sognefjord drainage basin is representative, a rough calculation regarding the total Quaternary glacial erosion in the fjord region of western Norway is about 35,000 km3 of rock.

  • Erosive processes

The present landscape features along the Sognefjord are the result of several erosive processes; glacial abrasion, glacial plucking, subglacial meltwater abrasion, fluvial down-cutting, subaerial denudation and non-glacial downslope movement. Compared to fluvial erosion and subaerial denudation, glacial erosion processes are by far the most effective, though it is impossible to quantify the contribution from each of them. Since both fluvial downcutting and subaerial denudation during the Holocene are insignificant, this may also have been the case for preceeding interglacials. As the contribution from these processes probably is within the error limits of our volume calculations, the contribution from these erosive agents is ignored in our estimates.

In addition, the drainage basin of the Sognefjord consists of several geomorphological styles; the little-affected paleic surface, deep fjords, U-shaped valleys, and steep-walled gorges, which complicate higher-resolution quantification of Quaternary erosion rates.

  • Summary and conclusions

(1) The volume difference between the paleic and present landscape in the Sognefjord drainage basin is 7610 km3.

(2) The volume of 7610 km3 of eroded bedrock distributed over the Sognefjord drainage basin (12,518 km2), yields an average vertical erosion of 610m.

(3) Supposing that the erosion of the Sognefjord basin started at the beginning of the first significant Quaternary glaciations in Scandinavia at 2.57 Ma, the average erosion rate was 24 cm/1000 yr.

(4) Supposing that an ice ratfed detritus (IRD) curve from the Vøring Plateau reflects paleoglaciations in western Scandinavia , 600,000 years (23%) during the last 2.57 Ma were dominated by ice sheets reaching coastal regions. Based on this value and assuming only glacial erosion, the average rate of ice erosion in the Sognefjord drainage basin was 102 cm/1000 year (1.02 mm/yr).

(5) Using the average relief along the Sognefjord basin (2000m), the average erosion rate over the last 2.57 Ma was 80 cm/1000 years (0.8 mm/yr). Averaged over the last 600,000 years and assuming only glacial erosion, the rate of erosion was 330 cm/ 1000 year (3.3 mm/yr).

(6) The mean maximum erosion rate may be estimated from the maximum relief along the Sogneflord (2850 m). The average erosion rate over the last 2.57 Ma was then 110 cm/1000 year
(1.1 mm/yr). Averaged over 600,000 years and assuming only glacial erosion, the mean erosion rate was 475 cm/1000 year
(4.75 mm/yr).

(7) Based on the selective nature of glacial erosion along ice streams, annual erosion rates for ice streams in the Sognefjord drainage basin probably varied between 1.02 and 3.3 mm/yr.
An estimate of 2 ± 0.5 mm year seems most likely.

(8 ) If the average Quaternary erosion of 610m in the Sognefjord drainage basin is representative, about 35,000 km3 of rock was eroded from the fjord region of western Norway during the Quaternary.


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Atle Nesje – Professor in Quaternary Geology, University of Bergen

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