Glacier facts and information about Nigardsbreen

Transcription

1 Glacier facts and information about Nigardsbreen Fact sheet for Jostedalen Breførarlag made by Marthe Gjerde 1/1/2014 University of Bergen Marthe Gjerde J.C. Dahl

2 Time WHAT IS A GLACIER? A glacier is a mass of snow and ice mainly located on land and which is, or has been, in movement 1. Glaciers are made up of fallen snow that, over many years, compresses into large, thickened ice masses. Glaciers form when snow remains in one location long enough to transform into ice. What makes glaciers unique is their ability to move. Due to sheer mass, glaciers flow like very slow rivers. of the most glaciated countries in Europe, and earlier ice ages have influenced the development of the landscape drastically 1. Jostedalsbreen - Area: 474 km 2 the largest on mainlan Europe - Max altitude (m a.s.l.): Min. altitude (m a.s.l.): 350 Presently, glaciers occupy about 10 percent of the world's total land area, with most located in polar regions like Antarctica and Greenland 2. Norway is one FORMATION OF GLACIER ICE A glacier forms when snow accumulates over time, turns to ice, and begins to flow outwards and downwards under the pressure of its own weight. In polar and high-altitude alpine regions, glaciers generally accumulate more snow in the winter than they lose in the summer from melting, ablation, or calving. If the accumulated snow survives one melt season, it is considered to be firn. The snow and firn are compressed by the overlying snow, and the buried layers slowly grow together to form a thickened mass of ice. The pressure created from the overlying snow compacts the underlying layers, and the snow grains become larger ice crystals randomly oriented in connected air spaces. These ice crystals can eventually ( grow to become several centimeters in diameter. As compression continues and the ice crystals grow, the air spaces in the layers decrease, becoming small and isolated. This dense glacial ice usually looks somewhat blue. Each year, new layers of snow bury and compress the previous layers. This compression forces the snow to re-crystallize, forming grains similar in size and shape to grains of sugar. Gradually the grains grow larger and the air pockets between the grains get smaller, causing the snow to slowly compact and increase in density 2. Why is glacier ice blue? Years of compression gradually make the ice denser over time, forcing out the tiny air pockets between crystals. When glacier ice becomes extremely dense, the ice absorbs all other colors in the spectrum and reflects primarily blue, which is what we see. When glacier ice is white, that usually means that there are many tiny air bubbles still in the ice 2.

3 GLACIER TYPES DEFINED BY SHAPE AND TEMPERATURE Glaciers can be classified in different ways depending on their shape and temperature regime. Some of the different types based on shape: Ice caps or plateau glaciers can have any size, but cover a smaller area than ice sheets. They are a domeshaped ice body covering the underlying surface topography and with radial flow. Jostedalsbreen and Hardangerjøkulen are typical Norwegian examples. Outlet glaciers drain an ice cap, often of valley glacier type, but the accumulation area may be difficult to define. Nigardsbreen and Briksdalsbreen are wellknown outlet glaciers from Jostedalsbreen, the largest ice cap in Norway. Cirque glaciers are located in a cirque, which is created by glacial erosion producing a basin shaped depression in the mountain with steep sides and back wall. Valley glaciers follow a valley and may originate from one or more cirque glaciers. Temperature glaciers can be temperate, polythermal or cold: A temperate glacier consists of ice that is at its pressure-melting point (so-called wet ice) except for a surface layer of about m thickness which may experience seasonal cooling. Nigardsbreen is a temperate glacier. In Norway most glaciers are considered to be temperate. A cold glacier is a glacier whose ice is below the melting point. Thus, there is no liquid water within the glacier or at its bed. Such a glacier is often called a cold-based glacier. A polythermal glacier contains both cold and temperate parts 3. Nigardsbreen -Outlet glacier from the plateau glacier Jostedalsbreen -Reaction time: 25 years -Area: 47.2 km 2 -Max altitude (m a.s.l.): Min. altitude (m a.s.l.): 345 -Temperate/ warm glacier MASS BALANCE = ACCUMULATION + ABLATION Accumulation is the supply of mass in the form of snow and ice, and ablation is the sum of processes which altogether decrease the mass of the glacier (melting). Winter balance is the total accumulation throughout the winter season (winter season: 1.oct.- 30.April). Summer balance is the total ablation throughout the summer season (1.May 30.Sept.). Mass balance is accumulation minus ablation at the end of the melting season. It is positive if the glacier mass is increased, and negative if it is decreased 1. The mass balance of a glacier is the result of changes in ablation and accumulation, which, in turn, leads to changes in volume. The equilibrium line altitude is the zone where the accumulation is equal to the ablation: where added amount of precipitation (snow) to the glacier equals the amount melted away. Here we have just as much input as output. Most often we either have a positive (lots of precipitation and less melting) or a negative (more melting than snow fall) situation for the glacier. The glacier will grow if the numbers are positive (much precipitation in winter and a cold summer), and retreat if the numbers are negative (less precipitation in winter and a warm summer with lots of melting). The amount of precipitation (snowfall, freezing rain, avalanches, or wind-drifted snow) is important to glacier survival 2.

4 Earth: Portrait of a Planet, 2 nd Ed. MOVING FORWARD Under the pressure of its own weight and the forces of gravity, a glacier will begin to move, or flow, outwards and downwards. Glaciers move by internal deformation and/or by sliding at the base. Internal deformation occurs when the weight and mass of a glacier causes it to spread out due to gravity. Sliding occurs when the glacier slides on a thin layer of water at the bottom of the glacier. Glaciers can also readily slide on a soft sediment bed that has some water in it 4. Glaciers periodically retreat or advance, depending on the amount of snow accumulation or ablation that occurs. This retreat or advance refers only to the position of the terminus, or snout, of the glacier. Even as it retreats, the glacier still deforms and moves down slope, like a conveyor belt. Several visible features are common to most glaciers. At locations where a glacier flows rapidly, crevasses are created, which may make travel across a glacier treacherous 2. Crevasses are typical features of glaciers and may be up to 30 meters deep on Norwegian glaciers 3. In Antarctica they can be 300 m or even deeper. Beneath this point, the plasticity of the ice is too great for cracks to form 4. How old is the glacier ice? The Jostedalsbreen ice cap and the other main ice caps in Norway came gradually into being during a cold period from 4100 BC to 500 BC. Because the ice is constantly moving, the ice present there now is, however, not that old. The time that elapsed from glacier ice being formed from snow high up in these glaciers until the mass flow has transported it to the glacier margin is measured in centuries, not millennia 5.

5 HOW DO GLACIERS AFFECT LAND? A glacier is always transporting and carrying lots of material both on the top, inside and at the bottom of the glacier. When a glacier is growing, it is pushing material in front of it. This material is then transported away by glacial rivers, and eventually ends up in a fjord/the ocean. Because the glaciers are eroding lots of material when it acts like a sandpaper on the bedrock (the ice itself cannot erode the bedrock, but smaller rocks and sands which freeze in the ice at the bed can erode just like a sandpaper), all this material which is then carried away by rivers is the reason for the beautiful colors that glacial rivers and fjords have. Lots of sediments (sand, silt and clay) are kept in suspension in the waters, and they reflect the sunlight to make the colors we see as emerald green and greyish. When a glacier has grown for a period, and then starts retreating, it leaves moraines as footprints. The moraines show the extent the glacier had at a given time, and by looking at the moraines we can therefore reconstruct how far out the glacier was in the past, and when it was there. Terminal/end moraines are formed at the foot of a glacier. Lateral moraines are formed on the sides of the glacier. Medial moraines are formed when two different glaciers merge and the lateral moraines of each coalesce to form a moraine in the middle of the combined glacier 4. In front of Nigardsbreen, many of the moraines have been dated and mapped. Why is the glacier lake green? Silt (smaller particles than sand grains) in suspension reflect the light which makes the glacier lake in front of Nigardsbreen appear emerald green. Nesje et al., 2007 Glaciated valleys are easily visible glacial landforms. Similar to fjords, they are u-shaped, often with steep vertical cliffs where entire mountainsides were removed by glacial action. Fjords are long, narrow coastal valleys that were originally carved out by glaciers 2. Many glaciers deepen their valleys more than their smaller tributaries. Therefore, when glaciers recede, the valleys of the tributary glaciers remain above the main glacier's depression and are called hanging valleys 4. Krundalen is a local example of a hanging valley, where the glacier which occupied Krundalen during the last ice age fed the larger glacier occupying the main valley of Jostedalen. Fåbergstølsgrandane is the largest active sandur (glacier river plain) on mainland Europe! U-shaped valley Stordalen (Øy). Photo: Marthe Gjerde

6 A layer of clay, sand or rocks on top of the glacier ice will absorb more solar radiation than the lighter layers. Cryoconite holes are formed where the debris layer is quite thin. The debris (small rock particles, soot and microbes) will melt down into the ice, and smaller chunks of loose material can get stuck in the bottom of holes which may get up to cm thickness 1. A pothole is a cavity or hole which has been drilled in the surrounding rocks by eddying currents of water bearing stones, gravel and other detrital matter 4. w WHAT WAS THE LITTLE ICE AGE? Despite the term, this period from about AD 1400 to 1920 in Norway (varying from one country to another) is no real ice age. It is merely a chilly and rainy time span, albeit a long one, with 1-2 C lower mean annual temperature than earlier in medieval times. Glaciers did advance, most rapidly during the period AD They then reached their maximum size. Historical evidence shows that Nigardsbreen reached its maximum extent in AD The moraine deposited from the glacier at the time can be viewed on this picture to the right and gives you an understanding of the glacier size at the time. The rapid glacier advance in the early 18th century in western Norway is best explained by increased winter precipitation and thus high snowfall on the glaciers due to mild and humid winters 6. From AD Nigardsbreen advanced 2800 m, and this gives an average advance of 112 m per year! From AD Nigardsbreen advanced 150 m. In total, Nigardsbreen advanced 2950 m from AD , which gives a mean annual advance of ~80 m. Nigardsbreen and its glacier front variations is uniquely well documented for this period.this is due to the large amount of historical sources that date and describe the glacier variations for Jostedalen and Nigardsbreen from ministers and vicars living in the valley as well as paintings and sketches from artists visiting the valley at the time 7. The glacier had retreated approximately 700 meters from its 1743 Little Ice Age (LIA) maximum extent. From 1930 to 1975 the glacier retreated 2.3 km, the retreat speeded up in the 1940s when the glacier calved into lake Nigardsvatn. In 1959 the glacier was almost out of the lake. After a period of only small changes the glacier advanced 250 meters in the 1990s, culminating in Since then the glacier has retreated, and between 2004 and 2011 it retreated 136 m 3. The LIA changed the way of life in different ways for the inhabitants in Jostedalen. Because of the quickly advancing Nigardsbreen in , the farm Nigard (gard=farm) was destroyed by the advancing glacier. Thankfully, no lives were lost since the glacier

7 approached the farm in a pace so that the family could move out, but nevertheless the entire farm was crushed and overrun by the river of ice. The glacier did not only crush the farm Nigard, but it also closed in on other farms and nearly destroyed other farm houses at Bjørkehaugen and in Mjølversgrendi 8. The conditions during LIA were especially bad in the uppermost farms in the valley where the glacier advances also led to severe damages on graze lands and cropped fields. Therefore, nine farmers in Krundalen, Mjølverdalen and Fåberg, asked to be exempted from paying taxes. Five farms in Jostedalen received message from the King in Copenhagen that they were allowed lower tax payment because of flooding, slides and the advancing glaciers. After several years of crop failure, 200 barrels of grain arrived to the suffering population (and minister) of Jostedalen in spring This emergency aid was royally approved by the King of Denmark-Norway 7. USE OF THE GLACIER The people in Jostedalen have been interacting with the glacier in their everyday life for centuries. Trading across the glacier with other villages in e.g. Nordfjord is an example of the use of the glacier. The dealing with people in the Nordfjord region is even reflected in the dialect spoken in Jostedalen. Woodwork, snowshoes, bowls, ladles, wooden spoons, etc. were carried across the glacier in exchange for money, oat, herring and grain as well as buying livestock. Men who knew the safest routes across the glacier were used as guides to ensure as as safe as possible travel. The inhabitants in the valley even travelled over the glacier when attending wedding parties in villages located across the glacier. The picture shows old routes crossing the glacier 9. REFERENCES 1) Nesje, A.: Brelære 2) National Snow and Ice Data Center 3) NVE Breatlas ) 5) Breheimsenteret/Gagarin/Marthe Gjerde 6) Nesje, A. et al. 2008: The Little Ice Age glacial expansion in western Scandinavia: summer temperature or winter precipitation? 7) 8) Matthias Foss, 1750 (vicar in Jostedalen): Justedalens Kortelige Beskrivelse 9) T.O. Eide: Breden og bygda (downloaded from the webpages of Jostedal historielag) Marthe Gjerde, 2014