A chilly history

Snow transforms gradually into glacier ice in highlands that rise above a certain elevation which is commonly termed the glaciation limit or firn line. A common public term is snow line. The late 20th-century glaciation limit in Iceland varied from around 700-750 m above sea level in the northwest, to 1,100 m in the Southeast and 1,600 m in the Northeast. The elevation varies according to the annual preciptitation and summer/winter temperature and has been rising during the last decade due to global warming and changes in precipitation. The limit was lower down during the "Little Ice Age” from 1400 to 1900, resulting in advancing glaciers. In our times, about 11% of Iceland  (approx. 11,000 square kilometres) is sufficiently high to sustain glacier cover. Some 6,000-7,000 years ago, a warmer climate kept the glaciers very much smaller. Still further back in time lie peculiar climatic fluctuations which are collectively termed the (Quaternary) Ice Age. The Ice Age comprises at least 20 cold glacial periods, interspersed by much warmer interglacial periods resembling our present climate. Large temperature oscillations were common during these different climatic periods.

During glacial periods, most of Iceland was covered by ice of varying volume and the snow line was mainly at very low altitude. The fjords, bays and coves of Iceland, as well as the valleys and many steep-sided mountains are carved out of  high lava plateaux by succesive glaciers during the local "Icelandic" Ice Age which spanned just over 3,000,000 years each glacial period lasting about 100,000 years or even longer, while the interglacial period lasted for about 10,000-20,000 years. The last glacial period came to a very abrupt end about 9,700 years ago but fluctuations in glacer extent started a few thousand yars earlier. The extensive ice cover almost vanished but started to grow some 5,000 years ago and has attained its present size over the past 2,000-2,500 years, albeit with substantial variations.

Not all alike

Glaciers may be classified according to their appearance, size and location. To most people glaciers mean steep Alpine ice curtains and cracked ice streams that drape high mountains and crawl through mountain valleys. Such glaciers or ice falls are also found in Iceland, for example at Hrútfjallstindar and Þverártindsegg. A few high and rather steep mountains are draped with thin ice aprons that cover the upper sections, much like a parallel, interconnected row of Alpine glaciers. These are volcanoes like Eyjafjallajökull and Snæfelsjökull. In a few cases, like at Svínafellsjökull, steep (Alpine) glaciers from Hrútfjallstindar and Öræfajökull merge and form one ice stream resembling a valley glacier. Another morphological type of glacier is more common than the Alpine. They are dome-shaped and cover varied landscape or even vast highlands. These more or less circular ice-masses feed steep glaciers (like ice falls) or gently sloping outlets and are termed ice caps. The massive ice blanket in the larger ice caps measures up to 900 m thick. In addition, much smaller and thinner ice caps occur on large, individual mountains like Eiríksjökull and Hrútfell. A different glacier type is found in the small cirque glaciers, located in deep glacially eroded bowls and hanging valleys. They are most numerous in the Tröllaskagi highlands between Eyjafjörður and Skagafjörður. Yet another group is found in rock glaciers in steep terrain: the piedmont glacier category can be said to have one representative, Breiðamerkurjökull (at Vatnajökull Ice Cap). As with many a classification of this kind, some glaciers are hard to put in a definitive category.

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Icelandic glaciers are temperate, that is the ice temperature below 20-30 m remains at 0°C throughout the year. In most polar regions, glaciers are frozen to the bedrock. A few glaciers partly float on deep water, like the eastern section of Breiðamerkurjökull. 



Another way of classifying glaciers is to look at their physical environment and behaviour. Then the Icelandic glacier would classify as non-surging glaciers, surging glaciers (with periodic, very rapid advances), tidewater and floating glaciers, debris-covered glaciers, glaciers on top of volcanoes or volcanic vents and finally glaciers that cover active geothermal fields. The last two categories are subjected, respectively, to fast and sudden or gradual but long-term melting in addition to the normal weather-induced melting processes.



The ice domes

There are five ice caps in Iceland, ranging from the 150-square-kilometre Drangajökull to the approximately 8,100 square kilometre Vatnajökull. A large ice cap like Vatnajökull has over 20 outlet glaciers. The steep ones resemble Alpine ice falls or valley glaciers (like Morsárjökull, Skaftafellsjökull, Brókarjökull and Fláajökull) but the flat ones are broad lobes with a gentle surface angle (like Síðujökull or Brúarárjökull).

Ice caps have formed in areas of extensive highland when local glaciers have gradually coalesced, especially when the firn line falls to a lower level during long cold periods. The glacier covers the land surface underneath the ice for the greater part. Large gently sloping glacial domes form over the highlands and shallow depressions where valleys occur underneath. Only sharp, high peaks and the very highest mountain ranges protrude from the ice as nunataks. Ice flows slowly from the ice divide  at the top of each ice cap, downwards and outwards towards the edges. Valleys underneath the ice or the the broad slopes of subglacial highlands channel the ice into outlet glaciers. Here the main ablation (or melting) takes place below the so-called equilibrium line. Warm airstreams usually cause most melting in the mild island climate of Iceland and direct solar radiation is, on the whole, of less importance. Ice and snow melt results in a small amount of water vapour escaping to the atmosphere but the most of the water is runoff from the glacier edge in the form of streams and rivers.

From half to two thirds of large ice caps can be found above the glaciation limit when the climate has again become warmer - after the main period of glacial formation. Much of the land underneath the glaciers does not reach the present day glaciation limit. Thus, it can be said that in our time, large ice caps are in a way self-sustainable and would not form in the highlands under present day conditions. Today, new glacier ice would only form at the highest elevated landforms underneath the ice caps.



All the largest glaciers in Iceland are ice caps of the type mentioned above. They are in fact small versions of Ice Ageice fields and ice caps or the present day ice cap of Greenland. Similar glaciers can be found, for example, on Spitsbergen or Ellesmere Island in Canada in the northern hemsphere and in Patagonia and South Georgia in the southern hemisphere. The smaller ice caps and glacier domes on large and high, individual mountains are probably up to 300 m thick. They shed ice, mainly through steep outlet glaciers or at more gentle sloping margins on the plateaux that form the mountain tops.



The ice caps of Iceland probably started theri present day growth period in the early part of a Holocene cold period that began around 500 years B.C. Glacier ice formed before the Settlement is most likely all accounted for at the edges of the largest ice caps and has thus flowed to sea. The oldest ice in Vatnajökull is thought to be just over 1.000 years old and for example that on the edge of Tungnaárjökull outlet glacier is 700-750 years old, based on the age of tephra layers found there. Maximum extent was attained by the end of the 19th century. Here are approximate figures for their area (some since 2000):



Large ice caps

• Vatnajökull   8.100 sq. km
• Langjökull 900 sq.km
• Hofsjökull 880 sq. km
• Mýrdalsjökull 580 sq.km
• Drangajökull 150 sq.km



Small ice caps

• Tungnafellsjökull 43 sq.km
• Þórisjökull 22 sq.km
• Eiríksjökull 23 sq.km
• Þrándarjökull 25 sq.km
• Hrútfell  8 sq.km

The smaller ones
Within and close to the active volcanic zones most of the mountains are recent volcanic formations. Many of them are little weathered and eroded compared to the older parts of the country. The accumulation of the mountains in eruptions as well as external factors have controlled their shape more than anything else. Outside the active zones, in western Iceland, the western fjords, the west of northern Iceland and in the eastern fjords the crust is very old by Icelandic standards. Widespread lava piles form a layered and eroded bedrock. The mountains are either smooth on the top and with steep sides or in other words they are the remains of an ancient high plain, or they form sharp, eroded peaks.



Ice Age glaciers covered the country many times over during the past three million years. On each occasion they scoured and eroded the bedrock for at least 50-100,000 years. The ice cap was both large and thick during glacial periods and was especially thick along the central axis of the country but much thinner towards the coast. In coastal highlands, the mountains protruded from the ice and many peaks had small local cirque glaciers. Glaciers of this type were also common for a time during the few thousand years and centuries it took for the huge ice cap to retreat inland and finally disappear as each glacial period drew to a close. At the end of the glacial periods many small glaciers occupied cirques and depressions on the sides of the valleys and fjords. These are now ice-free or occupied by small glaciers i.e. cirque glaciers. There are about 170 such glaciers in the highland area between Skagafjörður and Eyjafjörður (Tröllaskagi) and in the area between Fnjóskadalur and Skjálfandi, Tiny cirque glaciers may still exist on the Hornstrandir area and, until recently, at the edges of the Gláma highlands, while small glaciers have prevailed east of some of the fjords in the east of Iceland.

Glacial ice, covered by rock debris/moraine in the highland areas just mentioned can form a moving rock glacier. They resemble large morainic piles but always with a definite creep form, for example curved waves and even glacier crevasses.  The seat of a cirque glaciers is usually a bowl-like depression or valley bottom in the side of a mountain. The ice movement downwards and forwards erodes the cirque still deeper through time. While glacial erosion is occurring, repeated freeze-thaw activity results in debris falling from the mountainsides directly onto the glacier or the process produces screes that slide onto the ice. Debris is also being added to the sole of a glacier while it moves. With time a rock glacier forms. In some cases rock glaciers are remains of old valley glaciers or the constitute the lower part of active glaciers.



Moves and budgets

Glacier ice moves down and outward from the high point of a glacier. Many metres of winter snow, added onto the glacier above the equilibrium line ensures that more ice forms but below the line, all the winter snow melts each summer. Even some of the underlying, older firn and ice melts too, especially close to the glacier margin. At the same time, ice is constantly fed to the outlet glaciers and ice margins. The ice flow is a combined internal deformation of ice plus a slip-like movement of ice across the surface of the earth. A thin layer of water at the glacier base and watersogged sediments facilitate the latter type of movement. The normal ice velocity in most Icelandic glaciers is probably between 10 cm and 1-2 metres per day, equivalent to a few dozen or few hundred metres per year. The steeper and thicker a glacier is, the faster it flows on land.



If, on the whole, more snow and ice melts than is compensated for by new ice formation and the ice flow,  glaciers retreat and vice versa. Increased accumulation of snow and less extensive melting induces advancing glaciers. The glacier budget is said to be positive (negative in the former case). At present, Icelandic glaciers and ice caps are subjected to a strong negative annual balance. This is indicated by measurements of dozens of retreating outlet glaciers and thinning of ice in all regions. Balance measurements are also made directly at the glacier surface. Model calculations show that the volume of an ice cap like Hofsjökull may diminsh by half in a century and that most of the glacier ice will have vanished within 100-200 yeras, given the present climatic conditions and forecasts.

New SAR-technology (time-lapse comparison of air-born radar images) enables scientists to map glacier elevations and ice margins with accuracy as well as to assess volume changes and analyze morphological and subglacial features. Laser based airborne surveys (LiDAR) are used to map glaciers and assess ice volume changes.



Shortly after the Settlement, the climate seems to have favoured retreating or stagnant glaciers and they were smaller than at the present. But in late medieval times, the glaciers began to advance and did so for more than five centuries (during the "Little Ice Age"). Grazing areas and framlands were overrun by glaciers or burried in fluvial sediments. This went on until 1910-1920. From then on, a very warm period (approx. 1920-1970) resulted in a different scenario. Large areas were laid bare in front of almost every glacier snout in Iceland.  The Breiðamerkurjökull tidewater glacier is almost at sea level in southeast Iceland. It overrode, for example, the Breiðamörk farm in the late 17the century and almost reached the seashore but started to retreat in the early 20th century. It has retreated continously, revealing a deep proglacial lake and a flat debris fan.



From the 1970's and into the early1990´s, however, many other glaciers showed signs of advance, that is positive budgets, especially narrow and steep outlets from ice caps and steep glaciers on high mountains, while a few others were more or less in balance. This points toward a drop in the annual mean temperature, verified by meteorological data from this somewhat chilly period. However, for about the past two decades the trend has become reversed again, as already mentioned. Almost all of the close to 50 position-monitored glaciers and glacier outlets show signs of retreat or stagnation, year after year.



Surging glaciers

Glacier ice is brittle in the uppermost 20-40 m slice of each ice cap or glacier. Here, the ugly crevasses appear. Deeper down, pressure (or weight) makes the "solid water" more plastic and it behaves somewhat like ductile, half-molten metal. The normal flow velocity is barely noticeable unless observed for quite some time. That is the normal state of many Icelandic glaciers.



However, many individual outlet glaciers, especially those flat lobes radiating from the big ice caps, behave from time to time in a completely different manner. After years or decades of retreat of these "abnormal" glaciers, a bulge is observed, growing above or at the equilibrium line. Somehow, the glacier is not able to transport ice fast enough to the snout. At a given time, the bulge cannot be upheld or halted anymore and it moves down-glacier like a slow wave. The whole upper part of the glacier sinks in and the glacier multiplies its ice velocity manyfold. The snout surges forward, and broken-up domes, crevasses and ice towers appear suddenly all over the lower region of the glacier, instead of the rather smooth surface. This phenomenon is termed  glacier surge.  The glacier belches brown water, indicating that maybe the water channels in the ice have been closed off and lots of water forced to flow under the ice, enabling the ice to slide faster. Whatever the mechanism, incredible ice flow velocities have been observed. At 20-100 m per day, one can actually see and feel how giant ice masses move, groaning, squeeking and thundering from time to time. During the surge of the Síðujökull outlet glacier (Vatnajökull ice cap) in 1994, over 200 bilion tons of ice surged forward. The glacier advanced 1-2 km and looked like a monstruous labyrinth. The same glacier surged in 1964 and 1934. The surface became smooth again within a few years. The broad lobes of Vatnajökull are all surging glaciers but the steep ones are non-surging. Surging glaciers are known at the southern margins of Langjökull and Hofsjökull but are probably not of this kind at Mýrdalsjökull. Some of Drangajökull´s outlet glaciers do surge. Most of the smaller glaciers and ice caps to not show this behaviour but a few small cirque glaciers at Tröllaskagi are known to surge. Surges are known at present in some glaciated areas of the world, for example in Alaska and Yukon, but surges do not happen in Scaninavia, New Zealand or in the European Alps.



Fire and ice

The relatively rare surges are observed in Iceland, but in addition the uncommon coexistence of volcanoes and thick glaciers makes the Icelandic world of perenial ice quite special. Four large and high volcanic cones (stratovolcaonoes) are active in Iceland: Hekla, Eyjafjallajökull, Snæfellsjökull and Öræfajökull, the latter being Iceland´s largest single volcano. They are all capped by glaciers like similar volcanoes in many parts of the world (ranging from 2 to 78 sq.km). The other central volcanoes in Iceland are mountain massifs set with calderas. Most of them are glaciated and the majority hide calderas under very thick blankets of ice. Mýrdalsjökull, Langjökull and Hofsjökull contain a caldera each but Vatnajökull has at least five. The Mýrdalsjökull ice cap conceals the very active Katla volcano with a large caldera. The huge volcanic centre beneath the ice of Hofsjökull was mapped in the 1980's. Among the volcanic centres that lurk under the cover of the Vatnajökull ice cap are Kverkfjöll, Bárðarbunga and Iceland´s most active central volcano, Grímsvötn. It erupts every 5-10 years on average.



As in 1996 (Gjálp, Vatnajökull), 1998, 2004 and 2011 (Grímsvötn) all eruptions which occur beneath ice, produce unconsolidated eruptives (explosive tephra eruptions) and may lead to sudden or prolonged floods (glacier bursts, jökulhlaups). The floods have created black deserts like Mýrdalssandur and Skeiðarársandur on the southern coast of Iceland. There are geothermal areas in highlands rising from the icefields or hidden beneath ice, like in Kverkfjöll, Lokahryggur and Grímsvötn which affect glacier melting. If ice-free, they create stark contrasts such as boiling mud pots surrounded by snow or ice. If under the ice, they may facilitate ice flow through melting the glacier sole. They also make their presence known through fuming crevasses and steaming ice caves or circular ice cauldrons. Meltwater can be temporarily stored in ice cavities which appear as such cauldrons in the glacier surface. Periodic flooding is an evident  result (jökulhlaups). Thinning glaciers cause isostatic rise of the crust which in turn may strengthen volcanic activity due to the deloading of the crust where magma forms or is stored.



Sources of water

Icelandic ice caps and glaciers store at least 3,600 cubic kilometres of ice. One cubic kilometre of ice weighs almost one billion tons (one thousand million) or 900 million tons to be more specific. If all glacier ice in Iceland is melted, the water would cover the island with 33-34 metres of stored water.



The daily meltwater from Icelandic glaciers is still the main source of energy in the country. Hydropower is utilized to produce electricity. Many glacial rivers discharge 50-700 tons per second in the spring and summer and have been considered as good power sources for middle-sized hydropower stations. Because of the much lower winter discharge such power plants need dammed water reservoirs.



In Iceland, however, glaciers are not a vital source of water for food production,

In some cases, melting is not controlled by the weather. Sudden melting or the storing of water induce sudden flood events. These glacier bursts or jökulhlaups may be confined to tunnels or they may disperse water over large areas beneath the ice (or do both) before they emerge at the glacier margin. Glacier thickness, the angle of the flow bed and water temperature are among the general factors controlling the course of events. In Iceland, the causes of jökulhlaups are more varied than in many other countries. Geothermal fields are concealed beneath glacier ice and subglacial volcanic eruptions produce vast volumes of meltwater. In other instances subglacial lakes or lakes at glacier margins manage to lift overlying ice and release water into otherwise managable rivers which then overflow and may even threaten life or property.

The floods from glacial lakes seldom discharge more than a few hundred or up to 2,000 cubic metres per second. Floods caused by geothermal activity tend to be somewhat larger; around 1,000-5,000 cubic metres per second. Such events have occurred for decades, for example in the Skaftá river (from the western part of Vatnajökull) every second year on the average and in Skeiðará every 4-5th year due to flooding in the Grímsvötn volcano after intense melting by geothermal activity.

Volcanic eruptions produce the largest floods where a few cubic kilometres are discharged in one or two days. Discharge may vary from 2,500 (Eyjafjallajökull)   to 300,000 cubic metres per second of turbulent water (Katla), heavily laden with sediments or fresh tephra and icebergs. The discharge is controlled by a few factors. Very high discharge results from a high output of magma at the base of a thick glacier and where most of the magma is effectively fragmented (strong hydro-magmatic, phreatic activity), to mention two aspects. At Gjálp in Vatnajökull (1996) the initial ice melting rate was probably about 5,000 cubic metres per second. Recent studies of the course of events during the initial stage of the more powerful Katla eruption in 1918 indicate that higher magma output and more effective granulation of magma caused about ten times more rapid ice melting. This helps to explain why meltwater floods (jökulhlaup) from Katla occur a few hours after an eruption starts and why the discharge is 5-10 times larger than most floods from the Grímsvötn area in Vatnajökull.



On land, the sediments and glassy tephra from glacier bursts form black deserts. In the ocean they add to other marine sediments on the shelf around Iceland and to the chemical content of sea water. The suspended sediment load in glacial rivers is important when it comes to fixinging carbon dixoxide in sea water. In historical times, glacier bursts have often caused damage to fertile land, to farms and rural areas as well as to modern constructions like roads, bridges and powerlines. Potential sources of large glacier bursts are monitored.  



Out of the icy womb

The bottom (or sole) of a glacier contains rock debris. The ice constantly plucks and grinds the bedrock, thereby not only creating landforms but also producing gigantic volumes of lose material, glacial sediment. This ranges from minute-particle clay and silt to gravel and heavy boulders.  Most particles become somewhat rounded and scratched. Such sediments can be found in front of any active glaciers. Commonly, some of the material forms moraine ridges and hills.  Another part is unveiled along the fringes of ice fields and glaciers, lying here and there on top of the ice. But much of the bottom sediments are revealed as the glaciers retreat. This was the case when the extensive cover during the last glacial period of the Ice Age suddenly disappeared. It is still possible to study these glacial remains almost everwhere in Iceland. The loose material that covers much of the highlands is the product of Ice Age glaciers.



The bedrock itself bears the claw-marks of the ice. Valleys, fjords, steep-sided mountains and whaleback forms occur in many parts of the country. The pressure at the bottom of a 500 m thick glacier is the equivalent of 45 kg for every square centimetre. Rock debris freezes to the bottom of the glacier as it creeps forward. The debris erodes the underlying surface at the same time as it breaks down itself. Thus glacial ice on the move is like a huge file which moulds the landscape through time.

Loose rubble and rock surfaces which have suffered the onslaught of glaciers bear various signs when they are uncovered after glacier rtetreat. Boulders and rock outcrops are covered in scratches or striations. On solid outcrops the striations are parallel and, in addition to other environmental features, indicate the direction of flow of the glacier. Large boulders and rocks can leave even larger glacier grooves behind. The rock outcrops seem to occur in waves. The surface itself and the upper parts of the cliffs are domed and are smooth on the side facing the direction of glacier flow and steep and uneven on the down flow side. The term whaleback has been used to describe this landform. Whaleback forms are often surrounded by moraine deposits, gravel and rocks while on top of them there are sometimes single rocks (erratics).



Ice caps which cover large land areas do not cover them completely for the highest peaks protrude from the ice. Nunatak has become the most favoured technical term for these “islands in the ice sheets”. Features of this type in Iceland are different. Some are small peaks, 0,5-2 km in diameter and rising 50-250 m above the ice. Examples are Eyjólfsfell in southern Vatnajökull and Pálsfjall, southwest of Grímsvötn. The Hásteinar peaks in Hofsjökull are of the same type and also Þursaborg in Langjökull and Goðasteinn in Eyjafjallajökull. Other nunataks are in fact highland areas with a few peaks such as Esjufjöll in Vatnajökull and the area including Fjallkirkja and Péturshorn in Langjökull.



During the Pleistocene glacial periods many peaks rose above the ice surface, in particular close to the high upland coasts. Ice movement sculpted the peaks into sharp serrated ridges and pointed horns and these are a common feature of the Icelandic landscape. Rock falls and large debris slides have been common in Iceland, formed when glaciers retreat, sometime causing floods as proglacial lakes are drained.



If glacial sediment forms mounds or thick layers containing considerable amounts of coarse materials it is known as moraine or glacial till. Such moraine deposits either occur as a debris blanket on flat or sloping land or form high, steep linear features of various types. Moraine deposits remain when the glaciers retreat as ground moraine. Moraine which is deposited at the margins of outlet glaciers, either from material which falls on to them or remains on retreat is called lateral moraine. Moraine can also be deposited at the end of active glaciers when material is transported there by glacier movement (terminal moraine). Long, narrow moraine tongues or stripes extend the length of outlet glaciers on the down glacier side of nunataks where two glaciers combine to form medial moraines. Finally moraine can be pushed up by a sudden surge of the glacier (push moraine). Rounded moulded ridges orientated in the direction of glacier flow and alternating with ground moraine are known as drumlins, while eskers are meandering ridges that are chiefly sediments from fluvial channels in glaciers. All these features occur widely in Iceland, but especially so along the margins of Vatnajökull.



Various periglacial phenomena

Cryoturbation features are prominent in polar areas. Where there is constant frost in the soil (permafrost areas) frost activity forms the most typical features of the landscape. These are less conspicuous in marginal areas like Iceland, but are nevertheless quite widespread.

Icelandic soil is unusually active material. This is the result of considerable precipitation in the country, the high water content of the soil, little cohesion and frequent frost activity. This involves freezing of water in the soil and sediments with an associated volume increase. The uppermost part of the soil or skriðuset swells. Sudden melting results in the movement of material within the unconsolidated soil while at the same time it subsides when the ice melts. Many features can be seen in unvegetated or vegetated areas, both on sloping land or level.



Unconsolidated sediment on sloping land automatically moves downslope. It makes no difference whether there is vegetation or not. Nevertheless vegetation succeeds to some extent in binding fragmental sediment or soil on slopes. Mostly the movement is gradual, the vegetation moving slowly along with it. Sometimes another more rapid kind of event occurs, mainly in sudden thaw, torrential rain or as a result of repeated thaw between periods of frost, especially if only the uppermost part of the soil melts on top of the frost underneath. Rock or mixed debris flows can occur under all these conditions.



On vegetated grassy slopes can be seen in many areas long parallel steps or small terraces. They resemble folds when viewed from the side. These are solifluction terraces and they are very widespread. They are formed by a combination of frost heaving and downslope movement of the soil. This solifluction is of the order of a few centimetres per year. On scree slopes the movement is much more or several centimetres or tens of centimetre per year.

On unvegetated gravel slopes, stripes are formed by solifluction in which bands of pebbles and finer grained soil alternate.

In gently sloping soil the soil grains, both coarse grained and fine, move due to continual frost and thaw. Frost activity results in patterned ground forming on the surface with varying height and characteristics. It occurs in both vegetated and unvegetated ground.

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Unvegetated gravel originates mainly as ground moraine in which clay, gravel and boulders are mixed together. Repeated freeze-thaw cycles result in pores forming underneath the stones during frost. They are then filled by finer grained material which sinks into them during thaw. Gradually the soil is lifted upwards from the surface. Thus the stoney sediment is uppermost in most Icelandic gravel areas, with finer grained material underneath. In addition the uneven grainsize distribution means that the gravel is covered in small hummocks and hollows during frost. The volume increase of water forces the soil grains laterally and vertically by different amounts. Pebbles are pushed to the side and also sink downsloope into the hollows during thaw. Over a long period of time a polygon pattern develops and the finer grained materials remain in the polygons. This is the phenomenon of patterned ground.



On vegetated, fairly flat land a similar process leads to small hummocks alternating with elongated depressions. When the temperature falls below freezing point the soil and vegetation is pushed up, in places higher than in others. In thaws between frost periods the soil sinks and the grains become displaced to new positions. Vegetated hummocks formd in this manner become more pronounced with time and the intervening depressions deeper. Sometimes the tops of the hummocks are eroded. Such hummocks are common in all peaty areas, bogs and pastures. In Icelandic tundra areas the hummocks contain an ice core all year and can become larger than usual (e.g. Þjórsárver). The formation of such hummocks follows a well defined evolutionary path in which small tarns or pools which form with time in the permafrost bogs play a part.

The author is a geoscientist with a long record of dissemination of scientific knowledge to the public.

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Selected bibliography
Surnames are written with inclined letters. The Icelandic letters é, ó, á, ú, í and ý should be written as e, o, a, u, i and y, ö as ö, oe or simply o, æ as æ or ae, ð as d and þ as th when searching for references and papers in most databases.

H. Björnsson 1988.  Hydrology of ice caps in volcanic regions. Socieats Scientarium Islandica. 45. Reykjavík. 139pp.

H. Björnsson H. and P. Einarsson 1990. Volcanoes beneath Vatnajökull, Iceland: Evidence from radio echo sounding, earthquakes and jökulhlaups. Jökull 40, p. 147-169.

H. Björnsson, F. Pálsson,  M.T. Guðmundsson and H. Haraldsson 1998. Mass balance of western and northen Vatnajökull, Iceland 1991-1995. Jökull 45, p. 35-38.

H. Björnsson, F. Pálsson and  M.T. Guðmundsson 2000. Surface and bedrock topography of Mýrdalsjökull, Iceland: The Katla caldera, recent eruption sites and routes of jökulhlaups. Jökull 49.

H. Björnsson and F. Pálsson 2008. Icelandic glaciers. Jökull 58, p. 343-386.

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Þ. Einarsson 1994. Geology of Iceland - rocks and landscape. Mál og menning, Reykjavík.

M.T. Guðmundsson, F. Sigmundsson, H.Björnsson  and Þ. Högnadóttir 2004.
The 1996 eruption at Gjálp, Vatnajökull ice cap, Iceland: efficiency of heat transfer, ice deformation and subglacial water pressure. Bulletin Vocanol. 66, p. 46-65.

T. Jóhannesson, G. Aðalgeirsdóttir, He. Björnsson, C.E. Böggild, H. Elvehöy, S. Guðmundsson, R. Jóhannesson, Ha. Björnsson, P. Crochet, F. Pálsson, O. Sigurðsson and Þ. Þorsteinsson 2006. Mass balance modeling of the Vatnmajökull, Hofsjökull and Langjökull ice caps. Proceedings Europ. Conf. of Impacts of Climate Change on Renewable Energy Resources. Reykjavik,  Iceland.

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