ICELANDIC GLACIAL LANDSCAPES
James S. Aber
|Ice Cap||Area (1958)||Area (1980)|
Iceland is, thus, a natural laboratory in which to observe the interaction of glaciers, climate, volcanoes, and the resulting sediments and landforms. The modern elements of glacial dynamics and glacial geomorphology in Iceland provide excellent models for conditions that existed for larger Pleistocene ice sheets in Europe and North America.
|Tephra (volcanic ash) bed melting out from northern margin of Mýrdalsjökull ice cap, southern Iceland. Such volcanic deposits are very common within glaciers of Iceland. Photo date 8/94; © by J.S. Aber.|
|Mýrdalssandur outwash plain, southern Iceland. Sandur plains are formed by massive flows of sediment-rich glacial melt water. This plain was innudated in 1918 by a jökulhlaup flood during the eruption of Katla volcano beneath Mýrdalsjökull ice cap (visible in distance). Photo date 8/94; © by J.S. Aber.|
Vatnajökull, which means literally waters glacier, is situated on a volcanic plateau in southeastern Iceland.
Modern study of the ice cap began in 1875 with a crossing by Scotish explorer, W.L. Watts, and his
Icelandic guide, Páll Pálsson--see Fig. 7-1. Several
other expeditions took place early in this century, and observations have
been more or less continuous since the 1930s. Vatnajökull is the largest
ice cap in Europe and is reasonably accessible today.
|Overview from southeastern edge of Vatnajökull ice cap, Iceland. The accumulation zone is visible in the foreground, a large outlet glacier can be seen in the middle distance, and the Atlantic Ocean appears on the far horizon. Elevation of foreground is about 1100 m a.s.l. Photo date 8/94; © by J.S. Aber.|
|A snowmobile tour resting on the southeastern edge of Vatnajökull ice cap, Iceland. Access to the glacier is routine during the summer months, and many tourists now visit Europe's largest ice cap. Photo date 8/94; © by J.S. Aber.|
The main part of Vatnajökull has a surface elevation of 1400 to 1600 m with highest parts reaching about 2000 m. The ice cap rests mainly on a base some 800-1000 m in elevation; ice thickness is generally 600 to 800 m with greatest thickness about 1000 m. Vatnajökull's firn line varies from 1000-1100 m elevation in the south to around 1200-1400 m on the northern side. On this basis, Vatnajökull should be regarded as a climatic relict; if removed, an ice cap could not reform under current climatic conditions (Bárdarson 1991).
Vatnajökull possesses all manner of large and small outlet glaciers. Brókarjökull is a dramatic glacier on the southeastern margin of the ice cap. This glacier begins as an icefall from the edge of Vatnajökull and is reconstituted as a small valley glacier at the bottom of the ice fall. The vertical relief from head of the ice fall to the glacier's snout is nearly 1 km. Hoffellsjökull is typical of mid-sized valley glaciers on the eastern side. It moves up to 630 m per year and is about 250 m thick (Bárdarson 1991). Several outlet glaciers have surged during modern times; for example, Sídujökull underwent a massive surge during the spring of 1994. Most outlet glaciers terminate on land, but Breidamerkurjökull ends in a lake just short of the sea.
|Overview of Brókarjökull, a spectacular small outlet glacier on the southeastern side of Vatnajökull, Iceland. The glacier begins at an ice fall on the left and descends into the valley on the right. The vertical drop of this glacier is about 1 km. Photo date 8/94; © by J.S. Aber.|
|Telephoto view of the snout of Brókarjökull, as seen from above. Notice the beautiful display of crevasses near the ice margin. Photo date 8/94; © by J.S. Aber.|
|Lower portion of Hoffellsjökull, a typical large outlet glacier on the eastern side of Vatnajökull. This glacier ends partly on land and partly in an ice-marginal lake. Photo date 8/94; © by J.S. Aber.|
|Hoffellsjökull ice margin and proglacial lake. Much glacially derived sediment accumulates in such lakes. Photo date 8/94; © by J.S. Aber.|
Grímsvötn is a large depression in the west-central portion of Vatnajökull--see Fig. 7-1. It is about 35 km² in area and up to 500 m deep. Grímsvötn is an ice-dammed lake within a volcanic caldera. Prior to 1934, Grímsvötn drained nearly every tenth year via jökulhlaups that flooded Skeidarársandur. Maximum discharge is estimated at 50,000 m³/sec (Bárdarson 1991). Following a volcanic eruption in 1934, the draining of Grímsvötn has been less regular, often at intervals of about 5 years. On Sept. 30, 1996, a major volcanic eruption began at Bardarbunga volcanic center under Vatnajökull. Melt water generated by the eruption drained into Grímsvötn and collected throughout the month of October. As much as 3 km³ of melt water was trapped. A jökulhlaup began on Nov. 5; it quickly increased in magnitude reaching a peak discharge of 45,000 m³/s about 15 hours after starting. This was one of the largest jökulhlaups of this century in Iceland. Bridges, roads, electric lines and communication cables were washed away. Total damage is estimated at $10-15 million.
The Skaftafell region, south of Vatnajökull, contains a long stratigraphic record of volcanic eruptions and glacial deposits. Subaerial eruptions produced lava flows, whereas subglacial eruptions resulted in pillow lavas and breccias. The composite sequence exposed in cliffs records nearly five million years of strata, which have been dated by magnetic-reversal and potassium-argon techniques. This sequence demonstrates at least 16 glacial-interglacial intervals (Helgason and Duncan 2001). The earliest glaciations were limited in duration; after 2.6 million years ago the frequency and intensity of glaciations increased significantly, and a further increase took place since about 800,000 years ago. These major stages in glaciation intensity correspond to similar conditions throughout the North Atlantic and worldwide.
Mýrdalsjökull is located on the active volcano Katla, which is part
of the Eldgjá volcanic trend in southern Iceland. The ice cap has two
summits >1400 m high and continues in an unbroken cover down to 1000-1300 m
elevation, below which the ice splits into several outlet glaciers of
various types--see Fig. 7-2. The southern and central portions of
the ice cap are located within the Katla caldera, where ice thickness
reaches 600-800 m. The northern part of the ice cap is situated outside
the caldera and is a largely independent glacial system.
|Haabunga dome of Mýrdalsjökull on horizon and a portion of Mýrdalssandur in foreground. The ice-cap dome reaches an altitude of nearly 1500 m, and the sandur is <100 m a.s.l. The ice dome buries Katla, an active volcano that last erupted in 1918. Photo date 8/94; © by J.S. Aber.|
The two largest outlet glaciers--Sléttjökull and Kötlujökull--display quite different dynamic conditions. Sléttjökull is a broad, relatively clean glacier that has fluctuated during this century in response to climatic events. It is fed by accumulation from outside the caldera. Kötlujökull, on the other hand, has experienced several asynchronous advances and retreats during this century. It drains from the center of Katla caldera, and its advances may have more to do with volcanic activity than with climatic conditions.
The glacial foreland of Sléttjökull provides an excellent scale model for glacial landscapes of Pleistocene ice sheets. The 20-km-long ice front slopes gently down to a low-relief plain, on which fresh glacial
deposits and landforms are well preserved--see Fig. 7-3. The
glacier ice is remarkably clean, and most sediment is transported in the
lowermost few cm of the glacier. Near its margin, the glacier moves about
20 m per year by a combination of basal sliding and subglacial shearing
|Marginal zone of Sléttjökull, northern edge of Mýrdalsjökull, Iceland. A terminal moraine is visible in the foreground and Sléttjökull can be seen on the left horizon; Mælifell is the small volcanic cone to right. Photo date 8/94; © by J.S. Aber.|
Beds of volcanic ash outcrop about ½ km upice from the margin. Ash from these beds is reworked by surface runoff and deposited in crevasses and moulins. Wherever ash/sediment cover exists on the surface, it protects the ice from ablation, and dirt cones form on the glacier. Surficial sediment is transported by runoff eventually to the base or margin of the glacier, where it becomes mixed with sediment from subglacial sources.
|Large dirt cone next to crevasse on margin of Sléttjökull. The dirt cone consists mainly of clean glacier ice with a cover of reworked volcanic ash (tephra) only a few cm thick. The dirt cone stands about 3 m (10 feet) high. Photo date 8/94; © by J.S. Aber.|
A terminal end-moraine is located about 1.2-1.5 km beyond the present ice margin, and an overridden end moraine is found about halfway between. The plain between the ice margin and the terminal moraine is a fluted ground moraine with small drumlins; the plain is locally dissected by rain/melt-water channels. Beneath the ground moraine, remnants of three sheets of lodgement till are preserved locally. These tills represent separate ice-front advances around AD 1750, 1800, and 1900--see Fig. 7-4.
|Terminal moraine of Sléttjökull; Mælifell volcanic cone in background. This moraine was constructed at the glacier margin in the latest 1800s and early 1900s. It consists of bouldery, poorly sorted sediment deposited at the ice margin. During this century, Sléttjökull has retreated to its present position about 2 km away (left of this view). Photo date 8/94; © by J.S. Aber.|
|The field party is standing on the overridden push moraine in front of Sléttjökull; Mælifell volcanic cone in background. This moraine is located about halfway between the terminal end moraine and the modern margin of Sléttjökull. The push moraine was constructed during glacier advance around 1800. Notice shallow source depressions to left. The moraine was overrun during a subsequent readvance in the late 1800s, when the glacier smoothed the moraine and left flutes--parallel stone stripes running left to right. Photo date 8/94; © by J.S. Aber.|
|Fluted ground moraine in front of Sléttjökull, Iceland. Flutes consist of slightly higher, more stony stripes a few m in width. Here, a flute begins at the boulder and continues several 100 m into the distance, and many parallel flutes can be seen. Ice axe for scale. Photo date 8/94; © by J.S. Aber.|
The present ice margin has been more or less stable in position since the mid-1980s. Each winter the thin ice margin freezes onto subglacial sediment (till) and transports the sediment forward as the ice margin advances. During the summer the ice margin melts back and sediment is released. Over a period of several years, a marginal moraine has accumulated. The moraine consists of stacked sediment layers that were transported from a shallow basin just behind the ice margin--see Fig. 7-5.
|Ice margin of Sléttjökull, Iceland. Field party is standing on the small marginal moraine that built up in front of Sléttjökull during the 1980s. The low-sloping, dirt-covered ice margin is visible to the right. Ice margin of Sléttjökull, Iceland. Field party is standing on the small marginal moraine that built up in front of Sléttjökull during the 1980s. The low-sloping, dirt-covered ice margin is visible to the right. Photo date 8/94; © by J.S. Aber.|
The Kötlujökull ice tongue transports a huge mass of sediment debris of subglacial and englacial origin. The outer 1-3 km
of the glacier is almost completely covered by sediment--see Fig. 7-6.
Volcanic ash is derived from a thick bed that crops out at the upper limit of
dirt-covered ice. This ash was deposited during Katla's last
major eruption in 1918, during which up to 8 m of pumice accumulated on
Mýrdalssandur south of Kötlujökull.
The 1918 tephra became buried within the accumulation zone and since has gradually migrated into the the ablation zones of several outlet glaciers. The 1918 tephra serves as a marker bed to track movements of Kötlujökull (Krüger 1994). The limit of dirt-covered ice has migrated downglacier with decreasing velocity as it has approached the ice margin: 225 m/yr (1918-45), 100 m/yr (1945-60), 80 m/yr (1960-80).
|Northern margin of Kötlujökull, a large outlet glacier on the southeastern side of Mýrdalsjökull, Iceland. Note dark color of ice margin zone caused by heavy cover of reworked tephra and glacial sediment. The upper border of the dark zone marks the position of the 1918 tephra bed within the glacier. Photo date 8/94; © by J.S. Aber.|
Bouldery gravel caps high ice-cored ridges near the glacier terminus; this sediment was originally deposited by englacial and subglacial melt-water streams. These gravels presumably were formed by englacial and subglacial outburst flooding during Katla's 1918 eruption. Active and abandoned melt-water tunnels are preserved at all levels along the ice margin at close intervals. Some of these tunnels are up to 10 m in diameter. Katla has a history of producing tremendous jökulhlaups that may exceed 100,000 m³/sec peak discharges and may carry equally massive sediment loads onto the sandur and adjacent coast (Krüger 1994). During the 1918 eruption/jökulhlaup event, the coast was built out as much a 4 km by sediment accumulation.
|Active melt-water tunnel at base of Kötlujökull. The tunnel completely fills with water during periods of rapid melting and/or rainfall on the glacier. Note complete sediment (mud) cover on adjacent glacier ice. Jens-Ove Näslund (Sweden) poses for scale at mouth of tunnel. Photo date 8/94; © by J.S. Aber.|
|Remnant of large ice tunnel on upper margin of Kötlujökull. This tunnel is >10 m in diameter; note person standing to lower left, Seppo Hassinen (Finland). When active, this tunnel could have carried at least 200 m³/sec melt-water discharge. Photo date 8/94; © by J.S. Aber.|
|Remnant of small ice tunnel preserved beneath dirt cones on surface of Kötlujökull. This tunnel is about 1 m in diameter; note person sitting behind tunnel. The great number of large and small ice tunnels bears witness to tremendous melt-water discharge from Kötlujökull. Photo date 8/94; © by J.S. Aber.|
|The rock cliffs of Hjörleifshöfði rise > 200 m above Mýrdalssandur outwash plain, southern Iceland. In Viking times, Hjörleifshöfði was an offshore island. It is now part of the mainland as a result of sediment accumulation and expansion of Mýrdalssandur. During Katla's eruption of 1918 the coastline was built out as much as 4 km. Photo date 8/94; © by J.S. Aber.|
Systems of small push moraines were constructed by ice advances in the 1950s and 1980s. These moraines form festoon patterns along the ice margin between outwash fans--see Fig. 7-7. The push moraines consist of stacked slabs of outwash gravel and till that are partly covered by and interlayered with slump and slide sediments derived from the ice surface.
|Small push moraines in front of Kötlujökull date from minor ice advances of the 1950s (moss covered) and 1980s (bare). Kötlujökull has since retreated about 100 m (or less, to right). Mýrdalssandur outwash plain and the Atlantic Ocean are visible in the background. Photo date 8/94; © by J.S. Aber.|
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ES 331/767 © J.S. Aber (2006).