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Špicberky (Svaldbard) - učebnice geologie

Ekologická dovolená, cestování
Ekologické vzdělání a výchova
Špicberky (Svaldbard) - učebnice geologie

Špicberky - závislé území Norska, souostroví s cca 1000 ostrovy v blízkostí severního pólu s cca 2 600 obyvateli (Norové, Rusové, Ukrajinci, Poláci), významné zejména těžbou černého uhlí, představuje zároveň zajímavou učebnici geologie.

Svalbard has a very rich, diverse and long geologic history that makes it some­what of a 'mecca' for geologists; a place where a great variety of geology can be studied and where much can be learned about the development of tectonic features and sedimentary environ­ments through time as well as the drift of continents across the Earths surface. The oldest till deposits on Svalbard were probably deposited when Spitsbergen was situated close to the South Pole, some 600 million years (MY) ago, and the red Devonian sandstone of northern Spitsbergen was deposited when Svalbard was close to the Equator, some 350-400 MY ago. Since the vegetation cover is so sparse and the glacially eroded landscape so fresh, there are continuous bedrock sections that span more than 11 km and that are more than 1 km high! Even students of Quaternary geology should be familiar with the outlines of the bedrock geology of Svalbard - it makes it so much more fun to be in the field!

Simplified geological map and stratigraphical column for Svalbard. Source: Hjelle 1993.

The Basement - The large-scale features of the Svalbard geology are relatively simple. The oldest formations, the so-called Basement, including Pre­cambrian, Cambrian and Ordovician rocks, occur as a belt along the west-coast of Spits­bergen, in a large area between Wijde­fjorden and Hinlopenstretet in the north, as well as in the northern part of Nordaust­landet and in the southern part of Bjornoya. The Precambrian bedrock of Svalbard is more than 570 MY old, and there is evidence of repeated mountain-building and metamorphic episodes (orogenies) dating from ca. 1700, 1000 and 600 MY ago. The mountain chains have long time ago disappeared due to erosion, and what remains are rocks from their deeper, inner parts. The Precambrian rocks on north-western Spitsbergen are generally very strongly metamorphosed. This means that they have once been buried deep in the crust, and high pressure and temperatures have caused them to partially melt and recrystallize. The oldest rocks dated on Svalbard are zircon minerals from NW-Spitsbergen, giving the age of 3200 MY! Precambrian tillites, bearing witness to glacial coverage about 600 MY ago occur both on the west- and north-coasts of Spitsbergen. Other Pre­cambrian rocks include volcanic rocks found on both sides of Forlandsundet.

Precambrian (650-600 MY) tillite, Scottbreen. Photo: Jon Landvik 2003.
The Cambrian and Ordovician calcareaous and limestone strata of Svalbard often contain fossils of marine creatures that once upon a time lived in the so-called Lapetus Ocean. These were primitive invertebrates, such as trilobites and graptolites.

Svalbard stratigraphical column and paleomagnetic record reflecting northwards drift of Svalbard since the Devonian. Source: Aga 1986.
Silurian: mountain building - During the Silurian, two continental plates collided, causing large-scale folding and fault­ing in what is now the North Atlantic region (the Caledonian Orogeny). All rocks on Svalbard that are older than late-Silurian (see the stratigraphic column) are therefore folded and metamorphosed. Granites intruded in the older rocks during late Silurian. The highest peak on Svalbard, Newtontoppen, consists of late Silurian granite. Palaeomagnetic research has shown that at this time Svalbard was located at the Equator. The system of metamorphic and igneous rocks, which was the result of the Caledonian Orogeny, is traditionally called the Heckla Hoek series. The name Spitsbergen, meaning the Pointed Peaks, comes from the wild and rugged Heckla Hoek terrain on north-western Spitsbergen.

Ragged peaks of Hekla Hoek rocks, Smeerenburgfjorden, NW Spitsbergen. Photo: Ólafur Ingólfsson 2001
Devonian: erosion and sedimentation -During the Devonian Period, northern Svalbard began sinking and rivers eroded down the Caledonian mountains. A total of more than 8000 m thick sandstones, conglomerates and shales were deposited in near-shore environments, deltas and lakes. The Devonian beds often consist of characteristic red sandstone. The red color implies periods of dry, desert-like climate. At this time, 360-400 MY ago, Svalbard was located just north of the Equator. The Devonian is called "the Age of Fishes", and fossils of primitive fish - the first known vertebrates - have been found on Svalbard. As have fossils of primitive plants.

Examplesof fossil primitive fish from the Devonian beds on Svalbard. Source: Palaeontological Museum, University of Oslo
Carboniferous: tropical swamps - During the early part of the Carboniferous, Svalbard was a relatively flat and undulating terrain, with lakes, lagoons and alluvial plains. Svalbard was drifting from the desert climate of Devonian time, entering wetter, more tropical climate. Carboniferous sandstones in the central parts of Spitsbergen - which were mined by the Russians in the settlement of Pyramiden until a few years ago - contain coal seams with plant fossils, showing a luxorious swamp vegetation. Life on Earth was developing rapidly: this was the Age of the Amphibians, and the swamps teamed with life. Amphibians were common, but fish, insects and scorpions were also numerous. The swamps of Carboniferous Svalbard have no modern analogue anywhere in the world: not only were seedless plants and ferns extremely important in this community, but there were no birds singing or insects with humming wings. The first birds did not emerge in the evolution of life until in Jurassic times, more than 100 million years later, and all insects of the Carboniferous swamps had fixed wings.

Carboniferous rocks in Zittelberget, Central Spitsbergen. Source: Aga, 1986.

Later, during Middle and Upper Carboniferous, Svalbard was subject to alternating deposition on land and in shallow marine seas. Strong evaporation in a dry, temperate climate caused deposition of gypsum, anhydrate and dolomite. Typical rocks from this time in Svalbards history are breccias, limestone and dolostone (containing fossil mollusks and snails), and beds of gypsum or anhydrite.

Intensely folded Permian strata, Axeloya, Van Mijenfjorden. Photo: Ólafur Ingólfsson, 2004.
Permian: sub-tropical shallow seas - The Permian beds on Svalbard resemble the upper­most Carboniferous, with shallow-water marine deposits in areas that periodically became dry. Intense evaporation during dry periods resulted in renewed formation of gypsum, anhydrate and dolomite. The Permian beds contain numerous fossils of shells, as well as siliceous sponges and bryozoans. These beds can be seen e.g. along the north-coast of Isfjorden, in Linnédalen and at Axeloya in Bellsund.

The Mesozoic: Age of the Reptiles - During the Triassic, Jurassic and Cretaceous Periods, Svalbard drifted from about 45oN to about 65oN. The Mesozoic was a "greenhouse world", considerably warmer globally than at present, and with higher sea levels. The Svalbard climate throughout this time was temperate and humid. The Mesozoic deposits on Svalbard are mostly marine, but they alternate with terrestrial deposits, and volcanic intrusions towards the latter part of the Era. Among fossils from the Triassic Period found on Svalbard are bivalves and ammonites, but reptile remains also occur in the Lower Triassic strata.
During the Jurassic and lowermost Cretaceous, most of the Svalbard area was covered by a shal­low sea. The sedimentary rocks consist mostly of marine shales, often very rich in fossil ammonites, belemnites (squids) and bivalves. Remains of Plesiosaurus, a swan-necked marine reptile that swam the Jurassic seas have been found in the Isfjorden area and on Kong Karls Land. The Plesiosaurus remains from Kong Karls Land, discovered in 1973, are spectacular. The ca. 200 MY old fossil contained the remnants of its last meal in its stomach: a healthy diet of squids and plants. Recently, fossil Ichthyosaur and Plesiosaur remains of Late Jurassic age have been discovered on Bohemanflya, in the Isfjorden area.

During the early Cretaceous Period, Svalbard had extensive alluvial plains of sand. There are abundant plant remains preserved in the sandstones. Fossilized footprints of the Cretaceous (ca 130 MY) plant-eating Iguanodon were discovered at Festningen, close to Kapp Linné in the outer Isfjorden area. Fossil foot­prints of the carnivorous Allosaurus were found at Kvalvagen, in south-eastern Spitsbergen. The dinosaur remains suggest that the Cretaceous climate was mild, with luxorious vegetation. 130 MY ago, Svalbard was located between 50o and 60o N. The upper part of the lower Cretaceous is characterized by alter­nating marine and ter­restrial deposits, deposited in shallow seas and deltas. The fossil record contains bivalves, squids, snails and tree-trunks. During the later part of the Cretaceous, Svalbard was subject to a tectonic uplift causing erosion and a hiatus in the sedimentary record.

The Tertiary: folding and sedimentation - At the transition between the Cretaceous and Tertiary periods, some 65-60 MY ago, Sval­bard and Greenland collided, causing intense folding of the Svalbard west-coast strata, as well as the formation of a depression basin to the east where sedimentation occurred.

Plate configuration in the Arctic prior to opening of the Atlantic and the Polar Basin. Source: Worsley 1986

The Central Tertiary Basin dominates the geology of the central-southern part of Spitsbergen, from the Isfjorden area to Storfjorden. Lower Tertiary deposits in the basin mainly consist of sandstones, with numerous coal seams which form the basis for commercial mining of coal in Svalbard today. The vegetation was luxuriant, and beautiful plant-fossils are frequent in the strata. The Longyearbyen glacier today erodes a Tertiary coal seam, and plant fossils can be sampled from its frontal moraine. Towards the latter part of the Tertiary, when the plate pressure started to diminish, Svalbard was subject to intense faulting and volcanism. Lava flows of basalt occur spotwise on northern Spitsbergen. The climate of Svalbard was getting successively cooler partly as a consequence of the slow northward drift, and partly because of a late Tertiary global cooling.

The Quaternary of Svalbard: repeated glaciations - Svalbard had largely attained its present shape and size by late Tertiary times. The sculpturing of the landscape with its fjords, bays, valleys, cirques, mountains, horns and ar^etes was largely carried out by the repeated glaciations during the Quaternary. No one knows how many glaciations have occurred on Svalbard since the onset of the glacial period at these high latitudes some 3 MY ago, but there may have been 50 or more glaciations! The preservation of Quarternary sediments is poor since subsequent glaciations have removed signs of earlier glaciations.

The Kongsvegen tidewater glacier. Photo: Ólafur Ingólfsson 2004.
During the last glacial cycle, ca. 115,000-10,000 years before present (BP), Svalbard was subject to two or three major glaciations, where a major ice sheet grew in the Barents Sea and extended over, or confluenced with, an ice sheet over Svalbard. During their peak, glaciers extended in fjords and troughs out to the shelf break west of Spitsbergen. The last major glaciation on Svalbard, during the so called Late Weichselian (~25,000-10,000 BP) ended with a rapid deglaciation during the period ca. 14,000-10,000 BP. Transgression by the sea subsequent to deglaciation left marine terraces and flights of raised beaches around Svalbard. The isostatic fingerprinting, as expressed in well-dated relative sea-level curves, suggests heaviest ice-loading over central-northern Barents Sea during the last glaciation. During early-mid Holo­cene, Svalbard glaciers were probably smaller than at present, and the early Holocene climate was considerably milder. Some of the present cirque- and valley glaciers probably did not exist prior to ca. 2500 BP. Glaciers expanded considerably during the so-called Little Ice Age, which culminated on Svalbard during the first decade of the 20th Century. Since then most glaciers have retreated, probably as a consequence of a considerable summer warm­ing occurring in the period after ca. 1915.
- Continental drift will continue to move Svalbard northwards and east­wards, and in 50 MY time it may lie at or close to the North Pole. Global warming may cause its glaciers to melt away - but again, Svalbard may well plunge into new series of extensive glaciations. Regardless, nothing the future may hold will be more dramatic than the environmental changes through time recorded in the geological record of Svalbard today.

ZDROJ: http://notendur.hi.is

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