Geology of Iceland
Kverkfjoll
Source: Iceland Tourist Board
Iceland lies astride the Mid-Atlantic Ridge and is an integral part of the global mid-oceanic ridge system. It is the largest supramarine part of the mid-oceanic ridge system. Iceland has developed on the Mid-Atlantic Ridge as a landmass between the submarine Reykjanes Ridge to the southwest and the Kolbeinsey Ridge to the north, and has been active during the last 20-25 million years, broadly coinciding with the time-span of active volcanism in Iceland. Being a hot spot above a mantle plume, Iceland has been piled up through voluminous emissions of volcanic material with a much higher production rate per time unit than in any region in the world. It has grown by rifting and crust accretion through volcanism along the axial rift zone, the volcanic zones, which in terms of the plate tectonic framework marks the boundary between the Eurasian and North American plates. Accordingly the western part of Iceland, west of the volcanic zones, belongs to the North American plate and the eastern part to the Eurasian plate, with the oldest rocks outcropping in northwest and in eastern Iceland. To complicate the picture there are also rocks of similar age in Western Iceland and in the centre of northern Iceland, due to movements of the hot spot and the volcanic zones. The rate of spreading is calculated as 1 cm in each direction per year.
Geology and Geological History
Iceland is built almost exclusively of volcanic rocks, predominantly basalts. Silicic and intermediate rocks - rhyolites, dacites and andesites - constitute about 10% and sediments another 10% of it.
The main rock formations. The rocks of Iceland can be divided into four main formations: (1) The Upper Tertiary Plateau Basalt Formation, (2) The Upper Pliocene and Lower Pleistocene Grey Basalt Formation, (3) The Upper Pleistocene Palagonite (Hyaloclastite) Móberg Formation and (4) The Postglacial Formation, which besides Postglacial lavas includes sediments as till and glacial sediments from the retreat of the last ice cover and marine, fluvial and lacustrine sediments and soils of Late Glacial and Holocene age.
The Tertiary Basalt Formation comprises eastern and southeastern Iceland, the main part of Western Iceland and the western part of northern Iceland, altogether about half the country's area. In eastern Iceland, basalt lava flows, mainly tholeiitic, form about 80% of the volcanic pile above sea-level, which has a stratigraphic thickness of about 10,000 m. Silicic (rhyolitic) and intermediate rocks and detrital beds form the rest. Dykes are common and intrusions of gabbro and fine-grained granite (granophyre) occur, especially within ruins of differentiated central volcanos. Beds of tephra and ignimbrite are found in and around many of the central volcanoes.
The oldest rocks so far K/Ar dated above sea level are about 14 million years old. Thus, the oldest basalts are no older than the Middle Miocene and much younger than the basalts in Britain, Greenland and the Faroes. This accords with the theory of ocean-floor spreading.
In the Tertiary Icelandic basalts, lava vesicles are usually filled with quartz minerals such as rock crystal, jasper and chalcedony or with zeolites. Zeolites from Teigarhorn in Berufjörður are found in museums all over the world. The Helgustaðir mine in Reydarfjörður remained the world's main supplier of the transparent Iceland spar (optical calcite) for centuries.
Intercalated between the plateau basalts, especially in northwestern Iceland, are plant-bearing sediments and thin layers of lignites. Species found include beech, maple, vine, liriodendron and conifers. The mixed forests of conifers and warmth-loving broad-leaf trees indicate a warm-temperate climate. The warmth-loving trees gradually disappeared during the Pliocene when the climate slowly grew cooler and the first glacial sediments, tillites, turned up. The thin layers of lignites are inferior in quality as fuel seams, although they have been used on a small scale in some places.
The Pleistocene rocks are confined mainly to a broad SW-NE trending zone between the Tertiary plateau basalt areas, and they are also exposed on the peninsulas Tjörnes, Snæfellsnes and Skagi. The Pleistocene rocks are divided into two formations and the limit between them is the last magnetic reversal, occurring about 700,000 years ago. In the Pleistocene Formation there are three main facies:
1) Interglacial basalt lava flows which are generally grey in colour and of coarser texture than the Tertiary ones. The Grey Basalt Formation, where the interglacial basalt layers remain a dominating facies, is mainly exposed along the inner border of the Tertiary basalt areas and in the central part of southern Iceland.
2) Subglacially formed pillow lavas, breccias and brownish tuffs, known as palagonite (móberg), rich in hydrated and otherwise altered basaltic volcanic glass. The share of silicic and intermediary rocks in the Pleistocene Formation is similar to that in the Tertiary ones. The main rhyolitic massifs are the Torfajökull area and Kerlingarfjöll.
3) Glacial, fluvial, lacustrine and marine sediments are interbedded between lava flows. The thickest series of marine strata are found on the Tjörnes peninsula in northern Iceland, where the arrival of Pacific molluscs commenced about 3 million years ago, after the first opening of the Bering Strait.
Stratigraphic studies indicate about fourteen Upper Pliocene and Pleistocene glacial periods. During the main ones the country was almost completely covered by ice. Broad-leaf and conifer forests disappeared during the Lower Pleistocene, but birch, willow and mountain ash survived all theglacial periods, and alder all except the last two.
During the Pleistocene glacial periods, thick ice blanketed the volcanic activity, which consequently took place mainly under water (meltwater) and thus under conditions similar to the submarine parts of the World Rift System. The volcanos which built up subglacially in the volcanic zones mainly depict two types, ridges and table mountains. The ridges are steep-sided and serrated and run in parallel lines, NE-SW in southern Iceland, N-S in northern Iceland. The table mountains are isolated mountains, circular to sub-rectangular. They consist of a shield volcano resting on a socle of pillow lavas and palagonite tuffs and breccias. The lava shields were formed by subaerial outflow of lava when the socles had grown high enough to protrude through the ice cover. The prototype of such table mountains is Herðubreið (1,682 m), north of Vatnajökull.
