European geology

Europe is divided by three mountain belts that run roughly northeast-southwest. In the far north, the highlands of Scandinavia and northwestern Britain formed about 400 million years ago above the lowland gneisses and granites of Karelia, Finland, and the Scandinavian peninsula. The broad highland belt that fans out westwards across central Europe from southern Poland to northern Spain and southern Ireland was formed from a deep-sea trough that was raised about 300 million years ago. Europe's most southerly mountains were formed when Africa moved northwards, enclosing the Mediterranean and raising the seabed to form the mountains of southern Spain, the Pyrenees, the Alps, the Carpathians, and the mountains of the Balkan region. The first ice age was 2-3 million years ago; Britain was cut off from mainland Europe by the rise in sea level when the ice melted at the end of the last ice age 8,000-10,000 years ago.

The oldest rocks in Europe - Precambrian rocks more than 1,000 million years old - are those of the Baltic Shield. It forms the northern border of the continent and passes southeast through Sweden, Finland, and Russia, below a cover of flat-lying Phanerozoic rocks forming the Russian platform. Precambrian rocks reappear to the south of the Russian platform in the Ukraine area. All these areas taken together form the European craton, an area that has remained stable throughout Phanerozoic time.

Part of another ancient stable shield area is seen in northwestern Scotland; the Precambrian rocks here are a small fragment of a much larger shield area disrupted by continental drift. The other, larger fragments are seen in central and western Greenland.

Another major component in the geology of Europe is the younger Caledonian system of orogenic belts (see orogeny), developed over the period 900-400 million years ago. This system extends from western Ireland through Britain and Scandinavia to northern Norway; another branch of it runs through central Europe, but here it is largely obliterated by the younger Hercynean or Variscan system of orogenic belts, developed 400-280 million years ago. These run west-east through southern and central Europe. In its northern part this belt is in turn overlaid by younger sedimentary rocks, and in its southern part the more recent Alpine orogenic belt has deformed and metamorphosed the pre-existing rocks. The Alpine orogenic belt, formed between 300 million years ago and the present day, is itself an intricate system of fold-mountain belts running roughly west-east across southern Europe, the Mediterranean region, and North Africa.

The eastern limit of the European continent in terms of its geology can be taken as the Ural Mountains, the site of a north-south orogenic belt which became stabilized at the end of the Palaeozoic.

The Baltic Shield occupies most of Sweden and Finland, southern Norway, and Karelia, an area of some 500,000 sq km/200,000 sq mi. The oldest rocks, gneisses more than 3,000 million years old, are found in the Kola Peninsula and in the Ukraine. They occur as discrete masses within a larger area of rocks of middle Precambrian age in the northeastern part of the shield. The bulk of the central and western part of the shield is composed of rocks belonging to the Sveccofennid group; these are metamorphosed sediments and volcanics, the remnants of a well-defined mobile belt active in late Precambrian times. This belt became stabilized around 1,700 million years ago; from that time on the greater part of the shield remained as a stable area. A belt of younger rocks occurs in southern Norway and Sweden; these are the remains of the Grenville mobile belts, again of Precambrian age, consisting of gneisses and high-grade metamorphic rocks, together with granites and anorthosites.

The Caledonian mobile belts were initiated in Precambrian times and continued their activity well into the Phanerozoic. Beginning around 800 million years ago, thick accumulations of sediment were laid down in a number of basins of deposition extending from Ireland through Scotland, England, and Wales and across to Sweden and Norway. These sediments range in age from Precambrian to Silurian, reaching thicknesses of up to 15 km/10 mi. To the east and west of the main sedimentary basins, thinner successions of Lower Palaeozoic rocks were deposited on the stable continental platform areas; the base of these deposits is marked by a widespread marine transgression at the base of the Cambrian. Episodes of orogenic folding and metamorphism began in the Late Precambrian in the earlier-formed basins of deposition, and continued over a long period until the Silurian. In the later-formed basins of deposition, the metamorphism and folding were generally less marked in character. The final stages of the orogenic cycle were marked in the Caledonian belts by regional metamorphism and uplift, forming a new mountain chain in Silurian times. During this final metamorphism, granites were emplaced at depth in the orogenic belt, while on the surface the thick deposits of the Old Red Sandstone were laid down in a desert environment as the rapidly rising mountain chain was eroded.

The metamorphic rocks of the Scottish Highlands represent the deposits originally laid down in one of the early basins of deposition.

The Moine series (7 km/4.5 mi thick) and the Dalradian series (8 km/5 mi thick) form a thick pile of metamorphosed sandstones, turbidites, and volcanics. In the extreme northwest a different series of sediments were laid down on the stable foreland bordering the mobile belt; these are the shallow-water sandstones of the Torridonian series (about 4 km/2.5 mi thick), and these are succeeded by Cambro-Ordovician quartzites, shales, and limestones.

The remains of other early Caledonian basins of deposition are seen as fragmented remnants further to the south, underlying the later Caledonian basins of southeastern Ireland, Wales, and the Welsh Borders. In Scandinavia, early basins of deposition occur immediately alongside the eastern foreland of the Baltic Shield. The succession on the foreland ranges from Cambrian to Silurian in age. Within the early basins, the lowest unit is the Sparagmite group, sandstones of Precambrian age, which are followed by Lower Palaeozoic sediments of considerable thickness in the centre of the mobile belt.

In a number of areas in and around the Caledonian belts there are deposits from an ancient glacial period in the late Precambrian. The later basins of deposition in the Caledonian belts are a number of relatively narrow basins which formed in or alongside the early basins. They include the Southern Uplands basin of Scotland and northeastern Ireland, the Mayo basin of western Ireland, the Lake District basin, and the Welsh basin. All the Upper Palaeozoic rocks that accumulated in these basins contain thick turbidites and volcanics.

The Hercynean mobile belts represent parts of the earlier Caledonian system which remained active throughout Upper Palaeozoic times. In Western Europe the west-east Hercynean belt diverges sharply in trend from the northeast-southwest Caledonian belt, to run as a broad belt along the southern edge of a stable area composed of the Baltic Shield, the Russian platform, and the Old Caledonides forming the Old Red Sandstone continent.

In much of Europe the Hercynean rocks have been fragmented and disturbed either by the later Alpine activity or by later sedimentary cover. In the centre of the belt, however, pre-Hercynean crystalline rocks are preserved. To the south is a Mediterranean facies of deep-water Palaeozoic rocks, while to the north lies the broad tract of Devonian and Carboniferous geosynclinal deposits in the Rhine and Ardennes. These are a largely detrital facies, often containing volcanics. The Hercynian cycle proper includes rocks ranging in age from Devonian to Permian. The base of the Devonian is marked by a marine transgression which initiated the geosynclinal phase of deposition.

In Lower Carboniferous times there was a further transgression of the sea onto the Old Red Sandstone continent, while to the south, sandy and shaly culm sediments were laid down in the geosyncline. At the end of the Lower Carboniferous there was a phase of deformation and metamorphism leading to the formation of a new mountain range known as the Hercynides, running east-west through central Europe. To the north, in Upper Carboniferous times, sediments accumulated on the foreland and in a narrow foredeep zone between the foreland and the Hercynides. In the last stages of the Carboniferous, coal was deposited in narrow basins in the foredeep and also within the new mountain range to the south. It is the thick deposits of the foredeep that provide the economically important coalfields of southern Wales, northern France, the Namur basin in Belgium, and the Ruhr in Westphalia, Germany. To the northwest lie further coal-bearing rocks laid down in basins on the foreland.

The final stage of the Hercynean activity is the deposition of continental Permian and Triassic sediments, eroded from the newly risen Hercynean mountain chain. These form the New Red Sandstone. Granites were again emplaced at the end of the Hercynean phase in late Carboniferous times. In Western Europe they are associated with an important suite of ore minerals, including tin, tungsten, silver, copper, lead, and zinc.

The Ural belt , which runs north-south and continues north to Novaya Zemlya, is similar to the Hercynean belt in that it was a long-lived mobile belt which underwent several orogenic episodes and finally became stabilized in early Mesozoic times. It separated the Russian and Siberian platforms, and represents the site of a wide Palaeozoic ocean.

A wide range of economically important deposits such as iron, chromium, and aluminium has made it an important industrial region.

The Alpine mobile belt is the youngest orogenic belt in Europe, and was initiated in the early Mesozoic. It overlies the earlier Palaeozoic and late Precambrian mobile belts. The Alpine belt proper stretches, in a series of linked arcs, from the west and east Alps through the Carpathians and Balkanides in the north, and through the Dinarides and Hellenides to the southwest.

The Alpine cycle began with the advance of the Triassic seas over the old Hercynean complexes, and new basins of deposition began to be formed. In the Alpine region, 2-3 km/nearly 2 mi of Triassic quartzites, limestones, and dolomites were laid down; to the north and south of this central region nonmarine sediments and evaporites were deposited.

By the end of the Triassic, marine conditions predominated throughout the whole belt. Deposition of limestones, dolomites, and marls continued through the Jurassic until mid-Cretaceous times. At this point the first major orogenic events began: marine sedimentation ceased in the central part of the belt, and as compression and uplift of the central zone occurred, thick deposits of flysch (unsorted turbidite deposits) were laid down outside the central orogenic zone along with Upper Cretaceous and Palaeogene sediments.

By Miocene times folding, metamorphism, and uplift had brought much of the belt above sea level to form a new mountain chain. As the mountain chain rose, thick deposits of molasse (unsorted continental-type sandstones and breccias) were laid down. There was intense deformation and regional metamorphism within the mobile belt, with the formation of huge nappes, recumbent folds of rock which were often detached from their original root zones and moved for long distances. Late orogenic igneous intrusions are found in the Carpathians and the Hellenides. The Apeninnes in Italy are a still-active branch of the Alpine mobile belt, containing very late orogenic and still-active volcanoes such as Vesuvius, Stromboli, and Etna.

The Alpine belt passes east into the Caucasus, Cyprus, and the Zagros range of Iran. In these areas the main orogenic activity occurred in late Jurassic and early Cretaceous times, and was accompanied by the intrusion of granodiorites and related volcanicity. Widespread uplift occurred in Miocene and Pliocene times.

The Mesozoic and Tertiary cover rocks on the European craton to the north of the Alpine mobile belt are generally less than 1 km thick, except in basins such as the North Sea. They consist of continental sediments at the base - Permo-Triassic sediments derived from the old Hercynean landmass. These were followed by various clays, limestones, ironstones, and greensands in Jurassic and Lower Cretaceous times. A widespread marine transgression in Upper Cretaceous times led to the deposition of the chalk over southern England and the northern part of continental Europe. At the same time the initial rifting of the Atlantic was beginning to the northwest, with the development first of narrow basins of deposition which were rapidly widened by continental rifting and seafloor spreading. There was extensive volcanic activity associated with this rifting and the remains of a vast Brito-Arctic volcanic province can be seen in the Tertiary volcanic rocks of western Scotland and Ireland.

This phase was followed by gradual uplift of the craton, associated with the Alpine mountain-building movements to the south, and sedimentation became restricted to a number of marine and nonmarine basins. During the Pleistocene much of Britain, all of Fennoscandia, and the Alps were covered by the advancing polar ice sheets, and the landforms we now see in these areas are a direct result of this glaciation.