МИНИСТЕРСТВО ОБРАЗОВАНИЯ И НАУКИ УКРАИНЫ

ОДЕССКИЙ ГОСУДАРСТВЕННЫЙ ЭКОЛОГИЧЕСКИЙ УНИВЕРСИТЕТ

Доклад

На научно-студенческую конференцию

На тему: ”Геология Великобритании”

ВЫПОЛНИЛ

СТУДЕНТ ГРУППЫ Э-14

МАСЛОВСКИЙ МИХАИЛ АЛЕКСАНДРОВИЧ

Одесса,2014

The geology of Great Britain is renowned for its diversity. As a result of its eventful geological history, Great Britain shows a rich variety of landscapes across the constituent countries of England, Wales and Scotland. Rocks of almost all geological ages are represented at outcrop, from the Archaean onwards. Seismographical research shows that the crust of the Earth below Great Britain is from 27 to 35 km (17 to 22 miles) thick. The oldest rocks are found at the surface in north west Scotland and are more than half as old as the planet. These rocks are thought to underlie much of Great Britain and Ireland (although boreholes have only penetrated the first few kilometres), but next appear extensively at the surface in Brittany and the Channel Islands. The youngest rocks are found in south east England. The bedrock geology consists of a complex mix of generally older metamorphic rocks overlain by varying sequences of sedimentary rocks, into both of which igneous rocks have been intruded at different times. The complexity of Britain's geology is due ultimately to its being subject to a variety of plate tectonic processes over a very extended period of time. Changing latitude and sea levels have been important factors in the nature of sedimentary sequences, whilst successive continental collisions have affected its geological structure with major faulting and folding being a legacy of each orogeny (mountain-building period), often associated with volcanic activity and the metamorphism of existing rock sequences. Overlain on this bedrock geology is a varied distribution of unconsolidated material of more recent origin. It includes material deposited by glaciers (boulder clay, and other forms of glacial drift such as sand and gravel). "Drift" geology is often more important than "solid" geology when considering building works, drainage, siting water boreholes, sand and gravel resources and soil fertility. Although "drift" strictly refers to glacial and fluvio-glacial deposits, the term on geological maps has traditionally included other material including alluvium, river terraces, etc.

Geological history

Archaean eon

The Lewisian gneiss, the oldest rocks in Great Britain or Ireland, date from at least 2,700 Ma (Ma = million years ago) in the Archaean eon, the Earth itself being about 4,600 Ma old. They are found in the far north west of Scotland and in the Hebrides, with a few small outcrops elsewhere. Formed from rock originally deposited at the surface of the planet, the rocks were later buried deep in the Earth's crust and metamorphosed into crystalline gneiss.

Proterozoic eon

South of the gneisses are a complex mixture of rocks forming the North West Highlands and Grampian Highlands in Scotland. These are essentially the remains of folded sedimentary rocks that were originally 25 km thick, deposited over the gneiss on what was then the floor of the Iapetus Ocean. The process started in about 1,000 Ma, with a notable 7 km thick layer of Torridon Sandstone being deposited about 800 Ma, as well as the debris deposited by an ice sheet 670 Ma.

Palaeomagnetic evidence indicates that 520 Ma, what is now Great Britain was split between two continents, separated by 7,000 km (4,300 mi) of ocean. The north of Scotland was located at about 20° south of the equator on the continent of Laurentia near the Tropic of Capricorn, while the rest of the country was at about 60° south on the continent of Gondwana near the Antarctic Circle.

In Gondwana, England and Wales were near a subduction zone. Both countries were largely submerged under a shallow sea studded with volcanic islands. The remains of these islands underlie much of central England with small outcrops visible in many places. Around 600 Ma, the Cadomian Orogeny (mountain building period) caused the English and Welsh landscape to be transformed into a mountainous region, along with much of north west Europe.

Cambrian period

In the early Cambrian period the volcanoes and mountains of England and Wales were eroded as the land became flooded by a rise in sea level, and new layers of sediment were laid down. Much of central England formed a stable block of crust which has remained largely undeformed ever since. Sandstones were deposited in the north of Scotland. The first animals with hard shells evolved at this time, consequently, fossils become much more common in rocks formed during this and later periods.

Ordovician period

500 million years ago, in the Ordovician period, southern Britain, the east coast of North America and south-east Newfoundland broke away from Gondwana to form the continent of Avalonia, which by 440 Ma had drifted (by the mechanisms of plate tectonics) to about 30° south.

During this period north Wales was subject to volcanic activity. The remains of these volcanoes are still visible, one example of which is Rhobell Fawr dating from 510 Ma. Large quantities of volcanic lava and ash known as the Borrowdale Volcanics covered the Lake District and this can still be seen in the form of mountains such as Helvellyn and Scafell Pike.

The Ordovician also saw the formation of the Skiddaw slate deposits around 500 Ma.

Silurian period

Deposition continued into the early part of the Silurian period, with mudstones and sandstones being laid down, notably in Wales.

Avalonia had now joined with the continent of Baltica, and the combined landmass collided with Laurentia at about 20° south between 425 and 400 Ma, joining the southern and northern halves ofGreat Britain together. The resulting Caledonian Orogeny produced an Alpine-style mountain range in much of north and west Britain. The continental collision was probably at an oblique angle rather than a head-on collision, and this probably led to movement along strike-slip faults trending north-east to south-west across Scotland, the Great Glen Fault being the best example (some of these fault zones may have been old lines of weakness from earlier earth movements).

Volcanic ashes and lavas deposited during the Silurian are still found in the Mendip Hills and in Pembrokeshire.

Devonian period

The collision between continents continued during the Devonian period, with continuing uplift, and more volcanic deposits such as those now forming Ben Nevis. Sea levels varied considerably, with the coastline advancing and retreating from north to south across England. The uplifted region was gradually eroded down, resulting in the deposition of numerous sedimentary rock layers in lowlands and seas. The rocks of marine origin in the county of Devon gave the period its name, though deposits of this age are found in many other places, such as the Brecon Beacons, the Midland Valley of Scotland, and the Orkney Islands. Most of these are of terrestrial origin and are informally known as the Old Red Sandstone.

The Caledonian mountains had largely been eroded away by the end of the period during which the country would have experienced an arid desert climate as it was located close to the equator at between 10° and 15° south.

Carboniferous period

Around 360 Ma at the start of the Carboniferous period Great Britain was lying at the equator, covered by the warm shallow waters of theRheic Ocean, during which time the Carboniferous Limestone was deposited, as found in the Mendip Hills, north and south Wales, in thePeak District of Derbyshire, north Lancashire, the northern Pennines and southeast Scotland. Caves developed more recently in the limestone of some of these areas by the action of carbonic acid and other organic acids in rainwater and groundwater.

These were followed by dark marine shales, siltstones, and coarse sandstones of the Millstone Grit. Later, river deltas formed and the sediments deposited were colonised by swamps and rain forest. It was in this environment that the cyclic Coal Measures were formed, the source of the majority of Britain's extensive coal reserves that powered the Industrial Revolution. Coal can be found in many areas of Britain, as far north as the Midland Valley of Scotland, as far south as Kent, although coal mining has largely been concentrated in the English Midlands, northern England and Wales.

Throughout the period, southwest England in particular was affected by the collision of continental plates. The mountain building period known as the Variscan orogeny, which occurred around 280 Ma, caused major deformation in south west England. Towards the end of this period granite was formed beneath the overlying rocks of Devon and Cornwall, now exposed at Dartmoor andBodmin Moor, giving rise to mineralised deposits of copper and tin. The general region of Variscan folding was south of an east–west line roughly from south Pembrokeshire to Kent. The maintectonic pressure was from the south or south-east, and may have resulted in dextral strike-slip faulting. The Devon-Cornwall massif may originally have been some distance further east, then to be moved westwards. Lesser Variscan folding took place as far north as Derbyshire and Berwick-upon-Tweed.

By the end of the Carboniferous period the various continents of the Earth had fused to form the super-continent of Pangaea. Britain was located in the interior of Pangea where it was again subject to a hot arid desert climate with frequent flash floods leaving deposits that formed beds of red sedimentary rock, somewhat similar to the later, Triassic New Red Sandstone.

Permian period

The Permian was characterised for 30 million years by arid desert conditions and the erosion of the land that had uplifted in the Variscan Orogeny, southwest England and adjacent areas of the present-day English Channel. Later, much of Great Britain was submerged in shallow waters as the polar ice sheets melted and the Tethys Ocean and Zechstein Sea formed, depositing shale,limestone, gravel, and marl, before finally receding to leave a flat desert with salt pans.

Mesozoic era

Triassic period

As Pangaea drifted during the Triassic, Great Britain moved away from the equator until it was between 20° and 30° north. Red beds, including sandstones and red mudstones form the main sediments of the New Red Sandstone. The remnants of the Variscan uplands in France to the south were eroded down, resulting in layers of the New Red Sandstone being deposited across central England, and in faulted basins in Cheshire and the Irish Sea. A basin developed in the Hampshire region around this time. Rifting occurred within and around Britain and Ireland, prior to the breakup of the super-continent in the Jurassic period.

Rock fragments found near Bristol appear to indicate that in 214 Ma Great Britain was showered with a fine layer of debris from an asteroid impact at the Manicouagan Impact Crater in Canada, although this is still being debated.