Glaciation

I

Introduction

Glaciation, state of being covered by ice, and the action of that ice as it moves over the surface of the Earth creating characteristic landforms. The impact of glaciation upon the landscape depends on a number of factors, including the kind of ice involved and the way it moves, and the nature of the land over which it moves. Also important are how many times a particular area has been subject to glaciation and how much time has passed since the ice melted. This is because at various times during the Earth's history glaciation has affected much wider areas than it does today, helping to create the landscape in parts of the world which are now ice free.

II

Glaciation in the Past

Ice Ages, periods when the climate was significantly colder and when ice cover expanded greatly, have affected the Earth on numerous occasions. Evidence indicates that major glaciations occurred about 950 million, 750 million, and 600 million years ago, during the Precambrian; about 450 million years ago, during the Ordovician period; about 280 million-290 million years ago, during the changeover between the Carboniferous and Permian periods; and about 15 million years ago, during the Miocene epoch of the Tertiary period. Evidence of the late Ordovician glaciation, for example, is particularly clear in the Sahara desert, where striations and other erosional features cut by ice have been found.

However, the impact of glaciation in relation to contemporary landscapes is most associated with the last 2 million years of Earth history, the geological period known as the Quaternary. The Quaternary is divided into the Pleistocene and the Holocene epochs. The Holocene, in which we live, covers only the last 10,000 years.

A

Glacials and Interglacials

During the Pleistocene epoch, the climate of the world alternated between glacial and interglacial periods, possibly passing through 20 or more cycles. Glacials are periods when average global climates cool and ice sheets expand. Interglacials are the periods between glacials when temperatures rise, and the ice sheets retreat and decay; the Holocene is considered by some scientists to be an interglacial. Climates of interglacial periods have probably been very similar to, or in some cases perhaps a little warmer than, those of today. It is likely that the two major existing ice bodies, the Antarctic Ice Sheet and the Greenland Ice Cap, persisted as stable glaciers during previous interglacials. The primary characteristic of the glacial periods was the growth and decay of two major ice sheets: the Laurentide Ice Sheet, centred on northern Canada, and the Scandinavian Ice Sheet. The expansion of these ice bodies, and of others elsewhere in the world, meant that many areas were glaciated which have no glaciers at present, for example the British Isles.

Our knowledge of the glacial-interglacial cycles is largely based on the very accurate measurement of two isotopes of oxygen, ¹⁸O and ¹⁶O, in the shells of dead foraminifera, predominantly marine-dwelling, unicellular animals, which form part of oceanic plankton. These foraminifera shells have accumulated on the ocean floor where sedimentation has been continuous over millions of years. During glacial periods ¹⁶O is selectively evaporated from the oceans and transferred to the growing ice sheets, and is returned on subsequent melting. Because the carbonate of the shells of living foraminifera contains the two oxygen isotopes in the same ratio as is found in the oceans, the changing ratio of ¹⁸O and ¹⁶O with depth in floor sediment can be determined, allowing an estimation of the volume of the world's oceans, and consequently of glacial ice on land, through time. Measurements are taken from sediment cores obtained from drilling through the ocean bed. One of the best cores was taken from the Solomon Plateau at a depth of about 3 km (1.9 mi) in the Pacific Ocean; it was about 15 m (49 ft) long and covered most of the Pleistocene.

The last glacial period began around 120,000 years ago and ended about 10,000 years ago. The decline of climate and the growth of ice sheets from the previous interglacial to maximum glaciation about 18,000-20,000 years ago was relatively slow. The subsequent melting of the ice sheets and the establishment of modern climate patterns was rapid, occurring over a 10,000-year period. In some ways, however, the Earth is still recovering from the last glacial. The huge weight of the ice sheets depressed the Earth's crust in a process known as isostasy. Since the melting of the ice sheets, the crust has been rebounding in areas such as the Baltic Sea, causing the land to rise (see Sea Level, Changes in).

At present glaciers cover about 14.9 million sq km (5.7 million sq mi) or almost 10 per cent of the Earth's land area. This increased to approximately 44.4 million sq km (17 million sq mi) or 30 per cent of the Earth's land area during the glacial periods. The Laurentide Ice Sheet, for example, is estimated to have covered more than 13.3 million sq km (5 million sq mi), compared with the present area of glacial coverage of 147,248 sq km (56,852 sq mi) in northern Canada; the comparable figures for Scandinavia are 6.7 million sq km (2.6 million sq mi) and 3,810 sq km (1,471 sq mi) respectively. In addition to presently glaciated areas, the parts of the Earth formerly covered by glaciers will have suites of landforms and sediments associated with these much more extensive glaciations. Glaciers also have indirect effects on landscapes, one of the more common of which is river diversion in pre-existing drainage systems. An example is the River Severn in Great Britain, the upper reaches of which were once the headwaters of the River Trent. Because the lower reaches of the Trent were blocked by ice, its headwaters were diverted south, cutting the Ironbridge gorge in Shropshire to connect with the present lower reaches of the river.

During the Pleistocene glacials, ice sheets covered most of North America, as far south as Chicago and New York, northern Europe, including almost all of the British Isles, and north-western Siberia. The only part of Britain which was not glaciated at some time during the Pleistocene lies south of a line running from just north of London to Bristol, including the south-west peninsula. However, at times of glaciation this area, like most of France, would have been tundra, underlain by permafrost and subject to very cold periglacial conditions.

III

Glacier Mass, Balance, and Motion

A glacier is a mass of snow and ice which, if it becomes sufficiently thick, deforms under its own weight and begins to flow. Glaciers are classified into a number of main types. They are: mountain glacier, one which is constrained by topography into a well-defined channel; piedmont glacier, a form of mountain glacier which spreads out into lowland areas; ice cap and ice sheet (also sometimes called continental glaciers), which form domes over the underlying topography and cover large areas; and ice shelf, which occurs when ice forms a floating sheet on the sea in an embayment in a land mass. For details of each type see Glacier.

All glaciers, however, exist in a delicate balance with the local climate. Mass is added to a glacier by precipitation (snow or rain), sublimation (the direct deposition of water vapour on to the ice surface), and condensation. Loss of mass from glaciers is primarily caused by melting and evaporation. These processes occur all over the glacier but in the upper reaches, known as the accumulation area or zone, gain by the addition of snow is greater than loss by melting. In the lower part of the glacier, the ablation zone, melting is high and there is a net loss over the period of a year. Between the two zones is the equilibrium line where the net change is zero. The mass balance of a glacier, the difference between the amounts of accumulation and ablation over the period of a budget year, is a crucial factor determining glacier behaviour. If the balance is positive, a glacier will thicken and advance; if negative, it will tend to thin and retreat.

The activity of a glacier is partly related to the absolute amounts of accumulation and ablation. Where both are high, for example on Vatnajökull in south-eastern Iceland, the glacier will flow relatively quickly compared with glaciers where the absolute amounts are small. The temperature of the ice is another factor which determines the type of flow and the speed of a glacier. In some glaciers, for example the Antarctic Ice Sheet, the temperature is almost everywhere well below melting point and the ice is termed cold. In many glaciers, however, the temperature is so close to melting point that liquid water exists within, or at the base of, the glacier and the ice is termed warm.

Glaciers are composed of discrete crystals of ice and achieve motion either by internal deformation, that is by slippage between or within ice crystals, or by basal sliding. If the ice is warm, up to 90 per cent of movement is likely to occur by basal sliding, with the free water acting as a lubricant, or through regelation creep. This latter process is related to pressure on the base of the glacier, and involves thawing of ice on the upstream (high-pressure) side of obstacles and re-freezing on the downstream (low-pressure) side. Basal movement is virtually zero in cold glaciers since they are frozen to their beds. Warm glaciers, therefore, are likely to be very effective agents of erosion as they slip over their beds, while cold glaciers probably form a protective cover over pre-existing landscapes.

The surface long-profile of a glacier is maintained by the vertical components of its motion, which are towards the bed in the accumulation zone. This will carry rocks or other debris which fall on the surface of the glacier towards the base, where it can be used as a tool of erosion. The vertical component of the glacier's motion in the ablation zone is away from the bed of the glacier and will bring basal material to the surface. Because of this, the ice in the lower sections of many glaciers appears dirty with a heavy sediment load; the snout area is often covered with debris. The motion of the ice is often reflected in patterns of cracks at the surface of the glacier, known as crevasses.