From Icehouse to Greenhouse
Almost all the present day landscape features of the British Isles were shaped during the last 2 million years. Most of us think of this period of time as the ‘ice age’, and there were indeed some very cold spells! But it was not continuously cold. The ice age is not a single age at all, but a series of cold periods separated by times when the climate was as warm as today, or even warmer. Sedimentary rocks deposited throughout this time show us that the climate was quite varied, swinging from cold ‘glacial periods’ with widespread ice, to warmer, temperate, humid and semi-dry spells which we call ‘interglacial periods’. The transitional time when the climate is switching from one extreme to the other is called a ‘periglacial period’, during which conditions were characterised by no permanent ice, but ground frozen solid all year round, like the tundra on the Russian Steppes today.
Why is the ice age so important and how did it begin?
Ice develops at the Earth’s poles when the Polar Regions become isolated from the Earth’s main atmospheric and oceanic currents, which cycle warm air and water up from the equator and cold air and water back down from the poles. The South Pole or Antarctica has had a permanent ice cap since about 38 million years ago, when the movement of the Earth’s plates split it away from Australasia, South America and India. The ice age is special because it is the only time when both the South and the North Pole were, and still are, covered by ice - we are still in an icehouse period right now.
In the northern hemisphere the spread of polar ice began about two and a half million years ago, when the passage of water between North and South America was cut off. This caused a whole new ocean current system to develop in the Atlantic, like the Gulf Stream which helps keep the British climate so wet and mild. At the same time water in the Bering Strait between Alaska and Asia became very shallow and at times cut off. This prevented the circulation of water from the warmer Pacific Ocean and the cold Arctic Ocean. So, with the North Pole now cut off from any warming influence, the climate in the northern hemisphere began to deteriorate. The first major glaciation in Europe was about two and a half million years ago when many of our local or indigenous trees became extinct. After that the ice sheets and glaciers spread out rapidly and began to carve out their imprint and shape our present landscape.
What is glacier ice and how does something so solid and heavy move?
Bodies of ice are the major stores of the Earth’s fresh water. If all the ice on our planet were to melt and flow into the oceans, the world sea level would rise by about 70m. Over time ice sheets expand and shrink, responding to changes in the environment and therefore they can give us valuable records of climate change.
Glacier ice is not the same as you would make in your ice tray in the refrigerator at home. To start with it does not begin as liquid water. Glacier ice begins life as snow. Fresh snow is a mass of fragile ice crystals, much lighter than water and far more delicate. As the snow lies on the ground, melting and re-freezing changes the delicate filigree crystals into round solid crystals. We call this melted, compressed snow 'névé':
Névé is much denser than real snow because all the ‘branches’ have been knocked off the original snow flakes. After about a year névé compacts into still denser snow called ‘firn’, which has very small rounded ice crystals. If you bury and squash firn even more it forms glacier ice. The whole process can take from 25 to 1,000 years.
How does solid glacier ice move?
When ice on a slope gets thick enough it begins to deform and spread outwards under its own weight. Gravity takes over and the glacier moves downslope. The ice crystals in the glacier start to slide along internal planes, like playing cards in a deck slide over each other. At the same time the ice crystals recrystallize into new shapes, moving downslope all the time.
The size (or mass) of a glacier changes as the environmental conditions change - if there is more snow to be turned into ice, the glacier will get bigger. On the other hand, in warmer periods there will be less snow, so the glacier will melt and its front end will retreat backwards. Glaciers and ice sheets stop spreading out when they meet the ocean. This is because they start ‘calving’ or breaking off into the sea forming icebergs and ice floes.
As glaciers move they carve out the landscape. They scoop up and remove soil and weathered rock fragments as they travel across the surface. They flow because the weight of the overlying ice causes the bottom layer of ice to melt, this liquid layer then freezes onto bedrock and plucks out bits of rock as the glacier moves forward, all the time melting and re-freezing at its base. The rock fragments are trapped in the ice as it moves, acting like coarse sandpaper, grinding out even deeper crevices and valleys. This is called ‘scouring’ – it’s a process that can form all kinds of features, like glacial ‘striations’ or scratches on the rocks, which show where the ice sheet passed over them.
Glaciers especially follow pre-existing river valleys, but they make them their own by turning them into wide ‘U’ shaped valleys:
The rocks and fragments that are carried along in the base of the ice are called the ‘load’ of the glacier. This consists of boulders, pebbles, gravel and very finely ground up rock called ‘rock flour’. The load cannot be sorted out according to size like a river would do, so it is deposited all mixed up together when the glacier melts. It is either plastered on the ground or released at the glacier margins in humps and lumps which we call ‘moraine’ – you may have heard it described as ‘boulder clay’.
Ice sheets are also capable of transporting huge boulders over great distances. When the ice melts the boulders are left behind, often in regions where the local rocks are quite different. They are called ‘erratics’ – look out for them in your local area.
What happens next?
Scientists have speculated on whether we are really out of the ice age, or if we are simply in an interglacial warm period. If this is the case, when will our present interglacial end? Most interglacials last for about 11,000 years. Our current interglacial has been going on for about 10,000 years, so perhaps the end is not far away. Using evidence from the past, it could end abruptly with rapid fluctuations between warm and cold conditions, and then prolonged cold. But what about ‘global warming’, won’t that help to keep us warm? No! In fact it might have the reverse effect. Global warming could cause more rain in the northern latitudes, which could lower the salinity of the surface sea water in the northern Atlantic. This would shut down the oceanic circulation which brings warm water northwards from the equator ... the result - rapid cooling of our climate and another big freeze!