What’s Beneath Our Feet?

From the Open University and BBC’s open2net site -

For the past 4,000 years we humans have been making an impact on our natural landscape – whether it be exploiting our natural resources, cutting down trees for building, firewood and to expose agricultural land, or searching for precious metals like gold and silver to make weapons, jewellery and coins. We have even used our landscape to make religious and political statements, like the Cerne Abbas Giant chalk figure.

While we can all understand ‘historical time’, like the Medieval period, the Romans, Saxons and Vikings, which takes us back to about 2,000 years ago; ‘archaeological time’ takes us back even further through the Iron, Bronze and Stone age, Neanderthals and back to the first Homo Sapiens, almost a 100 thousand years ago. But this is nothing, ‘geological time’, measured in millions of years, takes us right back to the beginning of the earth over 4,600 million years ago.

What’s beneath our feet?
Most of us have probably picked up an attractive pebble on the beach, or found an unusual rock in the garden, or maybe your house or local church is made out of a particular stone. Depending on where you live the rocks beneath your feet will be quite different, because they formed at different times and in different environments.

The rocks of Britain provide a physical record of the past and ancient history of our island and they have changed over time according to the climate and the movement of the earth’s plates. They show that the UK has been drifting like a ‘passenger’ on its tectonic plate, beginning somewhere south of the equator and ending up in the northern latitudes where we are now. On the way the UK has formed rocks and sediments typical of those environments. Going out and looking at the rocks in your local area will give you a clue to the ‘geological history’ of the UK. But how do you know what you’re looking at?

We mainly see rocks in cliffs and road cuttings, but rocks also lie below the vegetation and soil in every landscape. Some of these rocks are useful as building stones, others are the source of valuable and precious metals. Rocks can seem to be a permanent feature of our landscape, but in fact they are being created, destroyed and recreated all the time. Over millions of years, volcanoes erupt, mountains are built, these are eroded by water and ice, and the rock fragments are laid down in rivers and on the seabed, only to be crumpled up to form mountains and eroded again, in a continuous cycle of processes that goes on to the present day.

The different rocks making up the surface of the earth form in different ways, and the processes involved leave their mark on the rocks they produce.

At a basic level it’s important to be clear about what is a rock and a mineral.

A “mineral“ is a solid material, formed by natural processes and with a chemical composition that falls within certain narrow limits. Minerals are made up of atoms which are arranged in a regular pattern, so they form ‘crystals’ with characteristic shapes, like cubes, sheets or pyramids. A “rock“ is a solid collection of mineral grains. These may be fragments of crystals or whole crystals and they can be mm to cm in size. A rock may have only one type of mineral, but usually it consists of several different minerals. Look at this rock – it’s made of three different kinds of minerals – black, grey and white crystals:

New rocks are formed where ‘magma’ or molten rock flows out onto the surface of the earth, like lava flows on the volcanoes of Hawaii, but they can also be formed by the weathering and erosion of existing rocks. The earth is a dynamic planet and the rocks are continually being recycled. There are 3 basic types of rock which are produced by three different processes acting to form rocks on the earth.

‘Igneous rocks’ are formed from molten rock that becomes solid when it cools, either on a volcano or deep in the ground in the earth’s crust:

They usually contain crystals. The number and size of crystals depends on how long they took to grow. Rocks which cool slowly, deep underground, grow big crystals, like granite. Rocks which cool very quickly at the surface like lavas, have minute crystals and can even be glassy.

‘Sedimentary rocks’ are made up of grains which have been eroded from other rocks, like igneous rocks. The grains are small rock fragments or individual mineral grains, and are often rounded because they have been transported by water or wind. The grains are laid down as sediments, in layers, like sand on the beach or mud in a river. Over time they get buried, become compacted and cemented into solid rock. Sedimentary rocks can contain fossils of plants or animals which were living at the time the rock was being deposited, or in some cases they are made completely of the fossil skeletons of plants and animals, forming a rock like limestone.

‘Metamorphic rocks’ are existing rocks which have been ‘changed’ or ‘metamorphosed’ by high temperature and pressure, usually after being buried deep within the earth. These rocks are made up of crystals, and are often banded, contain veins and can be flaky or sugary. Metamorphic rocks make up some of our most useful and beautiful building materials, like slate and marble.

Slate is a metamorphic rock with very tiny crystals. It was originally laid down as a soft mud, but it has been recrystallized into a hard, water resistant rock that can be split into thin sheets, making excellent roofing tiles. Marble is formed from limestone, but unlike slate it is not flaky. Marble doesn’t break into sheets like slate, therefore it is a good material for statues, as smooth surfaces can be carved in any direction.

Igneous and metamorphic rocks give us some clues about what is happening deep within the earth, or at the surface, where magma forms new rocks. Sediments give us a record of what has been happening at the earth’s surface over the past thousands and millions of years.

Different sediments are laid down in different environments, with different climates. If we look at the rocks around us they give us a clue to the past ‘geological history’ and climate of Britain. The geology of Great Britain records the passage of our island from south of the equator to its current position.

• In the South East of Britain the rocks are mainly chalk, sandstone and clays. Chalk is a well known type of rock made up of minute skeletons of billions of tiny sea creatures.

• In the North of England, the commonest type of rock is limestone, which is also made up of the skeletons of sea creatures, shells and corals.

Both of these rocks were laid down in the warm seas of a tropical climate, imagine that England once had a climate like the Bahamas has today, with warm seas and coral reefs.

• Parts of Wales, Scotland and Ireland have rocks that indicate another period when the British climate was much hotter than it is today. Red sandstone in these areas was laid down in a hot, arid desert environment.

• At the other extreme, North Wales and the English Lake District have rocks like shales and mudstones, that were formed in deep cold water. Intruding into all these sedimentary rocks are igneous rocks like granite, and we even have ancient volcanoes and lava flows.

Of course our landscape is not only characterised by the rocks that form it, it has also been shaped by the weather. In the ice age, glaciers and great ice sheets carved out deep wide valleys in the landscape and left mounds of gravel and sheets of boulder clay when the ice melted and retreated.

From the OU & BBC’s pages on ’The Big Freeze – from Icehouse to Greenhouse’ -

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!