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FIG 5. Landscapes are changed by surface modifications (Chapter 2) and solid earth movements (Chapter 3).
Our next chapter deals with the timescales represented in the landscapes of Southern England and the processes that have been modifying them. Chapter 3 deals with the movements from below – from within the Earth’s crust – that are ultimately creating major landscape patterns.
CHAPTER 2 Time, Process Southern England’s Landscapes (#ulink_77560cca-d3bc-5dd8-9cc3-96fb80e32610)
BEDROCK AND SURFACE BLANKET
WALK AROUND THE COUNTRY IN SOUTHERN ENGLAND and the ground beneath your feet is very rarely solid rock. You are walking over soil made of weathered mineral grains and organic debris, along with other relatively soft and granular materials that make up the surface blanket. Beneath the surface blanket lies solid rock, the bedrock of the landscape.
Bedrock forms the bones of the land. From the colour of the soil, to the elevation of the hills, to the types of vegetation present, the landscape is profoundly influenced by the bedrock underlying it. For example, in Southern England the Lower Greensand (a distinctive layer of bedrock of Early Cretaceous age, see page 26) produces soil water with acidic chemical properties. The Lower Greensand was originally deposited as sand over a period of a few million years, more than 100 million years ago. This layer represents a different environment of deposition from the older sediments on which it lies, and was followed by another change of environment which produced the deposits that lie on top of it. Both the preceding and the following bedrock deposits have alkaline chemical properties. In certain regions the bedrock layers have now been brought to the surface of the landscape by erosion and movements within the Earth. The Greensand is harder than the layers above and below it (largely mudstones) and so is generally more resistant to weathering. In some areas the Lower Greensand lies just below the surface blanket and has resisted the general landscape erosion to form a distinct Greensand ridge running across the countryside, characterised by special vegetation adapted to the acidity of the soils.
It is only in cliffs or at man-made excavations such as quarries that we can see bedrock at the surface in most low-lying areas. By using those areas where the bedrock does outcrop at the surface, and the results of drillings (e.g. for wells), we can discover the types and arrangements of rock below any landscape.
THREE DIFFERENT TIMESCALES
More recent past events tend to be better known and of greater interest than distant past events. Figure 6 is plotted on a logarithmic timescale, so that the most recent times are given more space and greater ages are given less and less space.
FIG 6. Three different timescales, plotted to give more space to more recent events.
For the purposes of this book, we can distinguish three overlapping timescales to help us to understand the landscapes of Southern England:
The bedrock timescale (extending from 542 million years ago to about 2 million years ago)
The Ice Age timescale (covering roughly the last 1 million years)
The last 30,000 years timescale
We shall now review each of these, commenting on the sorts of episodes in each that are important in our exploration of Southern England.
THE BEDROCK TIMESCALE
Figure 7 is a simplified version of a generally accepted geological timescale relevant to the landscapes of Southern England. The names of the divisions are universally accepted in the geological world and, unlike the previous diagram, the passage of time is represented on a uniform (linear) timescale. The divisions have been selected, and sometimes grouped, to help in our analysis of the situation in Southern England, and these have been colour-coded for use in the rest of the book.
FIG 7. Bedrock timescale for Southern England.
The rocks at, or just below, the surface of Southern England range in age over hundreds of millions of years, and most of them were formed long before the present scenery began to appear. At the time of their origin, these rocks were deposited in a variety of different environments, mostly when mud or sand materials were transported into and/or around the seas that existed where England is now. Most of the bedrock of Southern England was formed in this way and is said to be of sedimentary origin. The depositional conditions varied from time to time: the climate varied, the geographical pattern of rising and sinking land movements changed, and the supply of mud and sand brought downstream by rivers changed also. Despite these fluctuations, it is possible to generalise the way that sediment has accumulated over an area the size of Southern England, and to offer a succession of layers of different composition, age and average thickness that can provide a general guide. This is shown in Figure 8.
For each of the Regions (and some of the Areas) discussed in Chapters 4 to 8, a rock column, generalised for that particular area, will show the main bedrock layers. Each column will be coloured using the standard colour codes of this book to represent the ages of the layers.
As an example, we will consider another particularly distinctive layer of bedrock, the Chalk, which ranges between 200 and 400 m in thickness. Chalk is visible quite widely at or just below the surface over perhaps a quarter of the area of Southern England (Fig. 9). Chalk is an easily recognised rock because it is made of very small fragments of lime (calcium carbonate) and is usually brilliant white. It formed from fine-grained limey mud deposited on the sea bed, but through many millions of years of burial below other sediments it has been compressed and altered into the hard rock we recognise today. The Chalk is a result of a unique combination of environmental conditions and the presence of particular algal organisms in the history of evolution. It is only found in northwest Europe, and was only formed in Late Cretaceous times.
The presence of Chalk near the surface in Southern England is almost always linked to the presence of hills and slopes in the scenery, clearly showing that Chalk is a tough material that resists landscape erosion more than most of the other rock types available. The Chalk is also noteworthy because it represents the most recent time when most of Southern England was covered uniformly with soft sediment and a shallow sea: in Late Cretaceous times, except for possible islands in the southwest, there was no emergent land across Southern England.
Like all sedimentary bedrock layers, the Chalk initially formed as flat layers or sheets of sediment, extending widely across the floor of the sea. As will be discussed in the next chapter, these sheets of sediment are generally characteristic of stretching movement episodes in the Earth’s surface. Such movements produce areas of collapsed and low-lying land that can accommodate large volumes of sediment, if it is available.
FIG 8. Generalised succession of the bedrock of Southern England, showing a typical thickness for each layer.
FIG 9. The Chalk and its topography. The darker areas represent the chalk uplands.
However, we do not see the Chalk at or near the surface everywhere across Southern England; instead the Chalk forms narrow bands across the land. This is due to later movements affecting the bedrock layers by folding and tilting them, so that some parts were raised (to be later removed by erosion) and other parts were lowered (Fig. 10, A and B). In the millions of years since the sediment layers were laid down, they have been buried, compacted, deformed by various processes, and finally uplifted to form part of the landscape that we know today (deformation processes are treated more fully in Chapter 3). The Chalk layer has been moved and folded as a result of mild compression or convergence, to form a downfold or syncline between the Chilterns and the North Downs, and an upfold or anticline between the North and South Downs (Fig. 10, C). Later, the central part of the anticline was eroded away to produce the bedrock pattern that we recognise today (Fig. 10, D). The vein-like river valleys visible on the elevated Chalk hills of Figure 9 are evidence of this continuing erosion.
FIG 10. Deposition and folding of the Chalk.
LANDSCAPE MODIFICATION BY RIVERS
Weathering of landscape surfaces and the production of soils by the action of rainwater, air and organisms are important factors in shaping landscapes. These processes affect the bedrock when it is very close to the surface, and most of them weaken the material that they work on. This is particularly so when tough silicate rock minerals are altered to soft clay minerals, which are then easily eroded. Freezing and thawing also works to weaken the bedrock as water in cracks freezes and expands, breaking the rocks into fragments.
Whilst weathering is a widespread and general process, most of the other important landscape processes involve the formation of discrete features that we shall call landforms. Rivers result in the formation of a number of important landforms that are described below.
The most important landforms resulting from river processes are the channels of rivers and streams (Fig. 11). When rain falls onto a land surface some of it soaks into the land (forming groundwater), whilst the remainder runs along the surface, collecting in topographical lows and producing stream and river channels. Today, many of Southern England’s river channels tend to be relatively narrow and shallow – only metres or tens of metres in width and less in depth – so they occupy an extremely small percentage of the area that they drain. However, they are still the dominant agents of landscape change, causing downwards and/or sideways erosion as well as acting as conduits to transport the eroded material out of their catchments.
Most river channels develop a sinuous course, becoming curved (or meandering) to varying degrees, or developing a number of channels separated by islands of sediment (becoming braided). The positions of the curves or islands change with time as sediment is shifted downstream, and the position of a river channel will change with time correspondingly.
Because of their ability to erode material and remove the resulting debris, river channels create valleys. The sides of a river valley are referred to as slopes. When a channel cuts downwards the valley sides generally become steeper and slope material (generated by ongoing weathering processes) moves down-slope towards the channel. The material is transported either as small individual fragments or as larger mass flows. Where down-slope movements involve the collapse of large areas of material, the terms landslip or slump are often used. Slope material is then deposited in the channel and removed downstream by the river.
The simplest valleys result from down-cutting by a river or stream to yield a V-shaped profile in cross-section. The gradient of the valley sides depends on the strength of the material that the slopes are composed of in the face of erosion. Stronger materials are more difficult to erode and remove, and so can form steeper slopes than weaker materials. In some areas, the river channel is unable to form valley slopes as the material is too weak to form a noticeable gradient. In the Areas we will be investigating, it is clear that some of the slopes are largely the result of a particularly strong layer in the bedrock resisting erosion as the landscape has developed.
FIG 11. Landforms of rivers.
As the valley develops, its profile can become more complex. In some cases, slopes appear to have retreated across a landscape some distance from the position in which they were initially created by river down-cutting. A river with a wide valley floor is one of the most obvious examples of this, in which movements of the channel across the floor have caused the slopes to retreat as the valley floor has become wider. In some cases, slopes appear to have retreated over many kilometres from the original valley as numerous collapses of the slope took place.
Overall, therefore, the valley profile and the channel course reflect variations in the strength of the material being eroded, and in the strength and flood pattern of the river. Climate changes are likely to have a major effect on the strength of the river by altering the volume of water flowing through the channels. Additionally, the lowering or raising of the channel by Earth movement effects (see Chapter 3) can affect the evolution of the landscape by river processes. For example, both climate change and the vertical movement of the river channel can initiate the formation of river terraces. Different examples of all these river geometries will be discussed in greater detail in the Area descriptions in Chapters 4–8.
Over millions of years, river down-cutting, slope erosion and material transport tend to smooth and lower landscapes until they approximate plains, unless they are raised up again (rejuvenated) by large-scale Earth movements (Chapter 3) or are attacked by a new episode of channel erosion, perhaps due to climate or sea-level change. Southern England generally has a smoothed and lowered landscape, representing hundreds of thousands of years of this river and slope activity.
The branching, map-view patterns of river channels and valleys are an obvious feature of all landscapes. An approach to understanding how this forms is illustrated by a computer-based experiment (Fig. 12) in which a flat surface (plateau or plain) is uplifted along one of its edges, so that it has a uniform slope towards the edge that forms the bottom of the rectangle shown. Rain is then applied uniformly across the surface, causing the formation and down-cutting of channels that erode backwards from the downstream edge. As the experiment continues, the channels and their valleys extend into the uniform sloping surface by headward erosion, resulting in longer valleys, more branches and a greater dissection of the surface by those valleys.
FIG 12. Model showing upstream erosion by tree-like (dendritic) river patterns. (Provided by Dimitri Lague from the work of A. Crave and P. Davy)
As we consider the various Regions and Areas of Southern England, we will summarise the present-day river patterns of each by simplifying the main directions of drainage involved. We will also give an impression of the present-day relative size of the more important rivers by quoting their mean flow rates as estimated in the National River Flow Archive, maintained by the Centre for Ecology and Hydrology at Wallingford.
It seems surprising that today’s often sleepy southern English rivers have been the dominant agent in carving the English landscape. However, even today’s rivers can become surprisingly violent in what are often described as hundred-or thousand-year floods. Floods in the past were certainly more violent at times than those of today, particularly towards the ends of cold episodes, when melting of ice and snow frequently produced floods that we would now regard as very exceptional.
THE ICE AGE TIMESCALE AND LANDSCAPE MODIFICATION
The most recent Ice Age began about 2 million years ago, and is still continuing in Arctic areas. At various times during this period ice has thickly covered most of northwest Europe. Recent research, particularly measurements of oxygen isotopes in polar icecaps and oceanic sediment drill cores, has revealed much of the detail of how the climate has changed during the current Ice Age. It has been discovered that long cold periods have alternated with short warm periods in a complex but rather regular rhythm. Looking at the last half-million years, this alternation has occurred about every 100,000 years, and this is now known to have been a response to regular changes in the way the Earth has rotated and moved in its orbit around the sun. A closer look at the last million years (Fig. 13) reveals that for more than 90 per cent of the time conditions have been colder than those of today. Warm (interglacial) periods, like our present one, have been unusual and short-lived, though they have often left distinctive deposits and organisms.
FIG 13. The last million years of global temperature change. *the Oxygen Isotope Stages are an internationally agreed numbering sequence to label the succession of climatic cold (even numbers) and warm (odd numbers) episodes.
One of the most important cold episodes (glacials), just under half a million years ago, resulted in the Anglian ice sheet. This was up to several hundreds of metres thick and extended from the north southwards, well into Southern England, covering much of East Anglia and the north London area (Fig. 14). As the ice spread slowly southwards, it was constricted between the Chalk hills of Lincolnshire and those of Norfolk. A wide valley, later to become the Wash and the Fens, was filled with ice to a depth well below that of present sea level. As the ice spread outwards from this valley it dumped the rock material it was carrying, including blocks and boulders up to hundreds of metres across, giving some idea of the tremendous power of the ice sheet. The direct evidence for the presence of an ice sheet is material in the surface blanket called till, or boulder clay (Fig. 15). This often rather chaotic mixture of fragments of rock of all sizes (large boulders mixed with sand and mud) lacks the sorting of the fragments by size that would have occurred in flowing water, and so must have been deposited from the melting of ice sheets.
FIG 14. The Anglian ice sheet.
Much of the rest of the surface blanket that accumulated during the last 2 million years was deposited by the rivers that were draining the land or any ice sheets present. As ice sheets have advanced and retreated, so have the rivers changed in their size and in their capacity to carry debris and erode the landscape. Rivers have therefore been much larger in the past as melting winter snow and ice produced torrents of meltwater, laden with sediment, which scoured valleys or dumped large amounts of sediment. The gravel pits scattered along the river valleys and river terraces of Southern England, from which material is removed for building and engineering, are remnants of the beds of old fast-flowing rivers which carried gravel during the cold times.
There are no ice sheets present in the landscape of Figure 16. The scene is typical of most of the Ice Age history (the last 2 million years) of Southern England, in that the ice sheets lie further north. It is summer, snow and ice are lingering, and reindeer, wolves and woolly mammoths are roaming the swampy ground. The river is full of sand and gravel banks, dumped by the violent floods caused by springtime snow-melt. The ground shows ridges of gravel pushed up by freeze-thaw activity, an important process in scenery terms that we discuss below.
FIG 15. Boulder clay or till, West Runton, north Norfolk.
The present-day Arctic has much to tell us about conditions and processes in Southern England during the cold episodes of the Ice Age. Much of the present-day Arctic is ice-sheet-free, but is often characterised by permanently frozen ground (permafrost). When the ground becomes frozen all the cracks and spaces in the surface-blanket materials and uppermost bedrock become filled by ice, so that normal surface drainage cannot occur. In the summer, ice in the very uppermost material may melt and the landscape surface is likely to be wet and swampy. Ice expands on freezing, and so the continuous change between freezing and thawing conditions, both daily and seasonally, can cause the expansion of cracks and the movement of material, with corresponding movements in the surface of these landscapes. This movement can cause many problems in the present-day Arctic by disturbing the foundations of buildings and other structures.
FIG 16. Artist’s impression of Southern England, south of the ice sheet, during the Ice Age. (Copyright Norfolk Museums and Archaeology Service & Nick Arber)
Remarkable polygonal patterns, ranging from centimetres to tens of metres across, are distinctive features of flat Arctic landscapes, resulting from volume changes in the surface blanket on freezing and thawing (Fig. 17). In cross-section the polygon cracks and ridges correspond to downward-narrowing wedges (often visible also in the walls of gravel pits in Southern England). Thaw lakes are also a feature of flat areas under conditions of Arctic frozen ground (Fig. 18). They appear to be linked to the formation of the polygonal features, but can amalgamate to become kilometres across and may periodically discharge their muddy soup of disturbed sediments down even very gentle slopes.
Not only can these frozen ground processes be studied in Arctic areas today, but they have left characteristic traces in many of the landscapes of Southern England. Some examples from Norfolk are illustrated in Chapter 8 (Figs 306 and 307), and these provide specific examples of the result of ancient freeze-thaw processes on a small scale. However, the more we examine the wider features of present-day landscapes across Southern England, the more it becomes clear that most have been considerably modified by the general operation of frozen ground processes during the last 2 million years. These processes are likely to have been responsible for the retreat of significant slopes and even for the lowering of surfaces that have almost no perceptible slope.
FIG 17. Polygonal frozen ground patterns on the Arctic coastal plain near Barrow, Alaska. (Copyright Landform Slides – Ken Gardner)
FIG 18. Thaw lakes, the larger ones several kilometres long, on the Arctic coastal plain near Barrow, Alaska. (Copyright Landform Slides – Ken Gardner)
THE LAST 30,000 YEARS TIMESCALE AND RECENT MODIFICATION
The timescale shown in Figure 19 covers a period during which various episodes have changed the landscapes of Southern England, creating our present-day world. These episodes include the dramatic rise in sea level and landward movement of the coastline caused by the warming of the climate following the last cold episode of the Ice Age. They also include the progressive changing of the countryside by people, leading up to the domination of some landscapes by man-made features.
FIG 19. Time divisions for the last 30,000 years (Late Pleistocene to Holocene).
The last 30,000 years have been warm, on average, relative to the previous 2 million years of the Ice Age. However, the higher level of detail available in this timescale makes it clear that climate change has not been one of uniform warming during this period. Short periods of colder climate, temporarily involving ice-sheet growth in the north of Britain (sometimes called stadials) have alternated with short periods of warmer climate (referred to as interstadials).
SEA-LEVEL CHANGE
The coastline is the most recently created part of the landscape, and the most changeable. This is due, in large part, to the rise in sea level over the last 20,000 years, since the last main cold episode of the Ice Age (the Devensian). Twenty thousand years ago sea level was 120 m lower than it is today because of the great volumes of water that were locked away on land in the world’s ice sheets (Fig. 20). Land extended tens or hundreds of kilometres beyond the present-day coastline, and Southern England was linked to northern France by a large area of land (Fig. 21). Global climate started to warm about 18,000 years ago (Fig. 13) and the world’s ice started to melt, raising global sea level. The North Sea and the Channel gradually flooded, and Britain became an island between 10,500 and 10,000 years ago. This flooding by the sea is known as the Flandrian transgression and was a worldwide episode.
FIG 20. Graph of sea-level rise over the last 18,000 years.
During the period of most rapid sea-level rise (between 12,000 and 8,000 years ago), areas of low-lying land were swamped and some local features of the coastal scenery moved great distances geographically towards their present positions. The sea cliffs, beach barriers, salt marshes, spits and estuaries that can be seen today have only taken up their present positions over the last few thousand years, as sea-level rise slowed.
In the treatment of the Regions and Areas in the rest of this book, maps are presented that distinguish a coastal flooding zone. This presentation is based on the simplifying assumption that the solid Earth movement of Southern England (i.e. any uplift or subsidence, see Chapter 3) has been very small compared with global sea-level changes. The coastal flooding zone is defined as extending between the submarine contour 120 m below present sea level and the contour 20 m above present sea level, and it can be used to identify parts of landscapes which are likely to have been areas of coastline activity in the recent past. Areas of land with an elevation between present sea level and 120 m below sea level correspond to the land submerged during the last 18,000 years of sea-level rise. Areas lying at, or up to, 20 m above present sea level may have been subjected to coastal processes during the highest sea levels of earlier interglacial periods, such as the Ipswichian (see Fig. 13). The coastal flooding zone also defines areas of land that are most likely to become submerged during predicted future rises of sea level.
FIG 21. Two episodes (17,000 and 12,000 years ago) in the rise of sea level around the North Sea area. (Redrawn and simplified from Current Archaeology207, 2006, Gaffney)
Drowned valleys (Figs 22 and 23) are present on the coastlines of Southern England as a result of recent sea-level rise. Formerly, the rivers draining the majority of these valleys would have transported mud and sand to the sea, where it would have been deposited on the sea bed. However, with the rise in sea level mud and sand are now often deposited in the flooded valleys or estuaries instead, and some have developed carpets of sediment, transported down-valley by rivers or brought up-valley by the sea where tides and storms have been effective.
Coastlines with low seaward slopes and a soft surface blanket and/or bedrock may develop beach barriers when flooded by rising sea level. These barriers are ridges of sand or gravel parallel to the general trend of the coastline (Fig. 24). They are created by the impact of storm waves on the gently sloping and soft landscape. They tend to develop a cap of wind-blown sand which is very vulnerable to storm wave erosion, but may eventually become stabilised by vegetation. Behind the barrier a low-lying area of more sheltered conditions develops and regular flooding at high tide may bring in muddy sediment from the sea that can settle and build up salt marshes.
FIG 22. The drowning of a valley by sea-level rise.
FIG 23. Drowned valley of the Deben, Suffolk, viewed from above the sea off Felixstowe Ferry. (Copyright London Aerial Photo Library)
FIG 24. Cross-section of a beach barrier formed as sea level rises over a very gently sloping landscape.
FIG 25. Beach barrier on Scolt Head Island, Norfolk. (Photograph held at Cambridge University Collection of Air Photographs, Unit for Landscape Modelling)
The aerial photograph of part of Scolt Head Island (Fig. 25) in north Norfolk shows the succession of zones parallel to the coastline typical of a recently flooded, gently sloping landscape. On the beach, coast-parallel ridges and hollows (runnels) have been created during recent storms, and are draining water as the photograph was taken at low tide. The crest of the barrier is capped by wind-blown dunes, which have been stabilised by marram grass, but also shows signs of erosion during recent storms. Behind the barrier are salt marshes, generally sheltered from storm waves and developing tidal channels. The salt marshes are forming around the remains of various sand and gravel spits that date from a landscape before the present beach barrier was there. The far side of the salt marsh is marked by a gently curved sea wall built within the last two centuries to reclaim some land by keeping high tides out. Behind that is the boundary between the present flat seaward zone of young sediment and the older terrain, marked by a complex field pattern that is underlain by Chalk bedrock.
DEVELOPMENT BY PEOPLE
My concern in this book is primarily with natural landscapes, and I will tend to comment on the development by people since the Bronze Age only where this relates to the natural features in an interesting way. However, in reviewing the appearance of the whole of Southern England, I have been struck by an intriguing distinction made by some landscape historians: the distinction between ancient and planned countryside (Figs 26–28). I have based my approach on the discussions offered by Oliver Rackham, ecologist and landscape historian, and these are summarised below.
Ancient countryside (Fig. 26) consists of many hamlets, small towns, ancient farms and hedges (of mixed varieties of shrubs and trees), along with roads that are not straight, numerous footpaths and many antiquities.
Planned countryside (Fig. 27) has distinct villages, much larger than the hamlets, along with larger eighteenth- and nineteenth-century farms, hedges of hawthorn and straight roads. Footpaths are less common and the few antiquities that are present are generally prehistoric.
I have re-examined the same areas used by Oliver Rackham as examples of these two countryside types, and compared the early Ordnance Survey maps with maps of the same area generated by me using the data and methods used in this book (see Chapter 1). The shading and ‘hachured’ patterning used in the earlier maps represents the hills and slopes rather clearly – better than the contour representation used in the present-day Ordnance Survey Landranger maps, although these show man-made features much more clearly. My map representation is a compromise in that it represents elevations and slopes using colours and hill-shading, but also allows the patterns of roads and settlements to be seen.
Oliver Rackham’s conclusion is that many of the distinctive features of planned countryside were created by the general parliamentary enclosure of land during the eighteenth and nineteenth centuries. This involved the wholesale conversion of commonly held land with open fields into enclosed fields awarded to individuals and institutions. Many landscape historians have claimed earlier origins for the difference between ancient and planned countryside, believing that historical and cultural differences in the people who settled and developed the two areas played an important role. Variations in the bedrock geology also seem to be important here. For example, the ancient countryside shown in Figure 26 is underlain by strongly deformed Variscan bedrock that has been eroded into small hills and valleys (see Chapter 4).
FIG 26. Example of ancient countryside at the Devon-Somerset border, near Tiverton, with 1809 and recent mapping compared. (Upper part from Cassini Old Series map 181, copyright Cassini Publishing 2007/www.cassinimaps.co.uk)
FIG 27. Example of planned countryside at the Berkshire-Oxfordshire border, around Didcot, with 1830s and recent mapping compared. (Upper part taken from Cassini Old Series maps 164 and 174, copyright Cassini Publishing 2007/www.cassinimaps.co.uk)
FIG 28. Generalised map distinguishing ancient and planned countryside across Southern England.
In contrast, the planned countryside covered by Figure 27 consists of only gently tilted Mesozoic bedrock that has formed a much flatter and more open landscape.
CHAPTER 3 Movement of the Earth’s Surface from Within (#ulink_b8f1aa59-56b9-58ee-ad3a-c811b38a295b)
WIDESPREAD MOVEMENTS OF THE EARTH’S SURFACE
TO UNDERSTAND THE CHANGES and movements affecting the appearance of the landscape on large scales we need to review some geological systems, especially plate tectonics. Many of the large changes that have created landscapes over long periods of time can now be understood using this discovery.
Knowledge of the processes causing the movement of large (10–1,000 km length-scale) areas of the Earth’s surface has been revolutionised by scientific advances made over the last 40 years. During this time, scientists have become convinced that the whole of the Earth’s surface consists of a pattern of interlocking tectonic plates (Fig. 29). The word ‘tectonic’ refers to processes that have built features of the Earth’s crust (Greek: tektōn, a builder). The worldwide plate pattern is confusing – particularly when seen on a flat map – and it is easier to visualise the plates in terms of an interlocking arrangement of panels on the Earth’s spherical surface, broadly like the panels forming the skin of a football.
Tectonic plates are features of the lithosphere, the name given to the ≈125 km thick outer shell of the Earth, distinguished from the material below by the strength of its materials (Greek: lithos, stone). The strength depends upon the composition of the material and also upon its temperature and pressure, both of which tend to increase with depth below the Earth’s surface. In contrast to the mechanically strong lithosphere, the underlying material is weaker and known as the asthenosphere (Greek: asthenos, no-strength). Note that on figure 30 the crustal and outer mantle layers are shown with exaggerated thickness, so that they are visible.
FIG 29. World map showing the present pattern of the largest lithosphere plates.
Most of the strength difference between the lithosphere and the asthenosphere depends on the temperature difference between them. The lithosphere plates are cooler than the underlying material, so they behave in a more rigid way when subjected to the forces generated within the Earth. The asthenosphere is hotter and behaves in a more plastic way, capable of deforming without fracturing and, to some extent, of ‘flowing’. Because of this difference in mechanical properties and the complex internal forces present, the lithosphere plates can move relative to the material below. To visualise the motion of the plates, we can use the idea of lithospheric plates floating on top of the asthenosphere.
Looking at the surface of the Earth (Fig. 29), the largest plates show up as relatively rigid areas of the lithosphere, with interiors that do not experience as much disturbance as their edges. Plates move relative to each other along plate boundaries, in various ways that will be described below. The plate patterns have been worked out by investigating distinctive markers within the plates and at their edges, allowing the relative rates of movement between neighbouring plates to be calculated. These rates are very slow, rarely exceeding a few centimetres per year, but over the millions of years of geological time they can account for thousands of kilometres of relative movement.
It has proved to be much easier to measure plate movements than to work out what has been causing them. However, the general belief today is that the plates move in response to a number of different forces. Heat-driven circulation (convection) occurs within the mantle, but other forces are also at play. Where plates diverge, warm, new material is formed that is elevated above the rest of the plate, providing a pushing force to move the plate laterally, around the surface of the Earth. At convergent boundaries, cold, older material ‘sinks’ into the asthenosphere, providing a pulling force which drags the rest of the plate along behind it. Deep within the Earth, the sinking material melts and is ultimately recycled and brought back to the surface to continue the process.
FIG 30. Diagram of the internal structure of the Earth.
Knowledge of how tectonic plates interact provides the key to understanding the movement history of the Earth’s crust. However, most people are much more familiar with the geographical patterns of land and sea, which do not coincide with the distribution of tectonic plates. From the point of view of landscapes and scenery, coastlines are always going to be key features because they define the limits of the land; we make no attempt in this book to consider submarine scenery in detail.
The upper part of the lithosphere is called the crust. Whereas the distinction between the lithosphere and the asthenosphere is based upon mechanical properties related to temperature and pressure (see above), the distinction between the crust and the lower part of the lithosphere is based upon composition. Broadly speaking, there are two types of crust that can form the upper part of the lithosphere: continental and oceanic. An individual tectonic plate may include just one or both kinds of crust.
Continental crust underlies land areas and also many of the areas covered by shallow seas. Geophysical work shows that this crust is typically about 35 km thick, but may be 80–90 km thick below some high plateaus and mountain ranges. The highest mountains in Britain are barely noticeable on a scale diagram comparing crustal thicknesses (Fig. 31). Continental crust is made of rather less dense materials than the oceanic crust or the mantle, and this lightness is the reason why land surfaces and shallow sea floors are elevated compared to the deep oceans. Much of the continental crust is very old (up to 3–4 billion years), having formed early in the Earth’s life when lighter material separated from denser materials within the Earth and rose to the surface.
Oceanic crust forms the floors of the deep oceans, typically 4 or 5 km below sea level. It is generally 5–10 km thick and is distinctly denser than continental crust. Oceanic crust only forms land where volcanic material has been supplied to it in great quantity (as in the case of Iceland), or where other important local forces in the crust have caused it to rise (as is the case in parts of Cyprus). Oceanic crust is generally relatively young (only 0–200 million years old), because its higher density and lower elevation ensures that it is generally subducted and destroyed at plate boundaries that are convergent (see below).
Figure 29 shows the major pattern of tectonic plates on the Earth today. The Mercator projection of this map distorts shapes, particularly in polar regions, but we can see that there are seven very large plates, identified by the main landmasses located on their surfaces. The Pacific plate lacks continental crust entirely, whereas the other six main plates each contain a large continent (Eurasia, North America, Australia, South America, Africa and Antarctica) as well as oceanic crust. There are a number of other middle-sized plates (e.g. Arabia and India) and large numbers of micro-plates, not shown on the world map.
Figures 29 and 32 also identify the different types of plate boundary, which are distinguished according to the relative motion between the two plates. Convergent plate boundaries involve movement of the plates from each side towards the suture (or central zone) of the boundary. Because the plates are moving towards each other, they become squashed together in the boundary zone. Sometimes one plate is pushed below the other in a process called subduction, which often results in a deep ocean trench and a zone of mountains and/or volcanoes, as well as earthquake activity (Fig. 32). The earthquake that happened on the morning of 26 December 2004 under the sea off western Sumatra was the strongest anywhere in the world for some 40 years. It seized world attention particularly because of the horrifying loss of life caused by the tsunami waves that it generated. This earthquake was the result of a sudden lithosphere movement of several metres on a fault in the convergent subduction zone where the Australian plate has been repeatedly moving below the Eurasian plate.
FIG 31. Scale diagram comparing average thicknesses of oceanic and continental crust and lithosphere.