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In 1755, the German philosopher Immanuel Kant proposed that a slowly rotating cloud of gas, now termed a nebula, condensed into a number of discrete globular bodies. By this nebular hypothesis he explained the consistency of revolution of the planets about the Sun, and the rotation directions of the planets and the Sun as a legacy of the rotation of the parent nebula. The French mathematician Laplace proposed essentially the same theory in 1796. According to Kant and Laplace, the original mass of gas cooled and began to contract -which increased its rotational speed until successive rings of gaseous material were spun off from the central mass by centrifugal force which eventually condensed into planets.

This theory was refuted 100 years later by two British physicists, James Clerk Maxwell and Sir James Jeans, who showed that there was not enough mass in the rings to provide the gravitational attraction for condensation into planets. Also at the close of the nineteenth century, American astronomer F.R. Moulton discounted the earlier ideas of Kant and Laplace in that in their theory the sun, which collected most of the mass, should have gathered up most of the angular momentum of the system and be rotating faster than it actually does.

Geologist T.C. Chamberlain and Moulton, both of the University of Chicago, worked on the collision hypothesis of the origin of the planets, which was a revival of an earlier theory, proposed in 1749 by the Frenchman Count Buffon. This hypothesis proposes that tongues of material were wrenched from the pre-existing Sun by the gravitational attraction of a passing star. These small planetismals went into orbits around the Sun in the same plane as the passing star, and by collision and gravitational attraction agglomerated into larger planetismals t6 finally become planets. Collision theories of this nature have, according to many astronomers, fundamental flaws in that material, from the Sun would probably, have dispersed through space with explosive violence rather than condensed into planets. Also the vastness of space makes the probability of two stars passing so close fairly remote.

More recent astronomical observations have detected in interstellar space and in nebulas the existence of rarefied matter consisting of gases and small amounts of dust. The gases are mostly hydrogen and helium, whilst the dust particles have similar compositions to terrestrial materials, such as silicon compounds, iron oxides and ice crystals. As a result of these observations recent theories are related to that of Laplace in that they revive the idea of a rotating cloud of gas and dust which flattened into a disk where matter began to drift towards its centre to accumulate as the primitive Sun. The proto-Sun then collapsed under its own gravity, along with the nebula to form the Sun and the planets, possibly triggered by a nearby supernova. This collapse caused the internal temperature of the proto-Sun to rise, initiating a thermonuclear reaction which released huge amounts of energy; in other words the Sun began to shine.

A number of questions remained unanswered. How did the planets form from the disk of gases around the sun, how did they get their angular momentum and why are they of differing compositions? Although there is little overall agreement to the questions above, one theory called the chemical-condensation-sequence model seems to predict the variation in chemical composition and density of the planets. At first the nebula disk was hot and composed of gases, but as it cooled solid compounds and minerals condensed out of the gas that gradually agglomerated to form planetismals. Planets grew by accretion of planetismals, but if it grew close to the Sun that it was too hot for certain materials to condense, those gases would be blown away by radiation. Near to the Sun, where temperatures were highest, the first materials to condense out were those with high boiling points and densities. Therefore Mercury, the planet closest to the Sun, is the densest. The lighter, rock-forming compounds, such as magnesium, silicon and oxygen, condensed more readily in cooler environments to form the terrestrial planets further from the Sun. Easily evaporated materials condensed in the cold outer reaches of the solar system to form the giant planets.

The Earth as an Evolving Planet

Although the precise mechanisms involved in the origin of the solar system remain largely conjectural, the accumulation of planet Earth was probably triggered by accretion of planetismals about 4.7 billion years ago. The newly formed planet was probably an unsorted agglomeration of silicon compounds, iron and magnesium oxides, and smaller amounts of all the other natural chemical elements. Although the planetismals were relatively cold, three different mechanisms began to heat up the growing planet. In the process of initial planetary accretion, the newly forming Earth was bombarded by planetismals and their energy of motion was converted to heat. Although much of this heat was radiated back to space, a fraction was retained by the growing planet. Gravitational compression of the Earth into a smaller volume also caused its interior to heat up, because the energy expended in compressing the interior was converted to heat. This heat did not flow out of the interior, but accumulated because heat is conducted only very slowly in rocks. The Earth also heated up by the slow decay of radioactive elements, releasing particles and radiation which became absorbed by the surrounding rocks, heating it. The disintegration of radioactive elements is a heat source that has persisted for billions of years, the heat generated warmed the newly formed Earth and initiated planetary differentiation, where a distinct internal layering was initiated.

Although the timing of the event may differ, it is generally accepted that by 1 billion years after the Earth was formed, internal heating would have elevated the temperature at depths of 400-800 km to the melting point of iron. When iron began to melt, it began to 'fall' towards the centre of the planet, because it is heavier than the other common elements of the Earth, so displacing lighter materials upwards. Iron accounts for about 1/3rd of the mass of the Earth, and the melting and sinking of iron to form a liquid core released a large amount of gravitational energy which was converted to heat sufficient enough to raise the temperature by some 2000'C, causing a large fraction of the Earth to melt.

Differentiation of the Earth

After the Earth warmed to the melting point of iron, it underwent a reorganization in that approximately 1/3rd of the Earth's primitive material sank to the centre and in the process a large part of the body was converted to a partially molten state. Formation of an iron-based core was the initial stage of differentiation of the Earth, converting it from a homogenous body to a dense iron-nickel core, which can itself be divided into a solid inner core (4980-6370 km depth) and a liquid outer core (2900-4980 km); and a surficial crust of relatively light material, divided into a thicker less dense continental crust (0-40 km), and a thinner but heavier oceanic crust (0-10 km). Between the crust and core is a residual mantle. The crust and upper mantle can be differentiated into two zones which are important in explaining many geological phenomena; comprising an outer rigid and strong lithosphere (0-70 km) underlain by a partially molten, weak asthenosphere (70-250 km). Beneath the asthenosphere is a transition zone (350-700 km) and the lower mantle (700-2900 km). These lower layers in the mantle have also been termed the mesosphere.

According to the best estimates, initial differentiation took place between 3.7 and 4.5 billion years ago. Differentiation probably also initiated the escape of gases from the interior which lead to the formation of the primitive atmosphere and the oceans. Flow of heat to the Earth's surface became more efficient after the interior was transformed into a molten state due to the development of convection cells which began to dissipate heat rapidly, quickly cooling the Earth. The mantle solidified but the core has largely remained molten. Convective overturn also produced a chemically zoned Earth. About 90% of the Earth is made up of four elements, iron, oxygen, silicon and magnesium, but these elements are unevenly distributed which is what is meant by chemical zonation. Differentiation of the planet was influenced primarily by the relative abundances.of the elements and the compounds they formed. Differentiation did not lead to a -vertical arrangement of the elements based entirely on relative weight because the various elements formed compounds, and it was the chemical and physical properties of these compounds that governed the distribution of elements. Calcium, sodium, potassium and aluminium silicates (feldspars) melt at temperatures as low as 700- 1000 degrees C and when molten are relatively light. They would have risen early to the surface by convection to form the most common minerals in the Earth's crust.

The mantle became the, reservoir for iron and magnesium silicates which are heavier and melt less easily, the principal minerals of the mantle are believed to be olivine and pyroxene. Many heavier elements probably sank to the Earth's core, but other radioactive elements, such as uranium and thorium, have a strong tendency to form oxides and silicates which are light and could rise to the crust. Before chemical differentiation, these radioactive elements Were evenly distributed, but as part of the process, they became concentrated in the outer layers where the heat that was generated by their decay could be more effectively conducted the shorter distances to the Earth's surface and be lost more easily acting to slow down the operation of the Earth's heat engine.

Formation of the Continents, Oceans and Atmosphere

The continents may have formed by lava from the interior of the Earth spreading over the surface to form a thin 'protocrust'. This crust melted and solidified repeatedly separating the lighter compounds which became distributed at the top. Weathering and erosion also modified the protocrust to produce primitive continents. The continents began to grow initially after core-mantle differentiation and it is generally accepted that it was nearly complete by about 2.5 billion years ago.

The oceans were a product of heating up and differentiation, water being released and carried to the surface along with lava, much of the water escaped as hot vapour clouds. The primitive atmosphere was the product of outgassing which was a part of the process of differentiation. The volcanic gases consisted mainly of water vapour, hydrogen, hydrogen chloride, carbon monoxide, carbon dioxide and nitrogen. Much of the hydrogen would have escaped to space as it does today, whilst some of the water vapour would have broken down to hydrogen and oxygen by the action of sunlight. Much of the oxygen formed in this way would not have remained free but combined with other gases and metals to form new compounds. Significant amounts of free oxygen in the atmosphere probably occurred only after life had evolved at least to the complexity of green algae, as a by-product of photosynthesis. Oxygen could not have accumulated in the atmosphere until its production exceeded its loss by chemical combination with other gases and metals.

Internal Motion of the Earth

Before the late 1960's there we re various disparate theories of mountain building, volcanism and other major geological phenomena, though no single theory was generalized enough to satisfactorily explain the entire range of geological phenomena. Since that time, geologists have generally accepted an all encompassing concept which seems to interpret many global geological and geomorphological features, especially the distribution and characteristics of volcanic activity, -seismic belts and mountain building, whilst the disposition of other global topographic features such as the deep ocean basins and trenches can also be explained by the theory. The concept of plate tectonics supposes that the outer rigid layer of the planet, the lithosphere, is riding, conveyor belt style, on a weaker, partially molten asthenosphere. The continents are raft-like inclusions of thicker, lighter crust at the top of the lithosphere, with only a thin oceanic crust elsewhere. The underpinning theory is that the lithosphere is broken into a number of rigid plates, where each plate moves as a distinct unit relative to others, driven by convection cells within the upper mantle. Plates may spread apart from each other along divergent margins, typified by a rift characterized by earthquake activity and volcanism. The gap between the spreading plates is filled by the addition of new molten material that extrudes from below the lithosphere and leads to sea floor spreading at divergent plate margins beneath the oceans.

The relative motion of two plates may also lead to convergence where the plates grind together. There is a profusion of geological phenomena associated with converging plate margins, such as crumpled mountain ranges, deep-sea trenches, shallow and deep-seated earthquakes and volcanic activity. This array of features is related to t he different types of convergent plate margin and whether the leading edges of the plate are formed of oceanic or continental crust. Where at least one of the converging plates is composed of heavy oceanic lithosphere, a subduction zone is formed such that one of the plate edges forms a slab which descends into the underlying asthenosphere. If both plate edges are composed of lighter continental crust, neither will subduct but a collision zone will be formed of intense buckling and folding.

Regions of convergence where subduction is taking place causes downbuckling at the plate margin to form deep-sea trenches, whilst the edge of the overriding plate is often crumpled to form mountain ranges and is the site of volcanic and seismic activity. Subduction zones are sinks in which lithospheric material is being consumed, and are also sites where rock and sediment are squeezed and heated. At plate margins where one plate subducts into the hot asthenosphere, parts of it begin to melt and become assimilated due to intense heat and pressure. This melting leads to the production of magma which tends to 'float' upwards through the overriding plate, some of it reaching the Earth's surface to erupt as lava and pyroclastic material at volcano vents, whilst much is intruded into the overriding plate to form igneous intrusions which adds to the crustal material. Plates can separate and collide, but they can also slide past each other at conservative plate margins, or transform faults, where material is neither being created nor destroyed.

Motions of the plates are related to how the Earth generates and gets rid of its heat. They are a product of the general pattern of the heat engine's work output, but the specific dynamics are yet to be fully understood by geologists. We can only infer that the internal heat engine probably drives convection currents within the upper mantle, causing movements of the rigid lithospheric skin of the Earth, movements which have been responsible for the generation of many global topographic features over extended geological time.

The Earth's external heat engine, powered by solar radiation, has also been responsible for the modification and formation of landscapes. As soon as the proto-atmosphere formed by outgassing, the external heat engine began to grind out the products of weathering and erosion to modify the newly formed continents.

How does the Earth's Landscape change?

Once the primitive continents, oceans and atmosphere developed how did the Earth's landscape change and evolve further? The energy required for landscape change since the initial formation of the continents, oceans and atmosphere has been derived from four principal sources; geothermal heat, solar radiation, rotational energy of the solar system, and gravitational attraction.

The Earth's interior is a heat engine fueled by radioactivity and producing geothermal heat. This internal heat drives convection cells in the mantle below the more rigid crustal surface. These convection currents are considered to be the mechanism by which rigid plates of rock are separated, pushed together or rotated, causing great rifts in the crust where the plates separate, or high mountain chains where they collide. Plate motions also give rise to earthquakes and a high heat-flow towards the surface, especially along the plate boundaries, which produces volcanoes. The major source of geothermal heat is through the radioactive decay of long-lived isotopes of uranium, thorium and potassium. About 83% of the geothermal heat produced can be attributed to isotope decay, whilst the remainder comes from Earth cooling since its formation 4.7 billion years ago. Geothermal heat flux has remained, relatively constant over the last several hundred million years as half-lives of common isotopes decay at similar rates (10x(9) - 10x(11) years ). This internal energy drives landscape forming processes which are said to be endogenetic and is the ultimate source of energy for seismic, volcanic and diastrophic ( horizontal and vertical movements of the Earth's crust ) activity.

A second major source of energy for landscape change comes from solar radiation which drives the Earth's atmosphere by setting up convection currents within it to generate the hydrological cycle, continuous movement of water between and within the atmosphere, oceans and land surface. Solar radiation drives exogenetic landscape forming processes. An additional energy source for landscape change comes from the momentum of the Earth's rotation and the gravitational attraction of the Sun, Moon and Earth which leads to the occurrence of tidal forces which are most noticeable in water bodies and affect especially coastal landscapes. Gravitational forces also provide energy less directly, but attract all earth material towards its centre, so imparting a potential energy to the rock and soil.

These principal energy sources are not constant, especially the external processes which are influenced by environmental and climatic changes, whilst geothermal heat flow has not been uniform throughout geological time. Therefore landscape forming processes have not always proceeded with the same intensity and distribution as they do at the present time. Changes in the intensity and distribution of processes may leave an imprint on the land surface as relict landforms or deposits and shows that the modern landscape has a history which has at least in part influenced its form.

Therefore the landscapes of the Earth represent the net effect of two sets of natural forces which seemingly constantly act against each other. Endogenetic processes driven by geothermal heat may cause the injection of new material; into the crust, or the spilling of molten magma onto the surface to form volcanoes and lava flows; and lead to earth movements producing large scale uplift, warping and folding. Endogenetic processes are generally constructional in that they lead to an increase in elevation and relief. They provide much of the initial relief of the landscape and are said to be responsible for the production of primary landforms. Primary landforms are modified by exogenetic processes leading to denudation of the landscape and powered by the agents of weathering and erosion, namely water, wind, ice and gravity. Landforms modified and formed by exogenetic processes are referred to as secondary landforms.

With the formation of the primitive continents and the initiation of plate movements after chemical differentiation and cooling of the surface to form a rigid lithosphere, the Earth became very variable topographically. On Earth today, the oceans occupy 70.8% of its surface, whilst the remainder is land. The most striking aspect of the form of the solid surface of the Earth is the dominance of two distinct levels. About 30% ties between +2000m and -200m, called the continental level, and another 50% lies between -3000m and -6000m, or the oceanic level. The intermediate slopes between these two levels, the high mountain chains and the ocean deeps are of very much smaller extent. Only 1.6% lies above +3 000m and about I% lies below -6000m, giving the Earth a maximum relief of some 20km, which is about 50% of the average thickness of the continental crust.

This simple division of relief is complicated by the fact that a part of the continental level is submerged, which means that the present shoreline is of little importance topographically. The outer edge of the continental shelf is more significant in terms of global topography, About 34.8% of the solid surface of the Earth lies between the high mountains and the edge of the continental shelf, which is closely related to the 34% of the global surface composed of continental crust. A fundamental topographic distinction, based on endogenetic processes, could be between the deep ocean basins and the continents. However, in order to distinguish between subaerial and submarine environments a three-fold division of global topography is commonly recognized, in the form of continents, continental margins and ocean basins.

The continents display a wide variety of relief and topography, but for simplification we can subdivide it into two simple elements. Fold mountains are curvilinear belts of pressure associated with a range of geological processes and can be themselves roughly subdivided into younger and older groups. The young fold mountains include the highest elevations on the planet and are tectonically unstable. The older. group occupy medium scale elevations and are more stable tectonically. Continental platforms are generally 'plain' areas of low relief and elevation and often occur in central continental locations. They are commonly composed of ancient igneous and metamorphic rocks, overlain by thick accumulations of largely undeformed sedimentary strata. Core areas of continental platforms, called shields, have been tectonically stable for very long periods of geological time and often display ancient landforms.

The submerged continental margins can be subdivided into shelf, slope and rise. The continental shelf is a gently sloping topographic feature which has an average width of 78km, but is at its widest opposite large rivers. The continental slope is a distinct relief feature which marks the outer edge of the continents and descends down to the -2000m isobath.. Below is the continental rise which is formed of coalescing submarine fans of sediment which slope at very gentle gradients to depths of -2.5 to -5km. In places, especially opposite large rivers, the continental shelf and slope are cut by submarine canyons, formed through erosion by density and turbidity currents. These canyons furnish the material for fan accumulation on the continental rise.

The ocean basins all display a common relief pattern consisting of a broad, roughly central ridge or rise, flanked by low flat plains. The ocean rises are broad, transversely fractured, linear ridges which are tectonically unstable and the sites of shallow earthquake activity, high heat flow and volcanic action.- They are a major feature of the Earth’s topography -and rise by over 1-3km above the surrounding ocean floor. Some rises are irregular, whilst others form broad smooth arcs. Many crests of ocean ridges are broken by rift valley type features, whilst their outer flanks are fractured in a series of steps to the ocean basin floor. In some locations, such as Iceland, the oceanic ridges break the surface to form islands dominated by volcanic activity. The ocean basin floors which flank the rises comprise the deep flat abyssal plains which extend to depths of 45005500m below sea-level. The abyssal plains are interrupted by seamounts and abyssal hills. The former are isolated submarine volcanoes which may occasionally also reach the surface to form volcanic islands. The deepest parts of the ocean are the trenches or troughs which are not centrally placed but lie close to coasts with narrow and complex continental shelves. Trenches are narrow elongated basins and are commonly paralleled by island arcs. Island arcs occur on the landward side of ocean trenches and are seismically and volcanically active, consisting often of two parallel arcs of islands about 50-150km apart. Marginal sea basins are often associated with island arcs.

The broad global topographic elements described above are largely the product of endogenetic processes, though some have been greatly modified by exogenetic forces.

The lithosphere is composed of a series of rigid plates varying in size from 10x(5) - 10x(8) km'. There are seven major plates ( 10' km' ), North American, South American, Pacific, Eurasian, Antarctic, African, Australian-Indian, eight intermediate plates (10x(6) - 10x(7) km2) and more than twenty smaller plates (10x(5) - 10x(6) km2). There are three different types of boundary between the plates, divergent, convergent and conservative margins. These rigid lithospheric plates fide on a semiplastic asthenosphere. By the workings of the Earth's internal heat engine, magma rises from the asthenosphere at oceanic ridges and either side of the ridge the plates move apart or diverge. At margins where two plates meet it is common to find one subducting beneath the other. The subducting slab undergoes change because of the massive increase in temperature and pressure, and at about 400-700 km it becomes absorbed into the asthenosphere. Both melting and friction in the subduction zone release. pockets of magma which intrude up into the overlying plate to form volcanoes or large igneous intrusions or both. Pressure and stress formed by subduction also lead to the occurrence of earthquakes generated by the bending and final melting of the slab.

The concept of plate tectonics can convincingly explain many of the gross relief features of the globe. It can explain many topographical features, especially the distribution of volcanic activity and mountain building which are related to the patterns of plate movement. Since the early development of the rigid plates after chemical differentiation and cooling, the Earth could be subdivided into areas of relative stability and zones of instability. Over geological time, the relatively stable areas have been the continental surfaces dominated by low relief and modified by exogenetic processes. Also the older parts of ocean floors away from the main spreading and subduction centres have been stable for a period of geological time, though the oldest ocean floors are only up to 200 million years old, whilst continents may go back billions of years. In contrast, the unstable zones of the Earth have always been the plate margins where topography reflecting deformation is present.

Many divergent plate margins are where two plates composed of oceanic lithosphere diverge, displaying oceanic ridge topography. Convergent margins of similar oceanic plate contacts are characterized by island arcs and deep-sea trenches, where the subducting slab reaches sufficient temperatures to cause upwelling of molten material onto the overriding plate to generate a regular line of volcanoes at a distance of about 200 km from the trench.

The mountains along the western border of North and South America are the best present day examples of geomorphological features and geological structures resulting from an oceanic plate thrusting beneath the edge of a continental block. Along the Andean Cordillera, the Peru-Chile trench descends to about 8 km deep, and the ' highest Andean ranges are >5 km high. At the trench is a subducting slab which dips beneath the South American continent which has lead to buckling and folding of the overriding slab to produce the Andes mountain range superimposed on which is a chain of volcanoes. Initial subduction began about 195 million years ago and through geological time continuous subduction formed large igneous intrusions from the upwelling of magma which breaks the surface periodically to form the Andes volcanoes. Intense folding and compression of the overlying plate lead top the western Cordillera being raised, whilst the same forces uplifted the older rocks of the eastern Cordillera. The intervening altiplano is the result of sediments generated from the large scale erosion of the two uplifted areas by exogenetic processes.

When two continental lithosphere plate margins converge, neither is readily subducted because they are less dense, rather they collide. The results of this type of collision are complex because the crust does not readily subduct. The best modern examples are the Himalayas which developed between the Eurasian and Indian plates, now fused, and by the European Alps which lie between the Eurasian and African plates. Large volumes of sediment were laid down in a sea, called the Tethys, between the Europe and Africa, which upon closure of the sea due to the northwards drift of Africa, lead to the sediments becoming uplifted, overfolded and overthrusted into huge geological structures termed nappes. These structures were later subject to intense erosion to form the present Alpine range. Closure of the Tethys sea culminated in the Indian and Eurasian plates colliding about 45 million years ago, which lead to underthrusting of large slices of continental crust to form the Himalayan range-

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