Sunday, 3 February 2019

What is Geomorphology

What is Geomorphology



INTRODUCING GEOMORPHOLOGY

The word geomorphology derives from three Greek words: gew (the Earth), morfh (form), and logoV (discourse). Geomorphology is therefore ‘a discourse on Earth forms’. It is the study of Earth’s physical land surface featues, its landform rivers, hills, plains, beaches, sand dunes, and myriad others. Some workers include submarine landforms within the scope of geomorphology. And some would add the landforms of other terestrial type planets and satellites in the Solar System Mars, the Moon,Venus, and so on. Land forms are conspicuous features of the Earth and occur every where. They range in size from molehills to mountains to major tectonic plates, and their ‘lifespans’ range from days to millennia to aeons.

Geomorphology was first  used as aterm to describe the morphology of the Earth’s surface in the 1870s and 1880s (e.g. de Margerie 1886, 315). It was originally defined as ‘the genetic study of topographic forms’ (McGee 1888, 547),and was  used in popular parlance by 1896. Despite the modern acquisition of its name, geomorphology is a venerable discipline (Box 1.1). It investigates land forms and the processes that fashion them. A large corpus of geomorphologists expends much sweat in researching relationships between land forms and the processes acting on them now. These are the process or functional geomorphologists. Many geomorphic processes affect, and are affected by, human activities. Applied geomorphologists explore this rich area of enquiry, which is largely an extension of process geomorphology. Many landforms have a long history, and their present form does not always relate to the current processes acting upon them. The nature and rate of geomorphic process eschange with time, and some landforms were produced under different environmental conditions, surviving today as relict features. In high latitudes, many land forms are relicts from the Quaternary glaciations; but, in part sof the world,some land forms survive from millions and hundreds of millions of years ago. Geomorphology, then, has an important historical dimension,which is the domain of the historical geomorphologists. In short, modern geomorphologists study three chief aspects of
landforms form, process, and history. The first two are sometimes termed functional geomorphology, the last historical geomorphology (Chorley 1978). Process studies have enjoyed hegemony for some three or four decades. Historical studies were sidelined by process studies but are making a strong comeback. Although process and historical studies dominate much modern geomorphological enquiry, particularly in English speaking nations, other types of study exist. For example, structural geomorphologists, who were once a very influential group, argued that underlying geological structures are the key to understanding many landforms. Climatic geomorphologists, who are found mainly in France and Germany, believe that climate exerts a profound influence on landforms, each climatic region creating a distinguishing suite of landforms.



THE ORIGIN OF GEOMORPHOLOGY

Ancient Greek and Roman philosophers wondered how mountains and other surface features in the natural landscape had formed. Aristotle, Herodotus, Seneca, Strabo, Xenophanes, and many others discoursed on topics such as the origin of river valleys and deltas, and the presence of seashells in mountains. Xenophanes of Colophon (c. 580–480 BC ) speculated that, as seashells are found on the tops of mountains, the surface of the Earth must have risen and fallen. Herodotus (c. 484–420 BC ) thought that the lower part of Egypt was a former marine bay, reputedly saying ‘Egypt is the gift of the river’, referring to the year-by-year accumulation of river-borne silt in the Nile delta region. Aristotle (384–322 BC ) conjectured that land and sea change places, with areas that are now dry land once being sea and areas that are now sea once being dry land. Strabo (64/63 BC – AD 23?) observed that the land rises and falls, and suggested that the size of a river delta depends on the nature of its catchment, the largest deltas being found where the catchment areas are large and the surface rocks with in it are weak. Lucius Annaeus Seneca(4 BC –AD 65) appears to have appreciated that rivers possess the power to erode their valleys. About a millennium later, the illustrious Arab scholar ibn-Sina, also known as Avicenna (980–1037), who translated Aristotle,propounded the view that some mountains are produced by differential erosion, running water and wind hollowing out softer rocks. During the Renaissance,many scholars debated Earth history. Leonardo da Vinci (1452–1519) believed that changes in the levels of land and sea explained the presence of fossil marine shells in mountains. He also opined that valleys were cut by streams and that streams carried material from one place and deposited it elsewhere. In the eighteenth century, GiovanniTargioni-Tozzetti (1712–84) recognized evidence of stream erosion. He argued that the valleys of the Arno, Val di Chaina, and Ombrosa in Italy were excavated by rivers and floods resulting from the bursting of barrier lakes, and suggested that the irregular courses of streams relate to the differences in the rocks in which they cut, a process now called differential erosion. Jean-Étienne Guettard (1715–86) argued that streams destroy mountains and the sediment produce din the process builds floodplains before being carried to the sea. He also pointed to the efficacy of marine erosion, noting the rapid destruction of chalk cliffs in northern France by the sea, and the fact that the mountains of the Auvergne were extinct volcanoes. Horace-Bénédict de Saussure (1740–99) contended that valleys were produced by the streams that flow within them, and that glaciers may erode rocks. From the seearly idea son the origin of land forms arose modern geomorphology.(SeeChorleyet al.1964 and Kennedy 2005 for details on the development of the subject.

Historical geomorphology

Traditionally, historical geomorphologists strove to work out landscape history by mapping morphological and sedimentary features. Their golden rule was the dictum that ‘the present is the key to the past’. This was a warrant to assume that the effects of geomorphic processes seen in action today may be legitimately used to infer the causes of assumed landscape changes in the past. Before reliable dating techniques were available, such studies were difficult and largely educated guess-work. However, the brilliant successes of early historical
geomorphologists should not be overlooked.



William Morris Davis

The ‘geographical cycle’, expounded by William Morris Davis, was the first modern theory of landscape evolution (e.g. Davis 1889, 1899, 1909). It assumed that uplift takes place quickly. Geomorphic processes, without further complications from tectonic movements, then gradually wear down the raw topography. Furthermory lessen (though few field studies have substantiated this claim). So topography is reduced, little by little, to an extensive flat region close to baselevel – a peneplain  with occasional hills, called monadnocks after Mount Monadnock in New Hampshire, USA, which are local erosional remnants, standing conspicuously above the general level. The reduction process creates a time sequence of landforms that progresses through the stages of youth, maturity, and old age. However, these terms, borrowed from biology, are misleading and much censured(e.g.Ollier1967;OllierandPain1996,204–5). The ‘geographical cycle’ was designed to account for the development of humid temperate landforms produced by prolonged wearing down of uplifted rocks offering uniform resistance to erosion. It was extended to other landforms, including arid landscapes, glacial landscapes, periglacial landscapes, to landforms produced by shore processes, and to karst landscapes.

William Morris Davis’s‘ geographical cycle’–in which landscapes are seen to evolve through stages of youth, maturity, and old age – must be regarded as a classic work, even if it has been superseded (Figure 1.2). Its appeal seems to have lain in its theoretical tenor and in its simplicity (Chorley 1965). It had an all-pervasive influence on geomorphological thought and spawned the once highly influential field of denudation chronology. The work of denudation chronologists, who dealtmainly with morphological evidence,was subsequently criticized for seeing flat surfaces everywhere.

Walther Penck

A variation on Davis’s scheme was offered by Walther Penck. According to the Davisian model, uplift and planation take place alternately. But, in many landscapes, uplift and denudation occur at the same time. The continuous and gradual interaction of tectonic processes and denudation leads to a different model of landscape evolution, in which the evolution of individual slopes is thought to determine the evolution of the entire landscape (Penck 1924, 1953). Three main slope forms evolve with different combinations of uplift and denudation rates. First, convex slope profiles, resulting from waxing development (aufsteigende Entwicklung), form when the uplift rate exceeds the denudation rate. Second,
straight slopes, resulting from stationary (orsteady-state) development (gleichförmige Entwicklung), form when uplift and denudation rates match one another. And, third, concave slopes, resulting from waning development (absteigende Entwicklung), form when the uplift rate is less than the denudation rate. Later work has shown that valley-side shape depends not on the simple interplay of erosion rates and uplift rates, but on slope materials and the nature of slope-eroding processes. According to Penck’s arguments, slopes may either recede at the original gradient or else flatten, according to circumstances. Many textbooks claim that Penck advocated ‘parallel retreat of slopes’, but this is a false belief (see Simons 1962). Penck (1953, 135–6) argued that a steep rock face would move upslope, maintaining its original gradient, but would soon be eliminated by a growing basal slope. If the cliff face was the scarp of a tableland, however, it would take a long time to disappear. He reasoned that a lower-angle slope, which starts growing from the bottom of the basal slope, replaces the basal slope. Continued slope replacement then leads to a flattening of slopes, with steeper sections formed during earlier stages of development sometimes surviving in summit areas (Penck 1953, 136–41). In short, Penck’s complicated analysis predicted both slope recession and slope decline, a result that extends Davis’s simple idea of slope decline (Figure 1.3). Field studies have confirmed that slope retreat is common in a wide range of situations. However, a slope that is actively eroded at its base (by a river or by the sea) may decline if the basal erosion should stop. More over,at able land scarp retains its angle through parallel retreat until the erosion removes the protective cap rock, when slope decline sets in (Ollier and Tuddenham 1962).

Eduard Brückner and Albrecht Penck

Other early historical geomorphologists used geologically young sediments to interpret Pleistocene events.Eduard Brückner and Albrecht Penck’s (Walther’s father) work on glacial effects on the Bavarian Alps and their forelands provided the first insights into the effects of the Pleistocene ice ages on relief (Penck and Brückner 1901–9).Their classic river-terrace sequence gave names to the main glacial stages – Donau, Gunz, Mindel, Riss, and Würm – and sired Quaternary geomorphology.

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