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Friday 3 August 2018

Geography (Series ) By Tafadzwa Munhapa


                                                 




1 – Define geomorphology (4)
The word geomorphology derives from three Greek words: gew(the Earth), morfh(form), and logo~ (discourse). Geomorphology is therefore ‘a discourse on Earth forms’. The term was coined sometime in the 1870s and 1880s to describe the morphology of the Earth’s surface (e.g. de Margerie 1886, 315), was originally defined as ‘the genetic study of topographic forms’ (McGee 1888, 547) Geomorphology investigates landforms and the processes that fashion them. Form, process, and the interrelationships between them are central to understanding the origin and development of landforms. The earth is a dynamic place and nothing on earth is static, that’s where the study of geomorphology comes into play.
2- Discus the roll of historical and functional geomorphology in the study of different landforms (15)
Origins and life of the earth is staged by geographers into four eras and the eras are as follows Proterozoic, Palaeozoic, Mesozoic, and Cenozoic
Archean Eon, also spelled Archaean Eon, the earlier of the two formal divisions of Precambrian time (about 4.6 billion to 541 million years ago) and the period when life first formed on Earth. The Archean Eon began about 4 billion years ago with the formation of Earth’s crust and extended to the start of the Proterozoic Eon 2.5 billion years ago; the latter is the second formal division of Precambrian time. The start of the Archean Eon is only defined by the isotopic age of the earliest rocks. Prior to the Archean Eon, Earth was in the astronomical (Hadean) stage of planetary accretion that began about 4.6 billion years ago; no rocks are preserved from this stage. Archean oceans were likely created by the condensation of water derived from the outgassing of abundant volcanoes. Iron was released then (as today) into the oceans from submarine volcanoes in oceanic ridges and during the creation of thick oceanic plateaus. This ferrous iron (Fe2+) combined with oxygen and was precipitated as ferric iron in hematite (Fe2O3), which produced banded-iron formations on the flanks of the volcanoes. . The second oldest rocks are the 4-billion-year-old Acasta granitic gneisses in northwestern Canada, and a single relict zircon grain dated to 4.2 billion years ago was found within these gneisses. Archean rocks mostly occur in large blocks hundreds to thousands of kilometers across, such as in the Superior and Slave provinces in Canada; the Pilbara and Yilgarn blocks in Australia; the Kaapvaal craton in southern Africa; the Dharwar craton in India; the Baltic, Anabar, and Aldan shields in Russia; and the North China craton.
The Paleozoic is bracketed by two of the most important events in the history of animal life. At its beginning, multicelled animals underwent a dramatic "explosion" in diversity, and almost all living animal phyla appeared within a few millions of years. At the other end of the Paleozoic, the largest mass extinction in history wiped out approximately 90% of all marine animal species. The causes of both these events are still not fully understood and the subject of much research and controversy. The Paleozoic took up over half approximately 300 million years ago. . During the Paleozoic there were six major continental land masses; each of these consisted of different parts of the modern continents. For instance, at the beginning of the Paleozoic, today's western coast of North America ran east-west along the equator, while Africa was at the South Pole. These Paleozoic continents experienced tremendous mountain building along their margins, and numerous incursions and retreats of shallow seas across their interiors. Many Paleozoic rocks are economically important. For example, much of the limestone quarried for building and industrial purposes, as well as the coal deposits of western Europe and the eastern United States, were formed during the Paleozoic. The Paleozoic is divided into six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous. On a global scale, the Paleozoic was a time of continental assembly. The majority of Cambrian landmasses were gathered together to form Gondwana, a supercontinent made up of the present-day continents of Africa, South America, Australia, and Antarctica and the Indian subcontinent.
The Mesozoic Era is commonly subdivided into three geologic periods: Triassic (252 to 201.3 million years ago), Jurassic (201.3 to 145 million years ago), Cretaceous (145 to 66 million years ago).The Mesozoic Era begins in the wake of the largest extinction in Earth's history. This extinction took place 252 million years ago and resulted in 96% of marine life and 70% of terrestrial life dying out. The cause of the extinction is not fully understood, but eventually it led to dinosaurs dominating the planet for 135 million years. The Mesozoic Era began with today's continents combined into one large land mass known as Pangea, which was surrounded by a single global ocean called Panthalassa.
During the Jurassic Period, a rift between modern day Africa and South America began to split Pangea apart. This began the formation of the continents as we know them today. Even today, this same rift continues to spread the coasts of the Atlantic further apart.
It is believed that the Mesozoic Era was a dry climate for most of the time due to the abundance of evaporates, which is a type of mineral that only forms in dry climates. Fossils from the Mesozoic also indicate a warm and dry climate.

The Cenozoic Era began 65 million years ago with an asteroid impact that killed off a majority of the dinosaurs and ends at the present day. The Cenozoic is commonly divided into three periods:Paleogene (65.5 to 23.03 million years ago),Neogene (23.03 to 2.6 million years ago),Quaternary (2.6 million years ago to present). A picture of the Earth at the beginning of the Cenozoic Era would look somewhat similar to a picture of the Earth today. The supercontinent of Pangea that existed during the time of the dinosaurs had split apart by the Cenozoic, and the continents were on their paths to where they are today. In the Cenozoic Era, the Earth began a long period of cooling, caused in part by the continents shifting into their current positions. As South America separated from Antarctica, a global current of cold water was brought to the surface, cooling the surface temperature of the ocean and the atmosphere. The Quaternary Period is a geologic time period that encompasses the most recent 2.6 million years — including the present day. Part of the Cenozoic Era, the period is usually divided into two epochs — the Pleistocene Epoch, which lasted from approximately 2 million years ago to about 12,000 years ago, and the Holocene Epoch, which began about 12,000 years ago. The Quaternary Period has involved dramatic climate changes, which affected food resources and brought about the extinction of many species. The period also saw the rise of a new predator: man.
Historical geomorphology tends to focus around histories or trajectories of lands cape evolution and adopts a sequential, chronological view Largely, historical geomorphology and process geomorphology are complementary and go hand-in-hand, so that historical geomorphologists consider process in their explanations of landform evolution 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. Historical geomorphology is the study of landform evolution or changes in landforms over medium and long timescale.
Geomorphic processes are the multifarious chemical and physical means by which the Earth’s surface undergoes modification. They are driven by geological forces emanating from inside the Earth (endogenic or endogene processes), by forces originating at or near the Earth’s surface and in the atmosphere (exogenic or  exogene  processes), and by forces coming from outside the Earth (extraterrestrial processes, such as asteroid impacts). They include processes of transformation and transfer associated with weathering, gravity, water, wind, and ice. Although the study of geomorphology has been around since the ancient times,The first official geographic model was proposed between 1884 and 1899 by the American geographer William Morris Davis. His geographic cycle model was inspired by theories of Uniformitarianism and attempted to theorize the development of various landform features. The land surface of Earth is the consequence of specific, natural processes acting across some interval of time.  It follows that any landscape is not static, but is changing in some (probably) predictable way. So geomorphologists (and geomorph students) must acquire the habit of thinking historically when  trying  to interpret how a landscape came to be.
The processes that we take seriously as possible causes for landscapes are uniformitarian ones.  That is, they are processes we can actually see happening today or that are at least compatible with physical, chemical, and biological constraints that are well understood.  One idea for the evolution of a strange part of Montana (the Channeled Scablands) was rejected for many years, not because it was physically unreasonable, but because the source of the enormous energy for accomplishing it was not clear.  When it became obvious that such a source not only could exist, but must have existed, the hypothesis was greeted with a new respect.  The process itself was invisible, but it was physically possible to accomplish without recourse to magic, so eventually it was accepted. 
Geomorphic processes work in ways that are predictable, at least in their broad terms.  Tectonic events create structures that uplift and/or depress land in specific ways.  Weathering in specific climates will attack the rocks thus uplifted in pretty specific ways, and erosional agents will remove the loose regolith, transport it away, and eventually deposit it in fairly predictable ways.  Thus if we know how an area has been uplifted and what the weathering and transporting agents affecting it are, we can make a pretty good guess as to what the area will eventually look like.  In fact, we can predict what it will look like at different times in its evolutionary history.  Because we are convinced of this general predictability, we think of landscapes evolving under the influence of geomorphic systems
 Functional geomorphology are geomorphologic processes and are as follows: Fluvial geomorphologic processes, these are those related to rivers and streams. The flowing water found here is important in shaping the landscape in two ways. First, the power of the water moving across a landscape cuts and erodes its channel. As it does this, the river shapes its landscape by growing in size, meandering across the landscape, and sometimes merging with other rivers forming a network of braided rivers.The paths rivers take depend on the topology of the area and the underlying geology or rock structure found where it's moving.In addition, as the river carves its landscape it carries the sediment it erodes as it flows. This gives it more power to erode as there is more friction in the moving water, but it also deposits this material when it floods or flows out of mountains onto an open plain in the case of an alluvial fan
The mass movement process, also sometimes called mass wasting, occurs when soil and rock moves down a slope under the force of gravity. The movement of the material is called creeping, slides, flows, topples, and falls. Each of these is dependent on the speed of movement and composition of the material moving. This process is both erosional and depositional.
Glaciers are one of the most significant agents of landscape change simply because of their sheer size and power as they move across an area. They are erosional forces because their ice carves the ground beneath them and on the sides in the case of a valley glacier which results in a U-shaped valley. Glaciers are also depositional because their movement pushes rocks and other debris into new areas. The sediment created by the grinding down of rocks by glaciers is called glacial rock flour. As glaciers melt, they also drop their debris creating features like eskers and moraines.
Weathering is an erosional process that involves the chemical break down of rock (such as limestone) and the mechanical wearing down of rock by a plant’s roots growing and pushing through it, ice expanding in its cracks, and abrasion from sediment pushed by wind and water. Weathering can, for example, result in rock falls and eroded rock like those found in Arches National Park, Utah.
3 Outline reasons why explanation of most landforms needs element from different spectrum of approaches
When James Cook and his crew first saw New Zealand, in 1769, they probably believed the land had been shaped by the biblical Great Flood. But why was this dramatic landscape so different from England? A century later, science had begun to find the answers – in particular, it had become clear that the land was constantly changing.Traditionally the ‘geographical cycle’, expounded by William Morris Davis, was the first modern theory of landscape evolution, His geomorphic cycle model was inspired by theories of uniformitarianism and attempted to theorize the development of various landform features.
Austrian climatologist Alfred Wegener used the fit of opposing coastlines as one of the pieces of evidence to support his hypothesis of continental drift. Continental drift proposed that the continents were once assembled together as a single supercontinent Wegener named Pangaea. Wegener was unable to suggest a suitable mechanism to explain the motion of the continents across Earth's surface and his hypothesis received relatively little support until technology revealed the secrets of the ocean floor. Scientists gradually amassed additional data that would resurrect Wegener's hypothesis over 30 years after his death. By the 1960s the building blocks were in place to support a new hypothesis, Seafloor spreading, that would provide the mechanism for continental drift. Together these concepts would become the theory of plate tectonics.
The theory of plate tectonics provides an example of the evolution of scientific thought. The first two sections of the chapter reveal the basic observations that were used to make predictions on the geologic processes that shaped the face of Earth. The theory of plate tectonics links Earth’s internal processes to the distribution of continents and oceans
Rock cycle process also needs to be considered on explanation of different formations forms. Like landforms, many rocks do not remain in their original form indefinitely but instead, over a long term, tend to undergo processes of transformation. The rock cycle is a conceptual model for understanding processes that generate, alter, transport, and deposit mineral origin
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. 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.
Some geomorphologists, mainly the ‘big names’ in the field, have turned their attention to the long-term change of landscapes. Starting with William Morris Davis’s ‘geographical cycle,he stated that several theories to explain the prolonged decay of regional landscapes have been promulgated. Walther Penck offered a variation on Davis’s scheme. 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 inter action 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,
Gossman (1970) Slope recession, which produces a pediplain and slope decline, which produces a peneplain, resulting from waxing development form when the uplift rate exceeds the denudation rate.Second,straight slopes, resulting from stationary (or steady-state) development, form when uplift and denudation rates match one another. And, third, concave slopes, resulting from waning development form when the uplift rate is less than the denudation rate. According to Penck’s arguments, slopes may either recede at the original gradient or else flatten, according to circumstances.
 Many authers claim that Penck advocated ‘parallel retreat of slopes’, but this is a false belief (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. 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. Moreover, a tableland scarp retains its angle through parallel retreat until the erosion removes the protective cap rock, when slope decline sets in (Ollier and Tuddenham 1962). Common to all these theories is the assumption that, however the land surface may appear at the outset, it will gradually be reduced to a low-lying plain that cuts across geological structures and rock types. These planation surfaces or erosion surfaces are variously styled peneplains, panplains, etchplains, and so forth. Cliff Ollier (1991, 78) suggested that the term palaeoplain is preferable since it has no genetic undertones and simply means ‘old plain. It is worth bearing in mind when discussing the classic theories of landscape evolution that palaeoplain formation takes hundreds of millions of years to accomplish, so that during the Proterozoic aeon enough time elapsed for but a few erosion surfaces to form. In southeastern Australia, the palaeoplain first described by Edwin Sherbon Hills is still preserved along much of the Great Divide and is probably of Mesozoic
Lester Charles King  was known for his theories on scarp retreat. He offered a very different view of the origin of continental landscaping than that of William Morris Davis. King's ideas were an attempt at refuting Davis' cycle of erosion they were themselves of cyclical nature and contributed to what Cliff Ollier has called "Davis bashing" the ridicule of cyclical theories in geomorphology, in particular Davis' ones. Critics did however not propose alternative models. For him, the weathering of physical factors in arid areas causes the erosion of the hills, the deposition of the weathered material (pediments) and the deposition of these material in lower altitudes, contributing to the formation of the pediplain. King was a supporter of the Expanding Earth hypothesis. L C King said landforms evolved through out history in his pediplanation theory which explains the formation of inselbergs  Arthur N Strahler (1952) in his book (Dynamic basis of geomorphology) proposed a system of geomorphology grounded in basic principles of mechanics and fluid dynamics that he hoped would enable geomorphic processes to be treated as manifestations of various types of shear stresses. So in general processes of landform formations is filled with multitudes of approaches which means the only answer is the creator who knows when and how the landforms were created. learning how landforms evolved using approaches by some authors is good, but to some extent that can be opposed by some scientific researches and will leave us with no absolute answer

REFERENCES

N Strahler (1952) The Dynamic basis of geomorphology
Cliff Ollier (1991, 78)  Ancient  landforms
Richard John Huggett   (2002, 2007, 2011)   Fundamentals of Geomorphology
Robert E.Gabler,James F.Petersen,L.Michael Trapasso  Essentials of Physical Geography,Eighth Edition
Penck(1953,135–6)                                                                                                                                                              Journal by Jijo Sudarshan Endogenic Forces and Evolution of Land forms.
Hobart M King    the Geographic Time Scale


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