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
Hobart M King the Geographic Time Scale
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