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Landforms on the earth's surface

In general, the surface of the Earth can be classified as either rough or smooth. This is controlled by geological processes which mould and shape the Earth's surface. Most geological phenomena, such as the process which forms mountain ranges, takes place at an imperceptible rate, whereas at the other end of the spectrum there are event which can change the landscape in seconds, such as large volcanic eruptions or earthquakes. While separate continents have unique landforms, there are always similarities in the processes which build them and deconstruct them. For example, the Andes and Rockies of South and North America respectively are high mountain ranges which are distinct in their geology and form and yet were created due to same process of continental drift. A landform can be described as a geomorphologic unit, that is, a geographical entity which is morphed by constructive and destructive processes. Landforms are characterized and categorized by their soil type, stratification, orientation, elevation, rock exposure and slope. Different landforms are described with different names, such as cliffs, valleys, dykes, hills, mountains and so on. The main processes which control the formation of landforms are erosion, deposition, plate tectonics, and glacial movement.

The Rift Valley

For this essay i have chosen the Rift Valley for the landform of study. A rift valley is formed by a rift forming in an already present valley. They are made at divergent (or separating) plate boundaries, where the pulling force produces tension and openings in the ground. Today, the rift valley is characterized by flat and arid plains, dry desert, fertile land, and escarpments. It was first seeded around 20 million years ago, when in that area the Earth's crust pulled itself apart at a divergent plate boundary, creating a gaping rift in the earth, thousands of miles long down the eastern edge of the African continent across four countries. The tectonic forces present in the Cenozoic East African Rift System (Cenozoic meaning it formed since 65 million years ago) are very unique, where old plates are split apart to form new ones. The land on both sides of the newly formed rift was opened and tensioned in places, causing new volcanic mountains of vast scope, while the bottom of the valley slowly sank to form a flat plain. The Great Rift Valley, coined by the explorer John Walter Gregory, divides Kenya down the middle, creating an east-west divide; the rift is essentially a crack in the crust of the earth stretching over 6,000 miles. The Rift System is complex, and much is still not understood, though what is understood is that tectonic forces cause large areas of curst to sink in-between fault lines that ran parallel to each other, which forced magma upwards in other locations. This process is called rifting, and is essentially the Archimedes principle in action on a vast scale - the sinking crust forces the magma to be displaced elsewhere, in this case upwards. Evidence for rifting is clear in the volcanic activity around the rift.

The Cenozoic Rift System of East Africa reaches from the Afar Depression to beyond Lake Malawi. It lies very close to the equator, and is constructed of western and eastern rifts that are located on either side of Lake Victoria Basin. The largest and most widespread rift valley can be found running along the crest of the ocean ridge system, and is created due to seafloor spreading. Most rift valleys such as this are created due to the failed arm, or aulacogen of a triple junction. The Rio Grande Rift and the Mississippi embayment are other exemplary examples of this process. The rift valley is vast, spanning 60 miles in width at its widest point, and over 30 miles at the most narrow section. Lake Turkana is near where the rift is at its lowest, where little distinction can be made between the rift valley itself and the surrounding desert. As we travel south from Lake Turkana however, the valley walls become discernable, and turn into sheer cliff faces almost 2,000km in height at Lake Naivasha. After here, the rift valley sinks again to around 600 meters at the border of Tanzania. As evident by the high volcanic activity, with over 40 semi-active and active volcanoes, subterranean movement is a common feature of the Rift System. The volcanic activity is the reason for the alkaline lakes and hot springs present to the northwest of Nairobi. Hot or boiling springs include Lake Magadi, Lake Nakuru and Lake Bogoria. The alkaline is caused by the high content of sodium carbonate in the water, which in turn is the result of highly alkaline volcanic rocks, where water cannot escape thanks to the steep valley sides. The final piece in the puzzle is when the sun evaporates the lake water, forming sodium carbonate. Algae thrive in this environment, and so do certain specialized species of fish, such as tilapia. Vast quantities of migrating birds come to the alkaline lakes to feed on the abundant fish and algae.

Tectonic movement never ceases, and as such, the Rift Valley will continue to reshape and reform. One for the most interesting things about the East African Rift System is the thin crust in the area due to geothermal activity, which is 80km thinner than the average 100km of crust thickness, This means that the lithosphere may rupture and separate in a few million years, causing the eastern edge of Africa to split from the main continent to create a new landmass much bigger than Madagascar. If the divergence keeps on going, then a new mid-ocean ridge will form. Another feature of interest is the Ol Doinyo Lengai volcano, which is the only natrocarbonatite volcano currently present on Earth.

The Rift Valley is a wonderful geological place, and is a compelling source for anthropological discovery, especially in the Piedmont region. This is because highlands which easily erode have filled the rift valley with sediments ideal for the preservation of animals when they die. One of the most significant anthropological discoveries of recent times - Lucy - is an ancestor of homo sapiens sapiens, discovered by Donald Johanson. There is no telling what other amazing finds will occur in the years to come.

References

Geography. (2006). In Britannica Student Encyclopaedia. Retrieved July 4, 2006, from Encyclop?dia Britannica Premium Service: http://www.britannica.com/ebi/article-201425.

The Physical Geography of Africa.W. M. Adams - editor, A. S. Goudie - editor, A. R. Orme - editor. Publisher: Oxford University Press. Place of Publication: Oxford. Publication Year: 1999. Page Number: 18.

Population and Energy: A Systems Analysis of Resource Utilization in the Dominican Republic. Contributors: Gustavo A. Antonini - author, Katherine Carter Ewel - author, Howard M. Tupper - author. Publisher: Florida Presses. Place of Publication: Gainesville. Publication Year: 1975.

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