Landforms on the earth’s surface

The Earth's surface can be described as being rough or smooth. Various geological processes constantly remodel the earth’s surface. Some geological processes, such as those that make mountains or wear them down, typically take place at imperceptible rates. Sudden events, however, can change the landscape in a minute. Rates of these geological processes vary.

Each continent has its individual arrangement of landforms, though similarities do exist. For example, high mountain ranges are located along the western sides of both North and South America, since the two continents make up basically one land mass. A landform comprises a geomorphologic unit. Landforms are categorised by characteristics such as elevation, slope, orientation, stratification, rock exposure, and soil type. Landforms by name include mounds, hills, cliffs, valleys, and so forth. A number of factors, ranging from plate tectonics to erosion and deposition can generate and affect landforms.

In this article, I have chosen Rift Valley.

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A rift valley is a valley created by the formation of a rift. Rift valleys are produced by tensional tectonic forces, which occur at divergent plate boundaries. Uninhabitable desert and fertile farmland, flat arid plains and steep escarpments characterize today’s Rift Valley.

Some 20 million years ago, the earth's crust weakened and tore itself apart creating a jagged rift, thousands of kilometres long, across the African continent. The land on either side erupted creating great volcanic mountains, while the valley floor gradually sank into a low flat plain. This geologic phenomenon, dubbed the Great Rift Valley by the Scottish explorer John Walter Gregory, divides Kenya neatly down the length of the country essentially separating east from west. Africa's Great Rift Valley is a 6,000-mile crack (fissure) in the earth's crust, stretching from Lebanon to Mozambique.

Geologists know that violent subterranean forces that tore apart the earth’s crust formed the Rift Valley. These forces caused huge chunks of the crust to sink between parallel fault lines and force up molten rock in volcanic eruptions. Evidence that this process, called rifting, is still in progress comes from the many active and semi-active volcanoes, located along the Rift.

The Cenozoic rift system of Eastern Africa extends from the Afar Depression in the north to beyond Lake Malawi in the south, a distance of about 5600 km. Close to the Equator it is made up of eastern and western rifts to either side of the Lake Victoria Basin.

The most extensive rift valley is located along the crest of the mid-ocean ridge system and is the result of seafloor spreading. Existing continental rift valleys are usually the result of a failed arm (aulacogen) of a triple junction. Examples besides the Great Rift Valley include the Mississippi embayment and the Rio Grande Rift in North America. In some places this natural divide is up to 100 km (60 miles) wide, while it reaches its narrowest point just north of Nairobi at 45 km wide.

The valley floor is at its lowest near Lake Turkana where there is virtually no distinction between the Great Rift and the surrounding desert. As it heads south, however, the valley walls form sheer cliffs rising to 1,900 km (6,232 ft) at Lake Naivasha. After Naivasha, the valley descends again to 580 meters (1,902 feet) at the Tanzanian border. Subterranean movement is common today as the Rift Valley is home to thirty active and semi-active volcanoes and countless hot springs along its length. This string of alkaline lakes and boiling springs northwest of Nairobi includes Lake Baringo, Lake Bogoria, Lake Nakuru, Lake Elementaita, Lake Naivasha, and Lake Magadi in the south.

These lakes are unique because their water is highly concentrated sodium carbonate. This situation is caused by the high alkalinity from the surrounding volcanic rocks coupled with poor drainage outlets due to the steep sides of the valley. The high evaporation of the surface lake water results in sodium carbonate, which, in turn, creates an ideal breeding ground for algae.

Several species of fish, tilapia in particular, thrives in this environment. As a result, millions of birds flock to these soda lakes to feast on the abundant food supply of algae and fish. Each of the lakes in the Rift Valley string has a slightly different water composition ranging.

The formation of the Rift Valley continues, probably driven by mantle plumes and ultimately a result of the African supers well. The associated geothermal activity and spreading at the rift has caused the lithosphere to thin from a typical 100 km thickness for continents to a mere 20 km. Within a few million years, the lithosphere may rupture and eastern Africa will split off to form a new landmass. If spreading continues, this will lead to the formation of a new mid-ocean ridge.

The Ol Doinyo Lengai volcano remains active, and is currently the only natrocarbonatite volcano in the world.

The Rift Valley has been a rich source of anthropological discovery, especially in an area known as Piedmont. Because the rapidly eroding highlands have filled the valley with sediments, a favourable environment for the preservation of remains has been created. The bones of several hominid ancestors of modern humans have been found there, including those of "Lucy", a nearly complete australopithecine skeleton, which was discovered by anthropologist Donald Johanson. Richard and Meave Leakey have also done significant work in this region.

References

1. Geography. (2006). In Britannica Student Encyclopaedia. Retrieved July 4, 2006, from Encyclopædia Britannica Premium Service: http://www.britannica.com/ebi/article-201425.
2. 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.
3. 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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