Rift

In geology, a rift is a linear zone where the lithosphere is being pulled apart[1][2] and is an example of extensional tectonics.[3]

Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a rift valley, which may be filled by water forming a rift lake. The axis of the rift area may contain volcanic rocks, and active volcanism is a part of many, but not all active rift systems.

Major rifts occur along the central axis of most mid-ocean ridges, where new oceanic crust and lithosphere is created along a divergent boundary between two tectonic plates.

Failed rifts are the result of continental rifting that failed to continue to the point of break-up. Typically the transition from rifting to spreading develops at a triple junction where three converging rifts meet over a hotspot. Two of these evolve to the point of seafloor spreading, while the third ultimately fails, becoming an aulacogen.

Rift segmentation
Block view of a rift formed of three segments, showing the location of the accommodation zones between them at changes in fault location or polarity (dip direction)

Geometry

Most rifts consist of a series of separate segments that together form the linear zone characteristic of rifts. The individual rift segments have a dominantly half-graben geometry, controlled by a single basin-bounding fault. Segment lengths vary between rifts, depending on the elastic thickness of the lithosphere. Areas of thick colder lithosphere, such as the Baikal Rift have segment lengths in excess of 80 km, while in areas of warmer thin lithosphere, segment lengths may be less than 30 km.[4] Along the axis of the rift the position, and in some cases the polarity (the dip direction), of the main rift bounding fault changes from segment to segment. Segment boundaries often have a more complex structure and generally cross the rift axis at a high angle. These segment boundary zones accommodate the differences in fault displacement between the segments and are therefore known as accommodation zones.

Accommodation zones take various forms, from a simple relay ramp at the overlap between two major faults of the same polarity, to zones of high structural complexity, particularly where the segments have opposite polarity. Accommodation zones may be located where older crustal structures intersect the rift axis. In the Gulf of Suez rift, the Zaafarana accommodation zone is located where a shear zone in the Arabian-Nubian Shield meets the rift.[5]

Rift flanks or shoulders are elevated areas around rifts. Rift shoulders are typically about 70 km wide.[6] Contrary to what was previously thought, elevated passive continental margins (EPCM) such as the Brazilian Highlands, the Scandinavian Mountains and India's Western Ghats, are not rift shoulders.[6]

Rift development

Rift initiation

At the onset of rifting, the upper part of the lithosphere starts to extend on a series of initially unconnected normal faults, leading to the development of isolated basins.[7] In subaerial rifts, drainage at this stage is generally internal, with no element of through drainage.

Mature rift stage

As the rift evolves, some of the individual fault segments grow, eventually becoming linked together to form the larger bounding faults. Subsequent extension becomes concentrated on these faults. The longer faults and wider fault spacing leads to more continuous areas of fault-related subsidence along the rift axis. Significant uplift of the rift shoulders develops at this stage, strongly influencing drainage and sedimentation in the rift basins.[7]

During the climax of lithospheric rifting, as the crust is thinned, the Earth's surface subsides and the Moho becomes correspondingly raised. At the same time, the mantle lithosphere becomes thinned, causing a rise of the top of the asthenosphere. This brings high heat flow from the upwelling asthenosphere into the thinning lithosphere, heating the orogenic lithosphere for dehydration melting, typically causing extreme metamorphism at high thermal gradients of greater than 30 °C. The metamorphic products are high to ultrahigh temperature granulites and their associated migmatite and granites in collisional orogens, with possible emplacement of metamorphic core complexes in continental rift zones but oceanic core complexes in spreading ridges. This leads to a kind of orogeneses in extensional settings, which is referred as to rifting orogeny.[8]

Post-rift subsidence

Once rifting ceases, the mantle beneath the rift cools and this is accompanied by a broad area of post-rift subsidence. The amount of subsidence is directly related to the amount of thinning during the rifting phase calculated as the beta factor (initial crustal thickness divided by final crustal thickness), but is also affected by the degree to which the rift basin is filled at each stage, due to the greater density of sediments in contrast to water. The simple 'McKenzie model' of rifting, which considers the rifting stage to be instantaneous, provides a good first order estimate of the amount of crustal thinning from observations of the amount of post-rift subsidence.[9][10] This has generally been replaced by the 'flexural cantilever model', which takes into account the geometry of the rift faults and the flexural isostasy of the upper part of the crust.[11]

Multiphase rifting

Some rifts show a complex and prolonged history of rifting, with several distinct phases. The North Sea rift shows evidence of several separate rift phases from the Permian through to the Earliest Cretaceous,[12] a period of over 100 million years.

Magmatism

Many rifts are the sites of at least minor magmatic activity, particularly in the early stages of rifting.[13] Alkali basalts and bimodal volcanism are common products of rift-related magmatism.[14][15]

Recent studies indicate that post-collisional granites in collisional orogens are the product of rifting magmatism at converged plate margins.

Economic importance

The sedimentary rocks associated with continental rifts host important deposits of both minerals and hydrocarbons.[16]

Mineral deposits

SedEx mineral deposits are found mainly in continental rift settings. They form within post-rift sequences when hydrothermal fluids associated with magmatic activity are expelled at the seabed.[17]

Oil and gas

Continental rifts are the sites of significant oil and gas accumulations, such as the Viking Graben and the Gulf of Suez Rift. Thirty percent of giant oil and gas fields are found within such a setting.[18] In 1999 it was estimated that there were 200 billion barrels of recoverable oil reserves hosted in rifts. Source rocks are often developed within the sediments filling the active rift (syn-rift), forming either in a lacustrine environment or in a restricted marine environment, although not all rifts contain such sequences. Reservoir rocks may be developed in pre-rift, syn-rift and post-rift sequences. Effective regional seals may be present within the post-rift sequence if mudstones or evaporites are deposited. Just over half of estimated oil reserves are found associated with rifts containing marine syn-rift and post-rift sequences, just under a quarter in rifts with a non-marine syn-rift and post-rift, and an eighth in non-marine syn-rift with a marine post-rift.[19]

Examples

See also

References

  1. ^ Rift valley: definition and geologic significance, Giacomo Corti, The Ethiopian Rift Valley
  2. ^ Decompressional Melting During Extension of Continental Lithosphere, Jolante van Wijk, MantlePlumes.org
  3. ^ Plate Tectonics: Lecture 2, Geology Department at University of Leicester
  4. ^ Ebinger, C.J.; Jackson J.A.; Foster A.N.; Hayward N.J. (1999). "Extensional basin geometry and the elastic lithosphere" (PDF). Philosophical Transactions of the Royal Society A. The Royal Society. 357 (1753): 741–765. doi:10.1098/rsta.1999.0351. Retrieved 28 October 2012.
  5. ^ Younes, A.I.; McClay K. (2002). "Development of Accommodation Zones in the Gulf of Suez-Red Sea Rift, Egypt". AAPG Bulletin. 86 (6): 1003–1026. doi:10.1306/61EEDC10-173E-11D7-8645000102C1865D. Retrieved 29 October 2012.
  6. ^ a b Green, Paul F.; Japsen, Peter; Chalmers, James A.; Bonow, Johan M.; Duddy, Ian R. (2018). "Post-breakup burial and exhumation of passive continental margins: Seven propositions to inform geodynamic models". Gondwana Research. 53: 58–81. doi:10.1016/j.gr.2017.03.007. Retrieved February 10, 2018.
  7. ^ a b Withjack, M.O.; Schlische R.W.; Olsen P.E. (2002). "Rift-basin structure and its influence on sedimentary systems". In Renaut R.W. & Ashley G.M. Sedimentation in Continental Rifts (PDF). Special Publications. 73. Society for Sedimentary Geology. Retrieved 28 October 2012.
  8. ^ Zheng, Y.-F., Chen, R.-X., 2017. Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins. Journal of Asian Earth Sciences 145, 46-73.
  9. ^ McKenzie, D. (1978). "Some remarks on the development of sedimentary basins" (PDF). Earth and Planetary Science Letters. Elsevier. 40: 25–32. doi:10.1016/0012-821x(78)90071-7. Archived from the original (PDF) on 1 March 2014. Retrieved 25 October 2012.
  10. ^ Kusznir, N.J.; Roberts A.M.; Morley C.K. (1995). "Forward and reverse modelling of rift basin formation". In Lambiase J.J. Hydrocarbon habitat in rift basins. Special Publications. 80. London: Geological Society. pp. 33–56. ISBN 9781897799154. Retrieved 25 October 2012.
  11. ^ Nøttvedt, A.; Gabrielsen R.H.; Steel R.J. (1995). "Tectonostratigraphy and sedimentary architecture of rift basins, with reference to the northern North Sea". Marine and Petroleum Geology. Elsevier. 12 (8): 881–901. doi:10.1016/0264-8172(95)98853-W. Retrieved 29 October 2012.
  12. ^ Ravnås, R.; Nøttvedt A.; Steel R.J.; Windelstad J. (2000). "Syn-rift sedimentary architectures in the Northern North Sea". Dynamics of the Norwegian Margin. Special Publications. 167. London: Geological Society. pp. 133–177. ISBN 9781862390560. Retrieved 28 October 2012.
  13. ^ White, R.S.; McKenzie D. (1989). "Magmatism at Rift Zones: The Generation of Volcanic Margins and Flood Basalts" (PDF). Journal of Geophysical Research. American Geophysical Union. 94 (B6): 7685–7729. doi:10.1029/jb094ib06p07685. Retrieved 27 October 2012.
  14. ^ Farmer, G.L. (2005). "Continental Basaltic Rocks". In Rudnick R.L. Treatise on Geochemistry: The crust. Gulf Professional Publishing. p. 97. ISBN 9780080448473. Retrieved 28 October 2012.
  15. ^ Cas, R.A.F. (2005). "Volcanoes and the geological cycle". In Marti J. & Ernst G.G. Volcanoes and the Environment. Cambridge University Press. p. 145. Retrieved 28 October 2012.
  16. ^ United States Geological Survey (1993). "Lake Baikal - A Touchstone for Global Change and Rift Studies". Archived from the original on 29 June 2012. Retrieved 28 October 2012.
  17. ^ Groves, D.I.; Bierlein F.P. (2007). "Geodynamic settings of mineral deposit systems". Journal of the Geological Society. Geological Society. 164 (1): 19–30. doi:10.1144/0016-76492006-065. Retrieved 27 October 2012.
  18. ^ Mann, P.; Gahagan L.; Gordon M.B. (2001). "Tectonic setting of the world's giant oil fields". WorldOil Magazine. Retrieved 27 October 2012.
  19. ^ Lambiase, J.J.; Morley C.K. (1999). "Hydrocarbons in rift basins: the role of stratigraphy" (PDF). Philosophical Transactions of the Royal Society A. The Royal Society. 357: 877–900. doi:10.1098/rsta.1999.0356.

Further reading

African Great Lakes

The African Great Lakes (Swahili: Maziwa Makuu) are a series of lakes constituting the part of the Rift Valley lakes in and around the East African Rift. They include Lake Victoria, the third-largest fresh water lake in the world by area, and Lake Tanganyika, the world's second-largest freshwater lake by volume and depth. Collectively, they contain 31,000 km3 (7400 cu mi) of water, which is more than either Lake Baikal or the North American Great Lakes. This total constitutes about 25% of the planet's unfrozen surface fresh water.

The large rift lakes of Africa are the ancient home of great biodiversity; 10% of the world's fish species live there.

Countries in the African Great Lakes region (sometimes also called Greater Lakes region) include Burundi, the Democratic Republic of the Congo, Kenya, Rwanda, Tanzania and Uganda. A conference was held on the lakes region in 2017 in Uganda.The Great Lakes area, where colonial era borders cut through ethnic groups, has in the last 20 years been a crucible of conflict that has launched multiple uprisings and invasions. The United Nations, the United States, and several European countries have special envoys or representatives to the Great Lakes region. On June 18, 2013, Russ Feingold was appointed United States Special Representative for the region by United States Secretary of State John Kerry. He announced his departure from the position on February 24, 2015. Laurence D. Wohlers was appointed to the position in January 2017.

Against the Giants

Against the Giants is an adventure module written by Gary Gygax and published by TSR in 1981 for the Dungeons & Dragons fantasy role-playing game. It combines the G series of modules previously published in 1978: Steading of the Hill Giant Chief, Glacial Rift of the Frost Giant Jarl, and Hall of the Fire Giant King. All three were produced for use with the 1st edition Advanced Dungeons & Dragons rules. In 1999, to recognize the 25th anniversary of TSR, the company released an updated version, Against the Giants: The Liberation of Geoff. Later in 1999, Wizards of the Coast published a novelization of Against the Giants by Ru Emerson.

The plot of each of the three original modules focuses on a particular type of evil giant. Each can be played as a standalone adventure, or as a series. In Steading of the Hill Giant Chief, a tribe of hill giants have been raiding lands occupied by humans, and the humans hire the player characters to defeat them. Glacial Rift of the Frost Giant Jarl pits the player characters against the evil Jarl of the frost giants. Hall of the Fire Giant King concludes the series, this time against a group of fire giants. The first two modules disclose the existence of a secret force behind the giants, which in the third module is revealed to be evil drow elves. The plot involving the drow continues in four additional modules printed between 1978 and 1980.

The modules were well received by contemporary critics. In 1978, they earned a 9/10 overall rating from a White Dwarf magazine reviewer, who was impressed that Gygax found time to write them while also working on the Advanced Dungeons & Dragons (AD&D) rulebooks. White Dwarf also reviewed the re-released G module series in 1982, giving it a 10 out of 10. The Queen of the Spiders supermodule, which consisted of the three modules combined with the subsequent modules in the "D" series and Q1 Queen of the Demonweb Pits, was voted the single greatest adventure of all time by Dungeon magazine in 2004, on the 30th anniversary of the Dungeons & Dragons game.

Albertine Rift

The Albertine Rift is the western branch of the East African Rift, covering parts of Uganda, the Democratic Republic of the Congo (DRC), Rwanda, Burundi and Tanzania.

It extends from the northern end of Lake Albert to the southern end of Lake Tanganyika.

The geographical term includes the valley and the surrounding mountains.

Arabah

The Arabah (Arabic: وادي عربة‎, Wādī ʻAraba), or Arava / Aravah (Hebrew: הָעֲרָבָה‬, HaAravah, lit. "desolate and dry area"), as it is known by its respective Arabic and Hebrew names, is a geographic area south of the Dead Sea basin, which forms part of the border between Israel to the west and Jordan to the east.

The old meaning, which was in use up to the early 20th century, covered almost the entire length of what today is called the Jordan Rift Valley, running in a north-south orientation between the southern end of the Sea of Galilee and the northern tip of the Gulf of Aqaba at Aqaba/ Eilat. This included the Jordan River Valley between the Sea of Galilee and the Dead Sea, the Dead Sea itself, and what today is commonly called the Arava Valley. The contemporary use of the term is restricted to this southern section alone.

Arctic Ocean

The Arctic Ocean is the smallest and shallowest of the world's five major oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, although some oceanographers call it the Arctic Mediterranean Sea or simply the Arctic Sea, classifying it a mediterranean sea or an estuary of the Atlantic Ocean. It is also seen as the northernmost part of the all-encompassing World Ocean.

Located mostly in the Arctic north polar region in the middle of the Northern Hemisphere, the Arctic Ocean is almost completely surrounded by Eurasia and North America. It is partly covered by sea ice throughout the year and almost completely in winter. The Arctic Ocean's surface temperature and salinity vary seasonally as the ice cover melts and freezes; its salinity is the lowest on average of the five major oceans, due to low evaporation, heavy fresh water inflow from rivers and streams, and limited connection and outflow to surrounding oceanic waters with higher salinities. The summer shrinking of the ice has been quoted at 50%. The US National Snow and Ice Data Center (NSIDC) uses satellite data to provide a daily record of Arctic sea ice cover and the rate of melting compared to an average period and specific past years.

Divergent boundary

In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which eventually become rift valleys. Most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges. Divergent boundaries also form volcanic islands which occur when the plates move apart to produce gaps which molten lava rises to fill.

Current research indicates that complex convection within the Earth's mantle allows material to rise to the base of the lithosphere beneath each divergent plate boundary.

This supplies the area with vast amounts of heat and a reduction in pressure that melts rock from the asthenosphere (or upper mantle) beneath the rift area forming large flood basalt or lava flows. Each eruption occurs in only a part of the plate boundary at any one time, but when it does occur, it fills in the opening gap as the two opposing plates move away from each other.

Over millions of years, tectonic plates may move many hundreds of kilometers away from both sides of a divergent plate boundary. Because of this, rocks closest to a boundary are younger than rocks further away on the same plate.

East African Rift

The East African Rift (EAR) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. In the past, it was considered to be part of a larger Great Rift Valley that extended north to Asia Minor.

The rift is a narrow zone that is a developing divergent tectonic plate boundary, where the African Plate is in the process of splitting into two tectonic plates, called the Somali Plate and the Nubian Plate, at a rate of 6–7 mm (0.24–0.28 in) annually. As extension continues, lithospheric rupture will occur within 10 million years, the Somali plate will break off, and a new ocean basin will form.

Great Rift Valley

The Great Rift Valley is a contiguous geographic trench, approximately 6,000 kilometres (3,700 mi) in length, that runs from Lebanon's Beqaa Valley in Asia to Mozambique in Southeastern Africa. The name continues in some usages, although it is today considered geologically imprecise as it combines features that are today regarded as separate, although related, rift and fault systems.

Today, the term is most often used to refer to the valley of the East African Rift, the divergent plate boundary which extends from the Afar Triple Junction southward across eastern Africa, and is in the process of splitting the African Plate into two new separate plates. Geologists generally refer to these incipient plates as the Nubian Plate and the Somali Plate.

League of Legends

League of Legends (abbreviated LoL) is a multiplayer online battle arena video game developed and published by Riot Games for Microsoft Windows and macOS. The game follows a freemium model and is supported by microtransactions, and was inspired by the Warcraft III: The Frozen Throne mod, Defense of the Ancients.In League of Legends, players assume the role of an unseen "summoner" that controls a "champion" with unique abilities and battle against a team of other players or computer-controlled champions. The goal is usually to destroy the opposing team's "nexus", a structure that lies at the heart of a base protected by defensive structures, although other distinct game modes exist as well. Each League of Legends match is discrete, with all champions starting off fairly weak but increasing in strength by accumulating items and experience over the course of the game. The champions and setting blend a variety of elements, including high fantasy, steampunk, and Lovecraftian horror.

League of Legends was generally well received upon its release in 2009, and has since grown in popularity, with an active and expansive fanbase. By July 2012, League of Legends was the most played PC game in North America and Europe in terms of the number of hours played. In January 2014, over 67 million people played League of Legends per month, 27 million per day, and over 7.5 million concurrently during peak hours. League has among the largest footprints of any game in streaming media communities on platforms such as YouTube and Twitch.tv; it routinely ranks first in the most-watched hours. In September 2016, the company estimated that there are over 100 million active players each month. The game's popularity has led it to expand into merchandise, with toys, accessories, apparel, as well as tie-ins to other media through music videos, web series, documentaries, and books.

League of Legends has an active and widespread competitive scene. In North America and Europe, Riot Games organizes the League Championship Series (LCS), located in Los Angeles and Berlin respectively, which consists of 10 professional teams in each continent. Similar regional competitions exist in China (LPL), South Korea (LCK), Taiwan/Hong Kong/Macau (LMS), and various other regions. These regional competitions culminate with the annual World Championship. The 2017 World Championship had 60 million unique viewers and a total prize pool of over 4 million USD. The 2018 Mid-Season Invitational had an overall peak concurrent viewership of 19.8 million, while the finals had an average concurrent viewership of 11 million.

Oculus Rift

The Oculus Rift is a virtual reality headset developed and manufactured by Oculus VR, a division of Facebook Inc., released on March 28, 2016.

Oculus initiated a Kickstarter campaign in 2012 to fund the Rift's development, after being founded as an independent company two months prior. The project proved successful, raising US$2.5 million. In March 2014, Facebook purchased Oculus for $2 billion. In March 2017, after three years at the company, it was announced Oculus founder and creator Palmer Luckey was leaving Facebook.

The Rift has gone through various pre-production models since the Kickstarter campaign, around five of which were demonstrated to the public. Two of these models were shipped to backers, labelled as development kits; the DK1 in mid 2013 and DK2 in mid-2014, to give developers a chance to develop content in time for the Rift's release. However, both were also purchased by a large number of enthusiasts who wished to get an early preview of the technology.The Rift has two Pentile OLED displays, 1080×1200 resolution per eye, a 90 Hz refresh rate, and 110° field of view. The device also features rotational and positional tracking, and integrated headphones that provide a 3D audio effect.

Oculus VR

Oculus VR is an American technology company founded by Palmer Luckey, Brendan Iribe, Michael Antonov and Nate Mitchell in July 2012 in Irvine, California, now based in Menlo Park. It specializes in virtual reality hardware and software products.

In April 2012, Luckey announced the Rift, a virtual reality headset designed for video gaming, and launched a Kickstarter campaign in August to make virtual reality headsets available to developers. The campaign proved successful and raised $2.4 million, ten times the original goal of $250,000. Two pre-production models were released to developers; the Oculus VR DK1 (Development Kit 1) and Oculus VR DK2 (Development Kit 2). The consumer product was released on March 28, 2016 with an all-new design incorporating specialized VR displays, positional audio, and infrared tracking system.In March 2014, Facebook CEO Mark Zuckerberg agreed to acquire Oculus VR for US$2.3 billion in cash and stock. In 2015, Oculus VR acquired Surreal Vision, a British startup focused on 3D reconstruction and mixed reality, stating that it could be possible for Oculus VR to develop products with the concept of telepresence.

The company also partnered with Samsung to develop the Samsung Gear VR in November 2015 for the Samsung Galaxy smartphones.

Polygon (website)

Polygon is an American video game website that publishes news, culture, reviews, and videos. At its October 2012 launch as Vox Media's third property, Polygon sought to distinguish itself from competitors by focusing on the stories of the people behind the games instead of the games themselves. They also produced long-form magazine-style feature articles, invested in video content, and chose to let their review scores be updated as the game changed.

The site was built over the course of ten months, and its 16-person founding staff included the editors-in-chief of the gaming sites Joystiq, Kotaku and The Escapist. Its design was built to HTML5 responsive standards with a pink color scheme, and its advertisements focused on direct sponsorship of specific kinds of content. Vox Media produced a documentary series on the founding of the site.

Rift Valley Province

Rift Valley Province (Swahili: Mkoa wa Bonde la Ufa) of Kenya, bordering Uganda, was one of Kenya's eight provinces, before the Kenyan general election, 2013.

Rift Valley Province was the largest and one of the most economically important provinces in Kenya. It was dominated by the Kenya Rift Valley which passes through it and gives the province its name. According to the 2009 Census, the former province covered an area of 182,505.1 square kilometres (45,098,000 acres; 70,465.6 sq mi) and would have had a population of 10,006,805, making it the largest and most populous province in the country. The bulk of the provincial population is a strip between former Nairobi and Nyanza Province. The capital was the town of Nakuru.

Rift Valley fever

Rift Valley fever (RVF) is a viral disease that can cause mild to severe symptoms. The mild symptoms may include: fever, muscle pains, and headaches which often last for up to a week. The severe symptoms may include: loss of sight beginning three weeks after the infection, infections of the brain causing severe headaches and confusion, and bleeding together with liver problems which may occur within the first few days. Those who have bleeding have a chance of death as high as 50%.The disease is caused by the RVF virus, which is of the Phlebovirus type. It is spread by either touching infected animal blood, breathing in the air around an infected animal being butchered, drinking raw milk from an infected animal, or the bite of infected mosquitoes. Animals such as cows, sheep, goats, and camels may be affected. In these animals it is spread mostly by mosquitoes. It does not appear that one person can infect another person. The disease is diagnosed by finding antibodies against the virus or the virus itself in the blood.Prevention of the disease in humans is accomplished by vaccinating animals against the disease. This must be done before an outbreak occurs because if it is done during an outbreak it may worsen the situation. Stopping the movement of animals during an outbreak may also be useful, as may decreasing mosquito numbers and avoiding their bites. There is a human vaccine; however, as of 2010 it is not widely available. There is no specific treatment and medical efforts are supportive.Outbreaks of the disease have only occurred in Africa and Arabia. Outbreaks usually occur during periods of increased rain which increase the number of mosquitoes. The disease was first reported among livestock in Rift Valley of Kenya in the early 1900s, and the virus was first isolated in 1931.

Rift Valley lakes

The Rift Valley lakes are a group of lakes in the East African Rift that runs north-south through the eastern side of the African continent, from Ethiopia in the north to Malawi in the south. These lakes include some of the oldest, largest, and deepest lakes in the world. Many are freshwater ecoregions of great biodiversity, while others are alkaline "soda lakes" supporting highly specialised organisms.

The Rift Valley lakes are well known for the evolution of at least 800 cichlid fish species that live in their waters. More species are expected to be discovered.The World Wide Fund for Nature has designated these lakes as one of its Global 200 priority ecoregions for conservation.

In this article, the major lakes are listed, generally in order from north to south, and more detailed articles on each lake can be accessed through the linked names.

Rift sawing

Rift-sawing is a woodworking process that aims to produce lumber that is less vulnerable to distortion than flat sawn lumber. Rift-sawing may be done strictly along a log's radials—perpendicular to the annular growth ring orientation or wood grain—or as part of the quarter sawing process.

Rift valley

A rift valley is a linear shaped lowland between several highlands or mountain ranges created by the action of a geologic rift or fault. A rift valley is formed on a divergent plate boundary, a crustal extension or spreading apart of the surface, which is subsequently further deepened by the forces of erosion. When the tensional forces are strong enough to cause the plate to split apart, a center block drops between the two blocks at its flanks, forming a graben. The drop of the center creates the nearly parallel steeply dipping walls of a rift valley when it is new. That feature is the beginning of the rift valley, but as the process continues, the valley widens, until it becomes a large basin that fills with sediment from the rift walls and the surrounding area. One of the best known examples of this process is the East African Rift. On Earth, rifts can occur at all elevations, from the sea floor to plateaus and mountain ranges in continental crust or in oceanic crust. They are often associated with a number of adjoining subsidiary or co-extensive valleys, which are typically considered part of the principal rift valley geologically.

Valley

A valley is a low area between hills or mountains often with a river running through it. In geology, a valley or dale is a depression that is longer than it is wide. The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys. Most valleys belong to one of these two main types or a mixture of them, at least with respect to the cross section of the slopes or hillsides.

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.