Friday, April 2, 2010
Africa
Africa is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² (11.7 million sq mi) including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people in 61 territories, it accounts for about 14.72% of the World's human population. The continent is surrounded by the Mediterranean Sea to the north, both the Suez Canal and the Red Sea along the Sinai Peninsula to the northeast, the Indian Ocean to the southeast, and the Atlantic Ocean to the west. The continent has 54 states, including Madagascar, various island groups, and the Sahrawi Arab Democratic Republic, a member state of the African Union whose statehood is disputed by Morocco.
Africa, particularly central eastern Africa, is widely regarded within the scientific community to be the origin of humans and the Hominidae clade , as evidenced by the discovery of the earliest hominids and their ancestors, as well as later ones that have been dated to around seven million years ago – including Sahelanthropus tchadensis, Australopithecus africanus, A. afarensis, Homo erectus, H. habilis and H. ergaster – with the earliest Homo sapiens found in Ethiopia being dated to ca. 200,000 years ago.
Africa straddles the equator and encompasses numerous climate areas; it is the only continent to stretch from the northern temperate to southern temperate zones.
At the beginning of the Mesozoic Era, Africa was joined with Earth's other continents in Pangaea. Africa shared the supercontinent's relatively uniform fauna which was dominated by theropods, prosauropods and primitive ornithischians by the close of the Triassic period. Late Triassic fossils are found through-out Africa, but are more common in the south than north. The boundary separating the Triassic and Jurassic marks the advent of an extinction event with global impact, although African strata from this time period have not been thoroughly studied.
Early Jurassic strata are distributed in a similar fashion to Late Triassic beds, with more common outcrops in the south and less common fossil beds which are predominated by tracks to the north. As the Jurassic proceeded, larger and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa. Middle Jurassic strata are neither well represented nor well studied in Africa. Late Jurassic strata are also poorly represented apart from the spectacular Tendaguru fauna in Tanzania. The Late Jurassic life of Tendaguru is very similar to that found in western North America's Morrison Formation.
Midway through the Mesozoic, about 150–160 million years ago, Madagascar separated from Africa, although it remained connected to India and the rest of the Gondwanan landmasses.Fossils from Madagascar include abelisaurs and titanosaurs.
Later into the Early Cretaceous epoch, the India-Madagascar landmass separated from the rest of Gondwana. By the Late Cretaceous, Madagascar and India had permanently split ways and continued until later reaching their modern configurations.
By contrast to Madagascar, mainland Africa was relatively stable in position through-out the Mesozoic. Despite the stable position, major changes occurred to its relation to other landmasses as the remains of Pangea continued to break apart. By the beginning of the Late Cretaceous epoch South America had split off from Africa, completing the southern half of the Atlantic Ocean. This event had a profound effect on global climate by altering ocean currents.
During the Cretaceous, Africa was populated by allosauroids and spinosaurids, including the largest known carnivorous dinosaurs. Titanosaurs were significant herbivores in its ancient ecosystems. Cretaceous sites are more common than Jurassic ones, but are often unable to be dated radiometrically making it difficult to know their exact ages. Paleontologist Louis Jacobs, who spent time doing field work in Malawi, says that African beds are "in need of more field work" and will prove to be a "fertile ground ... for discovery."
Africa is considered by most paleoanthropologists to be the oldest inhabited territory on Earth, with the human species originating from the continent. During the middle of the twentieth century, anthropologists discovered many fossils and evidence of human occupation perhaps as early as 7 million years ago. Fossil remains of several species of early apelike humans thought to have evolved into modern man, such as Australopithecus afarensis (radiometrically dated to approximately 3.9–3.0 million years BC), Paranthropus boisei (c. 2.3–1.4 million years BC) and Homo ergaster (c. 1.9 million–600,000 years BC) have been discovered.
Throughout humanity's prehistory, Africa had no nation states, and was instead inhabited by groups of hunter-gatherers such as the Khoi and San.
At the end of the Ice Ages, estimated to have been around 10,500 BC, the Sahara had again become a green fertile valley, and its African populations returned from the interior and coastal highlands in Sub-Saharan Africa[citation needed]. However, the warming and drying climate meant that by 5000 BC the Sahara region was becoming increasingly dry and hostile. The population trekked out of the Sahara region towards the Nile Valley below the Second Cataract where they made permanent or semi-permanent settlements. A major climatic recession occurred, lessening the heavy and persistent rains in Central and Eastern Africa. Since this time dry conditions have prevailed in Eastern Africa, and increasingly during the last 200 years, in Ethiopia.
The domestication of cattle in Africa preceded agriculture and seems to have existed alongside hunter-gathering cultures. It is speculated that by 6000 BC cattle were already domesticated in North Africa. In the Sahara-Nile complex, people domesticated many animals including the donkey, and a small screw-horned goat which was common from Algeria to Nubia. In the year 4000 BC the climate of the Sahara started to become drier at an exceedingly fast pace. This climate change caused lakes and rivers to shrink significantly and caused increasing desertification. This, in turn, decreased the amount of land conducive to settlements and helped to cause migrations of farming communities to the more tropical climate of West Africa.
By the first millennium BC ironworking had been introduced in Northern Africa and quickly spread across the Sahara into the northern parts of sub-Saharan Africa and by 500 BC metalworking began to become commonplace in West Africa. Ironworking was fully established by roughly 500 BC in many areas of East and West Africa, although other regions didn't begin ironworking until the early centuries AD. Copper objects from Egypt, North Africa, Nubia and Ethiopia dating from around 500 BC have been excavated in West Africa, suggesting that trans-saharan trade networks had been established by this date.
Africa is the largest of the three great southward projections from the largest landmass of the Earth. Separated from Europe by the Mediterranean Sea, it is joined to Asia at its northeast extremity by the Isthmus of Suez , 163 km (101 miles) wide. From the most northerly point, Ras ben Sakka in Tunisia (37°21' N), to the most southerly point, Cape Agulhas in South Africa (34°51'15" S), is a distance of approximately 8,000 km (5,000 miles);[48] from Cape Verde, 17°33'22" W, the westernmost point, to Ras Hafun in Somalia, 51°27'52" E, the most easterly projection, is a distance of approximately 7,400 km (4,600 miles). The coastline is 26,000 km (16,100 miles) long, and the absence of deep indentations of the shore is illustrated by the fact that Europe, which covers only 10,400,000 km² (4,010,000 square miles) – about a third of the surface of Africa – has a coastline of 32,000 km (19,800 miles).
Africa's largest country is Sudan, and its smallest country is the Seychelles, an archipelago off the east coast.The smallest nation on the continental mainland is The Gambia.
According to the ancient Romans, Africa lay to the west of Egypt, while "Asia" was used to refer to Anatolia and lands to the east. A definite line was drawn between the two continents by the geographer Ptolemy (85–165 AD), indicating Alexandria along the Prime Meridian and making the isthmus of Suez and the Red Sea the boundary between Asia and Africa. As Europeans came to understand the real extent of the continent, the idea of Africa expanded with their knowledge.
Geologically, Africa includes the Arabian Peninsula; the Zagros Mountains of Iran and the Anatolian Plateau of Turkey mark where the African Plate collided with Eurasia. The Afrotropic ecozone and the Saharo-Arabian desert to its north unite the region biogeographically, and the Afro-Asiatic language family unites the north linguistically.
Africa, particularly central eastern Africa, is widely regarded within the scientific community to be the origin of humans and the Hominidae clade , as evidenced by the discovery of the earliest hominids and their ancestors, as well as later ones that have been dated to around seven million years ago – including Sahelanthropus tchadensis, Australopithecus africanus, A. afarensis, Homo erectus, H. habilis and H. ergaster – with the earliest Homo sapiens found in Ethiopia being dated to ca. 200,000 years ago.
Africa straddles the equator and encompasses numerous climate areas; it is the only continent to stretch from the northern temperate to southern temperate zones.
At the beginning of the Mesozoic Era, Africa was joined with Earth's other continents in Pangaea. Africa shared the supercontinent's relatively uniform fauna which was dominated by theropods, prosauropods and primitive ornithischians by the close of the Triassic period. Late Triassic fossils are found through-out Africa, but are more common in the south than north. The boundary separating the Triassic and Jurassic marks the advent of an extinction event with global impact, although African strata from this time period have not been thoroughly studied.
Early Jurassic strata are distributed in a similar fashion to Late Triassic beds, with more common outcrops in the south and less common fossil beds which are predominated by tracks to the north. As the Jurassic proceeded, larger and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa. Middle Jurassic strata are neither well represented nor well studied in Africa. Late Jurassic strata are also poorly represented apart from the spectacular Tendaguru fauna in Tanzania. The Late Jurassic life of Tendaguru is very similar to that found in western North America's Morrison Formation.
Midway through the Mesozoic, about 150–160 million years ago, Madagascar separated from Africa, although it remained connected to India and the rest of the Gondwanan landmasses.Fossils from Madagascar include abelisaurs and titanosaurs.
Later into the Early Cretaceous epoch, the India-Madagascar landmass separated from the rest of Gondwana. By the Late Cretaceous, Madagascar and India had permanently split ways and continued until later reaching their modern configurations.
By contrast to Madagascar, mainland Africa was relatively stable in position through-out the Mesozoic. Despite the stable position, major changes occurred to its relation to other landmasses as the remains of Pangea continued to break apart. By the beginning of the Late Cretaceous epoch South America had split off from Africa, completing the southern half of the Atlantic Ocean. This event had a profound effect on global climate by altering ocean currents.
During the Cretaceous, Africa was populated by allosauroids and spinosaurids, including the largest known carnivorous dinosaurs. Titanosaurs were significant herbivores in its ancient ecosystems. Cretaceous sites are more common than Jurassic ones, but are often unable to be dated radiometrically making it difficult to know their exact ages. Paleontologist Louis Jacobs, who spent time doing field work in Malawi, says that African beds are "in need of more field work" and will prove to be a "fertile ground ... for discovery."
Africa is considered by most paleoanthropologists to be the oldest inhabited territory on Earth, with the human species originating from the continent. During the middle of the twentieth century, anthropologists discovered many fossils and evidence of human occupation perhaps as early as 7 million years ago. Fossil remains of several species of early apelike humans thought to have evolved into modern man, such as Australopithecus afarensis (radiometrically dated to approximately 3.9–3.0 million years BC), Paranthropus boisei (c. 2.3–1.4 million years BC) and Homo ergaster (c. 1.9 million–600,000 years BC) have been discovered.
Throughout humanity's prehistory, Africa had no nation states, and was instead inhabited by groups of hunter-gatherers such as the Khoi and San.
At the end of the Ice Ages, estimated to have been around 10,500 BC, the Sahara had again become a green fertile valley, and its African populations returned from the interior and coastal highlands in Sub-Saharan Africa[citation needed]. However, the warming and drying climate meant that by 5000 BC the Sahara region was becoming increasingly dry and hostile. The population trekked out of the Sahara region towards the Nile Valley below the Second Cataract where they made permanent or semi-permanent settlements. A major climatic recession occurred, lessening the heavy and persistent rains in Central and Eastern Africa. Since this time dry conditions have prevailed in Eastern Africa, and increasingly during the last 200 years, in Ethiopia.
The domestication of cattle in Africa preceded agriculture and seems to have existed alongside hunter-gathering cultures. It is speculated that by 6000 BC cattle were already domesticated in North Africa. In the Sahara-Nile complex, people domesticated many animals including the donkey, and a small screw-horned goat which was common from Algeria to Nubia. In the year 4000 BC the climate of the Sahara started to become drier at an exceedingly fast pace. This climate change caused lakes and rivers to shrink significantly and caused increasing desertification. This, in turn, decreased the amount of land conducive to settlements and helped to cause migrations of farming communities to the more tropical climate of West Africa.
By the first millennium BC ironworking had been introduced in Northern Africa and quickly spread across the Sahara into the northern parts of sub-Saharan Africa and by 500 BC metalworking began to become commonplace in West Africa. Ironworking was fully established by roughly 500 BC in many areas of East and West Africa, although other regions didn't begin ironworking until the early centuries AD. Copper objects from Egypt, North Africa, Nubia and Ethiopia dating from around 500 BC have been excavated in West Africa, suggesting that trans-saharan trade networks had been established by this date.
Africa is the largest of the three great southward projections from the largest landmass of the Earth. Separated from Europe by the Mediterranean Sea, it is joined to Asia at its northeast extremity by the Isthmus of Suez , 163 km (101 miles) wide. From the most northerly point, Ras ben Sakka in Tunisia (37°21' N), to the most southerly point, Cape Agulhas in South Africa (34°51'15" S), is a distance of approximately 8,000 km (5,000 miles);[48] from Cape Verde, 17°33'22" W, the westernmost point, to Ras Hafun in Somalia, 51°27'52" E, the most easterly projection, is a distance of approximately 7,400 km (4,600 miles). The coastline is 26,000 km (16,100 miles) long, and the absence of deep indentations of the shore is illustrated by the fact that Europe, which covers only 10,400,000 km² (4,010,000 square miles) – about a third of the surface of Africa – has a coastline of 32,000 km (19,800 miles).
Africa's largest country is Sudan, and its smallest country is the Seychelles, an archipelago off the east coast.The smallest nation on the continental mainland is The Gambia.
According to the ancient Romans, Africa lay to the west of Egypt, while "Asia" was used to refer to Anatolia and lands to the east. A definite line was drawn between the two continents by the geographer Ptolemy (85–165 AD), indicating Alexandria along the Prime Meridian and making the isthmus of Suez and the Red Sea the boundary between Asia and Africa. As Europeans came to understand the real extent of the continent, the idea of Africa expanded with their knowledge.
Geologically, Africa includes the Arabian Peninsula; the Zagros Mountains of Iran and the Anatolian Plateau of Turkey mark where the African Plate collided with Eurasia. The Afrotropic ecozone and the Saharo-Arabian desert to its north unite the region biogeographically, and the Afro-Asiatic language family unites the north linguistically.
The Strait of Gibraltar
The Strait of Gibraltar is a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Spain from Morocco. The name comes from Gibraltar, which in turn originates from the Arabic Jebel Tariq , albeit the Arab name for the Strait is Bab el-Zakat or "Gate of Charity". It is also erroneously known as the Straits of Gibraltar, in naval use and as "Pillars of Hercules" in the ancient world.
Europe and Africa are separated by 7.7 nautical miles (14.24 km) of ocean at the strait's narrowest point. The Strait's depth ranges between 300 and 900 metres (980 and 3,000 ft) which possibly interacted with the lower mean sea level of the last major glaciation 20,000 years before present when the level of the sea was believed to be 110 to 120 metres (361 to 394 ft) lower. Ferries cross between the two continents every day in as little as 35 minutes. The Spanish side of the Strait is protected under El Estrecho Natural Park.
Around 5.9 million years ago, the connection between the Mediterranean Sea and the Atlantic Ocean along the Bethic and Rifan Corridor was progressively restricted until its total closure, effectively causing the salinity of the Mediterranean to periodically fall within the gypsum and salt deposition range, during what is known as the Messinian Salinity Crisis. In this water chemistry environment, dissolved mineral concentrations, temperature and stilled water currents combined properly and occurred regularly to precipitate many mineral salts in sea floor bedded layers. The resultant accumulation of various huge salt and mineral deposits about the Mediterranean basin are directly linked to this era. It is not believed this process took a long time geologically, lasting only 500-600 thousand-years, for it is further estimated that were the straits closed even at today's higher sea level, most water in the Mediterranean basin would evaporate within only a thousand years; as it is believed to have done then, and such an event would lay down similar mineral deposits as those such as the fabulous salt mines now found under the sea floor off Sicily. After a lengthy period of restricted intermittent or no water exchange between the Atlantic Ocean and Mediterranean basin, approximately 5.33 million years ago, the Atlantic-Mediterranean connection was completely reestablished through the Strait of Gibraltar, and has remained open ever since. It is expected that the strait will close again as the African Plate moves northward relative to the Eurasian Plate[citation needed], but on geological rather than human timescales.
Through the strait, water generally flows more or less continually in both an eastward and a westward direction. A smaller amount of deeper saltier and therefore denser waters continually work their way westwards , while a larger amount of surface waters with lower salinity and density continually work their way eastwards . These general flow tendencies may be occasionally interrupted for brief periods to accommodate temporary tidal flow requirements, depending on various lunar and solar alignments. Still, on the whole and over time, the balance of the water flow is eastwards, due to an evaporation rate within the Mediterranean basin higher than the combined inflow of all the rivers that empty into it. The shallow Camarinal Sill of the Strait of Gibraltar, which forms the shallowest point within the strait, acts to limit mixing between the cold, less saline Atlantic water and the warm Mediterranean waters. The Camarinal Sill is located at the far western end of the straits.
The Mediterranean waters are so much saltier than the Atlantic waters, that they sink below the constantly incoming water and form a highly saline layer of bottom water. This layer of bottom-water constantly works its way out into the Atlantic as the Mediterranean outflow. On the Atlantic side of the strait, a density boundary separates the Mediterranean outflow waters from the rest at about 100 metres (330 ft) depth. These waters flow out and down the continental slope, losing salinity, until they begin to mix and equilibrate more rapidly, much further out at a depth of about 1,000 metres (3,300 ft). The Mediterranean outflow water layer can be traced for thousands of kilometres west of the strait, before completely losing its identity.
Internal waves are often produced by the strait. Like traffic merging on a highway, the water flow is constricted in both directions because it must pass over a shallow submarine barrier, the Camarinal Sill. When large tidal flows enter the Strait and the high tide relaxes, internal waves are generated at the Camarinal Sill and proceed eastwards. Even though the waves may occur down to great depths, occasionally at the surface the waves are almost imperceptible, at other times they can be seen clearly using satellite imagery. These internal waves continue to flow eastward and to refract around coastal features. They can sometimes be traced for as much as 100 kilometres (62 mi), and sometimes create interference patterns with refracted waves.
Europe and Africa are separated by 7.7 nautical miles (14.24 km) of ocean at the strait's narrowest point. The Strait's depth ranges between 300 and 900 metres (980 and 3,000 ft) which possibly interacted with the lower mean sea level of the last major glaciation 20,000 years before present when the level of the sea was believed to be 110 to 120 metres (361 to 394 ft) lower. Ferries cross between the two continents every day in as little as 35 minutes. The Spanish side of the Strait is protected under El Estrecho Natural Park.
Around 5.9 million years ago, the connection between the Mediterranean Sea and the Atlantic Ocean along the Bethic and Rifan Corridor was progressively restricted until its total closure, effectively causing the salinity of the Mediterranean to periodically fall within the gypsum and salt deposition range, during what is known as the Messinian Salinity Crisis. In this water chemistry environment, dissolved mineral concentrations, temperature and stilled water currents combined properly and occurred regularly to precipitate many mineral salts in sea floor bedded layers. The resultant accumulation of various huge salt and mineral deposits about the Mediterranean basin are directly linked to this era. It is not believed this process took a long time geologically, lasting only 500-600 thousand-years, for it is further estimated that were the straits closed even at today's higher sea level, most water in the Mediterranean basin would evaporate within only a thousand years; as it is believed to have done then, and such an event would lay down similar mineral deposits as those such as the fabulous salt mines now found under the sea floor off Sicily. After a lengthy period of restricted intermittent or no water exchange between the Atlantic Ocean and Mediterranean basin, approximately 5.33 million years ago, the Atlantic-Mediterranean connection was completely reestablished through the Strait of Gibraltar, and has remained open ever since. It is expected that the strait will close again as the African Plate moves northward relative to the Eurasian Plate[citation needed], but on geological rather than human timescales.
Through the strait, water generally flows more or less continually in both an eastward and a westward direction. A smaller amount of deeper saltier and therefore denser waters continually work their way westwards , while a larger amount of surface waters with lower salinity and density continually work their way eastwards . These general flow tendencies may be occasionally interrupted for brief periods to accommodate temporary tidal flow requirements, depending on various lunar and solar alignments. Still, on the whole and over time, the balance of the water flow is eastwards, due to an evaporation rate within the Mediterranean basin higher than the combined inflow of all the rivers that empty into it. The shallow Camarinal Sill of the Strait of Gibraltar, which forms the shallowest point within the strait, acts to limit mixing between the cold, less saline Atlantic water and the warm Mediterranean waters. The Camarinal Sill is located at the far western end of the straits.
The Mediterranean waters are so much saltier than the Atlantic waters, that they sink below the constantly incoming water and form a highly saline layer of bottom water. This layer of bottom-water constantly works its way out into the Atlantic as the Mediterranean outflow. On the Atlantic side of the strait, a density boundary separates the Mediterranean outflow waters from the rest at about 100 metres (330 ft) depth. These waters flow out and down the continental slope, losing salinity, until they begin to mix and equilibrate more rapidly, much further out at a depth of about 1,000 metres (3,300 ft). The Mediterranean outflow water layer can be traced for thousands of kilometres west of the strait, before completely losing its identity.
Internal waves are often produced by the strait. Like traffic merging on a highway, the water flow is constricted in both directions because it must pass over a shallow submarine barrier, the Camarinal Sill. When large tidal flows enter the Strait and the high tide relaxes, internal waves are generated at the Camarinal Sill and proceed eastwards. Even though the waves may occur down to great depths, occasionally at the surface the waves are almost imperceptible, at other times they can be seen clearly using satellite imagery. These internal waves continue to flow eastward and to refract around coastal features. They can sometimes be traced for as much as 100 kilometres (62 mi), and sometimes create interference patterns with refracted waves.
Thursday, April 1, 2010
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