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, 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 associated with a number of adjoining subsidiary or co-extensive valleys, which are considered part of the principal rift valley geologically.
The most extensive rift valley is located along the crest of the mid-ocean ridge system and is the result of sea floor spreading. Examples of this type of rift include the East Pacific Rise. Many existing continental rift valleys are the result of a failed arm of a triple junction, although there are two, the East African Rift and the Baikal Rift Zone, which are active, as well as a third which may be, the West Antarctic Rift. In these instances, not only the crust, but entire tectonic plates, are in the process of breaking apart to create new plates. If they continue, continental rifts will become oceanic rifts. Other rift valleys are the result of discontinuities in horizontally-moving faults; when these bends or discontinuities are in the same direction as the relative motions along the fault, extension occurs. For example, for a right lateral-moving fault, a bend to the right will result in stretching and consequent subsidence in the area of the irregularity. In the view of many geologists today, the Dead Sea lies in a rift which results from a leftward discontinuity in the left lateral-moving Dead Sea Transform fault.
Where a fault breaks into two strands, or two faults run close to each other, crustal extension may occur between them, as a result of differences in their motions. Both types of fault-caused extension occur on a small scale, producing such features as sag ponds or landslides. Many of the world's largest lakes are located in rift valleys. Lake Baikal in Siberia, a World Heritage Site, lies in an active rift valley. Baikal is both the deepest lake in the world and, with 20% of all of the liquid freshwater on earth, has the greatest volume. Lake Tanganyika, second by both measures, is in the Albertine Rift, the westernmost arm of the active East African Rift. Lake Superior in North America, the largest freshwater lake by area, lies in the ancient and dormant Midcontinent Rift; the largest subglacial lake, Lake Vostok, may lie in an ancient rift valley. Lake Nipissing and Lake Timiskaming in Ontario and Quebec, Canada lie inside a rift valley called the Ottawa-Bonnechere Graben. Þingvallavatn, Iceland's largest natural lake, is an example of a rift lake.
Extraterrestrial rift valleys are known to occur on other terrestrial planets and natural satellites. The 4,000 km long Valles Marineris on Mars is believed by planetary geologists to be a large rift system; some features of Venus, most notably, the 4,000 km Devana Chasma and a part of the western Eistla, also Alta and Bell Regio have been interpreted by some planetary geologists as a rift valleys. Some natural satellites have prominent rift valleys; the 2,000 km long Ithaca Chasma on Tethys in the Saturn system is a prominent example. Charon's Nostromo Chasma is the first confirmed in the Pluto system, however large chasms up to 950 km wide observed on Charon have been tentatively interpreted by some as giant rifts, similar formations have been noted on Pluto. A recent study suggests a complex system of ancient lunar rift valleys, including Vallis Rheita and Vallis Alpes; the Uranus system has prominent examples, with large'chasma' believed to be giant rift valley systems, most notably the 1492 km long Messina Chasma on Titania, 622 km Kachina Chasmata on Ariel, Verona Rupes on Miranda, Mommur Chasma on Oberon.
Bonatti, E. "Punctiform initiation of seafloor spreading in the Red Sea during transition from a continental to an oceanic rift". Nature. 316: 33–37. Bibcode:1985Natur.316...33B. Doi:10.1038/316033a0. Mart, Y.. "Analogue experiments of propagation of oblique rifts". Tectonophysics. 316: 121–132. Bibcode:2000Tectp.316..121M. Doi:10.1016/s0040-195100231-0
The Eurasian Plate is a tectonic plate which includes most of the continent of Eurasia, with the notable exceptions of the Indian subcontinent, the Arabian subcontinent, the area east of the Chersky Range in East Siberia. It includes oceanic crust extending westward to the Mid-Atlantic Ridge and northward to the Gakkel Ridge; the eastern side is a boundary with the North American Plate to the north and a boundary with the Philippine Sea Plate to the south and with the Okhotsk Plate and the Amurian Plate. The southerly side is a boundary with the African Plate to the west, the Arabian Plate in the middle and the Indo-Australian Plate to the east; the westerly side is a divergent boundary with the North American Plate forming the northernmost part of the Mid-Atlantic Ridge, straddled by Iceland. All of the volcanic eruptions in Iceland, such as the 1973 eruption of Eldfell, the 1783 eruption of Laki, the 2010 eruption of Eyjafjallajökull, are caused by the North American and the Eurasian plates moving apart, a result of divergent plate boundary forces.
The geodynamics of central Asia is dominated by the interaction between the Eurasian and Indian Plates. In this area, many subplates or crust blocks have been recognized, which form the Central Asian and the East Asian transit zones. Sunda Plate
An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities; the seismicity, or seismic activity, of an area is the frequency and size of earthquakes experienced over a period of time. The word tremor is used for non-earthquake seismic rumbling. At the Earth's surface, earthquakes manifest themselves by shaking and displacing or disrupting the ground; when the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can trigger landslides, volcanic activity. In its most general sense, the word earthquake is used to describe any seismic event—whether natural or caused by humans—that generates seismic waves. Earthquakes are caused by rupture of geological faults, but by other events such as volcanic activity, mine blasts, nuclear tests.
An earthquake's point of initial rupture is called its hypocenter. The epicenter is the point at ground level directly above the hypocenter. Tectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane; the sides of a fault move past each other smoothly and aseismically only if there are no irregularities or asperities along the fault surface that increase the frictional resistance. Most fault surfaces do have such asperities and this leads to a form of stick-slip behavior. Once the fault has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface; this continues until the stress has risen sufficiently to break through the asperity allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, cracking of the rock, thus causing an earthquake.
This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior. There are three main types of fault, all of which may cause an interplate earthquake: normal and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas.
Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip. Reverse faults those along convergent plate boundaries are associated with the most powerful earthquakes, megathrust earthquakes, including all of those of magnitude 8 or more. Strike-slip faults continental transforms, can produce major earthquakes up to about magnitude 8. Earthquakes associated with normal faults are less than magnitude 7. For every unit increase in magnitude, there is a thirtyfold increase in the energy released. For instance, an earthquake of magnitude 6.0 releases 30 times more energy than a 5.0 magnitude earthquake and a 7.0 magnitude earthquake releases 900 times more energy than a 5.0 magnitude of earthquake. An 8.6 magnitude earthquake releases the same amount of energy as 10,000 atomic bombs like those used in World War II. This is so because the energy released in an earthquake, thus its magnitude, is proportional to the area of the fault that ruptures and the stress drop.
Therefore, the longer the length and the wider the width of the faulted area, the larger the resulting magnitude. The topmost, brittle part of the Earth's crust, the cool slabs of the tectonic plates that are descending down into the hot mantle, are the only parts of our planet which can store elastic energy and release it in fault ruptures. Rocks hotter than about 300 °C flow in response to stress; the maximum observed lengths of ruptures and mapped faults are 1,000 km. Examples are the earthquakes in Chile, 1960; the longest earthquake ruptures on strike-slip faults, like the San Andreas Fault, the North Anatolian Fault in Turkey and the Denali Fault in Alaska, are about half to one third as long as the lengths along subducting plate margins, those along normal faults are shorter. The most important parameter controlling the maximum earthquake magnitude on a fault is however not the maximum available length, but the available width because the latter varies by a factor of 20. Along converging plate margins, the dip angle of the rupture plane is shallow about 10 de
Pangaea or Pangea was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from earlier continental units 335 million years ago, it began to break apart about 175 million years ago. In contrast to the present Earth and its distribution of continental mass, much of Pangaea was in the southern hemisphere and surrounded by a superocean, Panthalassa. Pangaea was the most recent supercontinent to have existed and the first to be reconstructed by geologists; the name "Pangaea/Pangea" is derived from Gaia. The concept that the continents once formed a contiguous land mass was first proposed by Alfred Wegener, the originator of the scientific theory of continental drift, in his 1912 publication The Origin of Continents, he expanded upon his hypothesis in his 1915 book The Origin of Continents and Oceans, in which he postulated that, before breaking up and drifting to their present locations, all the continents had formed a single supercontinent that he called the "Urkontinent".
The name "Pangea" occurs in the 1920 edition of Die Entstehung der Kontinente und Ozeane, but only once, when Wegener refers to the ancient supercontinent as "the Pangaea of the Carboniferous". Wegener used the Germanized form "Pangäa", but the name entered German and English scientific literature in the Latinized form "Pangaea" due to a symposium of the American Association of Petroleum Geologists in November 1926; the forming of supercontinents and their breaking up appears to have been cyclical through Earth's history. There may have been several others before Pangaea; the fourth-last supercontinent, called Columbia or Nuna, appears to have assembled in the period 2.0–1.8 Ga. Columbia/Nuna broke up and the next supercontinent, formed from the accretion and assembly of its fragments. Rodinia lasted from about 1.1 billion years ago until about 750 million years ago, but its exact configuration and geodynamic history are not nearly as well understood as those of the supercontinents and Pangaea.
When Rodinia broke up, it split into three pieces: the supercontinent of Proto-Laurasia, the supercontinent of Proto-Gondwana, the smaller Congo craton. Proto-Laurasia and Proto-Gondwana were separated by the Proto-Tethys Ocean. Next Proto-Laurasia itself split apart to form the continents of Laurentia and Baltica. Baltica moved to the east of Laurentia, Siberia moved northeast of Laurentia; the splitting created two new oceans, the Iapetus Ocean and Paleoasian Ocean. Most of the above masses coalesced again to form the short-lived supercontinent of Pannotia; this supercontinent included large amounts of land near the poles and, near the equator, only a small strip connecting the polar masses. Pannotia lasted until 540 Ma, near the beginning of the Cambrian period and broke up, giving rise to the continents of Laurentia and the southern supercontinent of Gondwana. In the Cambrian period, the continent of Laurentia, which would become North America, sat on the equator, with three bordering oceans: the Panthalassic Ocean to the north and west, the Iapetus Ocean to the south and the Khanty Ocean to the east.
In the Earliest Ordovician, around 480 Ma, the microcontinent of Avalonia – a landmass incorporating fragments of what would become eastern Newfoundland, the southern British Isles, parts of Belgium, northern France, Nova Scotia, New England, South Iberia and northwest Africa – broke free from Gondwana and began its journey to Laurentia. Baltica and Avalonia all came together by the end of the Ordovician to form a minor supercontinent called Euramerica or Laurussia, closing the Iapetus Ocean; the collision resulted in the formation of the northern Appalachians. Siberia sat with the Khanty Ocean between the two continents. While all this was happening, Gondwana drifted towards the South Pole; this was the first step of the formation of Pangaea. The second step in the formation of Pangaea was the collision of Gondwana with Euramerica. By the Silurian, 440 Ma, Baltica had collided with Laurentia, forming Euramerica. Avalonia had not yet collided with Laurentia, but as Avalonia inched towards Laurentia, the seaway between them, a remnant of the Iapetus Ocean, was shrinking.
Meanwhile, southern Europe broke off from Gondwana and began to move towards Euramerica across the newly formed Rheic Ocean. It collided with southern Baltica in the Devonian, though this microcontinent was an underwater plate; the Iapetus Ocean's sister ocean, the Khanty Ocean, shrank as an island arc from Siberia collided with eastern Baltica. Behind this island arc was a new ocean, the Ural Ocean. By the late Silurian and South China split from Gondwana and started to head northward, shrinking the Proto-Tethys Ocean in their path and opening the new Paleo-Tethys Ocean to their south. In the Devonian Period, Gondwana itself headed towards Euramerica, causing the Rheic Ocean to shrink. In the Early Carboniferous, northwest Africa had touched the southeastern coast of Euramerica, creating the southern portion of the Appalachian Mountains, the Meseta Mountains and the Mauritanide Mountains. South America moved northward to southern Euramerica, while the eastern portion of Gondwana headed toward the South Pole from the equator.
North and South China were on independent continents. The Kazakhstania microcontinent had collided with Siberia. (Siberia had been a separate continent for millions of years since the deformation of the supercontinent Pannotia in the Middle Carbo
Hvannadalshnúkur or Hvannadalshnjúkur is a pyramidal peak on the northwestern rim of the summit crater of the Öræfajökull volcano in Iceland and is the highest in Iceland. An official measurement completed in August 2005 established the height of the mountain as 2,109.6 metres. The peak is part of the Vatnajökull National Park; the route to the top is a popular climb through numerous and hidden crevasses, because of this, the climb calls for experienced mountain guides. List of islands by highest point List of European ultra prominent peaks "Hvannadalshnúkur, Iceland" on Peakbagger
Ascension Island is an isolated volcanic island, 7°56' south of the Equator in the South Atlantic Ocean. It is about 1,600 kilometres from 2,250 kilometres from the coast of Brazil, it is governed as part of the British Overseas Territory of Saint Helena and Tristan da Cunha, of which the main island, Saint Helena, is around 1,300 kilometres to the southeast. The territory includes the sparsely-populated Tristan da Cunha archipelago, some 3,730 kilometres to the south, about halfway to the Antarctic Circle; the island is named after the day of Ascension Day. It was an important safe haven and coaling station to mariners and for commercial airliners during the days of international air travel by flying boats. During World War II it was an important naval and air station providing antisubmarine warfare bases in the Battle of the Atlantic. Ascension Island was garrisoned by the British Admiralty from 22 October 1815 to 1922; the island is the location of RAF Ascension Island, a Royal Air Force station, a European Space Agency rocket tracking station, an Anglo-American signals intelligence facility and the BBC World Service Atlantic Relay Station.
The island was used extensively as a staging point by the British military during the Falklands War. Ascension Island hosts one of four ground antennas that assist in the operation of the Global Positioning System navigational system. NASA operates a Meter Class Autonomous Telescope on Ascension Island for tracking orbital debris, hazardous to operating spacecraft and astronauts, at a facility called the John Africano NASA/AFRL Orbital Debris Observatory. In 1501, the Portuguese navigator Afonso de Albuquerque sighted the island on Ascension Day and named it Ilha da Ascensão after this feast day. Dry and barren, the island had little appeal for passing ships except for collecting fresh meat, was not claimed for the Portuguese Crown. Mariners could hunt for the numerous seabirds and the enormous female green turtles that laid their eggs on the sandy beaches; the Portuguese introduced goats as a potential source of meat for future mariners. In February 1701, HMS Roebuck, commanded by William Dampier, sank in the common anchoring spot in Clarence Bay to the northwest of the island.
Sixty men survived for two months. After a few days they found the strong water spring in the high interior of the island, in what is now called Breakneck Valley, it is possible that the island was sometimes used as an open prison for criminal mariners, although there is only one documented case of such an exile, a Dutch ship's officer, Leendert Hasenbosch, set ashore at Clarence Bay as a punishment for sodomy in May 1725. British mariners found the Dutchman's tent and diary in January 1726, his diary was published in translation in London that same year, under the title Sodomy Punish'd. Organised settlement of Ascension Island began in 1815, when the British garrisoned it as a precaution after imprisoning Napoleon I on Saint Helena to the southeast. Colonel Edward Nicolls was among those sent there as punishment for what the Crown perceived as excessive support for Seminole Indians during his posting on the future U. S. Gulf Coast during the War of 1812. On 22 October the Cruizer-class brig-sloops Zenobia and Peruvian claimed the island for King George III.
The Royal Navy designated the island as a stone frigate, HMS Ascension, with the classification of "Sloop of War of the smaller class". The location of the island made it a useful stopping-point for communications; the Royal Navy used the island as a victualling station for ships those of the West Africa Squadron working against the slave trade. A garrison of Royal Marines was based at Ascension from 1823. In 1836 the second Beagle voyage visited Ascension. Charles Darwin described it with nothing growing near the coast. Sparse vegetation inland supported "about six hundred sheep, many goats, a few cows & horses", large numbers of guineafowl imported from the Cape Verde islands, rats and land crabs, he noted the care taken to sustain "houses, gardens & fields placed near the summit of the central mountain", cisterns at roadsides to provide drinking water. The springs were managed, "so that a single drop of water may not be lost: indeed the whole island may be compared to a huge ship kept in first-rate order."
In commenting on this, he noted René Primevère Lesson's remark "that the English nation alone would have thought of making the island of Ascension a productive spot. Four years Hooker, with much encouragement from Darwin, advised the Royal Navy that with the help of Kew Gardens, they should institute a long-term plan of shipping trees to Ascension; the planted trees would capture more rain and improve the soil, allowing the barren island to become a garden. So, from 1850 and continuing year on year, ships came with an assortment of plants from botanical gardens in Argentina and South Africa. By the late 1870s Norfolk pines, eucalyptus and banana trees grew in profusion at the highest point of the is
The Azores the Autonomous Region of the Azores, is one of the two autonomous regions of Portugal. It is an archipelago composed of nine volcanic islands in the North Atlantic Ocean about 1,360 km west of continental Portugal, about 1,643 km west of Lisbon, in continental Portugal, about 1,507 km northwest of Morocco, about 1,925 km southeast of Newfoundland, Canada, its main industries are agriculture, dairy farming, livestock and tourism, becoming the major service activity in the region. In addition, the government of the Azores employs a large percentage of the population directly or indirectly in the service and tertiary sectors; the main capital of the Azores is Ponta Delgada. There are an islet cluster, in three main groups; these are Corvo, to the west. They lie in a northwest-southeast direction. All the islands have volcanic origins, although some, such as Santa Maria, have had no recorded activity since the islands were settled. Mount Pico, on the island of Pico, is the highest point in Portugal, at 2,351 m.
If measured from their base at the bottom of the ocean to their peaks, which thrust high above the surface of the Atlantic, the Azores are some of the tallest mountains on the planet. The climate of the Azores is mild for such a northerly location, being influenced by its distance from the continents and by the passing Gulf Stream. Due to the marine influence, temperatures remain mild year-round. Daytime temperatures fluctuate between 16 °C and 25 °C depending on season. Temperatures above 30 °C or below 3 °C are unknown in the major population centres, it is generally wet and cloudy. The culture, dialect and traditions of the Azorean islands vary because these once-uninhabited and remote islands were settled sporadically over a span of two centuries. A small number of alleged hypogea, earthen structures carved into rocks that were used for burials, have been identified on the islands of Corvo, Santa Maria and Terceira by Portuguese archaeologist Nuno Ribeiro, who speculated that they might date back 2000 years, implying a human presence on the island before the Portuguese.
These kinds of structures have been used in the Azores to store cereals and suggestions by Ribeiro that they might be burial sites are unconfirmed. Detailed examination and dating to authenticate the validity of these speculations is lacking, it is unclear whether these structures are natural or man-made and whether they predate the 15th-century Portuguese colonization of the Azores. Therefore, clear confirmation of a pre-Portuguese human presence in the archipelago has not yet been published; the islands were known in the fourteenth century, parts of them appear in the Catalan Atlas. In 1427, a captain sailing for Prince Henry the Navigator Gonçalo Velho, may have rediscovered the Azores, but this is not certain. In Thomas Ashe's 1813 work, A History of the Azores, the author identified a Fleming, Joshua Vander Berg of Bruges, who made landfall in the archipelago during a storm on his way to Lisbon, he stated that the Portuguese claimed it for Portugal. Other stories note the discovery of the first islands by sailors in the service of Henry the Navigator, although there are few documents to support the claims.
Although it is said that the archipelago received its name from the goshawk, a common bird at the time of discovery, it is unlikely that the bird nested or hunted in the islands. There were no large animals on Santa Maria, so after its discovery and before settlement began, sheep were let loose on the island to supply future settlers with food. Settlement did not take place however. There was not much interest among the Portuguese people to live in an isolated archipelago so far from civilization. Gonçalo Velho Cabral patiently gathered resources and settlers for the next three years and sailed to establish colonies first on Santa Maria and on São Miguel. Settlers cleared bush and rocks to plant crops—grain, grape vines, sugar cane, other plants suitable for local use and of commercial value, they brought domesticated animals, such as chickens, cattle, sheep and pigs and built houses and established villages. The archipelago was settled from mainland Portugal. Portuguese settlers came from the provinces of Algarve, Minho and Ribatejo as well as Madeira.
São Miguel was first settled in 1449, the settlers – from the Estremadura, Alto Alentejo and Algarve areas of mainland Portugal, under the command of Gonçalo Velho Cabral – landed at the site of modern-day Povoação. Many early settlers were Portuguese Sephardic Jews who fled the pressures of inquisition in mainland Portugal. In 1522, Vila Franca do Campo the capital of the island, was devastated by an earthquake and landslide that killed about 5,000 people, the capital was moved to Ponta Delgada; the town of Vila Franca do Campo was rebuilt on the original site and today is a thriving fishing and yachting port. Ponta Delgada received its city status in 1546. From the first settlement, the pioneers applied themselves to agriculture and by the 15th century Graciosa exported wheat, barley and brandy; the goods were sent to Terceira because of the proximity of the island. During the 18th and 19th centuries, Gra