Mean sea level is an average level of the surface of one or more of Earth's oceans from which heights such as elevation may be measured. MSL is a type of vertical datum – a standardised geodetic datum –, used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and aircraft flight levels. A common and straightforward mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location. Sea levels can be affected by many factors and are known to have varied over geological time scales; however 20th century and current millennium sea level rise is caused by global warming, careful measurement of variations in MSL can offer insights into ongoing climate change. The term above sea level refers to above mean sea level. Precise determination of a "mean sea level" is difficult to achieve because of the many factors that affect sea level. Instantaneous sea level varies quite a lot on several scales of space.
This is because the sea is in constant motion, affected by the tides, atmospheric pressure, local gravitational differences, salinity and so forth. The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and use it as a datum. For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point. Still-water level or still-water sea level is the level of the sea with motions such as wind waves averaged out. MSL implies the SWL further averaged over a period of time such that changes due to, e.g. the tides have zero mean. Global MSL refers to a spatial average over the entire ocean. One measures the values of MSL in respect to the land. In the UK, the Ordnance Datum is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921, the vertical datum was MSL at the Victoria Liverpool. Since the times of the Russian Empire, in Russia and other former its parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge.
In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 1.230m below the average sea level. In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level, it is used for main part of Africa as official sea level. As for Spain, the reference to measure heights below or above sea level is placed in Alicante. Elsewhere in Europe vertical elevation references are made to the Amsterdam Peil elevation, which dates back to the 1690s. Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES, TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008. Height above mean sea level is the elevation or altitude of an object, relative to the average sea level datum, it is used in aviation, where some heights are recorded and reported with respect to mean sea level, in the atmospheric sciences, land surveying.
An alternative is to base height measurements on an ellipsoid of the entire Earth, what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is used to define heights; the alternative is to use a geoid-based vertical datum such as NAVD88. When referring to geographic features such as mountains on a topographic map, variations in elevation are shown by contour lines; the elevation of a mountain denotes the highest point or summit and is illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet or both. In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see Amsterdam Airport Schiphol. To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the geoid. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field.
In reality, due to currents, air pressure variations and salinity variations, etc. this does not occur, not as a long-term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as ocean surface topography, it varies globally in a range of ± 2 m. Adjustments were made to sea-level measurements to take into account the effects of the 235 lunar month Metonic cycle and the 223-month eclipse cycle on the tides. Several terms are used to describe the changing relationships between sea level and dry land; when the term "relative" is used, it means change relative to a fixed point in the sediment pile. The term "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps; the term "steric" refers to global changes in sea level due to thermal expansion and salinity variations. The term "isostatic" refers to changes in
Krafla is a caldera of about 10 km in diameter with a 90 km long fissure zone, in the north of Iceland in the Mývatn region. Its highest peak reaches up to 818 m and it is 2 km in depth. There have been 29 reported eruptions in recorded history. Krafla includes the crater one of two well-known craters by this name in Iceland; the Icelandic word "víti" means "hell". In former times, people believed hell to be under volcanoes. Víti has a green lake inside of it. South of the Krafla area, but not within the caldera is Námafjall, a mountain, beneath, Hverir, a geothermal area with boiling mudpools and steaming fumaroles; the Mývatn fires occurred between 1724 -- 1729. The lava fountains could be seen in the south of the island and a lava flow destroyed three farms near the village of Reykjahlíð, although nobody was harmed. Between 1975 and 1984 there was a volcanic episode within the Krafla volcano, it involved fifteen uplift and subsidence events. This interrupted some of the Krafla drillfields. During these events a large magma chamber emerged.
This has been identified by analysing the seismic activity. Since 1977 the Krafla area has been the source of the geothermal energy used by a 60 MWe power station. A survey undertaken in 2006 indicated high temperatures at depths of between 3 and 5 kilometres and these favourable conditions have led to the development of the first well from the Iceland Deep Drilling Project, that found magma only 2.1 km deep. Geography of Iceland List of lakes of Iceland List of volcanoes in Iceland Volcanism in Iceland Geothermal power in Iceland Krafla in the Catalogue of Icelandic Volcanoes Volcanism Photos of Krafla and Reykjahlíð Univ. of Iceland: Information about Krafla Energy from magma at Krafla
Greenland is an autonomous constituent country of the Kingdom of Denmark between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for more than a millennium; the majority of its residents are Inuit, whose ancestors began migrating from the Canadian mainland in the 13th century settling across the island. Greenland is the world's largest island. Three-quarters of Greenland is covered by the only permanent ice sheet outside Antarctica. With a population of about 56,480, it is the least densely populated territory in the world. About a third of the population live in the capital and largest city; the Arctic Umiaq Line ferry acts as a lifeline for western Greenland, connecting the various cities and settlements. Greenland has been inhabited at intervals over at least the last 4,500 years by Arctic peoples whose forebears migrated there from what is now Canada.
Norsemen settled the uninhabited southern part of Greenland beginning in the 10th century, having settled Iceland to escape persecution from the King of Norway and his central government. These Norsemen would set sail from Greenland and Iceland, with Leif Erikson becoming the first known European to reach North America nearly 500 years before Columbus reached the Caribbean islands. Inuit peoples arrived in the 13th century. Though under continuous influence of Norway and Norwegians, Greenland was not formally under the Norwegian crown until 1262; the Norse colonies disappeared in the late 15th century when Norway was hit by the Black Death and entered a severe decline. Soon after their demise, beginning in 1499, the Portuguese explored and claimed the island, naming it Terra do Lavrador. In the early 18th century, Danish explorers reached Greenland again. To strengthen trading and power, Denmark–Norway affirmed sovereignty over the island; because of Norway's weak status, it lost sovereignty over Greenland in 1814 when the union was dissolved.
Greenland became Danish in 1814, was integrated in the Danish state in 1953 under the Constitution of Denmark. In 1973, Greenland joined the European Economic Community with Denmark. However, in a referendum in 1982, a majority of the population voted for Greenland to withdraw from the EEC, effected in 1985. Greenland contains the world's largest and most northerly national park, Northeast Greenland National Park. Established in 1974, expanded to its present size in 1988, it protects 972,001 square kilometres of the interior and northeastern coast of Greenland and is bigger than all but twenty-nine countries in the world. Greenland is divided into five municipalities – Sermersooq, Qeqertalik and Avannaata. Greenland does not have an independent seat at the United Nations. In 1979, Denmark granted home rule to Greenland, in 2008, Greenlanders voted in favor of the Self-Government Act, which transferred more power from the Danish government to the local Greenlandic government. Under the new structure, in effect since 21 June 2009, Greenland can assume responsibility for policing, judicial system, company law and auditing.
It retains control of monetary policy, providing an initial annual subsidy of DKK 3.4 billion, planned to diminish over time. Greenland expects to grow its economy based on increased income from the extraction of natural resources; the capital, held the 2016 Arctic Winter Games. At 70%, Greenland has one of the highest shares of renewable energy in the world coming from hydropower; the early Norse settlers named the island as Greenland. In the Icelandic sagas, the Norwegian-born Icelander Erik the Red was said to be exiled from Iceland for manslaughter. Along with his extended family and his thralls, he set out in ships to explore an icy land known to lie to the northwest. After finding a habitable area and settling there, he named it Grœnland in the hope that the pleasant name would attract settlers; the Saga of Erik the Red states: "In the summer, Erik left to settle in the country he had found, which he called Greenland, as he said people would be attracted there if it had a favorable name."The name of the country in the indigenous Greenlandic language is Kalaallit Nunaat.
The Kalaallit are the indigenous Greenlandic Inuit people. In prehistoric times, Greenland was home to several successive Paleo-Eskimo cultures known today through archaeological finds; the earliest entry of the Paleo-Eskimo into Greenland is thought to have occurred about 2500 BC. From around 2500 BC to 800 BC, southern and western Greenland were inhabited by the Saqqaq culture. Most finds of Saqqaq-period archaeological remains have been around Disko Bay, including the site of Saqqaq, after which the culture is named. From 2400 BC to 1300 BC, the Independence I culture existed in northern Greenland, it was a part of the Arctic small tool tradition. Towns, including Deltaterrassern
A mantle plume is a proposed mechanism of convection of abnormally hot rock within the Earth's mantle. Because the plume head melts on reaching shallow depths, a plume is invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, large igneous provinces such as the Deccan and Siberian traps; some such volcanic regions lie far from tectonic plate boundaries, while others represent unusually large-volume volcanism near plate boundaries or in large igneous provinces. The hypothesis of mantle plumes from depth is not universally accepted as explaining all such volcanism, it has required progressive hypothesis-elaboration leading to variant propositions such as mini-plumes and pulsing plumes. Another hypothesis for unusual volcanic regions is the "Plate model"; this proposes shallower, passive leakage of magma from the mantle onto the Earth's surface where extension of the lithosphere permits it, attributing most volcanism to plate tectonic processes, with volcanoes far from plate boundaries resulting from intraplate extension.
The theory was first proposed by J. Tuzo Wilson in 1963 and further developed by W. Jason Morgan in 1971. A mantle plume is posited to exist where hot rock nucleates at the core-mantle boundary and rises through the Earth's mantle becoming a diapir in the Earth's crust. In particular, the concept that mantle plumes are fixed relative to one another, anchored at the core-mantle boundary, would provide a natural explanation for the time-progressive chains of older volcanoes seen extending out from some such hot spots, such as the Hawaiian–Emperor seamount chain. Two independent convective processes are proposed: the broad convective flow associated with plate tectonics, driven by the sinking of cold plates of lithosphere back into the mantle asthenosphere the mantle plume, driven by heat exchange across the core-mantle boundary carrying heat upward in a narrow, rising column, postulated to be independent of plate motions; the plume hypothesis was studied using laboratory experiments conducted in small fluid-filled tanks in the early 1970s.
Thermal or compositional fluid-dynamical plumes produced in that way were presented as models for the much larger postulated mantle plumes. On the basis of these experiments, mantle plumes are now postulated to comprise two parts: a long thin conduit connecting the top of the plume to its base, a bulbous head that expands in size as the plume rises; the entire structure is considered to resemble a mushroom. The bulbous head of thermal plumes forms because hot material moves upward through the conduit faster than the plume itself rises through its surroundings. In the late 1980s and early 1990s, experiments with thermal models showed that as the bulbous head expands it may entrain some of the adjacent mantle into the head; the sizes and occurrence of mushroom mantle plumes can be predicted by transient instability theory developed by Tan and Thorpe. The theory predicts mushroom shaped mantle plumes with heads of about 2000 km diameter that have a critical time of about 830 Myr for a core mantle heat flux of 20 mW/m2, while the cycle time is about 2 Gyr.
The number of mantle plumes is predicted to be about 17. When a plume head encounters the base of the lithosphere, it is expected to flatten out against this barrier and to undergo widespread decompression melting to form large volumes of basalt magma, it may erupt onto the surface. Numerical modelling predicts that eruption will take place over several million years; these eruptions have been linked to flood basalts, although many of those erupt over much shorter time scales. Examples include the Deccan traps in India, the Siberian traps of Asia, the Karoo-Ferrar basalts/dolerites in South Africa and Antarctica, the Paraná and Etendeka traps in South America and Africa, the Columbia River basalts of North America. Flood basalts in the oceans are known as oceanic plateaus, include the Ontong Java plateau of the western Pacific Ocean and the Kerguelen Plateau of the Indian Ocean; the narrow vertical pipe, or conduit, postulated to connect the plume head to the core-mantle boundary, is viewed as providing a continuous supply of magma to a fixed location referred to as a "hotspot".
As the overlying tectonic plate moves over this hotspot, the eruption of magma from the fixed conduit onto the surface is expected to form a chain of volcanoes that parallels plate motion. The Hawaiian Islands chain in the Pacific Ocean is the type example, it has been discovered that the volcanic locus of this chain has not been fixed over time, it thus joined the club of the many type examples that do not exhibit the key characteristic proposed. The eruption of continental flood basalts is associated with continental rifting and breakup; this has led to the hypothesis that mantle plumes contribute to continental rifting and the formation of ocean basins. In the context of the alternative "Plate model", continental breakup is a process integral to plate tectonics, massive volcanism occurs as a natural consequence when it onsets; the current mantle plume theory is that material and energy from Earth's interior are exchanged with the surface crust in two distinct modes: the predominant, steady state plate tectonic regime driven by upper mantle convection, a punctuated, intermittently dominant, mantle overturn regime driven by plume convection.
This second regime, while discontinuous, is periodically significant in mountain building and continental breakup. The chemical and isotopic composition of basalts found at hotspots differs subtly from mid-ocean-ridge basalts; this geochemical signature arises from the mixing of near-surface materials such as subducted
Basalt is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt. Basalt lava has a low viscosity, due to its low silica content, resulting in rapid lava flows that can spread over great areas before cooling and solidification. Flood basalt describes the formation in a series of lava basalt flows. By definition, basalt is an aphanitic igneous rock with 45–53% silica and less than 10% feldspathoid by volume, where at least 65% of the rock is feldspar in the form of plagioclase; this is as per definition of the International Union of Geological Sciences classification scheme. It is the most common volcanic rock type on Earth, being a key component of oceanic crust as well as the principal volcanic rock in many mid-oceanic islands, including Iceland, the Faroe Islands, Réunion and the islands of Hawaiʻi. Basalt features a fine-grained or glassy matrix interspersed with visible mineral grains.
The average density is 3.0 g/cm3. Basalt is defined by its mineral content and texture, physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is grey to black in colour, but weathers to brown or rust-red due to oxidation of its mafic minerals into hematite and other iron oxides and hydroxides. Although characterized as "dark", basaltic rocks exhibit a wide range of shading due to regional geochemical processes. Due to weathering or high concentrations of plagioclase, some basalts can be quite light-coloured, superficially resembling andesite to untrained eyes. Basalt has a fine-grained mineral texture due to the molten rock cooling too for large mineral crystals to grow; these phenocrysts are of olivine or a calcium-rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt. Basalt with a vesicular texture is called vesicular basalt, when the bulk of the rock is solid; this texture forms when dissolved gases come out of solution and form bubbles as the magma decompresses as it reaches the surface, yet are trapped as the erupted lava hardens before the gases can escape.
The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic groundmass are referred to as diabase or, when more coarse-grained, as gabbro. Gabbro is marketed commercially as "black granite." In the Hadean and early Proterozoic eras of Earth's history, the chemistry of erupted magmas was different from today's, due to immature crustal and asthenosphere differentiation. These ultramafic volcanic rocks, with silica contents below 45% are classified as komatiites; the word "basalt" is derived from Late Latin basaltes, a misspelling of Latin basanites "very hard stone", imported from Ancient Greek βασανίτης, from βάσανος and originated in Egyptian bauhun "slate". The modern petrological term basalt describing a particular composition of lava-derived rock originates from its use by Georgius Agricola in 1556 in his famous work of mining and mineralogy De re metallica, libri XII. Agricola applied "basalt" to the volcanic black rock of the Schloßberg at Stolpen, believing it to be the same as the "very hard stone" described by Pliny the Elder in Naturalis Historiae.
Tholeiitic basalt is rich in silica and poor in sodium. Included in this category are most basalts of the ocean floor, most large oceanic islands, continental flood basalts such as the Columbia River Plateau. High and low titanium basalts. Basalt rocks are in some cases classified after their titanium content in High-Ti and Low-Ti varieties. High-Ti and Low-Ti basalts have been distinguished in the Paraná and Etendeka traps and the Emeishan Traps. Mid-ocean ridge basalt is a tholeiitic basalt erupted only at ocean ridges and is characteristically low in incompatible elements. E-MORB, enriched MORB N-MORB, normal MORB D-MORB, depleted MORB High-alumina basalt may be silica-undersaturated or -oversaturated, it has greater than 17% alumina and is intermediate in composition between tholeiitic basalt and alkali basalt. Alkali basalt is poor in silica and rich in sodium, it may contain feldspathoids, alkali feldspar and phlogopite. Boninite is a high-magnesium form of basalt, erupted in back-arc basins, distinguished by its low titanium content and trace-element composition.
Ocean island basalt Lunar basalt The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can be a significant constituent. Accessory minerals present in minor amounts include iron oxides and iron-titanium oxides, such as magnetite and ilmenite; because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, paleomagnetic studies have made extensive use of basalt. In tholeiitic basalt and calcium-rich plagioclase are common phenocryst minerals. Olivine may be a phenocryst, when
Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century; the geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s. The lithosphere, the rigid outermost shell of a planet, is broken into tectonic plates; the Earth's lithosphere is composed of many minor plates. Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, oceanic trench formation occur along these plate boundaries; the relative movement of the plates ranges from zero to 100 mm annually. Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust.
Along convergent boundaries, subduction, or one plate moving under another, carries the lower one down into the mantle. In this way, the total surface of the lithosphere remains the same; this prediction of plate tectonics is referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual expansion of the globe. Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography and density changes in the crust. At subduction zones the cold, dense crust is "pulled" or sinks down into the mantle over the downward convecting limb of a mantle cell. Another explanation lies in the different forces generated by tidal forces of the Moon; the relative importance of each of these factors and their relationship to each other is unclear, still the subject of much debate.
The outer layers of the Earth are divided into the asthenosphere. The division is based on differences in mechanical properties and in the method for the transfer of heat; the lithosphere is more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere transfers heat by convection and has a nearly adiabatic temperature gradient; this division should not be confused with the chemical subdivision of these same layers into the mantle and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like asthenosphere. Plate motions range up to a typical 10–40 mm/year, to about 160 mm/year; the driving mechanism behind this movement is described below. Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust and continental crust.
Average oceanic lithosphere is 100 km thick. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km thick at mid-ocean ridges to greater than 100 km at subduction zones. Continental lithosphere is about 200 km thick, though this varies between basins, mountain ranges, stable cratonic interiors of continents; the location where two plates meet is called a plate boundary. Plate boundaries are associated with geological events such as earthquakes and the creation of topographic features such as mountains, mid-ocean ridges, oceanic trenches; the majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being the most active and known today. These boundaries are discussed in further detail below; some volcanoes occur in the interiors of plates, these have been variously attributed to internal plate deformation and to mantle plumes.
As explained above, tectonic plates may include continental crust or oceanic crust, most plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans; the distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is fo
Katla is a large volcano in southern Iceland. It is active, it has not erupted violently for 101 years, although there may have been small eruptions that did not break the ice cover, including ones in 1955, 1999, 2011. Prior eruptions have had a Volcanic Explosivity Index of between 4 and 6 on a scale of 0 to 8. In comparison, the Eyjafjallajökull 2010 eruption had a VEI of 4. Bigger VEI-5 eruptions are comparable to Mount Pinatubo's 1991 eruption, while rarer VEI-6 eruptions would be comparable to Krakatoa's 1883 eruption. Katla is one of the largest volcanic sources of carbon dioxide on Earth, accounting for up to 4% of total global volcanic carbon dioxide emissions. Katla is one of the largest volcanoes in Iceland, it is situated to the east of the smaller glacier Eyjafjallajökull. Its peak reaches 1,512 metres and is covered by the Mýrdalsjökull glacier; the system has an area of 595 km2. The Eldgjá canyon is part of the same volcanic system; the caldera of the Katla volcano is covered with 200 -- 700 metres of ice.
The volcano erupts every 40–80 years. The flood discharge at the peak of an eruption in 1755 has been estimated at 200,000–400,000 m3/s, comparable to the combined average discharge of the Amazon, Mississippi and Yangtze rivers; the name Katla derives from the word ketill. Katla is used as a female first name, it is thought that Katla is the source of Vedde Ash, more than 6 to 7 cubic kilometers of tephra dated to 10,600 years BP found at a number of sites including Vedde in Norway, Denmark and North Atlantic cores. Sixteen eruptions have been recorded for Katla since 930; the last major eruption lasted for 24 days. It was a VEI-5 level eruption; the 1918 eruption resulted in extending the southern coast by 5 km due to laharic flood deposits. Its present dormancy is among the longest in known history. Minor eruptions occurred in