In geology, the places known as hotspots or hot spots are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. Their position on the Earth's surface is independent of tectonic plate boundaries. There are two hypotheses that attempt to explain their origins. One suggests that hotspots are due to mantle plumes that rise as thermal diapirs from the core–mantle boundary. The other hypothesis is that lithospheric extension permits the passive rising of melt from shallow depths. This hypothesis considers the term "hotspot" to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. Well-known examples include the Hawaii, Iceland and Yellowstone hotspots.
The origins of the concept of hotspots lie in the work of J. Tuzo Wilson, who postulated in 1963 that the formation of the Hawaiian Islands resulted from the slow movement of a tectonic plate across a hot region beneath the surface. It was later postulated that hotspots are fed by narrow streams of hot mantle rising from the Earth's core–mantle boundary in a structure called a mantle plume. Whether or not such mantle plumes exist is the subject of a major controversy in Earth science. Estimates for the number of hotspots postulated to be fed by mantle plumes have ranged from about 20 to several thousands, over the years, with most geologists considering a few tens to exist. Hawaii, Réunion, Yellowstone, Galápagos, and Iceland are some of the most active volcanic regions to which the hypothesis is applied.
Most hotspot volcanoes are basaltic (e.g., Hawaii, Tahiti). As a result, they are less explosive than subduction zone volcanoes, in which water is trapped under the overriding plate. Where hotspots occur in continental regions, basaltic magma rises through the continental crust, which melts to form rhyolites. These rhyolites can form violent eruptions. For example, the Yellowstone Caldera was formed by some of the most powerful volcanic explosions in geologic history. However, when the rhyolite is completely erupted, it may be followed by eruptions of basaltic magma rising through the same lithospheric fissures (cracks in the lithosphere). An example of this activity is the Ilgachuz Range in British Columbia, which was created by an early complex series of trachyte and rhyolite eruptions, and late extrusion of a sequence of basaltic lava flows.
Hotspot volcanoes are considered to have a fundamentally different origin from island arc volcanoes. The latter form over subduction zones, at converging plate boundaries. When one oceanic plate meets another, the denser plate is forced downward into a deep ocean trench. This plate, as it is subducted, releases water into the base of the over-riding plate, and this water mixes with the rock, thus changing its composition causing some rock to melt and rise. It is this that fuels a chain of volcanoes, such as the Aleutian Islands, near Alaska.
The joint mantle plume/hotspot hypothesis envisages the feeder structures to be fixed relative to one another, with the continents and seafloor drifting overhead. The hypothesis thus predicts that time-progressive chains of volcanoes are developed on the surface. Examples are Yellowstone, which lies at the end of a chain of extinct calderas, which become progressively older to the west. Another example is the Hawaiian archipelago, where islands become progressively older and more deeply eroded to the northwest.
Geologists have tried to use hotspot volcanic chains to track the movement of the Earth's tectonic plates. This effort has been vexed by the lack of very long chains, by the fact that many are not time-progressive (e.g. the Galápagos) and by the fact that hotspots do not appear to be fixed relative to one another (e.g. Hawaii and Iceland.)
In geology, anorogenic magmatism is the formation, intrusion or eruption of magmas not directly connected with orogeny. This contrasts with orogenic magmatism that occurs at convergent plate boundaries where continental collision, subduction and orogeny are common.Bermuda hotspot
The Bermuda hotspot is a supposed midplate hotspot swell in the Atlantic Ocean 500-1000 km southeast of Bermuda, proposed to explain the extinct volcanoes of the Bermuda Rise as well as the Mississippi Embayment and the Sabine Uplift southwest of the Mississippi Embayment.A 2002 paper by Roy B. Van Arsdale and Randel Cox of the University of Memphis proposes that the Bermuda hotspot generated the Mississippi Embayment in the Early Cretaceous period, when the hotspot strengthened and uplifted the present-day Mississippi Valley. The resulting highland eroded over time, and when North American plate motion moved the valley away from the hotspot the resulting thinned lithosphere subsided forming a trough. As evidence, Van Arsdale and Cox cite the seismic zones centered on New Madrid, Missouri and Charleston, South Carolina, and the volcanic kimberlite pipes in Arkansas.
Other papers argue that the lack of a chain of age-progressive seamounts (as in the Hawaiian-Emperor seamount chain), the absence of present-day volcanism, and the elongation of the Bermuda Rise oblique to plate motion are evidence against a hotspot origin for the Bermuda Rise. Peter R. Vogt and Woo-Yeol Jung alternatively attribute the Bermuda Rise to a reorganization of mantle convection associated with the closing of the Tethys Sea, though noting that shallow processes may not explain the source of the magmatism. A more recent paper finds a thinning in the mantle transition zone under Bermuda, apparently consistent with mantle upwelling and a hot lower mantle below Bermuda.Geography of Antarctica
The geography of Antarctica is dominated by its south polar location and, thus, by ice. The Antarctic continent, located in the Earth's southern hemisphere, is centered asymmetrically around the South Pole and largely south of the Antarctic Circle. It is washed by the Southern (or Antarctic) Ocean or, depending on definition, the southern Pacific, Atlantic, and Indian Oceans. It has an area of more than 14 million km².
Some 98% of Antarctica is covered by the Antarctic ice sheet, the world's largest ice sheet and also its largest reservoir of fresh water. Averaging at least 1.6 km thick, the ice is so massive that it has depressed the continental bedrock in some areas more than 2.5 km below sea level; subglacial lakes of liquid water also occur (e.g., Lake Vostok). Ice shelves and rises populate the ice sheet on the periphery.
In September 2018, researchers at the National Geospatial-Intelligence Agency released a high resolution terrain map (detail down to the size of a car, and less in some areas) of Antarctica, named the "Reference Elevation Model of Antarctica" (REMA).Guyot
In marine geology, a guyot (pronounced ), also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than 200 m (660 ft) below the surface of the sea. The diameters of these flat summits can exceed 10 km (6.2 mi). Guyots are most commonly found in the Pacific Ocean, but they have been identified in all the oceans except the Arctic Ocean.Hot spring
A hot spring is a spring produced by the emergence of geothermally heated groundwater that rises from the Earth's crust. While some of these springs contain water that is a safe temperature for bathing, others are so hot that immersion can result in an injury or death.Lava balloon
A lava balloon is a gas-filled bubble of lava that floats on the sea surface. It can be up to several metres in size. When it emerges from the sea, it is usually hot and often steaming. After floating for some time it fills with water and sinks again.
Lava balloons can form in lava flows entering the sea and at volcanic vents, but they are not common. They have been observed in the Azores, Canary Islands, Hawaii, Japan, Mariana Islands and Mexico. Apparently, they are generated when gases trapped within magma form large bubbles that eventually rise to the sea surface. In the Canary Islands, balloons containing sediments were used to infer the age of the basement on which the volcano is constructed; these sediments were also at first misinterpreted as evidence of an impending large explosive eruption.Louisville Ridge
The Louisville Ridge, also known as the Louisville Seamount Chain, is an underwater chain of over 70 seamounts in the Southwest Pacific Ocean. As one of the longest seamount chains on Earth it stretches some 4,300 km (2,700 mi) from the Pacific-Antarctic Ridge northwest to the Tonga-Kermadec Trench, where it subducts under the Indo-Australian Plate as part of the Pacific Plate. The movement of the Pacific Plate over the Louisville hotspot formed the chain.
Depth-sounding data first revealed the existence of the seamount chain in 1972.Meiji Seamount
Meiji Seamount, named after Emperor Meiji, the 122nd Emperor of Japan, is the oldest seamount in the Hawaiian-Emperor seamount chain, with an estimated age of 82 million years. It lies at the northernmost end of the chain, and is perched at the outer slope of the Kuril-Kamchatka Trench. Like the rest of the Emperor seamounts, it was formed by the Hawaii hotspot volcanism, grew to become an island, and has since subsided to below sea level, all while being carried first north and now northwest by the motion of the Pacific Plate. Meiji Seamount is thus an example of a particular type of seamount known as a guyot, and some publications refer to it as Meiji Guyot.
Meiji Seamount will eventually be destroyed by subduction into the Kuril-Kamchatka Trench where it is carried by the ongoing plate motion, although this will not fully occur for several million more years if the current rate of motion is maintained. Although Meiji is the oldest extant seamount in the Hawaii-Emperor chain, the question of whether there were older seamounts in the chain which have already been subducted into the trench remains open, and is the subject of ongoing scientific research.
The Deep Sea Drilling Project (DSDP) Leg 19, Hole 192A, recovered 13 m (43 ft) of pillow lava from near the summit of Meiji. The lavas were initially classified as alkali basalts on the basis of their mineralogy, but subsequent microprobe analyses of glass and pyroxene suggested that they are tholeiitic in origin. At least five flows were found.Plume tectonics
Plume tectonics is a geoscientific theory that finds its roots in the mantle doming concept which was especially popular during the 1930s and initially did not accept major plate movements and continental drifting. It has survived from the 1970s until today in various forms and presentations. It has slowly evolved into a concept that recognises and accepts large scale plate motions such as envisaged by plate tectonics, but placing them in a framework where large mantle plumes are the major driving force of the system.
The initial followers of the concept during the first half of the 20th Century are scientists like Beloussov and van Bemmelen, and recently the concept has gained interest especially in Japan, through new compiled work on palaeomagnetism, and is still advocaded by the group of scientists elaboration upon Earth expansion. It is nowadays generally not accepted as the main theory to explain the driving forces of tectonic plate movements, although numerous modulations on the concept have been proposed.
The theory focuses on the movements of mantle plumes under tectonic plates, viewing them as the major driving force of movements of (parts of) the Earth's crust. In its more modern form, conceived in the 1970s, it tries to reconcile in one single geodynamic model the horizontalistic concept of Plate tectonics, and the verticalistic concepts of mantle plumes, by the gravitational movement of plates away from major domes of the Earth's crust. The existence of various supercontinents in Earth history and their break-up has been associated recently with major upwellings of the mantle.
It is classified together with mantle convection as one of the mechanism that are used to explain the movements of tectonic plates. It also shows affinity with the concept of hot spots which is used in modern day plate teconics to generate a framework of specific mantle upwelling points that are relatively stable throughout time and are used to calibrate the plate movements using their location together with paleomagnetic data. Another affinity is the concept of surge tectonics which envisage flows through the mantle as major driving forces of Plate Tectonics.Portugal
Portugal (Portuguese: [puɾtuˈɣal]), officially the Portuguese Republic (Portuguese: República Portuguesa [ʁɛˈpuβlikɐ puɾtuˈɣezɐ]), is a country located mostly on the Iberian Peninsula in southwestern Europe. It is the westernmost sovereign state of mainland Europe, being bordered to the west and south by the Atlantic Ocean and to the north and east by Spain. Its territory also includes the Atlantic archipelagos of the Azores and Madeira, both autonomous regions with their own regional governments.
Portugal is the oldest nation state on the Iberian Peninsula and one of the oldest in Europe and the world, its territory having been continuously settled, invaded and fought over since prehistoric times. The pre-Celtic people, Iberians,
Celts, Carthaginians and Romans were followed by the invasions of the Visigoths and Suebi Germanic peoples. After the Muslim conquests of the Iberian Peninsula, most of the territory was part of Al-Andalus for several centuries. Portugal as a country was established during the early Christian Reconquista. Founded in 868, the County of Portugal gained prominence after the Battle of São Mamede in 1128. The Kingdom of Portugal was later proclaimed following the Battle of Ourique in 1139, and independence from León was recognised by the Treaty of Zamora in 1143.In the 15th and 16th centuries, Portugal established the first global empire, becoming one of the world's major economic, political and military powers. During this period, today referred to as the Age of Discovery, Portuguese explorers pioneered maritime exploration, notably under royal patronage of Prince Henry the Navigator and King John II, with such notable voyages as Bartolomeu Dias' sailing beyond the Cape of Good Hope (1488), Vasco da Gama's discovery of the sea route to India (1497–98) and the European discovery of Brazil (1500). During this time Portugal monopolized the spice trade, divided the world into hemispheres of dominion with Castille, and the empire expanded with military campaigns in Asia. However, events such as the 1755 Lisbon earthquake, the country's occupation during the Napoleonic Wars, the independence of Brazil (1822), and a late industrialization compared to other European powers, erased to a great extent Portugal's prior opulence.After the 1910 revolution deposed the monarchy, the democratic but unstable Portuguese First Republic was established, later being superseded by the Estado Novo authoritarian regime. Democracy was restored after the Carnation Revolution in 1974, ending the Portuguese Colonial War. Shortly after, independence was granted to almost all its overseas territories. The handover of Macau to China in 1999 marked the end of what can be considered the longest-lived colonial empire.
Portugal has left a profound cultural, architectural and linguistic influence across the globe, with a legacy of around 250 million Portuguese speakers, and many Portuguese-based creoles. It is a developed country with an advanced economy and high living standards, which ranks 41st on the Human Development Index. Additionally, it is highly placed in rankings of moral freedom (2nd), peacefulness (3rd), democracy (8th), press freedom (12th), stability (15th), social progress (18th), prosperity (24th), and LGBT rights (7th in Europe). A member of the United Nations and the European Union, Portugal was also one of the founding members of NATO, the eurozone, the OECD, and the Community of Portuguese Language Countries.Seamount
A seamount is a mountain rising from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly and are usually found rising from the seafloor to 1,000–4,000 m (3,300–13,100 ft) in height. They are defined by oceanographers as independent features that rise to at least 1,000 m (3,281 ft) above the seafloor, characteristically of conical form. The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea. During their evolution over geologic time, the largest seamounts may reach the sea surface where wave action erodes the summit to form a flat surface. After they have subsided and sunk below the sea surface such flat-top seamounts are called "guyots" or "tablemounts".There are more than 14,500 seamounts, of which 9,951 seamounts and 283 guyots, covering a total of 8,796,150 km2 (3,396,210 sq mi) have been mapped but only a few have been studied in detail by scientists. Seamounts and guyots are most abundant in the North Pacific Ocean, and follow a distinctive evolutionary pattern of eruption, build-up, subsidence and erosion. In recent years, several active seamounts have been observed, for example Loihi in the Hawaiian Islands.
Because of their abundance, seamounts are one of the most common marine ecosystems in the world. Interactions between seamounts and underwater currents, as well as their elevated position in the water, attract plankton, corals, fish, and marine mammals alike. Their aggregational effect has been noted by the commercial fishing industry, and many seamounts support extensive fisheries. There are ongoing concerns on the negative impact of fishing on seamount ecosystems, and well-documented cases of stock decline, for example with the orange roughy (Hoplostethus atlanticus). 95% of ecological damage is done by bottom trawling, which scrapes whole ecosystems off seamounts.
Because of their large numbers, many seamounts remain to be properly studied, and even mapped. Bathymetry and satellite altimetry are two technologies working to close the gap. There have been instances where naval vessels have collided with uncharted seamounts; for example, Muirfield Seamount is named after the ship that struck it in 1973. However, the greatest danger from seamounts are flank collapses; as they get older, extrusions seeping in the seamounts put pressure on their sides, causing landslides that have the potential to generate massive tsunamis.Timeline of volcanism on Earth
This timeline of volcanism on Earth is a list of major volcanic eruptions of approximately at least magnitude 6 on the Volcanic Explosivity Index (VEI) or equivalent sulfur dioxide emission around the Quaternary period (from 2.58 Mya to the present).
Some eruptions cooled the global climate—inducing a volcanic winter—depending on the amount of sulfur dioxide emitted and the magnitude of the eruption. Before the present Holocene epoch, the criteria are less strict because of scarce data availability, partly since later eruptions have destroyed the evidence. Only some eruptions before the Neogene period (from 23 Mya to 2.58 Mya) are listed. Known large eruptions after the Paleogene period (from 66 Mya to 23 Mya) are listed, especially those relating to the Yellowstone hotspot, the Santorini caldera, and the Taupo Volcanic Zone.
Active volcanoes such as Stromboli, Mount Etna and Kīlauea do not appear on this list, but some back-arc basin volcanoes that generated calderas do appear. Some dangerous volcanoes in "populated areas" appear many times: so Santorini, six times and Yellowstone hotspot, 21 times. The Bismarck volcanic arc, New Britain, and the Taupo Volcanic Zone, New Zealand, appear often too.
In addition to the events listed below, are many examples of eruptions in the Holocene on the Kamchatka Peninsula, which are described in a supplemental table by Peter Ward.Vema
Vema may refer to:
Prolaya Vema Reddy, the first king of the Reddy dynasty in Andhra Pradesh, India
A Greek pace (unit of length)
Research Vessel Vema, a research ship of the Lamont-Doherty Earth Observatory used by J. Lamar Worzel and Marie Tharp, and some ocean phenomena discovered using it:
Vema (genus), a genus of Monoplacophoran molluscs
The Vema Seamount, a seamount in the South Atlantic Ocean at 31°38' S 8°20' E.
The Vema hotspot, a hotspot (geology)
The Vema Fracture Zone a fracture zone in the equatorial Atlantic OceanWest Antarctic Rift
The West Antarctic Rift system (WARS) is a series of rift valleys lying between East and West Antarctica. It encompasses the Ross Embayment, the Ross Sea, the area under the Ross Ice Shelf and a part of Marie Byrd Land in West Antarctica, reaching to the base of the Antarctic Peninsula. It has an estimated length of 3000 km and a width of approximately 700 km. Its evolution is due to lithospheric thinning of an area of Antarctica that resulted in the demarcation of East and West Antarctica. The scale and evolution of the rift system has been compared to that of the Basin and Range Province of the western U.S.
|History of geology|
|Сomposition and structure|