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77-689: Grenville orogenic crust of mid-late Mesoproterozoic age ( c. 1250—980 Ma ) is found worldwide, but generally only events which occurred on the southern and eastern margins of Laurentia are recognized under the "Grenville" name. These orogenic events are also known as the Kibaran orogeny in Africa and the Dalslandian orogeny in Western Europe . The problem of timing the Grenville orogeny

154-489: A continuous supply of magma to a hotspot. As the overlying tectonic plate moves over this hotspot, the eruption of magma from the fixed plume 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 archetypal example. It has recently been discovered that the volcanic locus of this chain has not been fixed over time, and it thus joined

231-414: A core mantle heat flux of 20 mW/m , while the cycle time (the time between plume formation events) is about 2000 million years. 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 then erupt onto

308-543: A high Sr/ Sr ratio. Helium in OIB shows a wider variation in the He/ He ratio than MORB, with some values approaching the primordial value. The composition of ocean island basalts is attributed to the presence of distinct mantle chemical reservoirs formed by subduction of oceanic crust. These include reservoirs corresponding to HUIMU, EM1, and EM2. These reservoirs are thought to have different major element compositions, based on

385-405: A long thin conduit connecting the top of the plume to its base, and a bulbous head that expands in size as the plume rises. The entire structure resembles 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

462-615: A lower temperature. Mantle material containing a trace of partial melt (e.g., as a result of it having a lower melting point), or being richer in Fe, also has a lower seismic wave speed and those effects are stronger than temperature. Thus, although unusually low wave speeds have been taken to indicate anomalously hot mantle beneath hotspots, this interpretation is ambiguous. The most commonly cited seismic wave-speed images that are used to look for variations in regions where plumes have been proposed come from seismic tomography. This method involves using

539-534: A mantle plume postulated to have caused the breakup of Eurasia and the opening of the North Atlantic, now suggested to underlie Iceland . Current research has shown that the time-history of the uplift is probably much shorter than predicted, however. It is thus not clear how strongly this observation supports the mantle plume hypothesis. Basalts found at oceanic islands are geochemically distinct from mid-ocean ridge basalt (MORB). Ocean island basalt (OIB)

616-675: A massive dome of Proterozoic rock on the New York-Canada border. Both the Elzevirian (c. 1250–1190 Ma) and Ottawan (c. 1080–1020 Ma) orogenic pulses are recorded in the Adirondacks, producing high-grade metamorphic rock. A northwest-trending high-strain shear zone separates the dome into the highlands to the southeast and the lowlands to the northwest. It is believed that the shear zone (the Carthage-Colton)

693-496: A natural explanation for the time-progressive chains of older volcanoes seen extending out from some such hotspots, for example, the Hawaiian–Emperor seamount chain . However, paleomagnetic data show that mantle plumes can also be associated with Large Low Shear Velocity Provinces (LLSVPs) and do move relative to each other. The current mantle plume theory is that material and energy from Earth's interior are exchanged with

770-647: A network of seismometers to construct three-dimensional images of the variation in seismic wave speed throughout the mantle. Seismic waves generated by large earthquakes enable structure below the Earth's surface to be determined along the ray path. Seismic waves that have traveled a thousand or more kilometers (also called teleseismic waves ) can be used to image large regions of Earth's mantle. They also have limited resolution, however, and only structures at least several hundred kilometers in diameter can be detected. Seismic tomography images have been cited as evidence for

847-562: A number of mantle plumes in Earth's mantle. There is, however, vigorous on-going discussion regarding whether the structures imaged are reliably resolved, and whether they correspond to columns of hot, rising rock. The mantle plume hypothesis predicts that domal topographic uplifts will develop when plume heads impinge on the base of the lithosphere. An uplift of this kind occurred when the North Atlantic Ocean opened about 54 million years ago. Some scientists have linked this to

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924-399: A plume developed into a weakly defined hypothesis, which as a general term is currently neither provable nor refutable. The dissatisfaction with the state of the evidence for mantle plumes and the proliferation of ad hoc hypotheses drove a number of geologists, led by Don L. Anderson , Gillian Foulger , and Warren B. Hamilton , to propose a broad alternative based on shallow processes in

1001-483: A quantity of 79 million years, without the meaning of "79 million years ago". Mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle , hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspots , such as Hawaii or Iceland , and large igneous provinces such as

1078-507: A separate causal category of terrestrial volcanism with implications for the study of hotspots and plate tectonics. In 1997 it became possible using seismic tomography to image submerging tectonic slabs penetrating from the surface all the way to the core-mantle boundary. For the Hawaii hotspot , long-period seismic body wave diffraction tomography provided evidence that a mantle plume is responsible, as had been proposed as early as 1971. For

1155-736: A single province separated by opening of the South Atlantic Ocean), and the Columbia River basalts of North America. Flood basalts in the oceans are known as oceanic plateaus, and include the Ontong Java plateau of the western Pacific Ocean and the Kerguelen Plateau of the Indian Ocean. The narrow vertical conduit, postulated to connect the plume head to the core-mantle boundary, is viewed as providing

1232-521: Is "Cretaceous", for good reason. But the counter argument is that having myr for a duration and Mya for an age mixes unit systems, and tempts capitalization errors: "million" need not be capitalized, but "mega" must be; "ma" would technically imply a milliyear (a thousandth of a year, or 8 hours). On this side of the debate, one avoids myr and simply adds ago explicitly (or adds BP ), as in: The Cretaceous started 145 Ma ago and ended 66 Ma ago, lasting for 79 Ma. In this case, "79 Ma" means only

1309-462: Is a strong thermal (temperature) discontinuity. The temperature of the core is approximately 1,000 degrees Celsius higher than that of the overlying mantle. Plumes are postulated to rise as the base of the mantle becomes hotter and more buoyant. Plumes are postulated to rise through the mantle and begin to partially melt on reaching shallow depths in the asthenosphere by decompression melting . This would create large volumes of magma. This melt rises to

1386-431: Is also used with Mya or Ma. Together they make a reference system, one to a quantity, the other to a particular point in a year numbering system that is time before the present . Myr is deprecated in geology , but in astronomy Myr is standard. Where "myr" is seen in geology, it is usually "Myr" (a unit of mega-years). In astronomy, it is usually "Myr" (Million years). In geology, a debate remains open concerning

1463-549: Is an area of some contention. The timescale outlined by Toby Rivers in 2002 is derived from the well-preserved Grenville Province and represents one of the most detailed records of the orogeny. This classification considers the classical Grenville designation to cover two separate orogenic cycles; the Rigolet, Ottawan and Shawingian orogenies compose the Grenville Cycle, and the Elzevirian orogeny stands on its own. Due to

1540-532: Is composed of meta-igneous gneisses including anorthosite massifs. Anorthosites form in plutons and are composed mostly of plagioclase. The rocks of the Grenville Province in Canada are included in this category. The oldest magmatism known in this area dates to 1.32 Ga approximately. Granulite facies metamorphism began around 1.15 Ga and continued for about 150 Ma after the onset, however the continuity of

1617-454: Is consistent with a system that tends toward equilibrium: as matter rises in a mantle plume, other material is drawn down into the mantle, causing rifting. In parallel with the mantle plume model, two alternative explanations for the observed phenomena have been considered: the plate hypothesis and the impact hypothesis. Since the beginning of the 21st century, a paradigm debate "The great plume debate" has developed around plumes, in which

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1694-874: Is dynamic. The cyclic compression and extension history of this area is similar to the Wilson Cycle . In this area of the world the Wilson Cycle would be creating the basin for the Iapetus Ocean . Today, the Grenville orogen is marked by northwest verging fold-and-thrust belts and high pressure metamorphic regimes, as well as distinctive AMCG suite magmatism. Metamorphism is commonly of amphibolite and granulite facies , that is, medium to high temperature and pressure alteration. Eclogitized metagabbros (very high pressure ultramafic metamorphic rocks) are found in some localities and likely represent areas of deepest burial and/or most intense collision. Throughout

1771-536: Is enriched in trace incompatible elements , with the light rare earth elements showing particular enrichment compared with heavier rare earth elements. Stable isotope ratios of the elements strontium , neodymium , hafnium , lead , and osmium show wide variations relative to MORB, which is attributed to the mixing of at least three mantle components: HIMU with a high proportion of radiogenic lead, produced by decay of uranium and other heavy radioactive elements; EM1 with less enrichment of radiogenic lead; and EM2 with

1848-427: Is known about the orogeny and its processes is derived from the Grenville Province. The Laurentian Mountains are a part of the province. Myr Million years ago , abbreviated as Mya , Myr (megayear) or Ma (megaannum), is a unit of time equal to 1,000,000 years (i.e. 1 × 10 years), or approximately 31.6 teraseconds . Myr is in common use in fields such as Earth science and cosmology . Myr

1925-481: Is more diverse compositionally than MORB, and the great majority of ocean islands are composed of alkali basalt enriched in sodium and potassium relative to MORB. Larger islands, such as Hawaii or Iceland, are mostly tholeiitic basalt, with alkali basalt limited to late stages of their development, but this tholeiitic basalt is chemically distinct from the tholeiitic basalt of mid-ocean ridges. OIB tends to be more enriched in magnesium, and both alkali and tholeiitic OIB

2002-635: Is predominantly sedimentary and volcanic rocks which have undergone greenschist to granulite facies metamorphism. Subdivisions of this belt include the Bancroft, Elzevir, Sharbot Lake, and Frontenac Domains and the Adirondack Lowlands. In this belt magmatism is known to have occurred between 1.42 and 1.04 Ga depending on location. As with the Gneiss Belt, metamorphism is believed to have occurred at approximately 1.16 Ga. The Granulite Terrane

2079-604: Is suggested that the regime of subduction under the Laurentian margin (currently in Texas, north of the accreted Mexican terrane ) ended around 1230 Ma, and that subduction polarity reversed to bring the colliding continent north, since the Llano Uplift , which records the history of the Grenville in Texas, bears no evidence of arc magmatism after this time. The Appalachian Mountains contain small, isolated exposures of

2156-412: Is the formation of sedimentary basins which means the margin was quiescent enough that sediments could accumulate. However, in some areas from 1.16 to 1.13 Ga, coeval with extension, there is evidence there was still thrusting and emplacement of terranes occurring. According to one model, westward thrusting occurred from 1.12 to 1.09 Ga and then extension was the primary tectonic activity until 1.05 Ga. It

2233-738: The Chagos-Laccadive Ridge , the Louisville Ridge , the Ninety East Ridge and Kerguelen , Tristan , and Yellowstone . While there is evidence that the chains listed above are time-progressive, it has been shown that they are not fixed relative to one another. The most remarkable example of this is the Emperor chain, the older part of the Hawaii system, which was formed by migration of the hotspot in addition to

2310-451: 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. Mantle plumes were first proposed by J. Tuzo Wilson in 1963 and further developed by W. Jason Morgan in 1971 and 1972. A mantle plume is posited to exist where super-heated material forms ( nucleates ) at

2387-569: The Sudbury Igneous Complex in Canada are known to have caused melting and volcanism. In the impact hypothesis, it is proposed that some regions of hotspot volcanism can be triggered by certain large-body oceanic impacts which are able to penetrate the thinner oceanic lithosphere , and flood basalt volcanism can be triggered by converging seismic energy focused at the antipodal point opposite major impact sites. Impact-induced volcanism has not been adequately studied and comprises

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2464-543: The Yellowstone hotspot , seismological evidence began to converge from 2011 in support of the plume model, as concluded by James et al., "we favor a lower mantle plume as the origin for the Yellowstone hotspot." Data acquired through Earthscope , a program collecting high-resolution seismic data throughout the contiguous United States has accelerated acceptance of a plume underlying Yellowstone. Although there

2541-406: The core-mantle boundary and rises through the Earth's mantle. Rather than a continuous stream, plumes should be viewed as a series of hot bubbles of material. Reaching the brittle upper Earth's crust they form diapirs . These diapirs are "hotspots" in the crust. In particular, the concept that mantle plumes are fixed relative to one another and anchored at the core-mantle boundary would provide

2618-465: The country rock . Polarities of subduction (which plate overrode which) vary by region and time. Some island arc remnants were emplaced on the Laurentian margin, and some were accreted during orogeny. Timing of these events is constrained by cross-cutting relations observed in the field as well as SHRIMP ( sensitive high-resolution ion microprobe ) and TIMS ( thermal ionization mass spectrometry ) uranium-lead dating . The first period of tectonic activity

2695-404: The lower mantle under Africa and under the central Pacific. It is postulated that plumes rise from their surface or their edges. Their low seismic velocities were thought to suggest that they are relatively hot, although it has recently been shown that their low wave velocities are due to high density caused by chemical heterogeneity. Some common and basic lines of evidence cited in support of

2772-476: The Earth's core, in basalts at oceanic islands. However, so far conclusive proof for this is lacking. The plume hypothesis has been tested by looking for the geophysical anomalies predicted to be associated with them. These include thermal, seismic, and elevation anomalies. Thermal anomalies are inherent in the term "hotspot". They can be measured in numerous different ways, including surface heat flow, petrology, and seismology. Thermal anomalies produce anomalies in

2849-543: The Grenville Province prior to opening of the Iapetus Ocean, the two share largely the same history. Texas and Mexico represent the southern margin of Laurentia and likely collided with a different continent than that involved in the eastern collision. The Zapotecan orogeny of Mexico is coeval with the later stages of the Grenville orogeny, and they are generally considered to be one and the same. Mesoproterozoic igneous protoliths (metamorphosed to granulite facies during

2926-833: The Grenville orogen. The largest of these, the Long Range Inlier, comprises the Long Range Mountains of Newfoundland. Other exposures include the Shenandoah and French Broad massifs , which comprise the Blue Ridge province of Virginia. Blue Ridge rocks consist of various gneisses of upper amphibolite and granulite facies, intruded by charnockite and granitoid rocks. These igneous rocks were intruded in three intervals: c. 1160–1140  Ma, c. 1112 Ma, and c. 1080–1050 Ma, and are massive to weakly foliated in texture. This region consists of

3003-545: The Ottawan (now 1090–1020 Ma) and Rigolet (still 1010–980 Ma) become phases which are grouped into the Grenvillian orogeny. Reconstruction of the events of the orogeny is ongoing, but the generally accepted view is that the eastern and southern margins of Laurentia were active convergent margins until the beginning of continental collision . This type of subduction (B-type) tends to emplace magmatic arcs on or near

3080-507: The bottom of the mantle transition zone at 650 km depth. Subduction to greater depths is less certain, but there is evidence that they may sink to mid-lower-mantle depths at about 1,500  km depth. The source of mantle plumes is postulated to be the core-mantle boundary at 3,000  km depth. Because there is little material transport across the core-mantle boundary, heat transfer must occur by conduction, with adiabatic gradients above and below this boundary. The core-mantle boundary

3157-514: The bottom portion of the lithosphere is stripped off. Both models have been proposed for the Grenville orogeny. The Grenville orogeny can be categorized into three sections based on structure, lithology, and thermochronology. The three sections, respectively called the Gneiss Belt, Metasedimentary Belt, and the Granulite terrane are all separated by shear zones. The Gneiss Belt is made up of felsic gneisses and amphibolites that were metamorphosed in

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3234-428: The bulbous head expands it may entrain some of the adjacent mantle into itself. The size and occurrence of mushroom mantle plumes can be predicted by the transient instability theory of Tan and Thorpe. The theory predicts mushroom-shaped mantle plumes with heads of about 2000 km diameter that have a critical time (time from onset of heating of the lower mantle to formation of a plume) of about 830 million years for

3311-589: The club of the many type examples that do not exhibit the key characteristic originally proposed. The eruption of continental flood basalts is often 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. The chemical and isotopic composition of basalts found at hotspots differs subtly from mid-ocean-ridge basalts. These basalts, also called ocean island basalts (OIBs), are analysed in their radiogenic and stable isotope compositions. In radiogenic isotope systems

3388-490: The collision driving modern-day growth of the Himalaya range. For some time one of the blocks was believed to be the continent of Amazonia, but paleomagnetic evidence has now proven that this is not the case. These periods of thrusting and metamorphism were not continuous but were interrupted by comparatively quiet periods, during which AMCG ( anorthosite / mangerite / charnockite / granite ) plutons were intruded into

3465-493: The continental crust was already taking place and thickening the lithosphere . By 1.19 Ga the Elzevir back arc basin was closing. From 1.18 to 1.14 Ga extension was occurring in the area. Whether from lithospheric cooling, also known as thermal subsidence, or the compressional activity in the area reactivated some extensional faults. The extension is marked by the isotopic ages of the previously mentioned rocks. Additionally there

3542-484: The core-mantle boundary (2900 km depth) to a possible layer of shearing and bending at 1000 km. They were detectable because they were 600–800 km wide, more than three times the width expected from contemporary models. Many of these plumes are in the large low-shear-velocity provinces under Africa and the Pacific, while some other hotspots such as Yellowstone were less clearly related to mantle features in

3619-467: The core-mantle boundary. Lithospheric extension is attributed to processes related to plate tectonics. These processes are well understood at mid-ocean ridges, where most of Earth's volcanism occurs. It is less commonly recognised that the plates themselves deform internally, and can permit volcanism in those regions where the deformation is extensional. Well-known examples are the Basin and Range Province in

3696-438: The correlation between major element compositions of OIB and their stable isotope ratios. Tholeiitic OIB is interpreted as a product of a higher degree of partial melting in particularly hot plumes, while alkali OIB is interpreted as a product of a lower degree of partial melting in smaller, cooler plumes. In 2015, based on data from 273 large earthquakes, researchers compiled a model based on full waveform tomography , requiring

3773-422: The edge of the overriding plate in modern subduction zones, and evidence of contemporary (c. 1300–1200 Ma) island arcs can be found throughout the Grenville orogen. The Andes of South America are considered a modern analogue. From about c. 1190–980 Ma (the actual timing varies by locality) two separate continental blocks collided with Laurentia. Both of these collision events are thought to be analogous to

3850-523: The equivalent of 3 million hours of supercomputer time. Due to computational limitations, high-frequency data still could not be used, and seismic data remained unavailable from much of the seafloor. Nonetheless, vertical plumes, 400 C hotter than the surrounding rock, were visualized under many hotspots, including the Pitcairn , Macdonald , Samoa , Tahiti , Marquesas , Galapagos , Cape Verde , and Canary hotspots. They extended nearly vertically from

3927-453: The great size of the area affected by Grenville events, there is some variance in timing across the orogenic belt . Ages are approximated from the magmatic activity associated with the individual cycles of the orogeny. The gaps in the ages of the compression cycles and isotope analysis of hornblende , biotite , and potassium feldspar suggest that extension was occurring when compression had momentarily ceased. Rivers' 2008 paper examines

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4004-535: The mantle source. There are two competing interpretations for this. In the context of mantle plumes, the near-surface material is postulated to have been transported down to the core-mantle boundary by subducting slabs, and to have been transported back up to the surface by plumes. In the context of the Plate hypothesis, subducted material is mostly re-circulated in the shallow mantle and tapped from there by volcanoes. Stable isotopes like Fe are used to track processes that

4081-688: The metamorphism cannot be determined. It is important to separate local from large-scale tectonic history of the orogenic belt in order to understand the orogeny. For this purpose, the Grenville orogen is generally broken into four localities: the southern extent in Texas and Mexico, the Appalachians , the Adirondacks , and the Grenville Province . A portion of the orogen can be found in Scotland, but because of Scotland's proximity to

4158-436: The model. The unexpected size of the plumes leaves open the possibility that they may conduct the bulk of the Earth's 44 terawatts of internal heat flow from the core to the surface, and means that the lower mantle convects less than expected, if at all. It is possible that there is a compositional difference between plumes and the surrounding mantle that slows them down and broadens them. Mantle plumes have been suggested as

4235-409: The originally subducted material creates diverging trends, termed mantle components. Identified mantle components are DMM (depleted mid-ocean ridge basalt (MORB) mantle), HIMU (high U/Pb-ratio mantle), EM1 (enriched mantle 1), EM2 (enriched mantle 2) and FOZO (focus zone). This geochemical signature arises from the mixing of near-surface materials such as subducted slabs and continental sediments, in

4312-504: The orogen, these sequences of high pressure metamorphic rocks are cut by intrusive AMCG suite plutons, generally interpreted as syn- or post-tectonic. AMCG plutonism is generally associated with asthenospheric upwelling under thinned lithosphere . This is derived from the theory that AMCG plutonism is driven by ponding of olivine tholeiite basalt at the base of the continental crust during tectonic extension. The lithosphere may be thinned either convectively or by delamination , in which

4389-457: The orogeny) fall into two age groups in Mexico; c. 1235–1115 Ma and c. 1035–1010 Ma. Rocks of the former group bear geochemical signatures implying island arc and back-arc basin provenance. The latter group represents AMCG magmatism. These AMCG rocks are somewhat anomalous throughout the Grenville orogen, there is no known orogenic event which immediately predates their emplacement. It

4466-620: The plate motion. Another example is the Canary Islands in the northeast of Africa in the Atlantic Ocean. Helium-3 is a primordial isotope that formed in the Big Bang . Very little is produced, and little has been added to the Earth by other processes since then. Helium-4 includes a primordial component, but it is also produced by the natural radioactive decay of elements such as uranium and thorium . Over time, helium in

4543-412: The plume hypothesis has been challenged and contrasted with the more recent plate hypothesis ("Plates vs. Plumes"). The reason for this is that the mantle-plume hypothesis has not been suitable for making reliable predictions since its introduction in 1971 and has therefore been repeatedly adapted to observed hotspots depending on the situation. Over time, with the growing number of models, the concept of

4620-463: The sense of columnar vertical features that span most of the Earth's mantle, transport large amounts of heat, and contribute to surface volcanism. Under the umbrella of the plate hypothesis, the following sub-processes, all of which can contribute to permitting surface volcanism, are recognised: In addition to these processes, impact events such as ones that created the Addams crater on Venus and

4697-550: The shallow asthenosphere that is thought to be flowing rapidly in response to motion of the overlying tectonic plates. There is no other known major thermal boundary layer in the deep Earth, and so the core-mantle boundary was the only candidate. The base of the mantle is known as the D″ layer , a seismological subdivision of the Earth. It appears to be compositionally distinct from the overlying mantle and may contain partial melt. Two very broad, large low-shear-velocity provinces exist in

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4774-734: The source for flood basalts . These extremely rapid, large scale eruptions of basaltic magmas have periodically formed continental flood basalt provinces on land and oceanic plateaus in the ocean basins, such as the Deccan Traps , the Siberian Traps the Karoo-Ferrar flood basalts of Gondwana , and the largest known continental flood basalt, the Central Atlantic magmatic province (CAMP). Many continental flood basalt events coincide with continental rifting. This

4851-425: The speeds of seismic waves, but unfortunately so do composition and partial melt. As a result, wave speeds cannot be used simply and directly to measure temperature, but more sophisticated approaches must be taken. Seismic anomalies are identified by mapping variations in wave speed as seismic waves travel through Earth. A hot mantle plume is predicted to have lower seismic wave speeds compared with similar material at

4928-411: The surface and erupts to form hotspots. The most prominent thermal contrast known to exist in the deep (1000 km) mantle is at the core-mantle boundary at 2900 km. Mantle plumes were originally postulated to rise from this layer because the hotspots that are assumed to be their surface expression were thought to be fixed relative to one another. This required that plumes were sourced from beneath

5005-413: The surface crust in two distinct and largely independent convective flows: The plume hypothesis was simulated by laboratory experiments 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. Based on these experiments, mantle plumes are now postulated to comprise two parts:

5082-750: The surface. Numerical modelling predicts that melting and 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 (less than 1 million years). 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 (formerly

5159-533: The theory are linear volcanic chains, noble gases , geophysical anomalies, and geochemistry . The age-progressive distribution of the Hawaiian-Emperor seamount chain has been explained as a result of a fixed, deep-mantle plume rising into the upper mantle, partly melting, and causing a volcanic chain to form as the plate moves overhead relative to the fixed plume source. Other hotspots with time-progressive volcanic chains behind them include Réunion ,

5236-461: The timing of the different periods of the orogeny and reconstructs the timeline based on the spatial and temporal metamorphism of the rocks present. According to this newer version of the timeline which is a composite of Rivers 1997 and Gower and Krogh 2002, the Elzevirian orogeny occurs from 1240 to 1220 Ma, the Shawinigan occurs from 1190 to 1140 Ma and is no longer part of the Grenville cycle,

5313-536: The upper amphibolite to granulite facies. Thrusting in this section was low angle but would have the potential to increase and rotate as it continued and evolved. Shear in this area is referred to as ductile shear meaning the material was cooling and becoming solid, but still behaving viscously or plasticly. The age of this belt is approximately 1.8 to 1.18 Ga. Regional metamorphism is believed to have deformed this area at approximately 1.4 Ga and metamorphic thrusting at approximately 1.16 to 1.12 Ga. The Metasedimentary Belt

5390-556: The upper atmosphere is lost into space. Thus, the Earth has become progressively depleted in helium, and He is not replaced as He is. As a result, the ratio He/ He in the Earth has decreased over time. Unusually high He/ He have been observed in some, but not all, hotspots. This is explained by plumes tapping a deep, primordial reservoir in the lower mantle, where the original, high He/ He ratios have been preserved throughout geologic time. Other elements, e.g. osmium , have been suggested to be tracers of material arising from near to

5467-480: The upper mantle and above, with an emphasis on plate tectonics as the driving force of magmatism. The plate hypothesis suggests that "anomalous" volcanism results from lithospheric extension that permits melt to rise passively from the asthenosphere beneath. It is thus the conceptual inverse of the plume hypothesis because the plate hypothesis attributes volcanism to shallow, near-surface processes associated with plate tectonics, rather than active processes arising at

5544-457: The uprising material experiences during melting. The processing of oceanic crust, lithosphere, and sediment through a subduction zone decouples the water-soluble trace elements (e.g., K, Rb, Th) from the immobile trace elements (e.g., Ti, Nb, Ta), concentrating the immobile elements in the oceanic slab (the water-soluble elements are added to the crust in island arc volcanoes). Seismic tomography shows that subducted oceanic slabs sink as far as

5621-433: The use of Myr (duration) plus Ma (million years ago) versus using only the term Ma . In either case, the term Ma is used in geology literature conforming to ISO 31-1 (now ISO 80000-3 ) and NIST 811 recommended practices. Traditional style geology literature is written: The Cretaceous started 145 Ma and ended 66 Ma, lasting for 79 Myr. The "ago" is implied, so that any such year number "X Ma" between 66 and 145

5698-590: The western USA, the East African Rift valley, and the Rhine Graben . Under this hypothesis, variable volumes of magma are attributed to variations in chemical composition (large volumes of volcanism corresponding to more easily molten mantle material) rather than to temperature differences. While not denying the presence of deep mantle convection and upwelling in general, the plate hypothesis holds that these processes do not result in mantle plumes, in

5775-480: Was a transpressional boundary during the Ottawan, when the highlands were thrust over the lowlands. The Grenville province is named for Grenville , Quebec, and constitutes the youngest portion of the Canadian Shield . Since the area has not undergone any regional metamorphic overprinting since the orogeny, it is considered an ideal study area for Grenville and pre-Grenville age tectonics. Hence, most of what

5852-496: Was at this point that the Central Granulite Terrane was exhumed and minor magmatism occurred. The reason for change from compression to extension is unknown but may be the result of gravitational collapse, mantle delamination, the formation of a plume underneath a supercontinent , changes in far-field drivers on the distribution of stress, or any combination of reasons originating from the fact that our planet

5929-470: Was the accretion of an island arc at some point during the Elzevirian Orogeny. Before the accretion of the island arc took place, subduction between a continental plate and presumably an oceanic plate was taking place. Slab pull and far-field drivers such as ridge push were aiding in closing the distance between the island arc and the continent. Depending on the angle of subduction, deformation of

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