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70-398: The Alexandra Volcanic Group (also known as Alexandra volcanic lineament or Alexandra Volcanics ) is a chain of extinct calc-alkalic basaltic stratovolcanoes that were most active between 2.74 and 1.60 million years ago but is now known to have had more recent activity between 1.6 and 0.9 million years ago. They extend inland from Mount Karioi near Raglan with Mount Pirongia being

140-533: A calc-alkaline magma is oxidized enough to (simultaneously) precipitate significant amounts of the iron oxide magnetite , causing the iron content of the magma to remain more steady as it cools than with a tholeiitic magma. The difference between these two magma series can be seen on an AFM diagram, a ternary diagram showing the relative proportions of the oxides of Na 2 O + K 2 O (A), FeO + Fe 2 O 3 (F), and MgO (M). As magmas cool, they precipitate out significantly more iron and magnesium than alkali, causing

210-669: A critical time of about 830 Myr for a core mantle heat flux of 20 mW/m , 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 then erupt onto 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

280-522: A definitive list. Some scientists suggest that several tens of plumes exist, whereas others suggest that there are none. The theory was really inspired by the Hawaiian volcano system. Hawaii is a large volcanic edifice in the center of the Pacific Ocean, far from any plate boundaries. Its regular, time-progressive chain of islands and seamounts superficially fits the plume theory well. However, it

350-518: A geo-stationary plate. Many postulated "hot spots" are also lacking time-progressive volcanic trails, e.g., Iceland, the Galapagos, and the Azores. Mismatches between the predictions of the hypothesis and observations are commonly explained by auxiliary processes such as "mantle wind", "ridge capture", "ridge escape" and lateral flow of plume material. Helium-3 is a primordial isotope that formed in

420-537: 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 those found at mid-ocean ridges and volcanoes associated with subduction zones (island arc basalts). " Ocean island basalt "

490-578: A model based on full waveform tomography , requiring 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

560-647: 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

630-490: 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

700-630: 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 "hot spots" with time-progressive volcanic chains behind them include Réunion , the Chagos-Laccadive Ridge , the Louisville Ridge , the Ninety East Ridge and Kerguelen , Tristan , and Yellowstone . An intrinsic aspect of

770-536: 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 "hot spots", 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

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840-498: 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, "hot spots". In mantle plume theory, 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. In the context of the Plate hypothesis, the high ratios are explained by preservation of old material in

910-421: 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 a lower temperature. Mantle material containing a trace of partial melt (e.g., as

980-532: A subduction-related origin but which include the still active Mount Taranaki at the southern end of this belt. The Taranaki Fault is between the two sets of volcanoes. To the south east are more back arc volcanoes including now the volcanoes of the Taupō Volcanic Zone which have now been continuously active for over 2 million years. Between Karioi and Pirongia the highland terrain of the Karioi horst block

1050-501: Is volcanism that takes place away from the margins of tectonic plates . Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics . However, the origins of volcanic activity within plates remains controversial. Mechanisms that have been proposed to explain intraplate volcanism include mantle plumes; non-rigid motion within tectonic plates (the plate model); and impact events . It

1120-499: 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. The plume hypothesis postulates that this melt rises to

1190-535: Is almost unique on Earth, as nothing as extreme exists anywhere else. The second strongest candidate for a plume location is often quoted to be Iceland, but according to opponents of the plume hypothesis its massive nature can be explained by plate tectonic forces along the mid-Atlantic spreading center. Mantle plumes have been suggested as 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

1260-608: Is also similar to basalts found throughout the oceans on both small and large seamounts (thought to be formed by eruptions on the sea floor that did not rise above the surface of the ocean). They are also compositionally similar to some basalts found in the interiors of the continents (e.g., the Snake River Plain ). In major elements, ocean island basalts are typically higher in iron (Fe) and titanium (Ti) than mid-ocean ridge basalts at similar magnesium (Mg) contents. In trace elements , they are typically more enriched in

1330-409: Is drawn down into the mantle, causing rifting. 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 theory . This proposes shallower, passive leakage of magma from

1400-617: Is interrupted by the mongenic volcanoes of Okete volcanic field. The lineament then extends into the Hamilton Basin , a major rift-related depression bound by the Waipa Fault Zone with the arc basaltic volcanoes of Pukehoua, Kakepuku , Te Kawa , Tokanui . Kairangi is the furtherist to the east and has been dated at 2.62 ± 0.17 million years ago. Other basaltic volcanic fields that are also now thought to represent Auckland Volcanic Province intraplate volcanism active in

1470-471: 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 the speeds of seismic waves, but unfortunately so do composition and partial melt. As

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1540-704: 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 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

1610-451: Is likely that different mechanisms accounts for different cases of intraplate volcanism. A mantle plume is a proposed mechanism of convection of abnormally hot rock within the Earth's mantle . Because the plume head partly 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

1680-463: 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. In the context of the alternative "Plate model", continental breakup is a process integral to plate tectonics, and massive volcanism occurs as a natural consequence when it starts. The current mantle plume theory is that material and energy from Earth's interior are exchanged with

1750-422: 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, and 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

1820-504: Is some confusion regarding what constitutes support, as there has been a tendency to re-define the postulated characteristics of mantle plumes after observations have been made. Some common and basic lines of evidence cited in support of 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

1890-473: 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 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

1960-528: 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. The hypothesis of mantle plumes has required progressive hypothesis-elaboration leading to variant propositions such as mini-plumes and pulsing plumes. Mantle plumes were first proposed by J. Tuzo Wilson in 1963 and further developed by W. Jason Morgan in 1971. A mantle plume

2030-953: 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 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

2100-610: The Earth's mantle . Trends in composition can be explained by a variety of processes. Many explanations focus on water content and oxidation states of the magmas . Proposed mechanisms of formation begin with partial melting of subducted material and of mantle peridotite (olivine and pyroxene) altered by water and melts derived from subducted material. Mechanisms by which the calc-alkaline magmas then evolve may include fractional crystallization, assimilation of continental crust , and mixing with partial melts of continental crust. Intraplate volcanism Intraplate volcanism

2170-518: The Hawaiian–Emperor seamount chain . However, paleomagnetic data show that mantle plumes can be associated with Large Low Shear Velocity Provinces (LLSVPs) and do move. Two largely independent convective processes are proposed: 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

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2240-659: The Pleistocene are adjacent in a more recent to the north trend from the Alexandra Volcanic Group through to the Ngatutura volcanic field which was active between 1,830,000 and 1,540,000 years ago, the South Auckland volcanic field which erupted between 550,000 and 1,600,000 years ago, and the very recently active but presently dormant younger Auckland volcanic field . These locations fit with

2310-421: 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. Various lines of evidence have been cited in support of mantle plumes. There

2380-535: The Alexandra volcanic lineament, an alignment striking north-west to south-east over 60 km (37 mi) in length and is an example of backarc, intraplate basaltic volcanism that is very rare on land. This is because the arc basalts are in a very close relationship to a basaltic intraplate monogenetic volcanic field , the Okete which also erupted in late Pliocene times (2.7-1.8 million years ago). The separation of

2450-448: 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 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

2520-507: 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

2590-416: The calc-alkaline series include volcanic types such as basalt , andesite , dacite , rhyolite , and also their coarser-grained intrusive equivalents ( gabbro , diorite , granodiorite , and granite ). They do not include silica-undersaturated , alkalic, or peralkaline rocks . Rocks from the calc-alkaline magma series are distinguished from rocks from the tholeiitic magma series by the redox state of

2660-484: 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

2730-417: The distinct geochemical signature of ocean island basalts results from inclusion of a component of subducted slab material. This must have been recycled in the mantle, then re-melted and incorporated in the lavas erupted. In the context of the plume hypothesis, subducted slabs are postulated to have been subducted down as far as the core-mantle boundary, and transported back up to the surface in rising plumes. In

2800-673: The eastern slopes of Pirongia. The small basaltic centre at Kairangi is likely the furthest east point of the Okete volcanic field, but there is the possibility from drill sampling in the Hamilton Basin that other basaltic volcanoes exist that are subsurface now. To its west, under the Tasman Sea are the even older volcanoes associated with the Northland-Mohakatino volcanic belt (Mohakatino Volcanic Arc) which are of

2870-455: The formation of island arc basalts. The subducting slab is depleted in these water-mobile elements (e.g., K , Rb , Th , Pb ) and thus relatively enriched in elements that are not water-mobile (e.g., Ti, Nb, Ta) compared to both mid-ocean ridge and island arc basalts. Ocean island basalts are also relatively enriched in immobile elements relative to the water-mobile elements. This, and other observations, have been interpreted as indicating that

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2940-577: The iron-magnesium ratio to remain relatively constant, so the magma moves in a straight line towards the alkali corner on the AFM diagram. Calc-alkaline magmas are typically hydrous . Calc-alkaline rocks typically are found in the volcanic arcs above subduction zones, commonly in island arcs , and particularly on those arcs on continental crust. Rocks in the series are thought to be genetically related by fractional crystallization and to be at least partly derived from magmas of basalt composition formed in

3010-487: The largest, with Pukehoua on the eastern slopes of Pirongia, Kakepuku , Te Kawa , and Tokanui completing the definitive lineament. The associated, but usually separated geologically basaltic monogenetic Okete volcanic field (also known as the Okete Volcanic Formation or Okete Volcanics ), lies mainly between Karioi and Pirongia but extends to the east and is quite scattered. The chain extends in

3080-446: The light rare-earth elements than mid-ocean ridge basalts. Compared to island arc basalts, ocean island basalts are lower in alumina (Al 2 O 3 ) and higher in immobile trace elements (e.g., Ti, Nb , Ta ). These differences result from processes that occur during the subduction of oceanic crust and mantle lithosphere . Oceanic crust (and to a lesser extent, the underlying mantle) typically becomes hydrated to varying degrees on

3150-520: The magma they crystallized from. Tholeiitic magmas are reduced, and calc-alkaline magmas are oxidized, with higher oxygen fugacities . When mafic (basalt-producing) magmas crystallize, they preferentially crystallize the more magnesium-rich and iron-poor forms of the silicate minerals olivine and pyroxene , causing the iron content of tholeiitic magmas to increase as the melt is depleted of iron-poor crystals. (Magnesium-rich olivine solidifies at much higher temperatures than iron-rich olivine.) However,

3220-431: The magmas to move towards the alkali corner. In tholeiitic magma, as it cools and preferentially produces magnesium-rich crystals, the magnesium content of the magma plummets, causing the magma to move away from the magnesium corner until it runs low on magnesium and begins to move towards the alkali corner as it loses iron and remaining magnesium. With the calc-alkaline series, however, the precipitation of magnetite causes

3290-406: 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 plate theory attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics . According to the plate theory,

3360-483: 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

3430-510: 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. Many different localities have been suggested to be underlain by mantle plumes, and scientists cannot agree on

3500-416: The much larger postulated mantle plumes. Based on these experiments, mantle plumes are now postulated to comprise two parts: 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 is considered to resemble a mushroom. The bulbous head of thermal plumes forms because hot material moves upward through the conduit faster than

3570-593: 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 is consistent with a system that tends toward equilibrium: as matter rises in a mantle plume, other material

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3640-409: 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

3710-477: The period that the South Auckland volcanic field and Mangakino caldera complex were active. The arc basalt volcano remnants at Tokanui are a small mound that rises about 30 m (98 ft) within higher rolling hills of the Puketoka and Karapiro Formations. There has been much progress over the last decade in characterising Karioi , Pirongia and a separate arc basaltic centre at Pukehoua incorporated into

3780-429: The plate hypothesis, the slabs are postulated to have been recycled at shallower depths – in the upper few hundred kilometers that make up the upper mantle . However, the plate hypothesis is inconsistent with both the geochemistry of shallow asthenosphere melts (i.e., Mid-ocean ridge basalts) and with the isotopic compositions of ocean island basalts. In 2015, based on data from 273 large earthquakes, researchers compiled

3850-553: The plume hypothesis is that the "hot spots" and their volcanic trails have been fixed relative to one another throughout geological time. Whereas there is evidence that the chains listed above are time-progressive, it has, however, 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 volcanic activity across

3920-455: 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 easily 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

3990-506: The presence of deep mantle convection and upwelling in general, the plate hypothesis holds that these processes do not result in mantle plumes, in 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: Lithospheric extension enables pre-existing melt in

4060-410: The principal cause of volcanism is extension of the lithosphere . Extension of the lithosphere is a function of the lithospheric stress field . The global distribution of volcanic activity at a given time reflects the contemporaneous lithospheric stress field, and changes in the spatial and temporal distribution of volcanoes reflect changes in the stress field. The main factors governing the evolution of

4130-407: The seafloor, partly as the result of seafloor weathering, and partly in response to hydrothermal circulation near the mid-ocean-ridge crest where it was originally formed. As oceanic crust and underlying lithosphere subduct, water is released by dehydration reactions, along with water-soluble elements and trace elements. This enriched fluid rises to metasomatize the overlying mantle wedge and leads to

4200-552: 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

4270-424: The shallow mantle. Ancient, high He/ He ratios would be particularly easily preserved in materials lacking U or Th, so He was not added over time. Olivine and dunite , both found in subducted crust, are materials of this sort. Other elements, e.g. osmium , have been suggested to be tracers of material arising from near to the Earth's core, in basalts at oceanic islands. However, so far conclusive proof for this

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4340-563: The stress field are: Beginning in the early 2000s, 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 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

4410-417: The surface and erupts to form "hot spots". 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 "hot spots" 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

4480-614: The surface crust in two distinct modes: the predominant, steady state plate tectonic regime driven by upper mantle convection , and a punctuated, intermittently dominant, mantle overturn regime driven by plume convection. This second regime, while often 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. These basalts, also called ocean island basalts (OIBs), are analysed in their radiogenic and stable isotope compositions. In radiogenic isotope systems

4550-416: 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 recently been discovered that the volcanic locus of this chain has not been fixed over time, and it thus joined the club of the many type examples that do not exhibit the key characteristic originally proposed. The eruption of continental flood basalts

4620-694: The trend being related to the opening of the Hauraki Rift in the Miocene and/or fracturing of the lithosphere. At the same approximate time the Alexandra Volcanic Group was initially active to its east in Zealandia the Tauranga Volcanic Centre was active. More age data is accessible for individual basalts/vents by enabling mouseover in the interactive map of the field in the infobox. Calc-alkalic The diverse rock types in

4690-649: The two fields because of the different basalt composition was first proposed in 1983. The arc-type lavas of the Alexandra Volcanic Group are mainly ankaramite , a type of basalt found typically in some South Pacific Ocean Islands and not within continental crust. There are at least 27 vents in the Okete volcanic field, with most being in the northwest near the eastern flanks of Karioi. Only a few sites globally have island arc basalt and intraplate ocean island basalt so associated. The first stage of activity that finished about 1.9 million years ago produced all

4760-457: 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

4830-440: The volcanoes of both the Alexandra volcanic lineament and the monogenetic Okete volcanic field. Karioi is the oldest at 2.48 to 2.28 ± 0.07 million years ago on unmodified chronology. Pirongia has at least six edifice-forming vents separated by features including those resulting from large volume collapse events. The second stage was confined to Pirongia and consisted of basaltic eruptions between 1.6 and 0.9 million years ago during

4900-536: 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, often referred to as a "hotspot". As the overlying tectonic plate (lithosphere) moves over this hotspot, the eruption of magma from the fixed conduit onto

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