Ocean island basalt (OIB) is a volcanic rock, usually basaltic in composition, erupted in oceans away from tectonic plate boundaries. Although ocean island basaltic magma is mainly erupted as basalt lava, the basaltic magma is sometimes modified by igneous differentiation to produce a range of other volcanic rock types, for example, rhyolite in Iceland, and phonolite and trachyte at the intraplate volcano Fernando de Noronha. Unlike mid-ocean ridge basalts (MORBs), which erupt at spreading centers (divergent plate boundaries), and volcanic arc lavas, which erupt at subduction zones (convergent plate boundaries), ocean island basalts are the result of intraplate volcanism. However, some ocean island basalt locations coincide with plate boundaries like Iceland, which sits on top of a mid-ocean ridge, and Samoa, which is located near a subduction zone.
In the ocean basins, ocean island basalts form seamounts, and in some cases, enough material is erupted that the rock protrudes from the ocean and forms an island, like at Hawaii, Samoa, and Iceland. Over time, however, thermal subsidence and mass loss via subaerial erosion causes islands to become completely submarine seamounts or guyots. Many ocean island basalts erupt at volcanic hotspots, which are thought to be the surface expressions of melting of thermally buoyant, rising conduits of hot rock in the mantle, called mantle plumes. Mantle plume conduits may drift slowly, but Earth’s tectonic plates drift more rapidly relative to mantle plumes. As a result, the relative motion of Earth’s tectonic plates over mantle plumes produces age-progressive chains of volcanic islands and seamounts with the youngest, active volcanoes located above the axis of the mantle plume while older, inactive volcanoes are located progressively farther away from the plume conduit (see Figure 1). Hotspot chains can record tens of millions of years of continuous volcanic history; for example, the oldest seamounts in the Hawaiian–Emperor seamount chain are over 80 million years old.
Not all ocean island basalts are the product of mantle plumes. There are thousands of seamounts that are not clearly associated with upwelling mantle plumes, and there are chains of seamounts that are not age progressive. Seamounts that are not clearly linked to a mantle plume indicate that regional mantle composition and tectonic activity may also play important roles in producing intraplate volcanism.
The geochemistry of ocean island basalts is useful for studying the chemical and physical structure of Earth’s mantle. Some mantle plumes that feed hotspot volcanism lavas are thought to originate as deep as the core–mantle boundary (~2900 km deep). The composition of the ocean island basalts at hotspots provides a window into the composition of mantle domains in the plume conduit that melted to yield the basalts, thus providing clues as to how and when different reservoirs in the mantle formed.
Early conceptual models for the geochemical structure of the mantle argued that the mantle was split into two reservoirs: the upper mantle and the lower mantle. The upper mantle was thought to be geochemically depleted due to melt extraction which formed Earth’s continents. The lower mantle was thought to be homogenous and “primitive”. (Primitive, in this case, refers to silicate material that represents the building blocks of the planet that has not been modified by melt extraction, or mixed with subducted materials, since Earth’s accretion and core formation.) Seismic tomography showed subducted slabs passing through the upper mantle and entering the lower mantle, which indicates that the lower mantle cannot be isolated. Additionally, the isotopic heterogeneity observed in plume-derived ocean island basalts argues against a homogenous lower mantle. Heavy, radiogenic isotopes are a particularly useful tool for studying the composition of mantle sources because isotopic ratios are not sensitive to mantle melting. This means that the heavy radiogenic isotopic ratio of a melt, which upwells and becomes a volcanic rock on the surface of the Earth, reflects the isotopic ratio of the mantle source at the time of melting. The best studied heavy radiogenic isotope systems in ocean island basalts are 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 176Hf/177Hf and, more recently, 187Os/188Os. In each of these systems, a radioactive parent isotope with a long half-life (i.e., longer than 704 million years) decays to a “radiogenic” daughter isotope. Changes in the parent/daughter ratio by, for example, mantle melting, results in changes in the radiogenic isotopic ratios of 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 176Hf/177Hf, and 187Os/188Os. Thus, these radiogenic isotopic systems are sensitive to the timing, and degree, of parent/daughter the changed (or fractionated) parent daughter ratio, which then informs the process(es) responsible for generating observed radiogenic isotopic heterogeneity in ocean island basalts. In mantle geochemistry, any composition with relatively low 87Sr/86Sr, and high 143Nd/144Nd and 176Hf/177Hf, is a referred to as “geochemically depleted”. High 87Sr/86Sr, and low 143Nd/144Nd and 176Hf/177Hf, is referred to as “geochemically enriched”. Lead isotopic compositions (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) in mantle derived rocks are described as unradiogenic (for relatively low 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) or radiogenic (for relatively high 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb).
These isotopic systems have provided evidence for a heterogenous lower mantle. There are several distinct “mantle domains” or endmembers that appear in the ocean island basalt record. When plotted in multi-isotope space, ocean island basalts tend to form arrays trending from a central composition out to an endmember with an extreme composition. The depleted mantle, or DM, is one endmember, and is defined by low 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and high 143Nd/144Nd and 176Hf/177Hf. The DM is therefore geochemically depleted (as the name states), and relatively unradiogenic. Mid-ocean ridge passively sample the upper mantle and MORBs are typically geochemically depleted, and therefore it is widely accepted that the upper mantle is composed mostly of depleted mantle. Thus, the term depleted MORB mantle (DMM) is often used to describe the upper mantle that sources mid-ocean ridge volcanism. Ocean island basalts also sample geochemically depleted mantle domains. In fact, most ocean island basalts are geochemically depleted, and <10% of ocean island basalts have lavas that extend to geochemically enriched (i.e., 143Nd/144Nd lower than the Earth’s building blocks) compositions.
There are two geochemically enriched domains, named enriched mantle 1 (EM1), and enriched mantle 2 (EM2). Though broadly similar, there are some important distinctions between EM1 and EM2. EM1 has unradiogenic 206Pb/204Pb, moderately high 87Sr/86Sr, and extends to lower 143Nd/144Nd and 176Hf/177Hf than EM2. Pitcairn, Kerguelen-Heard, and Tristan-Gough are the type localities of EM1. EM2 is defined by higher 87Sr/86Sr than EM1, and higher 143Nd/144Nd and 176Hf/177Hf at a given 87Sr/86Sr value, and intermediate 206Pb/204Pb. Samoa and Society are the archetypal EM2 localities.
Another distinct mantle domain is the HIMU mantle. In isotope geochemistry, the Greek letter µ (or mu) is used to describe the 238U/204Pb, such that ‘high µ’ (abbreviated HIMU) describes a high 238U/204Pb ratio. Over time, as 238U decays to 206Pb, HIMU Earth materials develop particularly radiogenic (high) 206Pb/204Pb. If an Earth material has elevated 238U/204Pb (HIMU), then it will also have elevated 235U/204Pb, and therefore will produce radiogenic Pb compositions for both the 206Pb/204Pb and 207Pb/204Pb isotopic systems (238U decays 206Pb, 235U decays to 207Pb). Similarly, Earth materials with high U/Pb also tend to have high Th/Pb, and thus evolve to have high 208Pb/204Pb (232Th decays to 208Pb). Ocean island basalts with highly radiogenic 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb are the products of HIMU mantle domains. St. Helena, and several islands in the Cook-Austral volcanic lineament (e.g., Mangaia) are the type localities for HIMU ocean island basalts.
The final mantle domain discussed here is the common composition that ocean island basalts trend toward in radiogenic isotopic multi-space. This is also most prevalent mantle source in ocean island basalts, and has intermediate to geochemically depleted 87Sr/86Sr, 143Nd/144Nd, and 176Hf/177Hf, as well as intermediate 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb. This central mantle domain has several names, each with slightly different implications. PREMA, or “Prevalent Mantle” was the first term coined by Zindler and Hart (1986) to describe the most common composition sampled by ocean island basalts. Hart et al. (1992) later named the location of the intersection of ocean island basalt compositions in radiogenic isotopic multi-space as the “Focus Zone”, or FOZO. Farley et al. (1992) in the same year described a high 3He/4He (a primitive geochemical signature) component in plumes as the “Primitive Helium Mantle”, or PHEM. Finally, Hanan and Graham (1996) used the term “C” (for common component) to describe a common mixing component in mantle derived rocks.
The presence of a particular mantle domain in ocean island basalts from two hotspots, signaled by a particular radiogenic isotopic composition, does not necessarily indicate that mantle plumes with similar isotopic compositions are sourced from the same physical reservoir in the deep mantle. Instead, mantle domains with similar radiogenic isotopic compositions sampled at different hotspot localities are thought to share similar geologic histories. For example, the EM2 hotspots of Samoa and Society are both thought to have a mantle source that contains recycled upper continental crust, an idea that is supported by stable isotope observations, including δ18O and δ7Li. The isotopic similarities do not imply that Samoa and Society have the same physical mantle source, as evidenced by their slightly distinct arrays in radiogenic isotopic multi-space. Thus, hotspots that are categorized as “EM1”, “EM2”, “HIMU”, or “FOZO”, may each sample physically distinct, but compositionally similar, portions of the mantle. Furthermore, some hotspot chains host lavas with wide range of isotopic compositions so that the plume source seems to either sample multiple domains which can be sampled at different times in the volcanic evolution of a hotspot.
Isotopic systems help to deconvolve the geologic processes that contributed to, and in some cases the timing of, the formation of these mantle domains. Some important examples include the presence of crustal fingerprints in enriched mantle sources that indicate that material from Earth’s continents and oceans can be subducted into the mantle and brought back up to the surface in buoyantly rising mantle plumes. Sulfur isotopic analyses have shown mass-independent-fractionation (MIF) in the sulfur isotopes in some plume-derived lavas. MIF of sulfur isotopes is a phenomenon that occurred in Earth’s atmosphere only before the Great Oxidation Event ~2.3 Ga. The presence of recycled material with MIF signatures indicates that some of the recycled material brought is older than 2.3 Ga, formed prior to the Great Oxidation Event and has resurfaced via mantle plume volcanism. Noble gas isotopic systems, such as 3He/4He, 20Ne/22Ne, and 129Xe/130Xe, have been used to demonstrate that parts of the lower mantle are relatively less degassed and have not been homogenized despite billions of years of mantle convective mixing. Some large, hot mantle plumes have anomalously high 3He/4He. Since 4He is being constantly produced within the Earth via alpha decay (of 235,238U, 232Th, and 147Sm), but 3He is not being generated in appreciable quantities in the deep Earth, the ratio of 3He to 4He is decreasing in the interior of the Earth over time. The early Solar System began with high 3He/4He and therefore the Earth first accreted with high 3He/4He. Thus, in plume-derived lavas, high 3He/4He is an “ancient” geochemical signature that indicates the existence of a well-preserved helium reservoir in the deep mantle. The timing of the formation of this reservoir is constrained by observed anomalies of 129Xe/130Xe in ocean islands basalts, because 129Xe was only produced by decay of 129I during the first ~100 My of Earth’s history. Together, high 3He/4He and 129Xe/130Xe indicate a relatively less degassed, primitive noble gas domain that has been relatively well preserved since the early Hadean.
There are various sources identified for ocean island basalt magma in Earth's mantle. These mantle sources are inferred from differences in radiogenic isotope ratios that magmas inherit from their source rock. Sources have been defined from a combined analysis of strontium (Sr), neodymium (Nd) and lead (Pb) isotopes. The sources as defined by radiogenic isotopes are:
|EMI||Enriched Mantle I||Probably mantle contaminated with material derived from subducted pelagic sediments. An alternative explanation is that this source derives from the sub-continental lithosphere which could also be contaminated by subducted pelagic sediments.|
|EMII||Enriched Mantle II||Likely mantle contaminated with material derived from the recycling[A] of terrigenous sediments from the continental crust into the mantle.|
|HIMU||High U/Pb ratio||Likely derived from subducted oceanic crust that has not been homogenized with the rest of the mantle. The lack of homogenization could be indebted to the accumulation of subducted oceanic crust in large-scale “megaliths” at the 670 km seismic discontinuity or near the core–mantle boundary.|
|PREMA||Prevalent Mantle||Possible formed by mixing of all the other mantle sources or a source formed early in Earth's history.|
|FOZO||Focus Zone||A source associated with mantle plumes. It is of intermediate composition between DMM and HIMU. The name Focus Zone derives from the apparent fanning out of compositions from this zone when displaying isotope composition data on tetrahedron chart. FOZO contains high contents of Helium-3. The FOZO source is associated with deep mantle plumes. FOZO has been proposed to be either the plume material that rises from the core–mantle boundary or material that becomes attached to the plume as a sheet as the plume it rises from the core–mantle boundary.|
A-type granites are a class of granite. They are characterised by low water and a lack of tectonic fabric. A stands for anorogenic or anhydrous. The granite class was proposed by Loiselle and Wones in 1979. It was in addition to I-, S- and M-type granites.The A-type granites form in an extensional tectonic setting. This could either be far from any orogeny, or after orogeny is completed.Chemical characteristics of A-type granites include high silica, alkalis, zirconium, niobium, gallium, yttrium and cerium. The ratio of gallium to aluminium is high, as is the ratio of iron to magnesium. There are lower levels of calcium and strontium. By using Ga/Al ratio, fractionated felsic I or S-type granites can overlap in apparent composition. Enriched alkalis include sodium, potassium, rubidium and caesium. Other depleted elements include barium, phosphorus, titanium and europium.Subtypes include A1, anorogenic, derived from ocean island basalt; and A2 post-orogenic, derived by crustal melting or crust and mantle mixing.The source could be dry granulite left over from the loss of wet magma during orogenies.Arctic Cordillera
The Arctic Cordillera is a vast, deeply dissected chain of mountain ranges extending along the northeastern flank of the Canadian Arctic Archipelago from Ellesmere Island to the northeasternmost part of the Labrador Peninsula in northern Labrador and northern Quebec, Canada. It spans most of the eastern coast of Nunavut with high glaciated peaks rising through icefields and some of Canada's largest ice caps, including the Penny Ice Cap on Baffin Island. It is bounded to the east by Baffin Bay, Davis Strait and the Labrador Sea while its northern portion is bounded by the Arctic Ocean.Basalt
Basalt (US: , UK: ) is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very 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.Bravo Lake Formation
The Bravo Lake Formation is a mafic volcanic belt and large igneous province located at the northern margin of the Trans-Hudson orogeny on central Baffin Island, Nunavut, Canada. It is exposed along a nearly continuous east-west passage for 120 km (75 mi) and changes in stratigraphic thickness from 1 to 2.5 kilometers. The formation is a rare alkaline-suite that formed as a result of submarine rifting during the Paleoproterozoic period. The Bravo Lake Formation is surprisingly undeformed by the Himalayan-scale forming event during the Trans-Hudsonian orogeny.
The stratigraphy of the Bravo Lake Formation starts with a basic section of iron-oxide rich sandstones, psammites, and semi-pelites which cover a series of deformed pillow lavas which expand in viscosity towards the west, and volcanic/clastic deposits and ultramafic sills. The lower volcanic section is covered by garnet and diopside bearing calc–silicate layers and finely layered metasediments composed of coarse-grained actinolite, hornblende and biotite followed by pelites and semi-pelites that are intruded by separate sills. In the Ridge Lake area, the volcanic belt includes an interlayered series of amphibolite, gabbro, iron formation, sulfidic schist and metasediments.Geochemical results of pillow lavas and chill boundaries along five transects across the volcanic belt suggest the existence of three chemically different magma types within the Bravo Lake Formation.Lavas of the volcanic belt display geochemical characteristics similar to modern ocean-island–basalt groups. They range from moderately to intensely fractionated. REE-profiles are similar to those from tholeiitic basalts to extremely alkaline lavas in Hawaii.Ch'iyar Qullu (Oruro)
Ch'iyar Qullu (Aymara ch'iyara black, qullu mountain, "black mountain", also spelled Chiar Kkollu) is a volcanic centre in Bolivia. It is located in the Oruro Department, Ladislao Cabrera Province, Salinas de Garci Mendoza Municipality, northeast of Salinas de Garci Mendoza, near a maar named Jayu Quta ("salt lake").It is a sill formed from primitive phyric alkali basalt that closely resembles ocean island basalt in composition and now appears as a hill. The rocks contain augite and olivine and the eruption site coincides with a local lineament and is of Miocene age, with dates of 22.51±0.45 mya by Ar-Ar dating and 25.2±0.5 mya by K-Ar dating. The Ch'iyar Qullu magmas are Central Andes intraplate magmas and originate from the upper mantle.Lunar Crater volcanic field
Lunar Crater volcanic field is a volcanic field in Nye County, Nevada. It lies along the Reveille and Pancake Ranges and consists of over two hundred vents, mostly small volcanic cones with associated lava flows but also several maars and Lunar Crater. Some vents have been eroded so heavily that the structures underneath the volcanoes have been exposed. Lunar Crater itself has been used as a testing ground for Mars rovers and as training ground for astronauts.
The volcanic field has formed on top of older, Oligocene to Miocene age volcanic rocks and calderas, but its own activity commenced only about 6 million years ago. The reasons for volcanism there are not well known. The volcanic field has produced various types of basaltic magma and also trachyte; the most recent eruption was about 38,000 years ago and renewed activity is possible.Lutetium–hafnium dating
Lutetium–hafnium dating is a geochronological dating method utilizing the radioactive decay system of lutetium–176 to hafnium–176. With a commonly accepted half-life of 37.1 billion years, the long-living Lu–Hf decay pair survives through geological time scales, thus is useful in geological studies. Due to chemical properties of the two elements, namely their valences and ionic radii, Lu is usually found in trace amount in rare-earth element loving minerals, such as garnet and phosphates, while Hf is usually found in trace amount in zirconium-rich minerals, such as zircon, baddeleyite and zirkelite.The trace concentration of the Lu and Hf in earth materials posed some technological difficulties in using Lu–Hf dating extensively in the 1980s. With the use of inductively coupled plasma mass spectrometry (ICP–MS) with multi-collector (also known as MC–ICP–MS) in later years, the dating method is made applicable to date diverse earth materials. The Lu–Hf system is now a common tool in geological studies such as igneous and metamorphic rock petrogenesis, early earth mantle-crust differentiation, and provenance.Mantle plume
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 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.North Fiji Basin
The North Fiji Basin (NFB) is an oceanic basin west of Fiji in the south-west Pacific Ocean. It is an actively spreading back-arc basin delimited by the Fiji islands to the east, the inactive Vitiaz Trench to the north, the Vanuatu/New Hebrides island arc to the west, and the Hunter fracture zone to the south.
Roughly triangular in shape with its apex located at the northern end of the New Hebrides Arc, the basin is actively spreading southward and is characterised by three spreading centres and an oceanic crust younger than 12 Ma. The opening of the NFB began when a slab roll-back was initiated beneath the New Hebrides and the island arc started its clockwise rotation.
The opening of the basin was the result of the collision between the Ontong Java Plateau and the Australian Plate along the now inactive Solomon–Vitiaz subduction system north of the NFB.
The NFB is the largest and most developed back-arc basin of the south-west Pacific. It is opening in a complex geological setting between two oppositely verging subduction systems, the New Hebrides/Vanuatu and Tonga trenches and hence it's ocean floor has the World's largest amount of spreading centres per area.Two opposite-facing systems of deformation partly overlap where the Australian and Pacific plates meet along a section of the andesite line in the south-west Pacific: east of the NFB the Kermadec-Tonga Arc stretches some 3,000 km (1,900 mi) north from New Zealand, and west of the NFB the New Hebrides subduction zone formed during the opening of the NFB back-arc basin.There are three microplates in the NFB: New Hebrides, Balmoral Reef, and Conway Reef.Little was known about the NFB before 1985 and in the 1970s the central part of the basin, the only mapped area, was called the North Fiji Plateau.Payún Matrú
Payún Matrú is a shield volcano located in the Reserva Provincial La Payunia of the Malargüe Department, south of the Mendoza Province in Argentina. It lies in the back-arc region of the Andean Volcanic Belt, and formed during the Nazca Plate beneath the South American Plate. Payún Matrú, along with the Llancanelo, Nevado and Salado Basin volcanic fields, form the Payenia province. It has been proposed as a World Heritage Site since 2011.
Payún Matrú developed on sediment and volcanic rocks ageing from the Mesoproterozoic to the Tertiary periods. It consists of a large shield volcano capped by a caldera, formed during a major eruption between 168,000 and 82,000 years ago, a high compound volcano (known as Payun or Payun Liso), and two groups of scoria cones and lava flows. The Pleistocene Pampas Onduladas lava flow reaches a length of 167–181 km (104–112 mi) and is the world's longest Quaternary lava flow.
Volcanic activity at Payún Matrú commenced during the Plio-Pleistocene period, and generated lava fields such as Pampas Onduladas, the Payún Matrú shield volcano and the Payun volcano. After the formation of the caldera, volcanism continued both within the caldera in as lava domes and flows, and outside of it with the formation of scoria cones and lava flows east and especially west of Payún Matrú. Volcanic activity continued into the Holocene until about 515 years ago; oral tradition of local inhabitants contains references to earlier eruptions.Researcher Ridge
Researcher Ridge is an underwater ridge in the Northern Atlantic Ocean. It appears to be a chain of seamounts named Gollum Seamount, Vayda Seamount, Bilbo Seamount, Gandalf Seamount, The Shire Seamount, Pippin Seamount, Merry Seamount, Molodezhnaya Seamount, Frodo Seamount, Sam Seamount and Mount Doom Seamount that were likely formed by a hotspot.Southwest Indian Ridge
The Southwest Indian Ridge (SWIR) is a mid-ocean ridge located along the floors of the south-west Indian Ocean and south-east Atlantic Ocean. A divergent tectonic plate boundary separating the African Plate to the north from the Antarctic Plate to the south, the SWIR is characterised by ultra-slow spreading rates (only exceeding those of the Gakkel Ridge in the Arctic) combined with a fast lengthening of its axis between the two flanking triple junctions, Rodrigues (20°30′S 70°00′E) in the Indian Ocean and Bouvet (54°17′S 1°5′W) in the Atlantic Ocean.Stanley Robert Hart
Stanley Robert Hart (born 20 June 1935 in Swampscott, Massachusetts) is an American geologist, geochemist, leading international expert on mantle isotope geochemistry, and pioneer of chemical geodynamics.Tonga-Kermadec Ridge
The Tonga-Kermadec Ridge is an oceanic ridge in the south-west Pacific Ocean underlying the Tonga-Kermadec island arc.
It is the most linear, fastest converging, and most seismically active subduction boundary on Earth, and consequently has the highest density of submarine volcanoes.The Tonga-Kermadec Ridge stretches more than 3,000 km (1,900 mi) north-northeast from New Zealand's North Island. The Pacific Plate subducts westward beneath the Australian Plate along the ridge. It is divided into two segments, the northern Tonga Ridge and southern Kermadec Ridge, by the Louisville Seamount Chain. On its western side, the ridge is flanked by two back-arc basins, the Lau Basin and Havre Trough, that began opening at 6 Ma and 2 Ma respectively. Together with these younger basins the ridge forms the eastward-migrating, 100 Ma-old Lau-Tonga-Havre-Kermadec arc/back-arc system or complex.The extension in the Lau-Havre basin results in a higher rate of subduction than convergence along the Australian-Pacific plate boundary. The rates of extension, subduction, and convergence all increase northwards in this complex, subduction at a rate of 24–6 cm/year (9.4–2.4 in/year) and extension at a rate of 91–159 mm/a (3.6–6.3 in/year). As a result, the Tonga-Kermadec Ridge moves independently of both tectonic plates and forms the Tonga-Kermadec Plate, in its turn fragmented into the Niuafo'ou, Tonga, and Kermadec microplates.The Samoa and Louisville mantle plumes both contribute to the lavas of two of the northern Tonga islands, Tafahi and Niuatoputapu; ocean island basalt (OIB) from the Samoa plume were introduced from 3-4 Ma when subduction in the Vitiaz Trench (north-west of Tonga) ceased. The lavas of the Louisville Seamount Chain were generated 80-90 Ma but began to subduct under the Tonga-Kermadec Ridge at c. 8 Ma.The Hikurangi and Manihiki plateaux, north and south of the Tonga-Kermadec Ridge respectively, form part of the Ontong Java-Hikurangi-Manihiki large igneous province (LIP), the largest volcanic event on Earth during the past 200 million years.
The Osbourn Trough, located just north of the Tonga-Kermadec and Louisville intersection, is the palaeo-spreading centre between the Hikurangi and Manihiki plateaux away from which the age of the Pacific Plate increases from c. 85 Ma to 144 Ma.
The subduction of the Hikurangi Plateau beneath New Zealand and the southern part of the Kermadec Arc has resulted in large volumes of lava and a high density of volcanoes in the arc. The initial Hikurangi-Kermadec collision, however, occurred 250 km (160 mi) to the north where a missing piece of the Ontong Java-Hikurangi-Manihiki LIP has already been subducted.Tropic Seamount
Tropic Seamount is a Cretaceous seamount southwest of the Canary Islands and north of Cape Verde, one of a number of seamounts (a type of underwater volcanic mountain) in this part of the Atlantic Ocean. It was probably formed by volcanic processes triggered by the proximity to the African continent. Tropic Seamount is located at a depth of 970 metres (3,180 ft) and has a summit platform with an area of 120 square kilometres (46 sq mi).
Tropic Seamount is formed by volcanic rocks including basalt and trachyte and was probably an island at first; for reasons unknown it sunk to its present day depth. Large landslides and late volcanic activity affected the seamount, cutting large scars into its flanks and forming cones on its summit plateau, respectively. Volcanic activity at Tropic Seamount commenced almost 120 million years ago and ended about 60 million years ago. Later, sedimentation commenced on the seamount leading to the deposition of manganese crusts and pelagic sediments; iron and manganese accumulated in crusts over time beginning a few tens of millions of years ago.
Types of basalts
|Basalts by tectonic setting|
|Basalts by form and flow|
|Basalts by chemistry|