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Magma is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural satellites. Besides molten rock, magma may also contain suspended crystals and gas bubbles.
Basalt is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron exposed at or very near the surface of a rocky planet or moon. More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro. The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is also an important rock type on other planetary bodies in the Solar System. For example, the bulk of the plains of Venus, which cover ~80% of the surface, are basaltic; the lunar maria are plains of flood-basaltic lava flows; and basalt is a common rock on the surface of Mars.
Andesite is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.
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 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.
Peridotite ( PERR-ih-doh-tyte, pə-RID-ə-) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.
Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa, and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.
Compatibility is a term used by geochemists to describe how elements partition themselves in the solid and melt within Earth's mantle. In geochemistry, compatibility is a measure of how readily a particular trace element substitutes for a major element within a mineral.
Fractional crystallization, or crystal fractionation, is one of the most important geochemical and physical processes operating within crust and mantle of a rocky planetary body, such as the Earth. It is important in the formation of igneous rocks because it is one of the main processes of magmatic differentiation. Fractional crystallization is also important in the formation of sedimentary evaporite rocks.
Boninite is an extrusive rock high in both magnesium and silica, thought to be usually formed in fore-arc environments, typically during the early stages of subduction. The rock is named for its occurrence in the Izu-Bonin arc south of Japan. It is characterized by extreme depletion in incompatible trace elements that are not fluid mobile but variable enrichment in the fluid mobile elements. They are found almost exclusively in the fore-arc of primitive island arcs and in ophiolite complexes thought to represent former fore-arc settings or at least formed above a subduction zone.
Adakites are volcanic rocks of intermediate to felsic composition that have geochemical characteristics of magma originally thought to have formed by partial melting of altered basalt that is subducted below volcanic arcs. Most magmas derived in subduction zones come from the mantle above the subducting plate when hydrous fluids are released from minerals that break down in the metamorphosed basalt, rise into the mantle, and initiate partial melting. However, Defant and Drummond recognized that when young oceanic crust is subducted, adakites are typically produced in the arc. They postulated that when young oceanic crust is subducted it is "warmer" than crust that is typically subducted. The warmer crust enables melting of the metamorphosed subducted basalt rather than the mantle above. Experimental work by several researchers has verified the geochemical characteristics of "slab melts" and the contention that melts can form from young and therefore warmer crust in subduction zones.
Pyrolite is a term used to characterize a model composition of the Earth's mantle. This model is based on that a pyrolite source can produce the Mid-Ocean Ridge Basalt by partial melting. It was first proposed by Ted Ringwood (1962) as being 1 part basalt and 4 parts harzburgite, but later was revised to being 1 part tholeiitic basalt and 3 parts dunite. The term is derived from the mineral names PYR-oxene and OL-ivine. However, whether pyrolite is representative of the Earth's mantle remains debated.
The Central Indian Ridge (CIR) is a north–south-trending mid-ocean ridge in the western Indian Ocean.
Crustal recycling is a tectonic process by which surface material from the lithosphere is recycled into the mantle by subduction erosion or delamination. The subducting slabs carry volatile compounds and water into the mantle, as well as crustal material with an isotopic signature different from that of primitive mantle. Identification of this crustal signature in mantle-derived rocks is proof of crustal recycling.
The San Quintín Volcanic Field is a collection of ten or eleven volcanic cinder cones situated along the Pacific coast of the Baja California peninsula in Mexico. The field formed by repeated eruptions beginning in the Pleistocene and ending about 3000 years ago. It is one of several known Quaternary period volcanic fields in Baja. The lava shields appear to have first grown as subaqueous volcanoes that emerged as islands.
The Fifteen-Twenty or 15°20' Fracture Zone (FTFZ), also known as the Cabo Verde Fracture Zone, is a fracture zone located on the Mid-Atlantic Ridge (MAR) in the central Atlantic Ocean between 14–16°N. It is the current location of the migrating triple junction marking the boundaries between the North American, South American, and Nubian plates. The FTFZ is roughly parallel to the North and South America—Africa spreading direction and has a broad axial valley produced over the last ten million years by the northward-migrating triple junction. Offsetting the MAR by some 175 km, the FTFZ is located on one of the slowest portions of the MAR where the full spreading rate is 25 km/Myr.
Mark S. Ghiorso is an American geochemist who resides in Seattle, Washington. He is best known for creating MELTS, a software tool for thermodynamic modeling of phase equilibria in magmatic systems.
The lower oceanic crust is the lower part of the oceanic crust and represents the major part of it. It is generally located 4–8 km below the ocean floor and the major lithologies are mafic which derive from melts rising from the earth's mantle. This part of the oceanic crust is an important zone for processes such as melt accumulation and melt modification. And the recycling of this part of the oceanic crust, together with the upper mantle has been suggested as a significant source component for tholeiitic magmas in Hawaiian volcanoes. Although the lower oceanic crust builds the link between the mantle and the MORB, and can't be neglected for the understanding of MORB evolution, the complex processes operating in this zone remain unclear and there is an ongoing debate in Earth Sciences about this. It is 6KM long.
The deep water cycle, or geologic water cycle, involves exchange of water with the mantle, with water carried down by subducting oceanic plates and returning through volcanic activity, distinct from the water cycle process that occurs above and on the surface of Earth. Some of the water makes it all the way to the lower mantle and may even reach the outer core. Mineral physics experiments show that hydrous minerals can carry water deep into the mantle in colder slabs and even "nominally anhydrous minerals" can store several oceans' worth of water.
Stanley Robert Hart is an American geologist, geochemist, leading international expert on mantle isotope geochemistry, and pioneer of chemical geodynamics.
The Madagascar flood basalt, also known as the Madagascar large igneous province (LIP), is one of the major magmatic events of the Late Cretaceous. They cover a large area of basaltic and rhyolitic lava flows that erupted during an episode of widespread basaltic volcanism during the Cretaceous period. The flood basalts are characterized by lava flows, dykes, sills, and intrusions, and other volcanic features include plugs, scoria, and spatter cones. Tholeiitic basalt constitutes the primary rock type.
References
- ↑ McKenzie, D.; O'Nions, R. K. (1 October 1991). "Partial Melt Distributions from Inversion of Rare Earth Element Concentrations". Journal of Petrology. 32 (5): 1021–1091. doi:10.1093/petrology/32.5.1021.
- 1 2 Kelemen, P. B.; Hirth, G.; Shimizu, N.; Spiegelman, M.; Dick, H. J. (15 February 1997). "A review of melt migration processes in the adiabatically upwelling mantle beneath oceanic spreading ridges". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 355 (1723): 283–318. doi:10.1098/rsta.1997.0010.
- 1 2 3 Asimow, P. D. (19 August 2004). "The Significance of Multiple Saturation Points in the Context of Polybaric Near-fractional Melting". Journal of Petrology. 45 (12): 2349–2367. doi: 10.1093/petrology/egh043 .
- ↑ Lundstrom, Craig (October 2000). "Models of U-series disequilibria generation in MORB: the effects of two scales of melt porosity". Physics of the Earth and Planetary Interiors. 121 (3–4): 189–204. doi:10.1016/S0031-9201(00)00168-0.
- ↑ Asimow, Paul D (June 1999). "A model that reconciles major- and trace-element data from abyssal peridotites". Earth and Planetary Science Letters. 169 (3–4): 303–319. doi: 10.1016/S0012-821X(99)00084-9 .
- ↑ Ghiorso, Mark S.; Hirschmann, Marc M.; Reiners, Peter W.; Kress, Victor C. (May 2002). "The pMELTS: A revision of MELTS for improved calculation of phase relations and major element partitioning related to partial melting of the mantle to 3 GPa". Geochemistry, Geophysics, Geosystems. 3 (5): 1–35. doi: 10.1029/2001GC000217 .
- ↑ Longhi, John (March 2002). "Some phase equilibrium systematics of lherzolite melting: I". Geochemistry, Geophysics, Geosystems. 3 (3): 1–33. doi: 10.1029/2001GC000204 .