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Ocean 454A Winter 2014 Some definitions relevant to hydrothermal vent topics Andesite: An extrusive volcanic rock intermediate in composition between basalt and granite; major component in subduction zone volcanism Anhydrite: A mineral, CaSO4, consisting of anhydrous calcium sulfate. Apatite: Calcium Phosphate + other stuff Ca5(PO4,CO3)3(F,OH,Cl) Arc: A string of volcanic or sedimentary islands lying behind a trench and above a subducting plate. Astenosphere: A hot, soft layer of the mantle, on which the lithosphere floats Backarc basin: A depression lying behind the volcanic arc above the subducted slab, and apparently spreading. Not clearly developed in subduction under continents, but common behind Pacific trenches. Basalt: Fine-grained, dark extrusive igneous rock rich in iron and magnesium-bearing minerals and relatively poor in silica; characteristic of ocean crust. Calcite: A mineral CaCO3, consisting of calcium carbonate crystallized in hexagonal form and including common limestone, chalk, and marble Clathrate (also called gas hydrates): Nonstoichiometric compound with a lattice of water molecules with gas molecules (examples CH4, CO2, H2S) occupying cavities (or cages) within the lattice. Crust: Surface layer of the earth, about 35 km thick in continental regions and of sialic (Si and Al rich) character, including granitoids, metamorphic rocks and sediments; and about 8 km thick in oceanic regions, where it is composed of basalts, gabbros and cumulate rocks, overlain by sediment. EMORB: Basalts which, relative to NMORB, are enriched in the so-called incompatible elements, such as Rb, Ba, Th, U, Nb and the light rare-earth elements. Eukaryotes: A cell containing a distinct nucleus bound by a membrane, and organelles; the cell type for all higher plants, animals and fungi. Evaporite: A sedimentary rock produced by evaporation of seawater and precipitation of its salts; contains a wide range of components, among which rock salt and gypsum are the most common. Facies: the characteristics of a rock, usually sedimentary, which reflect the environment of deposition in which it is formed. Feldspar: Very common, 60% of earth’s crust (KNaCaBaRbSrFe)Al(AlSi)3O8 Forearc: A zone at the boundary of colliding plates just behind the trench, where sediments and to some extant oceanic crust are being deformed. Fracture zone: A dislocation at right angles to a mid-ocean ridge, usually marked by a set of transverse ridges and troughs. Ga: 109 years (giga-anum) Gabbros: Rock resulting from the slow crystallization of a basalt at depth when the constituting minerals (plagioclase, pyroxenes and olivine) have time to grow to sizes visible to the naked eye. Gneiss: A coarsely crystalline, quartz-rich metamorphic rock usually formed from granite. Granite: A coarsely crystalling intrusive igneous rock rich in lighter elements and containing mainly quartz and feldspar; typical for continental crust. Hematite: Sulfide mineral of composition Fe2O3 Igneous: rock formed by cooling from a molten state (magma or lava) Island Arc: A system of volcanoew arranged above a subduction zone. The ring of fire around the Pacific is made up of such arcs. Lava: Molten rock, usually so called when it flows from a surface fissure or a volcano; when congealed, it forms an extrusive rock. Lithosphere: The crust plus the rigid upper mantle Mafic rock: Dark rock rich in magnesium and iron rich minerals such as olivine Magma: Molten rock existing deep in the earth; when congealed, it forms intrusive rocks. Magnetite: Sulfide mineral of composition Fe3O4 Mantle: Shell of the earth limited by the base of the crust at a depth of about 10-30 km and by the core at a depth of 2900 km. Below a depth of 400 km the peridotites, which make up the mantle, start to transform into denser rocks under the influence of increasing pressure. Metamorphism: A change in composition, mineralogy, or structure of a rock resulting from changes in temperature and pressure, common during deformation or very deep burial, as well as near intrusions of hot magma. Moho: Boundary between crust and mantle at a mean depth of 6 km below the oceans and about 30 km below the continents. MORB or NMORB: normal mid-ocean ridge basalt – common extrusive volcanic rock. Olivine: Mineral (Mg,Fe)2SiO4). The main constituent of upper mantle peridotites. A solid solution of fayalite (Fe2SiO4) and fosterite (Mg2SiO4) Ophiolite: Fragment of oceanic lithosphere (crust and mantle) emplaced on the continent and frequently part of mountain peaks. Peridotite: Rock with over 90% mafic minerals, usually dominated by olivine Plagioclase: Feldspars of general formula (NaCa)Al(SiAl)Si2O8 Pyroxene: Rock forming minerals of general formula (MgFeCaNa)(MgFeAl)Si2O6 Prokaryotes: A cell lacking a membrane-bound nucleus and organelles and include bacteria and archaea. Pyrite: Sulfide mineral of composition FeS2 Pyrrohotite: Sulfide mineral of composition Fe(1-x)S Quartz: SiO2 Rift: A trough between two fault zones, with a down-dropped central block; it often is the beginning of continental breakup and also occurs on mid-ocean ridge axes. Serpentinization: The alteration of olivine to serpentine and magnetite by adding water. Stromatolite: A finely layered mound of limestone produced by an algal mat (usually cyanobacteria); particularly common in the Precambrian, and one of the earliest signs of the presence of life on earth. Subduction: The process by which a plate, always one consisting of oceanic crust, is dragged under the adjoining plate; subduction compensates for the continuous creation of new oceanic crust on mid-ocean ridges. Tectonics: The study of the movement and deformation of the earth’s crust and mantle. Transform fault: A fault with horizontal movement connecting two segments of a midocean ridge; more broadly, a plate boundary along which plates more horizontally past one another without either convergence or divergence. Ultramafic rocks: rock with generally >90% magnesium and iron-rich minerals, such as olivine and pyroxenes