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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