Glossary

Gad Soffer
Lamont-Doherty Earth Observatory
and the Healy mission team
Fire + Ice: Exploring for Volcanoes Beneath the Arctic
The Arctic Mid-Ocean Ridge Expedition
July 31 - October 3, 2001

A – E

F – J

K – O

P – T

U – Z

A

Anhydrite A mineral consisting of anhydrous calcium sulfate, CaSO₄. It is essentially the mineral gypsum without its water of crystallization, and it alters readily to gypsum.

Asthenosphere Equivalent to the upper mantle. It is the weak layer of the Earth below the lithosphere. Many magmas are generated in the asthenosphere.

Axial valley The deep central cleft in the mid-ocean ridge (rift valley).

B

Barite A white, yellow, or colorless mineral with the chemical formula BaSO₄.

Basalt A general term for dark, mafic igneous rocks, commonly exstrusive but locally intrusive. Basalts are composed chiefly of calcium rich plagioclase and clinopyroxene. A basalt is the fine grained equivalent of a gabbro.

Bathymetric map A map of the bottom of the ocean, with water depths indicated by contours that link points of equal depth.

Bathymetry The measuring of ocean depths and the charting of the topography of the ocean floor.

Benthos Organisms that live on or in the bottom of the sea.

Black smoker A vent on the seafloor from which hydrothermal fluids are emitted. On mixing with seawater and cooling, the fluids precipitate a cloud of fine-grained sulfide minerals that resembles a cloud of black smoke.

C

Chemosynthesis The process by which bacteria use energy from chemicals, such as hydrogen sulfide, to combine water and carbon dioxide to form carbohydrates.

Chimney An area around a hydrothermal vent where minerals coalesce. As fluid is expelled from the vent, the hydrothermal fluid interacts with seawater. Chemical reactions then cause the precipitation of large amounts of iron-bearing minerals.

Chromite A brownish-black to iron-black mineral of the spinel group. It occurs as a primary accessory mineral in mafic and ultramafic igneous rocks.

Clinopyroxene A group name for pyroxenes crystallizing in the monoclinic crystal system and sometimes containing considerable amounts of calcium with or without aluminum. Examples include diopside, augite, and jadeite.

Continental crust The type of the Earth's crust that directly underlies the continents. It ranges in thickness from 35 km to as much as 70 km under mountain ranges. The density of the upper layer of the continental crust is about 2.7 g/cm³, and the velocity of compressional seismic waves through it are less than about 7.0 km/sec.

Convection The circulatory motion that occurs in fluids due to differences in temperature within the fluid. One example of convection in Earth Science is mantle convection that drives plate tectonics.

Crust The outermost layer or shell of the Earth, defined according to various criteria, including seismic velocity, density, and composition. There are two types of crust: continental and oceanic.

D

Diapir A dome in which the overlying rocks have been ruptured by the squeezing out of plastic (malleable) material at the center of the dome.

Diverging plate boundary Boundary between two lithospheric plates that are moving apart. The Mid-Atlantic Ridge is an example.

Dike A tabular igneous intrusion that cuts across the bedding of the country rock.

Dredge An ocean bottom sampler that scoops up rocks and sediment as it is dragged behind a moving ship. It is usually a heavy metal container.

E

Earthquake A sudden motion or trembling in the Earth caused by the abrupt release of slowly accumulated strain.

East Pacific Rise (EPR) A part of the mid-ocean ridge system extending northeasterly from near New Zealand to just off the coast of Mexico. The EPR is spreading extremely rapidly and is the site of a great deal of volcanic and hydrothermal activity.

F

Fault scarp A steep slope or cliff formed directly by movement around a fault and representing the exposed surface of the fault before modification by erosion and weathering.

Feldspar A group of abundant rock forming minerals of the general formula: MAl(Si,Al) ₃O₈, where M may represent K, Na, or Ca. Feldspars are the most widespread of any mineral group and constitute 60% of the Earth's crust. They occur as components of almost all kinds of rocks.

G

Gabbro Dark-colored intrusive igneous rock composed principally of plagioclase and clinopyroxene, with or without olivine and orthopyroxene. It is the intrusive equivalent of basalt.

Gakkel Ridge The slowest spreading mid-ocean ridge on the surface of the Earth and the subject of this expedition. It is located in the Arctic Ocean and extends for nearly 1800 km.

Garnet A group of minerals with the general formula A₃B₂ (SiO₄)₃, where A may represent Ca, Mg, or Fe and B may be Al or Fe. Garnet occurs as an accessory mineral in a wide range of igneous rocks. It is also commonly found in metamorphic rocks and is frequently used as a semiprecious stone.

Geochemistry The study of the distribution and amounts of the chemical elements in minerals and rocks. Geochemistry concerns how these elements circulate in nature on the basis of the properties of their atoms and ions.

Geophysics The field of Earth science that uses physics to explore structure and physical characteristics of the Earth.

Gravity anomaly The difference between the observed value of gravity at a point and the theoretically calculated value at this point.

H

Hot spot A volcanic center 100-200 km across and persistent for a few tens of millions of years, that is thought to be the surface expression of a persistent rising plume of hot mantle material. Hot spots may or may not be associated with oceanic ridges.

Hydrothermal Of or pertaining to hot water, to the action of hot water, or to the products of this action, such as a mineral deposit precipitated from a hot aqueous solution.

Hydrothermal deposit A mineral deposit formed from the precipitation of minerals from a hot fluid.

I

Igneous rock A rock that solidified from molten or partially molten material (i.e., from a magma). There are two main kinds of igneous rocks: extrusive and intrusive. Extrusive igneous rocks erupt onto the surface of the Earth and cool rapidly resulting in fine grained rocks that look fairly homogenous to the naked eye. Intrusive igneous rocks remain within the Earth's interior and cool relatively slowly resulting in course grained rocks with many clearly visible crystals. Igneous rocks represent one of the three main classes of rocks, the other two being metamorphic and sedimentary rocks.

Incompatible element An element that tends to remain in the liquid portion of a slowly crystallizing magma.

Isotope One of two or more species of the same chemical element having the same number of protons in the nucleus, but differing from one another by having different numbers of neutrons. The isotopes of an element have slightly different physical and chemical properties, owing to their mass differences.

L

Laptev Sea A body of water found at the eastern extent of the Gakkel Ridge.

Latitude The angular distance north or south of the Earth's equator. Latitude is measured in degrees, minutes, and seconds. One degree of latitude is equivalent to 60 nautical miles. A combination of latitude and longitude can be used to locate any point on the surface of the Earth.

Lava Melted or molten rock that is above the surface of the earth. Such rock is called magma if it is below the surface of the Earth.

Liquidus On a graph of temperature vs composition, a diagram showing the maximum solubility of a solid component or phase in the liquid phase.

Lithology The physical character of a rock. Lithologic characters include grain size, color, and mineralogic composition.

Lithosphere In plate tectonics, a strong layer relative to the underlying asthenosphere. It includes the crust and part of the upper mantle and is of the order of 100 km in thickness.

Longitude The angular distance east or west from a meridian drawn between the North and South Poles and passing through Greenwich, England. Longitude is measured in degrees, minutes, and seconds. A combination of latitude and longitude can be used to locate any point on the surface of the Earth.

M

Mafic An igneous rock composed primarily of one or more ferro-magnesian, dark minerals.

Magma Naturally occurring molten rock, generated within the Earth and capable of intrusion or extrusion. Igneous rocks are developed through the cooling and solidification of magma.

Magnetic anomaly The amount by which a local measurement of the magnet field intensity exceeds or falls below the intensity of the global magnetic field.

Magnetic susceptibility The ratio of the induced magnetic field to the strength of the magnetic field causing the magnetization.

Mantle The zone of the Earth below the crust and above the core, that is divided into the upper mantle and the lower mantle, with a transition zone between. The mantle serves as the primary source for nearly all volcanic rocks.

Mantle plume A column of hot, partially molten material that rises from an intermediate depth in the mantle.

Metamorphic rock A rock derived from pre-existing rocks by way of mineralogical, chemical, or structural changes. These changes come in response to marked changes in temperature, pressure, shearing stress, or the chemical environment. Metamorphism generally occurs at depth in the Earth's crust. Metamorphic rocks represent one of the three main classes of rocks, the other two being igneous and sedimentary rocks.

Mid-Atlantic Ridge (MAR) A part of the mid-ocean ridge system that extends from north to south through the center of the Atlantic Ocean.

Mid-Ocean Ridge A continuous, volcanically active mountain range extending through the North and South Atlantic Oceans, the Arctic Ocean, the Indian Ocean, and the South Pacific Ocean. It is a broad, fractured swell with a central rift valley, and usually extremely rugged topography. It is 1-3 km in elevation, about 1500 km in width, and over 84,000 km in length. According to the theory of sea-floor spreading, new oceanic crust is created by volcanic eruptions at the mid-ocean ridge.

Mid-Ocean Ridge basalt (MORB) A basalt produced at the Mid-Ocean Ridge. An MORB is often described as either depleted in light rare earth elements (N-MORB) or enriched in light rare earth elements (E-MORB).

Morphology The shape or form of a structure on the surface of the Earth.

N

Neovolcanic zone The zone consisting of the most recently erupted volcanic rocks.

Nephelometry The measurement of the cloudiness of a medium. In particular, we can determine the concentration of a suspension of particles by measuring, at more than one angle, the scattering of light transmitted or reflected by the medium.

O

Oblique spreading Seafloor spreading at an angle other than 90 degrees to the strike of the mid-ocean ridge.

Oceanic crust The type of the Earth's crust that underlies the ocean basins. The oceanic crust is generally about 5-10 km thick; it has a density of 3.0 g/cm³, and compressional seismic wave velocities traveling through it exceed 6.2 km/sec.

Olivine Rock of chemical composition (Mg,Fe)₂SiO₄. Olivine is a common rock forming mineral of mafic rocks such as basalt, gabbro, and peridotite. It is olive-green, grayish-green, or brownish in color.

Organic carbon Carbon (C) that was originally part of an organism. Organic carbon is typically composed mainly of the lighter of the two stable isotopes of carbon, C¹².

Orthopyroxene A group name for pyroxenes crystallizing in the orthorhombic crystal system and usually containing no calcium and little or no aluminum. Examples include enstatite and hypersthene.

P

Partial melting The process of preferentially melting certain components of a solid. Basalts are derived from varying degrees of partial melting of mantle material.

Peridotite A coarse-grained plutonic rock composed chiefly of olivine with or without other mafic minerals, such as pyroxenes, amphiboles, or micas, and containing little or no feldspar. Peridotite is commonly altered to serpentinite.

Petrology The branch of Earth science dealing with the origin, occurrence, structure, and history of rocks, especially igneous and metamorphic rocks.

Phenocryst A relatively large, conspicuous crystal in an otherwise mostly fine grained rock.

Pillow lava Lava that erupts underwater and forms a rounded, pillow-like shape.

Plagioclase feldspar A group of feldspars of the general formula (Na,Ca)Al(Si,Al)Si₂O₈. The sodium-bearing end of the plagioclase feldspars is known as albite, and the calcium-bearing end is known as anorthite. Plagiocloase minerals are among the commonest rock forming minerals.

Plate A rigid, thin segment of the Earth's lithosphere, which may be assumed to move horizontally and adjoins other plates along zones of seismic activity called plate boundaries.

Plate boundary Zone of seismic and tectonic activity along the edges of lithospheric plates.

Plate tectonics A theory in which the lithosphere is divided into a number of plates whose pattern of horizontal movement is that of rigid bodies that interact with each other at their boundaries, causing seismic activity and the deformation of the lithosphere along these boundaries.

Plutonic rock Igneous rock formed at great depth.

Pyroxene A group of dark rock forming silicate minerals having the general formula: ABSi₂O₆, where A may represent Ca, Na, Mg, or Fe and B may be Mg or Fe. Pyroxenes are a common constituent of igneous rocks. Pyroxenes can crystalize in the orthorhombic (orthopyropxene) or monoclinic (clinopyroxene) crystal systems.

Pyroxenite An ultramafic plutonic rock composed chiefly of pyroxene.

R

Radiogenic isotope A radioactive isotope of an element.

Ridge segments Discrete lengths of a mid-ocean ridge separated (or bounded) by transform faults.

Ridge shoulders The areas of highest elevation on the mid-ocean ridge found on either side of the axial valley.

Rift valley The deep central cleft in the mid-ocean ridge.

Rock corer An ocean-bottom sampler that is a metal or plastic cylinder or box, lowered by cable and driven into the ocean floor by impact of attached weights. Examples include piston corers, box corers, and gravity corers.

S

Seabeam Sonar depth-recording system.

Seafloor spreading Theory in which new oceanic crust is created by volcanic eruptions at the mid-ocean ridge. As new crust is created, it moves away from the ridge at a rate that varies from place to place along the ridge. This movement provides the source of dynamic thrust in the theory of plate tectonics.

Seamount An elevation of the seafloor, 1000 m or higher, either flat-topped (called a guyot) or peaked (called a seapeak).

Sedimentary rock A rock resulting from the consolidation of loose sediment (solid fragmental material that originates from the weathering of rocks) that has accumulated in layers. Sedimentary rocks represent one of the three main classes of rocks, the other two being metamorphic and igneous rocks.

Segmentation The process by which a mid-oceanic ridge is broken into smaller pieces or segments separated by transform faults.

Seismic wave A wave generated either artificially or by an earthquake that travels through the Earth.

Seismicity The phenomenon of Earth movements.

Serpentinite A rock consisting almost wholly of serpentine group minerals (such as antigorite and chrysotile) that are derived from the alteration of ferromagnesian silicate minerals such as olivine and pyroxene. Serpentinites are often derived from the alteration of peridotites.

Sessile Attached directly by the base, instead of raised on, say, a stalk.

Solidus On a graph of temperature vs composition, a diagram showing the temperatures above which solid and liquid are in equilibrium and below which the system is completely solid.

Southwest Indian Ridge (SWIR) A part of the mid-ocean ridge system extending northeasterly and separating the African plate from the Antarctic plate. The SWIR is spreading extremely slowly and serves as an interesting comparison to the Gakkel Ridge.

Sphalerite A yellow, brown, or black mineral with the chemical formula (Zn,Fe)S.

Spreading rate The rate at which material moves laterally from the crest of the mid-ocean ridge. Spreading rate varies from super slow (less than 1 cm per year) to fast (greater than 9 cm per year).

Spreading center a region of divergence on the Earth's surface (e.g. the mid-ocean ridge).

Stable isotope An isotope of an element that does not undergo radioactive decay.

Strike Compass direction of the intersection between a structural surface (such as a bedding plane or a fault plane) and the horizontal.

Sulfide A mineral compound characterized by the linkage of the element sulfur and a metal or semi-metal. Examples include galena, PbS; pyrite (or fool's gold), FeS2; and sphalerite, (Zn,Fe)S.

T

Trace element An element that is not essential in a mineral but that is found in small quantities in its structure or adsorbed on its surfaces.

Tracer Any substance that is used in a process to trace its course.

Transform fault A type of plate boundary where two plates slide past each other. Transform faults ideally shows pure strike-slip movement; that is, movement in the direction parallel to the strike of the fault.

U

Ultramafic rock An igneous rock composed chiefly of mafic minerals. Examples include monomineralic rocks composed of pyroxene or olivine.

V

Vein A thin sheet-like igneous intrusion into a fissure.

Vent fauna The organisms that inhabit the area immediately surrounding a hydrothermal vent.

Viscosity The property of a substance that offers internal resistance to flow. For instance, honey has a much higher viscosity than water, since it does not flow nearly as easily.

Volcanic glass A amorphous product of the extremely rapid cooling of magma. It may constitute the whole rock (e.g., obsidian) or only part of a groundmass.

W