ESS 212: Week I vocabulary

D''

A 100-300 km-thick layer immediately above the core-mantle boundary. It may be composed of subducted tectonic plates.

Forearc basin

A basin formed between an oceanic trench and a volcanic arc at a convergent plate boundary.

Divergent

A boundary at which tectonic plates move apart and new crust is created between them, as occurs at a mid-ocean ridge.

Isomorph

A close similarity in the crystal structure of two or more substances of similar chemical composition.

Passive margin

A continental margin created when plate divergence rifts a continent apart. As the plate cools on moving away from the divergent boundary, the passive continental margin slowly subsides.

Prismatic

A crystal bounded by a prism or prisms, commonly giving it an elongate appearance.

Euhedral

A crystal bounded by well-developed crystal faces.

Subhedral

A crystal that is only partly bounded by good crystal faces. The remaining faces are less well developed because of crowding by adjacent mineral grains.

Parting

A division of a crystal along a plane that is not a cleavage plane.

Basalt

A fine-grained igneous rock composed predominantly of equal proportions of plagioclase feldspar and pyroxene (or olivine). It is the most common igneous rock and covers most of the ocean floor.

Large igneous province (LIP)

A large region (>100,000 sq km) within a tectonic plate characterized by extensive and long lasting (50 million years) igneous activity, which is usually predominantly basaltic in composition. Believed to form over large mantle plumes and to often lead to the rifting apart of tectonic plates (e.g., the Central Atlantic Magmatic Province was the precursor to the breakup of Pangaea).

Asthenosphere

A layer in the upper mantle extending from depths between 20 km and 50 km beneath the oceans and 70 km and 220 km beneath continents that is marked by low seismic velocities. Compared with the lithosphere above, this layer is weak (plastically deformable) and provides the zone on which lithospheric plates can move.

Isotherm

A line on a map, cross-section, or phase diagram joining points of equal temperature.

Bragg equation

A mathematical equation that expresses the relationship between the angle of diffraction (theta), X-ray wavelength (lambda), and the atomic spacing (d) as in: n*(lambda)=2d*sin(theta)

Granite

A medium to coarse-grained igneous rock composed of approximately 1/3 quartz and 2/3 alkali feldspars.

Foliated

A mineral (or crystal) that consists of a stack of thin leaves or plates that can be separated from each other.

Anhedral

A mineral grain that lacks well-formed crystal faces as a result of having grown against adjacent minerals.

Tabular

A mineral or crystal mass with pronounced flat surfaces as in a board.

Acicular

A mineral with a needlelike shape.

Massive

A mineral, or mineral aggregate, without obvious crystal habit, or other distinguishable physical characteristics.

Migmatite

A mixed metamorphic and igneous rock formed when the rock was raised to a temperature at which partial melting occurred. The melt tends to be granitic in composition and forms light-colored layers in the darker refractory metamorphic rock.

Mineral

A naturally occurring solid with an ordered atomic arrangement and a definite (but commonly not fixed) chemical composition, and of an inorganic origin (with the exception of calcites, etc, that are excreted by certain mollusks, for example).

Rock

A naturally occurring, consolidated mixture of minerals. It is the solid material that makes up the Earth.

Transform

A plate boundary at which one plate grinds past the other, as, for example, along the San Andreas Fault.

Travertine

A porous boundstone formed by the precipitation of calcite from hot springs.

Mars-sized impactor

A proto-planet that impacted the Earth ~30 million years after its initial formation, removing material from the early Earth and leading to creation of the moon.

Hot spot

A region on the Earth's surface marked by a lengthy period of igneous activity. They have been interpreted to be located above plumes that have sources deep in the mantle.

Limestone

A sedimentary rock composed predominantly of calcite or more rarely aragonite.

Sandstone

A sedimentary rock composed predominantly of sand-sized grains (0.0625 mm - 2.0 mm), which are usually made of quartz.

Fracture

A separation in a geologic formation, as a joint or fault, that divides the rock into two or more pieces.

Epeiric sea

A shallow sea formed by the flooding of a continent during periods of high sea-level stand.

Chondrule

A small millimeter-size sphere that characterizes chondritic meteorites. The spheres are composed of extremely fine-grained grains of magnesium-iron silicates (olivine and pyroxene) and glass. They were formed by flash heating and rapid cooling of dust particles in the early solar nebula.

Amorphous

A solid that lacks an ordered crystalline structure.

Dendritic

A term describing the occurrence of a mineral (or crystal) aggregate in a treelike branching pattern.

Flood basalt

A type of basalt that is erupted in large volumes from long fissures and tends to form large flat lava flows. They are commonly associated with large igneous provinces (LIPs)

Chondrite

A type of stony meteorite (in contrast to an iron meteorite) that contains small spherical bodies known as chondrules

Rift valley

A valley or basin formed by the down faulting of a block along normal faults as a result of crustal extension (e.g. the East African Rift Valley).

Andesite

A volcanic rock that characterizes composite volcanoes formed along island arcs above subduction zones. It is composed of approximately equal proportions of pyroxene and plagioclase, whose average composition is less than An50, although more calcic compositions may occur in the cores of phenocrysts.

Transition zone

A zone in the lower part of the upper mantle between ~410 km and 660 km at which numerous mineral changes probably occur as a result of increasing pressure.

Volcanic arc

An arc of regularly spaced composite volcanoes that form above a subducting oceanic plate when it reached a depth of ~100 km.

Pegmatite

An exceptionally coarse-grained rock, usually of granitic composition and forming irregular dikes and lenses.

Shale

An extremely fine-grained (<0.004 mm) sedimentary rock that is characterized by a prominent foliation of fissility parallel to the bedding.

Batholith

An extremely large intrusive igneous body with an outcrop area exceeding 100 sq km. Usually composed of granite and granodiorite.

Anion

An ion with a negative charge

Cation

An ion with a positive charge

Labradorescence (shiller)

An optical phenomenon consisting of flashes of laminated iridescence of a single bright hue that changes gradually as the mineral is moved about in reflected light. Seen commonly in labradorite, a member of the plagioclase series.

Cubic cleavage

Breaking of a mineral along three different planar directions at 90 degrees to each other (as halite).

Granular

Composed of mineral grains of approximately the same size.

Nucleosynthesis

Cosmic formation of atoms more complex than the hydrogen atom.

Phenocryst

Crystals in igneous rocks that are significantly larger than most of the crystals in the rock. Igneous rocks containing phenocrysts are described as porphyritic.

Oolitic

Describes a mineral aggregate consisting of grains in rounded masses the size of fish roe.

Reniform

Describes a mineral having the outward appearance in the shape of a kidney.

Botryoital

Describes a mineral specimen that has the outward appearance of a bunch of grapes.

EMPA

Electron microprobe analysis is a chemical analytical instrument that deploys a finely focused electron beam that generates characteristic X-ray emissions from the chemical elements housed in the material under the beam.

EDS

Energy-dispersive X-ray spectrometer or detector; separates the characteristic X-rays of different elements (as from the emissions from a sample being studied using EMPA) into an energy spectrum, and relevant software is used to determine the abundance of specific chemical elements.

Crystal/crystalline

Having an ordered internal crystal structure. Applies to well-developed (euhedral) crystals as well as poorly formed (anhedral) mineral grains.

Texture

In describing rocks, it is used to indicate how individual mineral grains are related to one another.

Glass

Material that composes an amorphous solid--i.e. lacking in an ordered crystalline structure.

MORB

Mid-Ocean Ridge Basalt.

Magma

Molten rock. Most magma is not totally molten, but contains some solids that are carried along in the liquid.

Mantle plume

Narrow column of rock that is hypothesized to rise buoyantly from deep in the mantle (possibly as deep as the core-mantle boundary) and on reaching the lithosphere mushrooms out to form a plume head. Hot spots are believed to be located above mantle plumes (e.g., Hawaiian hot spot).

Meteorite

Natural objects from space that impact the Earth's surface. A meteor is a similar object that burns up while passing through the atmosphere and does not reach the Earth's surface.

Pelagic

Of the water column (of a sea or lake) rather than near the bottom (benthic) or the shore (littoral). Applied to organisms, for example.

Back-arc basin

Often, but not always coupling between subducting and overriding tectonic plates at a convergent plate boundary creates extensional stresses in the upper plate that results in formation of a rift valley, or even opening of new ocean floor.

Polymorph

Refers to minerals with the same chemical composition but different crystal structures. Quartz, tridymite, and cristobalite are three of the several polymorphs of SiO2.

Pillow basalt

Rocks with a basaltic composition and a pillow-shaped form that is attributed to the extrusion of lava underwater. Typically forms at mid-ocean ridges or at oceanic hot-spot volcanoes.

Igneous

Rocks, minerals, gases, textures, structures, and processes that are related to magmas--frequently used to describe a rock formed by the solidification of magma.

Metamorphic

Rocks, minerals, textures, structures, and processes that involve a change in the previously existing rock due to changes in the temperature, pressure, or fluid composition in the environment--frequently used to describe any rock created as a result of changes of temperature, pressure, directed stress or shear, and composition of fluid.

Sedimentary

Rocks, minerals, textures, structures, and processes that involve the deposition of sediment--frequently used to described rocks created by such deposition.

Asbestosform

Said of a highly fibrous mineral in which the fibers readily separate into thin, strong fibers that are flexible. Also, as defined by the National Institute for Occupational Safety and Health (NIOSH), includes six naturally occurring fibrous minerals: chrysotile, crocidolite, amosite, anthophyllite, tremolite, and actinolite.

SEM

Scanning electron microscopy is an instrumental electron beam technique that allows for magnifications as high as about 1 million times.

Silica tetrahedron

Silicon atom surrounded by 4 Oxygen atoms at the corners of the tetrahedral shape, is the fundamental unit for most silicate structures in the Earth's crust.

Heat flow

The amount of heat transferred in a unit of time--as with the Geothermal gradient.

Octahedral cleavage

The breaking of a mineral along four different planar directions that form the shape of an octahedron.

Habit

The characteristic appearance of a mineral reflecting crystal form, crystal intergrowth, or other physical aspects of its occurrence.

Streak

The color of the finely powdered mineral on an unglazed white porcelain plate, called a streak plate.

Plate tectonics

The deformation of the Earth as a result of the motion of lithospheric plates.

Heft

The estimation of the specific gravity (G (bolded)) of a mineral specimen by holding it in the hand.

Groundmass

The fine-grained crystals that surround phenocrysts in a porphyritic rock.

Inner core

The innermost, solid region of the Earth--with a radius of 1220 km, composed primarily of iron, but also containing smaller amounts of lighter elements.

Gas giant planet

The large outer planets in the solar system composed predominantly of condensed gases. They include Jupiter, Saturn, Uranus and Neptune.

Convection

The movement of material due to density differences that result from thermal expansion and contraction. As material cools, it shrinks and becomes denser, setting up a gravitational instability.

Magma ocean

The name given to the huge body of molten rock that is believed to have covered the outer part of the Earth early in its history.

Valence state

The number (or possible number) of electrons in the outermost shell of an atom. Controls the ability of an atom to bond with other atoms.

Crust

The outer part of the Earth above the Mohorovicic discontinuity. It is typically about 30 km thick on continents, but can be up to 70 km thick beneath mountain chains. Beneath oceans, it is 7-10 km thick.

Mantle

The part of the Earth lying between the Mohorovicic discontinuity and the core-mantle boundary. It is divided into the upper and lower mantle by the seismic discontinuity at a depth of ~660 km.

Hydrothermal alteration

The process by which minerals in a rock are altered, usually to hydrous minerals, by reaction with hot water.

Accretion

The process of growing or increasing in size, typically by the gradual addition of additional material or layers.

Foliation

The property of a rock to break into thin sheets that are bounded by planes along which typically platy minerals are aligned. This alignment may result from sedimentation of clay minerals or from the growth of micas in a preferred orientation in metamorphic rocks.

Specific gravity

The ratio of the weight of a volume of a substance to the weight of the same volume of water at 1 atm, 4 degrees Celsius. Designated by G (bold).

Outer core

The region of the Earth extending from a depth of 2888 km to 5151 km. It is composed primarily of molten iron, whose convection creates the Earth's magnetic field.

Lithosphere

The relatively rigid outer part of the Earth, including the crust and upper mantle down to the asthenosphere. It is broken into large plates that move relative to one another and generate plate tectonics.

Hardness

The resistance of a mineral to scratching by a sharp point, edge, or other mineral. Designated by H (bolded)

Mohorovicic (Moho) discontinuity

The seismic discontinuity defining the base of the Earth's crust. The sharp increase in seismic velocities across this boundary is believed to result from a change from crustal rocks to mantle peridotite.

Terrestrial planet

The small rocky planets closest to the Sun, including Mercury, Venus, Earth, and Mars. Though not large enough to be considered planets, the asteroids are commonly grouped with the terrestrial planets.

Isostasy

The state of buoyant equilibrium between masses of rock with different densities. Lower-density continental rocks, for example, float higher than denser oceanic rocks in the asthenosphere.

Big Bang

The term used in cosmology to describe the beginning of the universe ~14 billion years ago, when it suddenly started expanding from an extremely dense state. That expansion continues today.

Mid-ocean ridge

The topographic high produced by the creation of new hot oceanic crust at a divergent plate boundary. Ridges are often located near the center of an ocean, as in the case of the Mid-Atlantic Ridge, but they need not be, as in the case of the East Pacific Rise.

Conduction

The vibrational transfer of thermal energy from one atom to an adjoining atom down a temperature gradient.

Convergent

Where two tectonic plates converge, or collide, with one plate subducting beneath the other.

Planar cleavage

the breakage of a mineral along a single planar direction, as shown by mica.