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ferromagnesian mineral: dark-coloured minerals, such as augite and hornblende, which contain relatively high proportions of iron and magnesium and low proportions of silica.
fissure: a substantial crack in a rock.
fjord: a glaciated valley in a mountainous area coastal area that has been partly flooded by the sea.
focal depth: the depth of an earthquake focus below the surface.
focus: the origin of an earthquake, directly below the epicentre.
fold: arched or curved rock strata.
fold axis: line following the highest arching in an anticline, or the lowest arching in a syncline.
fold belt: a part of a mountain system containing folded sedimentary rocks.
foliation: a texture of a rock (usually schist) that resembles the pages in a book.
formation: a word used to describe a collection of related rock layers or beds.
A number of related beds make a member; a collection of related members makes up a formation. Formations are often given location names, e.g. Toroweap Formation, the members of which are a collection of dominantly limestone beds.
fossil: any evidence of past life, including remains, traces and imprints.
fossil fuel: any fuel that was formed in the geological past from the remains of living organisms. The main fossil fuels are coal and petroleum (oil and natural gas).
fraction: one of the components of crude oil that can be separated from others by heating and then by cooling the vapour.
fracture: a substantial break across a rock. fracture zone: a region in which fractures
are common. Fracture zones are particularly common in folded rock and near faults.
frost shattering: the process of breaking pieces of rock through the action of freezing and melting of rainwater
gabbro: alkaline igneous plutonic rock, typically showing dark-coloured crystals; plutonic equivalent of basalt.
gallery: a horizontal access tunnel in a mine. gangue: the unwanted mineral matter found
in association with a metal.
gem: a mineral, usually in crystal form,
that is regarded as having particular beauty and value.
genus: (pl. genera) the biological classification for a group of closely related species.
geode: a hollow lump of rock (nodule) that often contains crystals.
geological column: a columnar diagram showing the divisions of geological time (eons, eras, periods, and epochs).
geological eon: see eon.
geological epoch: see epoch.
geological era: see era.
geological period: a subdivision of a geological era (e.g. Carboniferous Period). A period is subdivided into epochs.
geological system: a term for an accumulation of strata that occurs during a geological period (e.g. the Ordovician System is the rocks deposited during the Ordovician Period). Systems are divided into series.
geological time: the history of the earth revealed by its rocks.
geological time scale: the division of geological time into eons, era, periods, and epochs.
geosyncline: a large, slowly subsiding region marginal to a continent where huge amounts of sediment accumulate. The rocks in a geosyncline are eventually lifted to form mountain belts.
gneiss: a metamorphic rock showing large grains.
graben: a fallen block of the earth’s crust forming a long trough, separated on all sides by faults. Associated with rift valleys.
grain: a particle of a rock or mineral. granite: an acidic, igneous, plutonic rock
containing free quartz, typically light in colour; plutonic equivalent of rhyolite.
grit: grains larger than sand but smaller than stones.
groundmass: see matrix.
group: a word used to describe a collection
of related rock layers, or beds. A number of related beds make a member; a collection of related members makes up a formation; a collection of related formations makes
a group.
gypsum: a mineral made of calcium sulphate.
halide minerals: a group of minerals
(e.g. halite) that contain a halogen element (elements similar to chlorine) bonded
with another element. Many are evaporite minerals.
halite: a mineral made of sodium chloride.
Hawaiian-type eruption: a name for a volcanic eruption that mainly consists of lava fountains.
hexagonal: a crystal system in which crystals have 3 axes all at 120 degrees to one another and of equal length.
hogback: a cuesta where the scarp and dip slopes are at about the same angle.
hornblende: a dark-green silicate mineral of the amphibole group containing sodium, potassium, calcium, magnesium, iron and aluminium.
horst: a raised block of the earth’s crust separated on all sides by faults. Associated with rift valleys.
hot spot: a place where a fixed mantle magma plume reaches the surface.
hydraulic action: the erosive action of water pressure on rocks.
hydrothermal: a change brought about
in a rock or mineral due to the action of superheated mineral-rich fluids, usually water.
hypocentre: the calculated location of the focus of an earthquake.
ice wedging: see frost shattering. Icelandic-type eruption: a name given to
a fissure type of eruption.
igneous rock: rock formed by the solidification of magma. Igneous rocks include volcanic and plutonic rocks.
impermeable: a rock that will not allow a liquid to pass through it.
imprint: a cast left by a former life form.
impurities: small amounts of elements or compounds in an otherwise homogeneous mineral.
index fossil: a fossil used as a marker for a particular part of geological time.
intrusive rock, intrusion: rocks that have formed from cooling magma below the surface. When inserted amongst other rocks, intruded rocks are called an intrusion.
invertebrate: an animal with an external skeleton.
ion: a charged particle.
island arc: a pattern of volcanic islands
that follows the shape of an arc when seen from above.
isostacy: the principle that a body can float in a more dense fluid. The same as buoyancy, but used for continents.
joint: a significant crack between blocks of rock, normally used in the context of patterns of cracks.
Jurassic, Jurassic Period: the second geological period in the Mesozoic Era, lasting from about 190 to 135 million years ago.
kingdom: the broadest division in the biological classification of living things.
laccolith: a lens-shaped body of intrusive igneous rock with a dome-shaped upper surface and a flat bottom surface.
landform: a recognisable shape of part of the landscape, for example, a cuesta.
landslide: the rapid movement of a slab of soil down a steep hillslope.