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60
Glossary
aa lava: a type of lava with a broken, bouldery surface.
abrasion: the rubbing away (erosion) of a rock by the physical scraping of particles carried by water, wind or ice.
acidic rock: a type of igneous rock that consists predominantly of light-coloured minerals and more than two-thirds silica (e.g. granite).
active volcano: a volcano that has observable signs of activity, for example, periodic plumes of steam.
adit: a horizontal tunnel drilled into rock.
aftershock: an earthquake that follows the main shock. Major earthquakes are followed by a number of aftershocks that decrease in frequency with time.
agglomerate: a rock made from the compacted particles thrown out by a volcano (e.g. tuff).
alkaline rock: a type of igneous rock containing less than half silica and normally dominated by dark-coloured minerals
(e.g. gabbro).
amygdule: a vesicle in a volcanic rock filled with secondary minerals such as calcite, quartz or zeolite.
andesite: an igneous volcanic rock. Slightly more acidic than basalt.
anticline: an arching fold of rock layers in which the rocks slope down from the crest. See also syncline.
Appalachian Mountain (Orogenic) Belt: an old mountain range that extends for more than 3000 km along the eastern margin of North America from Alabama in the southern United States to Newfoundland, Canada,
in the north. There were three Appalachian orogenies: Taconic (about 460 million years ago) in the Ordovician; Acadian (390 to 370 million years ago) in the Devonian; and Alleghenian (300 to 250 million years ago) in the Late Carboniferous to Permian. These mountain belts can be traced as the Caledonian and Hercynian orogenic belts in Europe.
Archean Eon: see eon.
arenaceous: a rock composed largely of
sand grains.
argillaceous: a rock composed largely of clay.
arkose: a coarse sandstone formed by the disintegration of a granite.
ash, volcanic: fine powdery material thrown out of a volcano.
asthenosphere: the weak part of the upper mantle below the lithosphere, in which slow convection is thought to take place.
augite: a dark green-coloured silicate mineral containing calcium, sodium, iron, aluminium and magnesium.
axis of symmetry: a line or plane around which one part of a crystal is a mirror image of another part.
basalt: basic fine-grained igneous volcanic rock; lava often contains vesicles.
basic rock: an igneous rock (e.g. gabbro)
with silica content less than two-thirds and containing a high percentage of dark-coloured minerals.
basin: a large, circular, or oval sunken region on the earth’s surface created by downwards folding. A river basin, or watershed, is the area drained by a river and its tributaries.
batholith: a very large body of plutonic rock that was intruded deep into the earth’s crust and is now exposed by erosion.
bauxite: a surface material that contains
a high percentage of aluminium silicate. The principal ore of aluminium.
bed: a layer of sediment. It may involve many phases of deposition, each marked by a bedding plane.
bedding plane: an ancient surface on which sediment built up. Sedimentary rocks often split along bedding planes.
biotite: a black-coloured form of mica.
body wave: a seismic wave that can travel through the interior of the earth. P waves and S waves are body waves.
boss: an upwards extension of a batholith. A boss may once have been a magma chamber.
botryoidal: the shape of a mineral that resembles a bunch of grapes, e.g. haematite the crystals of which are often arranged in massive clumps, giving a surface covered with spherical bulges.
butte: a small mesa.
calcareous: composed mainly of calcium
carbonate.
calcite: a mineral composed of calcium carbonate.
caldera: the collapsed cone of a volcano. It sometimes contains a crater lake.
Caledonian Mountain-Building Period, Caledonian Orogeny: a major mountain- building period in the Lower Paleozoic Era that reached its climax at the end of the Silurian Period (430 to 395 million years ago). An early phase affected only North America and made part of the Appalachian Mountain Belt.
Cambrian, Cambrian Period: the first
period of geological time in the Paleozoic Era, beginning 570 million years ago and ending 500 million years ago.
carbonate minerals: minerals formed with carbonate ions (e.g. calcite).
Carboniferous, Carboniferous Period: a period of geological time between about
345 and 280 million years ago. It is often divided into the Early Carboniferous Epoch (345 to 320 million years ago) and the Late Carboniferous Epoch (320 to 280 million years ago). The Late Carboniferous is characterised by large coal-forming swamps. In North America the Carboniferous is usually divided into the Mississippian (= Lower Carboniferous) and Pennsylvanian (= Upper Carboniferous) periods.
cast, fossil: the natural filling of a mould by sediment or minerals that were left when a fossil dissolved after being enclosed by rock.
Cenozoic, Cenozoic Era: the most recent era of geological time, beginning 65 million years ago and continuing to the present.
central vent volcano: see stratovolcano.
chemical compound: a substance made from the chemical combination of two or more elements.
chemical rock: a rock produced by chemical precipitation (e.g. halite).
chemical weathering: the decay of a rock through the chemical action of water containing dissolved acidic gases.
cinder cone: a volcanic cone made entirely of cinders. Cinder cones have very steep sides.
class: the level of biological classification below a phylum.
clast: an individual grain of a rock.
clastic rock: a sedimentary rock that is made up of fragments of pre-existing rocks, carried by gravity, water, or wind (e.g. conglomerate, sandstone).
cleavage: the tendency of some minerals to break along one or more smooth surfaces.
coal: the carbon-rich, solid mineral derived from fossilised plant remains. Found in sedimentary rocks. Types of coal include bituminous, brown, lignite, and anthracite. A fossil fuel.
complex volcano: a volcano that has had an eruptive history and which produces two or more vents.
composite volcano: see stratovolcano.
concordant coast: a coast where the geological structure is parallel to the coastline. See also discordant coastline.
conduction (of heat): the transfer of heat between touching objects.
conglomerate: a coarse-grained sedimentary rock with grains larger than 2 mm.
contact metamorphism: metamorphism that occurs owing to direct contact
with a molten magma. See also regional metamorphism.