Geoscience Reference
In-Depth Information
Glossary
This is a compilation of some of the technical terms used in this topic. For more formal
definitions, refer to J. A. Jackson, ed., 1997.
Glossary of Geology
, 4th edn, American
Geological Institute, Falls Church, Virginia.
a
abbreviation for year
abyssal plain
deep, old ocean floor; well sedimented.
accreted terrain (terrane)
terrane that has been accreted to a continent.
active margin
continental margin characterized by volcanic activity and earthquakes (i.e.,
location of transform fault or subduction zone).
adiabat
pressure-temperature path of a body that expands or contracts without losing or
absorbing heat.
aeon (eon)
longest division of geological time; also sometimes used for 10
9
years.
alpha decay
radioactive decay by emission of an alpha (
α
) particle.
alpha particle
nucleus of a helium atom (two protons and two neutrons).
altered rocks
rocks that have undergone changes in their chemical or mineral structure since
they were formed.
amphibolite
intermediate-grade metamorphic rock; temperature attained above 400-450
◦
C;
characterized by amphibole minerals such as hornblende.
andesite
extrusive igneous rock, usually containing plagioclase and mafic phases(s); about
55% SiO
2
. Usually associated with subduction zones.
anticline
a fold, convex upwards, whose core contains stratigraphically older rocks.
Archaean (Archean)
that division of geological time prior to
∼
2500 Ma ago.
aseismic region
region with very infrequent earthquakes.
asthenosphere
region beneath the lithosphere where deformation is dominantly plastic and
heat is transferred mainly by convection; now sometimes means the entire upper mantle
beneath the lithosphere. Literally, the 'sick' or 'weak' sphere.
atomic number
number of protons in the nucleus of an atom.
back-arc basin
basin on the overriding plate behind the volcanic arc of a subduction zone.
655
