Geology Reference
In-Depth Information
GLOSSARY
Most geomorphological terms are defined when used in the
text. This glossary provides thumbnail definitions of terms
that may be unfamiliar to students.
the valley sides), or by the covering of an old river terrace
with alluvium.
alluvium
An unconsolidated, stratified deposit laid
down by running water, sometimes applied only to fine
sediment (silt and clay), but more generally used to
include sands and gravels, too.
active margin
The margin of a continent that corre-
sponds with a tectonically active plate boundary.
aeolian
Of, or referring to, the wind.
alumina
Aluminium oxide, Al
2
O
3
; occurs in various
forms, for example as the chief constituent of bauxite.
aggradation
A building up of the Earth's surface by
the accumulation of sediment deposited by geomorphic
agencies such as wind, wave, and flowing water.
amphibole
A group of minerals, most of which are
mainly dark-coloured hydrous ferromagnesian silicates.
Common in intermediate rocks and some metamorphic
rocks.
alcrete
A duricrust rich in aluminium, commonly in
the form of hardened bauxite.
allitization
The loss of silica and concentration of
sesquioxides in the soil, with the formation of gibb-
site, and with or without the formation of laterite; more
or less synonymous with soluviation, ferrallitization,
laterization, and latosolization.
anaerobic
Depending on, or characterized by, the
absence of oxygen.
andesite
A grey to black, fine-grained, extrusive
igneous rock that contains the minerals plagioclase
and hornblende or augite. The extrusive equivalent of
diorite.
alluvial
Of, or pertaining to, alluvium.
alluvial fill
The deposit of sediment laid down by
flowing water in river channels.
anhydrite
Anhydrous calcium sulphate, CaSO
4
,
occurring as white mineral. Generally found in asso-
ciation with gypsum, rock salt, and other evaporite
minerals.
alluvial terrace
A river terrace composed of alluvium
and created by renewed downcutting of the floodplain
or valley floor (which leaves alluvial deposits stranded on
anion
A negatively charged ion.
