Geology Reference
In-Depth Information
chert
A cryptocrystalline form of silica, a variety of
chalcedony, often occurring as nodules and layers in
limestones.
corestone
A spheroidal boulder of fresh (unweathered)
rock, originally surrounded by saprolite, and formed by
subsurface weathering of a joint block.
cryosphere
All the frozen waters of the Earth (snow
chitons
Marine molluscs.
and ice).
chronosequence
A time sequence of landforms con-
structed by using sites of different ages.
cryostatic pressure
The pressure caused by ice on
water-saturated material sandwiched between an advanc-
ing seasonal layer of frozen ground and an impermeable
layer such as permafrost.
clasts
Rock fragments broken off a parent rock.
clastic sediment
Sediment
composed
of
particles
broken off a parent rock.
cyanobacteria
A group of unicellular and multicel-
lular organisms, formerly called blue-green algae, that
photosynthesize.
clay
A name commonly used to describe fine-grained
sedimentary rock, plastic when wet, that consists of
grains smaller than 0.004 mm, sometimes with a small
portion of silt- and sand-sized particles. The grains are
largely made of clay minerals but also of calcite, iron
pyrite, altered feldspars, muscovite flakes, iron oxides,
and organic material.
cyclic
Recurring at regular intervals; for example, lunar
cycles which occur twice daily, fortnightly, and so on.
dacite
A fine-grained rock, the extrusive equivalent
of granodiorite, with the same general composition as
andesite, though with less calcic plagioclase and more
quartz. Also called quartz andesite.
clay minerals
A group of related hydrous alumino-
silicates. The chief ingredients of clay and mudstone.
degradation
A running down or loss of sediment.
claystone
A sedimentary rock composed of clay-sized
dendrochronology
The study of annual growth rings
of trees. Used as a means of dating events over the last
millennium or so.
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particles (
0.002 mm in diameter) that does not split
into flakes or scales. Consolidated clay. Claystone is a
member of the mudstone group and is common in
Palaeozoic deposits. Most Precambrian claystones have
been metamorphosed to slates and schists.
denudation
The sum of the processes - weathering,
erosion, mass wasting, and transport - that wear away
the Earth's surface.
cohesion
In soils, the ability of clay particles to stick
together owing to physical and chemical forces.
diapir
A dome or anticlinal fold produced by an upris-
ing plume of plastic core material. The rising plume
ruptures the rocks as it is squeezed up.
colloids
Fine clay-sized particles (1-10 micrometres in
size), usually formed in fluid suspensions.
diatom
A unicellular organism (Kingdom Protista)
with a silica shell.
colluvium
An unconsolidated mass of rock debris at
the base of a cliff or a slope, deposited by surface wash.
diorite
A group of plutonic rocks with a composition
intermediate between acidic and basic. Usually composed
of dark-coloured amphibole, acid plagioclase, pyroxenes,
and sometimes a little quartz.
conglomerate
A bedded, rudaceous sedimentary rock
comprising rounded granules, pebbles, cobbles, or boul-
ders of other rocks lodged in a fine-grained matrix,
generally of sand.
dolerite
A dark-coloured, medium-grained, hypabyssal
igneous rock forming dykes and sills. Consists
of pyroxene and plagioclase in equal proportions or more
pyroxene than plagioclase, as well as a little olivine. An
intrusive version of gabbro and basalt.
connate water
Meteoric water trapped in hydrous
minerals and the pore spaces of sediments during deposi-
tion and out of contact with the atmosphere for perhaps
millions of years.
