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Aquiclude
Aquiclude
A rock that will not let water
flow through it. An aquiclude is
an impermeable rock. It can be a cap rock, preventing, for example, the escape of oil from rocks below it, or it can be below an aquifer, preventing water from flowing into the ground any further and forcing it to move to the surface as a spring. If an aquifer is sandwiched between two aquicludes, the
water in the aquifer is trapped and under pressure.
Aquifer
A rock that has a network of connecting spaces that will allow water to seep through it. An aquifer is made of a permeable rock. Common aquifers are limestone, chalk and sandstone. Springs occur where the saturated part of the aquifer reaches the surface. Water does not seep from an aquifer if the rocks below are watertight (aquicludes).
Arenaceous
A rock composed largely of sand grains. Sandstone is an arenaceous rock.
 Arenaceous – Detail of a sandstone, showing the sand grains.
 Argillaceous – Detail of a shale, showing the fineness of the grains and the way it breaks into sheets.
Argillaceous
A rock composed largely of clay. Shale is an argillaceous rock.
Arkose
A sandstone rock made from coarse grains. It is produced
by the disintegration of a granite.
Ash
Fine powdery material thrown out of a volcano. (See also: Igneous rock and Tuff.)
Augite
A dark green silicate mineral containing calcium, sodium, iron, aluminium and magnesium. (See also: Ferromagnesium mineral.)
Axis of symmetry
A line or plane around which one part of a crystal is a mirror image of another part. Crystal systems are based on their symmetry.
Augite
An igneous rock (for example, gabbro, basalt) with a high percentage of dark-coloured minerals and relatively little silica.
 Augite – Augite is the black crystal in this piece of rock.
Basic rock
6
B
Basalt
A black, basic, fine-grained, igneous volcanic rock. Basaltic lava often contains vesicles.
Basalt is the most common rock on the Earth’s surface, covering
all of the world’s ocean floors. It
is produced at the boundaries of
the world’s great tectonic plates and pours out onto the seabed as the plates pull apart. Hawaii and Iceland are volcanic islands that are made entirely from basalt.
Although basalts are found
mainly close to cracks in the ocean floors, they also exist on land,
for example, actively in Iceland and Hawaii. Historically, basalt has been pushed out from the mantle to form enormous streams of lava called flood basalts, which have solidified into vast black sheets. They are called basalt plateaux
or traps. The name trap comes from the world’s largest region of flood basalts, the Deccan Traps, India. Large areas of flood basalts also occur in the Columbia–Snake region of the northwestern United States and in the Paraña Basin of South America.