GeologyRocks

Andalusite

A polymorph whose other two minerals are kyanite and sillimanite. A particularly notable variety of andalusite is chiastolite, which has crosses of carbon in it.

Angular unconformity

An unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers that are at an angle to the overlying horizontal layers.

Hutton's unconformity at Siccar Point is a classic example where Devonian sediments are deposited on Silurian turbidites that are tilted at 70°.

Apatite

Phosphate mineral, which can be in three forms depending on its chemistry. Forms are Fluorapatite (fluorine-rich), Chlorapatite (chlorine-rich and Hydroxylapatite (rich in hyrdroxides). It is also what most teeth of mammals are made of!

Aquifer

An aquifer is a layer of porous rock that contains water. See Artisan Well

Aragonite

A polymorph of calcite. Produced by many organisms as a shell, test or other hard part. Unstable at the Earth's surface and is very rare in ancient sediments.

Artisan Well

An artesian aquifer or well (also known as a confined aquifer) is an aquifer that is trapped between two impermeable layers. An unconfined aquifer is recharged rapidly from surface water. A confined aquifer takes much longer to recharge, but this also means it is not responsive to daily, annual or even millennial changes. An example of an artesian aquifer is South Dakota, USA (see diagram below). Note how the Dakota sandstone (the aquifer) is bounded top and bottom by an impermeable rock.

Asthenosphere

The asthenosphere is the region of the inner structure of the Earth between 100-200 km below the surface that is the weak or "soft" zone in the upper mantle. It lies just below the lithosphere, which is involved in plate movements and isostatic adjustments. The layer is best thought of as "plastic" in that it is solid, but deforms over time.

Autopolyploidy

Chromosomes donated from one parental species. See also polyploidy and allopolyploidy.

Autozygous

When an individual possess’ two copies of the same allele that are identical by descent (see inbreeding coefficient).