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Aa

A solidified lava flow with a blocky surface, formed from a viscous lava. The name is of Hawaiian origin.

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Absolute Age

An age of a rock or formation given in years determined by radiometric means. See also Relative age.

Agpaitic

Alkaline rocks having the alkalis over Al ratio in excess of 1, and containing a suite of complex alkali titano-, niobo- and zirconosilicates (e.g. eudialyte).

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Alkaline

When applied to igneous rocks, this word implies the presence of Sodium (Na) and/or Potassium (K).

 

Allopatric

Where a population or species occupies a different geographic region to another population or species. See also sympatric and parapatric.

 

Allopatric Speciation

Where different populations are formed, without intermediates, if physical barriers restrict gene flow. There are two types: vicariant speciation and peripatric speciation.

 

Allopolyploidy

Chromosomes donated from >1 parental species. See also polyploidy and autopolyploidy.

 

Amethyst

Variant of quartz, purple in colour.

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Amphibole

A common rock forming (igneous or metamorphic) group of minerals with complex chemical compositions, but mostly ferro-magnesian silicates. They are composed of double chain SiO4 tetrahedra. Amphiboles show two cleavage planes at around 120 degrees to each other.

Anagenesis

The process by which evolutionary change along a single lineage creates a new species without any splitting of the phylogenetic tree (see cladogenesis).

 

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