Rock Group

Igneous (Mafic)

Grain Size

A rock with fine grains surrounding larger crystals like this is called porphyritic, for the phenocrysts of olivine in this case. This may also be referred to as olivine phenocrysts within a groundmass of very fine plagioclase and pyroxene crystals, along with basaltic glass, referring to the frozen basalt melt. The dark colour of many basalt samples is due to the abundant fine grained crystals in the groundmass.

Composition

Basalt is a mafic rock, meaning that it has high concentrations of magnesium and iron (one way to remember this is ‘ma’ for magnesium and ‘fic’ from ferrum, which is Latin for iron). Basalt is typically black or dark grey in colour. This sample also contains coarse olivine crystals.

Key Features

This basalt sample is porous, indicating it cooled quickly with gas bubbles still trapped within it. This samples comes from Hawaii.