Rock Group

Metamorphic

Grain Size

Like many gneisses, the Acasta Gneiss is medium- to coarse-grained.

Composition

The Acasta Gneiss is granitic in composition, including minerals like quartz, K-feldspar, plagioclase, biotite and amphibole. It is likely derived from an igneous rock precursor or protolith.

Key Features

The Acasta Gneiss shows gneissic banding, or gneissosity, defined by bands or lenses of light and dark coloured rock. This indicates that the rock has undergone metamorphism, specifically high-temperature regional metamorphism. Gneisses are hard due to the tightly interlocking minerals, a texture formed by metamorphism. The hardness is further aided by high quartz content in this sample. This gneiss comes from northern Canada and is famous for being the oldest dated rock formed on Earth (meteorites would be older). It is about 4 billion years old.