Volcanic rocks and textures are those associated with magmatic eruptions at the surface of the Earth. These eruptions can occur into air, water or ice. In each case cooling is very rapid, causing fine-grained textures to develop.

As a crystal grows slowly in a magma chamber the magma from which it is crystallising may evolve, changing its composition. Thus later crystallising regions or “zones” of the crystal will have a slightly different composition from earlier crystallising regions. The crystal thus displays zonation.
Zonation is seen under cross-polarised light by variations in the birefringence colours of the different zones.

Michel-Levy Chart

Chronostratigraphic Chart