Types of Metamorphism

Metamorphic rocks are formed through the tranformation of pre-existing rock into new rock from high pressures and/or temperatures, a process called metamorphism.  There are are two types of metamorphism:

Contact Metamorphism: a type of metamorphism where high temperature is the dominant factor.  It can occur surrounding a pluton where a body of magma intrudes into relatively cool rock baking the area (~1-100m) around the intrusion.  These types of metamorphic rocks are usually non-foliated.

Regional Metamorphism: a type of metamorphism that takes place at great depth (usually >5 km below the surface).  Pressure is high and temperatures are variable (~ 300-800°C). Regional metamorphic rocks are almost always foliated.

A metamorphic rock is identified based on it’s texture and mineral composition.

Texture

The texture of a metamorphic rocks refers to whether or not a rock exhibits foliation (a planar texture).

quartzite new2Non-Foliated metamorphic rocks do not show preferential mineral or grain alignment because the mineral shapes do not have a preferred orientation when subject to stress. This image of quartzite is a non-foliated metamorphic rock.

gneiss newFoliated metamorphic rocks occur when flat sided minerals crystallize perpendicular to the direction of stress. There are four different types of foliation: slaty, phyllitic, schistose and gneissic.  This image of gneiss is a foliated metamorphic rock exhibiting gneissic foliation.

Composition

The mineral composition of a metamorphic rock is controlled by the composition of the original parent rock.

Non-Foliated metamorphic rocks are named based on their composition.  For example, quartzite is made of quartz.

schist new

Foliated schists have variable mineral compositions and are named using the two most predominant minerals.  This sample of schist is a biotite kyanite schist.

Metamorphism Transition Mineralogy