Download Metamorphic Rocks - hrsbstaff.ednet.ns.ca

Geology 12 – Dalesandro
Notes – Metamorphic Rocks (Cont.)
-The original rock (before metamorphic processes take place) is called the
parent rock.

The metamorphic rock almost always has a different texture than the parent
rock.

Sometimes, texture and appearance are the only things that change, but more
commonly chemical composition also changes.

Recrystallization is the process whereby the original minerals of the parent
rock become the new minerals of the metamorphic rock.
-The four factors that influence the characteristics of a particular metamorphic
rock are:
1)
2)
3)
4)
The composition of the parent rock.
Temperature and pressure during the metamorphic process.
The effects of tectonic processes.
The presence or absence of water.
-Pressure from natural depth is known as confining pressure. It is generally
equal in all directions and causes new minerals formed during metamorphic
processes to be more compact (take up less space) than the original minerals.
-Pressure from tectonic processes is different. It is usually unequal and varies
by direction. It tends to squeeze and flatten minerals due to differential
stress, which is the stress placed on objects exposed to unequal forces.

Differential stress can cause foliation in the rock. Foliation refers to band-like
formations which give the rock a planar texture.
-There are three kinds of planar texture found in foliated rocks:
1) Slaty texture - the rock splits easily into flat, parallel planes, along
invisible boundaries between mineral grains or crystals.
2) Schistose texture – the rock splits along visible, parallel planes made of
needle-shaped mineral grains or crystals.
3) Gneissic texture – the visible planes appear as distinct, light and dark
layers or bands.
-Metamorphic rocks without planar texture are called non-foliated rocks.