Metamorphic Rock

Metamorphism

The changes or alterations of rock in terms of texture or mineralogy from the pre-existing rock is caused by:

◦Temperature

◦Pressure

◦Chemical environment changes

Another factor that affects the metamorphism is the original or parent rock.

Classification of Metamorphic Rock

There are numerous combinations of metamorphic rocks.

The first characteristic is to look at the texture of the rock:

◦Foliated

◦Non-foliated

Foliation

The alignment of minerals in a rock subjected to increased pressure is called foliation..

This involves rocks with two or more crystals.

Pressure Effects

Pressure will cause the material to compact.

The type of pressure will dictate the alignment of the minerals; foliation.

Mineralogy

The type of minerals will determine the type of new rock formed.

If only one mineral is found in the parent rock, when it undergoes metamorphism the composition remains the same but the crystals grow larger.

More that one mineral present, re-crystallization can take place making for new minerals.

Types of Metamorphism

The two main types are:

◦ Contact metamorphism, and

◦Regional metamorphism.

Contact Metamorphism

These rocks are formed during metamorphism usually without significant differential stress but with high temperature.

Contact metamorphism occurs in country rock (rock surrounding the magma) immediately adjacent to intruded magmas.

Regional Metamorphism

This is the most common source of metamorphic rock of the Earth’s crust.

It involves occurring at great depths.

Pressure, temperature and the chemical environment of the original rock are all altered.

Metamorphic Grade

This is the term given to the intensity of metamorphism that a mineral has been subjected.

Lots of heat, pressure and chemical change it would be considered a high metamorphic grade.

Shock Metamorphism

The impact of meteors can create a metamorphism called Shock metamorphism.

As the meteor strikes the earth it creates enough heat and melts local rock which are thrown up to 100km from the impact site. These rocks are called Tektites.