Metamorphic Rocks

Metamorphism?

• Metamorphism from the Greek μεταμορφώστε, meaning “changing form”– Is the solid-state

recrystallization of a pre-existing rock (a “parent” rock) though changes in temperature, pressure, and/or chemical conditions

– In layman’s terms: Cooking and Squishing a rock

Reacting to Change…a Practical Example

This is Bob→

ClassroomTemperature: 75 degrees F

Dressed the way he is now, Bob is adapted to his surroundings

HawaiiTemperature: 95 degrees F

Dressed in a long-sleeved shirt and jeans, Bob overheats

In order to adapt to his new environmenta change of clothes is in order…

Bob is now adapted to his environment

AntarcticaTemperature: -129 degrees F

Dressed the way he was in Hawaii, Bob would freeze to death In a matter of minutes

To adapt to his this new environment, anotherWardrobe change is needed

Though he can’t move,Bob has adapted to hisenvironment

Minerals Behave similarly

• Every mineral has a stability range—of Temperature and Pressure, that is– Ex. Graphite and diamond

• Metamorphism occurs because each mineral has a specific temperature/pressure/chemical environment stability range

• Remember, these changes occur while the rock is still solid

Making the Grade…

• Metamorphic grade—intensity of metamorphism undergone by a rock

• Low grade– High temp, low pressure--near surface

• High grade– High temp, high pressure--deep under ground

Changes…

• Two types of changes generally accompany metamorphism– Textural

• Changes in size and shape and orientation of minerals in a rock

– Mineralogical• Decomposition of

unstable minerals and crystallization of new ones

Changes in TextureMuscovite schist gneiss

Limestone marble

Foliation

• Texture change—mineral alignment perpendicular to primary stress– especially platy minerals

—like mica

• Any meta. rock lacking foliation is said to be non-foliated

Foliated or Non-foliated?

Foliated: gneiss

Non-foliated: marble

Changes in Minerology

Mostly clay minerals Micas, quartz and garnet

Slate Schist

More Jargon…w00t

• Four types of metamorphism – “Burial”– Contact– Regional– Hydrothermal

Burial Metamorphism

• The pseudo-metamorphism bordering on witchcraft

• The absolute lowest grade of metamorphism

• Caused by weight (pressure) of overlying rocks

• Creates non-foliated rocks– Ex. Bituminous coal

Contact Metamorphism

• Rocks surrounding a magma body literally cook

• Creates non-foliated rocks

• Highest grade occurs next to the magma body and decreases with distance

Hydrothermal Metamorphism

• occurs when mineral laden (and also, hot) water reacts with rock, changing the rock’s composition

• Created non-foliated rocks– Ex. Black smoker

Regional Metamorphism

• Occurs in regions where mountain building is occurring

• Rocks are being heated and deformed

• Produces rocks that are foliated—banded or layered

• Typical sequence as grade increases