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→

Classroom Temperature: 75 degrees F Dressed the way he is now, Bob is adapted to his surroundings

Hawaii Temperature: 95 degrees F Dressed in a long-sleeved shirt and jeans, Bob overheats In order to adapt to his new environment a change of clothes is in order… Bob is now adapted to his environment

Antarctica Temperature: -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, another Wardrobe change is needed Though he can’t move, Bob has adapted to his environment

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 Texture Muscovite schist gneiss Limestonemarble

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