metamorphic rocks. metamorphism? metamorphism from the greek μεταμορφώστε, meaning...
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Metamorphic Rocks
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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
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Reacting to Change…a Practical Example
This is Bob→
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ClassroomTemperature: 75 degrees F
Dressed the way he is now, Bob is adapted to his surroundings
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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
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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
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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
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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
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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
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Changes in TextureMuscovite schist gneiss
Limestone marble
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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
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Foliated or Non-foliated?
Foliated: gneiss
Non-foliated: marble
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Changes in Minerology
Mostly clay minerals Micas, quartz and garnet
Slate Schist
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More Jargon…w00t
• Four types of metamorphism – “Burial”– Contact– Regional– Hydrothermal
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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
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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
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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
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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
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