111lab-4 - GY 111L Lab Notes D Haywick(2008-09 1 GY 111L...

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GY 111L Lab Notes D. Haywick (2008-09) 1 1 GY 111L Lecture Note Series 4: Introduction to Metamorphic Rocks Lecture Goals: A) Metamorphism B) Foliated Metamorphic Rocks C) Non-foliated Metamorphic rocks D) Cataclastic Metamorphic Rocks References: GY 111; Earth Materials Lab Manual; Chapter 4. A) Metamorphism The term metamorphism means to change a rock (commonly called the parent rock ) from its original composition (e.g., a shale or a basalt) to something new (e.g., a metamorphic rock). The cause of these changes are the so called "agents of metamorphism”: 1. Heat (25 ˚ C to 800 ˚ C; up to the point of melting) 2. Pressure (1 Bar to 12,000 Bars) 3. Chemically active fluids (AKA hot water) Each can single-handedly induce metamorphism in rocks, or they can act collectively. Heat is associated with one of two tectonic situations. The first is with proximity to intrusions like plutons, batholiths or laccoliths. The country rock that is in direct contact with these hot bodies experiences the highest temperatures. The further away that you get from the pluton, the lower the temperature. In this tectonic setting, the highest grade heat-only (or thermal) metamorphism is in direct contact with the intrusion. Lower grade heat-only metamorphism occurs further into the country rock. Areas of equal temperature that surround the intrusion are called isotherms and they more or less delineate the extent of heat-only metamorphism within the country rock. Because this type of metamorphism only occurs in contact with intrusions, it is usually called contact metamorphism . Good name huh? The area of metamorphism around the intrusion is frequently called a contact metamorphic aureole . The other tectonic setting where heat can induce metamorphic changes is through burial . It is well known that the deeper you go in the Earth's interior, the hotter it gets. The rate of increase is called the geothermal gradient and although highly variable depending upon proximity to active plate boundaries, it averages about 25 -30 ˚ C/1000m. So about 10 km down, the temperature of the rocks is 300 ˚ C hotter than it would be at the surface. The deep you go the hotter it gets and by the time you are at the base of the lithosphere, it’s really quite toastie. By the way, at these depths, it is impossible to separate out the pressure component from the temperature component of metamorphism.
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GY 111L Lab Notes D. Haywick (2008-09) 2 2 Pressure is a metamorphic agent that requires a bit of explanation. We kind of touched on pressure when we were discussing lithification in one of our sedimentation lectures. Compaction is a type of horizontal pressure that occurs when muddy sediment is dewatered during shallow burial. That type of pressure occurs when more sediment is deposited on top of the wet mud. The overburden squeezes the water out of the mud ultimately converting it into shale. The pressure associated with metamorphism is far more extreme than compaction. Moreover, it need not be
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This note was uploaded on 02/04/2012 for the course GLY 111 taught by Professor Haywick during the Fall '11 term at S. Alabama.

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111lab-4 - GY 111L Lab Notes D Haywick(2008-09 1 GY 111L...

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