Metamorphic Rocks
Metamorphic Rocks Chemical and physical alterations of pre-existing rocks brought about by heat and pressure 2 Types Regional metamorphism Contact Metamorphism 4 Main Factors Temperature Fluids Pressure Deviatoric (directional) Stress
Regional Metamorphism Large scale deformation and alteration brought about by mountain building (orogeny) and related processes Usually on a larger scale
Contact Metamorphism Smaller scale alteration adjacent to intruding magma
Changes in Temperature and Pressure The heat source is Earth’s internal heat. Burial is inevitably accompanied by an increase in pressure due to the weight of the overlying rocks. Both temperature and pressure increase with depth in the Earth resulting in higher metamorphic “grades” with depth.
Once we have melting, it is no longer a metamorphic rock, it is igneous!!
Differential Stress Differential stress occurs if pressure is different in different directions. Sometimes called Deviatoric stress
Differential Stress Foliation vs Lineation
Foliation
Foliation--Slaty Cleavage Alters orientation of existing minerals
Foliation--Schistocity Alters orientation of existing minerals Crystallization of new minerals
Foliation--Schistocity Picture of schist Parent rock is often a fine grained rock Shale, mudstone, siltstone…
Foliation--Gneissic banding Alters orientation of existing minerals Crystallization of new minerals Minerals become segregated into bands
Foliation--Gneissic banding Parent rock is often a fine grained rock (as is both slate and schist) Shale, mudstone, siltstone…
Original Rock Metamorphism Low Grade High Grade Sedimentary rock with distinct bedding. Original Rock Metamorphism Slate Schist Gneiss Low Grade High Grade Foliation perpendicular to direction of maximum differential stress Foliation and crystallization perpendicular to direction of maximum differential stress Foliation, crystallization and differentiation perpendicular to direction of maximum differential stress http://www.tulane.edu/~sanelson/geol212/contactmeta.htm
Index Mineral – a mineral in a metamorphic rock which indicates a certain metamorphic grade Isograd – line marking the first appearance of an index mineral on a map or cross-section
Metamorphic Grade