Regional metamorphism The Rock Cycle Temperature, fluids, pressure, deviatoric stress Metamorphic grade Metamorphic facies Contact metamorphism Regional metamorphism The Rock Cycle

Rocks are classified by their mode of formation Rocks are classified by their mode of formation. There are three major rock forming processes on Earth, producing three kinds of rocks. Igneous Rocks Formed when magma (molten rocks) solidifies Environment: Hot enough to melt rock, pressure varies Sedimentary Rocks All non-igneous rocks formed by processes acting on the surface of the Earth Environment: normal for Earth’s surface Metamorphic Rocks Formed by chemically and physically altering rocks under heat and pressure deep within the Earth’s crust. Environment: High pressure, not hot enough to melt rock

Metamorphic Rocks Metamorphic rocks form deep within Earth’s crust under high pressure and temperature. The buried rock (protolith) undergoes solid state transformation, altering the mineral composition and texture of the rock. The rock can not melt, or it would become an igneous rock after cooling. Metamorphism that occurs around igneous intrusions is called contact metmorphism. Metamorphism occurring due to stresses caused by mountain building is called regional metamorphism. These complex metamorphic environments may include zones of contact metamorphism if igneous intrusions occur. The type of metamorphic rock produced is controlled by the composition of the protolith and the temperature, pressure and duration of the metamorphism. Metamorphic rocks come in two flavors – foliated and non-foliated.

Factors in Metamorphism Metamorphism is effected by four main factors: Fluids Temperature Pressure Deviatoric stress

Factors in Metamorphism Temperature Controlled by: Geothermal gradient (increasing temperature with greater depth in Earth) Tectonic setting (higher in areas of igneous activity) Most rocks melt at less than 2000oC, the upper limit of metamorphism (rocks formed from melt are igneous!)

Factors in Metamorphism Fluids Moving fluids within the rock body can effect: Pressure (increases with temperature) Heat (moves it around) Dissolved ions and gases (moves them around)

Factors in Metamorphism Pressure A uniform compressive stress created by Load (weight of overlying rocks) Fluid pressure Increases greatly deeper in the crust. With temperature, controls what minerals are stable.

Factors in Metamorphism Deviatoric Stress Stress imbalance causes compression in some directions and expansion in others. Controls: Foliation (orientation of minerals into layers) Lineation (linear orientation of minerals)

Hydrostatic Pressure vs Deviatoric Stress

Foliated Metamorphic Rocks Metamorphic rock with aligned flat minerals Foliation occurs when a protolith with flat minerals like mica is subjected to deviatoric stress. The flat minerals align perpendicular to the direction of maximum stress. Increasing metamorphism causes more severe reordering and alteration of the rock’s metamorphic texture.

Marble Quartzite Anthracite Amphibolite Marble Quartzite Anthracite Metamorphic rock without minerals that can be foliated Non- Foliated Metamorphic Rocks Marble Quartzite Anthracite Amphibolite quartz sandstone Protolith limestone coal mafic rocks Mineralogy carbonates quartz none amphiboles http://www.mii.org/ http://www.mii.org/ http://www.mii.org/ http://www.smccd.edu/ Marble Quartzite Anthracite Amphibolite

Deviatoric Stress Maximum crystal growth occurs perpendicular to direction of maximum stress http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

Deviatoric Stress Foliation - alignment of flat minerals Deviatoric stress is the primary control on foliation and lineation Foliation - alignment of flat minerals Slaty Cleavage – Alignment of small mica flakes Schistosity – Alignment of large mica flakes Gneissic Banding – segregation of different minerals into light and dark bands Lineation - alignment of pointy minerals

Foliation and lineation Deviatoric Stress Foliation and lineation http://www.aqd.nps.gov/grd/usgsnps/deform/gfoliation.html

Foliation

Foliation - Slaty Cleavage Orientation of existing minerals http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

Foliation - Slaty Cleavage Slate slaty cleavage.

Foliation - Slaty Cleavage Slate slaty cleavage. Protolith– fine grained rock like shale, mudstone, or siltstone

Foliation - Schistosity Orientation of existing minerals and crystallization of new minerals http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

Foliation - Schistosity exhibits schistosity Protolith – fine grained rock like shale, mudstone, or siltstone

Foliation - Schistosity New minerals crystallize - in this case garnets

Foliation - Gneissic Banding Orientation and crystallization continue, minerals become segregated into bands http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

Foliation - Gneissic Banding shows gneissic banding Arizona State University http://www.slu.edu

Low Grade High Grade Foliated Metamorphic Rocks Metamorphic rock with aligned flat minerals http://hyperphysics.phy-astr.gsu.edu/hbase/geophys 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

Metamorphic Grade Metamorphic grade increases with both increasing temperature and pressure, both of which increase with depth in the Earth http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

Metamorphic Grade Materials with the same chemical formula can have different crystalline structures, and thus different physical properties, depending on the conditions of formation Graphite Temperature (K) 1000 3000 Pressure Diamond Rocks containing diamonds formed under different P/T conditions than rocks containing graphite

Metamorphic Grade Polymorphs - minerals with the same chemical formula but different crystallize structures (=different chemical and physical properties) Finding one polymorph in a rock narrows down the P/T conditions at metamorphism

Metamorphic Grade kyanite sillimanite andalusite http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

Metamorphic Grade

Metamorphic Grade 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

Contact Metamorphism

Metamorphism adjacent to an igneous intrusion Contact Metamorphism Metamorphism adjacent to an igneous intrusion Shale Rock Igneous Intrusion

Contact Metamorphism Map Finds first specimens of “green” metamorphic index mineral Finds first specimens of “yellow” metamorphic index mineral Finds igneous intrusion Map igneous low grade high grade high grade low grade

Metamorphic Grade Map Contact Metamorphism Index Mineral – a mineral in a metamorphic terrain which indicates a certain metamorphic grade Isograd – line marking the first appearance of an index mineral on a map. A B C A B C Map Contact Metamorphism

Metamorphic Grade igneous high grade unmetamorphosed low grade

Metamorphic Grade 300oC 700oC http://www.sun.ac.za/geology/METCOURSE/2nd%20year/metamorphic%20zones.htm

Isograds of index minerals Contact Metamorphism Isograds of index minerals

Regional Metamorphism Large-scale metamorphism is usually related to tectonic events (rifting, mountain building, etc.) A single regional event can produce many different kinds of metamorphism, including smaller zones of contact metamorphism.

Metamorphic Facies Metamorphic Facies – Rocks that have been exposed to the same degree of metamorphism (P/T regime), and thus are the same metamorphic grade http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

Metamorphic Facies All rocks that have experienced this much metamorphism belong to Greenschist facies. http://www.dc.peachnet.edu/~janderso/images/metafaci.jpg

Minerals in Metamorphic Rock Metamorphic Facies Protoliths Mudstone Limestone Granite Gabbro Quartz K-feldspar Plagioclase Muscovite Biotite Chlorite Epidote Calcite Magnetite Actinolite Minerals in Metamorphic Rock Schist Marble Schist Schist Metamorphic Rocks However, all the rocks have undergone the same amount of metamorphism, and all belong to the Greenschist facies.

Carolina Slate Belt Greenschist facies Metamorphic Facies Carolina Slate Belt Greenschist facies http://www.tulane.edu/~sanelson/geol212/regionalmetamorph.htm

The Rock Cycle

The Rock Cycle Sedimentary Rocks Igneous Rocks Metamorphic Rocks

Igneous Process 1. Melt (form magma) 2. Cool (crystallize) making igneous rocks 1. Melt (form magma) 2. Cool (crystallize) geothermal gradient, Bowen’s Reaction Series, felsic, intermediate, mafic, magma, lava, texture, basalt, gabbro, andesite, diorite, rhyolite, granite

Sedimentary Processes making sedimentary rocks 1. Chemical and physical weathering (breaks down rock) (air/water) (ice) 2. Transport clasts and/or ions

Sedimentary Processes making sedimentary rocks 3. Deposition/ precipitation in basin 4. Lithification weathering, transportation, deposition, lithification, clasts, clastic rocks, chemical rocks, depositional environment

Metamorphic Processes making metamorphic rocks Add heat and pressure to pre-existing rock (protolith) metamorphism, temperature, fluids, pressure, deviatoric stress, metamorphic grade, metamorphic facies, index minerals, isograds, contact metamorphism, regional metamorphism

igneous rock metamorphic rock sedimentary rock Igneous Process 1. Heat (melt) 2. Cool (crystallize) original rock mass Metamorphic Process Add heat and pressure metamorphic rock Sedimentary Process The Rock Cycle 1. Breakdown (weather) 2. Transport 3. Deposit/precipitate 4. Lithify sedimentary rock