Metamorphic Rocks A.Evidence of metamorphism B.The ingredients of metamorphism C.Classifying metamorphic rocks D.Metamorphism of shale (& other parent rocks!) E.Metamorphic minerals

The Rock Cycle Rock Metamorphism (Increased T & P) Meta- morphic rock Partial Melting Magma Geological Materials Transformation Processes

Metamorphism  Metamorphic Rocks Definition Re-crystallization of minerals While still solid Into minerals that are stable at different temperatures and Different pressures

Metamorphism  Metamorphic Rocks WHY??? do the minerals recrystallize? To reach chemical equilibrium!!! Animation of metamorphic rock formation

A. Evidence of Metamorphism For example… Sedimentary Rocks Made of minerals derived from weathering of a parent rock Stable at atmospheric temperatures and pressures (low T & P) Originally horizontal, continuous and uniform layers 1. Bent (deformed) layers For example… Sedimentary Rocks Made of minerals derived from weathering of a parent rock Stable at atmospheric temperatures and pressures (low T & P) Originally horizontal, continuous and uniform layers 1. Bent (deformed) layers Shale Gneiss

Evidence of Metamorphism Differential pressure “squashes” rock and included features 2. Flattened Pebbles Conglomerate Metaconglomerate

3. Crystalline Texture Minerals tightly interlocking due to recrystallization under pressure Evidence of Metamorphism Quartz Sandstone Quartzite

Evidence of Metamorphism 4. New mineral assemblages Example: Shale: Clay minerals (some quartz)  Metamorphism (Mid-grade) Forms Schist: Mica, Feldspar and other silicate minerals 

B. Recipe of Metamorphism 1. Parent Rock Even though minerals will change Most elements are provided by parent rock Except water and some dissolved ions Shale Schist

B. Recipe of Metamorphism 2. Heat Geothermal gradient: more pressure = more heat!! Minerals stable at lower temperatures converted to minerals stable at higher temperatures Solid state chemical reactions are accelerated Temp. (ºC) ABC A C B

Recipe of metamorphism: Heat Geotherm: Geotherm: Temperature increases with depth beneath Earth’s surface Typical: 30° C/km Igneous intrusions: Igneous intrusions: Localized increases in temperature

Recipe of Metamorphism 3. Increased Pressure & Stress Increased Confining Pressure as rocks are buried Compression at convergent plate boundary or Sheared as plates slide past each other

Results of Stress Compressive Stress Shear Stress Causes Foliation

Ingredients of Metamorphism 4. Addition or removal of fluids (and elements) Water (and other fluids) within rocks and minerals Moving during metamorphism Accelerates solid-state chemical reactions and May change rock composition 5. Time  Millions of years!

Metamorphic Grade High grade (High P & T) Low grade (Low P & T) Intensity of Metamorphism: Intensity of Metamorphism:

Metamorphic Facies The minerals in a rock are clues to the (pressure and temperature) history of the rock Facies = a set of metamorphic conditions Each facies is characteristic of its Tectonic environment Source rocks in the environment (like shale)

Metamorphism of Shale Fig. Story 9.7b

Metamorphic Rocks of other Parent Rocks Limestone  Marble Bioclastic calcite  Crystalline calcite Nonfoliated

Metamorphic Rocks of other Parent Rocks Quartz Sandstone  Quartzite Granular quartz  Crystalline quartz Nonfoliated

Metamorphic Rocks of other Parent Rocks Basalt  Amphibole Schist Granite  Gneiss Uniform texture  Foliation (schistosity and gneissic banding)

Characteristic Metamorphic Minerals For more images: kyanite andalusite sillimanite staurolitegarnet

Distinct Mineral Assemblages Grade

Grade

Index Minerals Define the metamorphic conditions or zones Give information about the “metamorphic grade” Fig. Story 9.7a

Summary A.Evidence of metamorphism A.Deformed layers, flattened pebbles, crystalline texture, & new mineral assemblages B.The ingredients of metamorphism A.Parent rock, heat, pressure, fluids, & time C.Classifying metamorphic rocks A.Low grade --> high grade & facies D.Metamorphism of shale (& other parent rocks!) A.Shale --> Gneiss, Sandstone --> Quartzite, Limestone --> Marble E.Metamorphic minerals A.Muscovite and Garnet = Index minerals

To think about… Metamorphic Rocks & Plate Tectonics Tracing changes in pressure and temperature conditions that a rock experienced … … gives insight into plate tectonic settings!!