Chapter 8 Metamorphism and Metamorphic Rocks

Introduction  Metamorphism - The transformation of rocks without melting, usually beneath Earth's surface, as the result of heat, pressure, and/or fluid activity, produces metamorphic rocks.  During metamorphism, rocks are subjected to sufficient heat, pressure and/or fluid activity to change their mineral composition or texture, or both.  All this occurs below the melting point in the solid state.

Introduction  Metamorphism is responsible for producing a number of economically valuable materials, like marble, a favorite of sculptors throughout history.  Metamorphism is an important process that is closely related to plate tectonics, the growth of continents, and even climate change.

Introduction  Distribution of Metamorphic Rocks 1. Shields – oldest part of the continental crust 2. Cores of large mountain ranges

The Agents of Metamorphism  The three principal agents of metamorphism are heat, pressure and fluid activity.  Intrusive magmas or deep burial provide heat which causes metamorphism  Pressure is produced by overlying rocks (lithostatic) or differential pressure is produced by various stresses, especially from plate collisions.  Fluid activity increases the rate of metamorphism.

The Agents of Metamorphism  Heat  Heat is an important agent of metamorphism Heat increases the rate of reactions Sources of heat include: Lava Magma Deep burial  Temperature increases with depth. The geothermal gradient averages about 25 o C/km.

The Agents of Metamorphism  Pressure  What are lithostatic and differential pressures, and why are they important?  Lithostatic pressure is a uniform field of pressure experienced by most rocks beneath Earth’s surface. Like the hydrostatic pressure experienced by divers underwater, the pressure acting on a rock embedded in the crust “feels” the same from all directions. Fig. 8.2, p. 184

The Agents of Metamorphism  Pressure  What are lithostatic and differential pressures, and why are they important?  Differential pressure is a nonuniform field of pressure; the pressure acting on a rock in some directions is stronger than it is in others. Many metamorphic rocks form under conditions of differential pressure, which influences the development of metamorphic structures and textures in significant ways. Fig. 8.2, p. 184

The Agents of Metamorphism  Fluid Activity  Fluids within sedimentary rocks or issuing from magmas can accelerate chemical changes, which occur during metamorphism and can cause new minerals to form.  Common metamorphic fluids include water and carbon dioxide.

The Three Types of Metamorphism Contact Dynamic Regional

The Three Types of Metamorphism  Contact Metamorphism  Contact metamorphic rocks form under conditions of high temperature and low pressure. Metamorphic "baked zones" or aureoles occur around magmas. Fig. 8.5, p. 186Fig. 8.6, p. 187

Fig. 8.5, p. 186

Fig. 8.6, p. 187

The Three Types of Metamorphism  Contact Metamorphism  The size of an aureole depends on: Size of the intrusion Temperature of the intrusion, mafic magmas tend to be hotter than felsic ones Amount of fluids, water and other fluids generally increase the size of the metamorphic aureole

The Three Types of Metamorphism  Dynamic metamorphism  Dynamic metamorphism occurs when rocks are ground and crushed in faults, such as at convergent or transform plate boundaries. High pressure builds up, but the temperature is low. Fig. 8.7, p. 188

The Three Types of Metamorphism  Dynamic metamorphism  Mylonites are fine-grained rocks in faults that result from grinding and crushing during fault movements (earthquakes). Fig. 8.7, p. 188

The Three Types of Metamorphism  Regional metamorphism  The most common type of metamorphism Fig. 8.4, p. 186

The Three Types of Metamorphism  Regional metamorphism  As the name implies, regional metamorphism has a broad range. Temperature and pressure both act as driving forces for metamorphic reactions in regional metamorphism. Fig. 8.4, p. 186

The Three Types of Metamorphism  The type of metamorphism that results largely depends on which of the three agents was dominant. Fig. 8.4, p. 186

Fig. 8-4, p. 186 Stepped Art Environments of metamorphism (T = temperature, P = pressure) Country rock around active Plutons (high T, low P) Country rock in active mountain belts (moderate to high T, moderate to high P) Meteorite impacts (ultrahigh T and P) Deeply buried sediments (low T, moderate P) Active accretionary wedges (low T, high P) Subducting oceanic lithosphere (high T, high P) Boundary between upper and lower mantle high T, high P) Water-saturated seafloor lava flows near mid-ocean ridges (moderate T, low P)

The Three Types of Metamorphism  Index Minerals and Metamorphic Grade  Metamorphic grade – the degree of metamorphic change a rock has undergone, usually listed as low, intermediate, or high Fig. 8.8 p. 188

The Three Types of Metamorphism  Index Minerals and Metamorphic Grade  Index minerals – certain minerals are only known to form under specific ranges of metamorphic temperatures and pressures. The chemistries of the parent rock and metamorphic fluids also affect the formation of index minerals. Fig. 8.8 p. 188

The Three Types of Metamorphism  Index Minerals and Metamorphic Grade  Quartz, feldspars, and some other minerals occur in a wide variety of metamorphic rocks at different grades. They are not index minerals. Fig. 8.8 p. 188

Index and other Minerals during the Metamorphism of an Aluminum-rich Shale

How are Metamorphic Rocks Classified?  Metamorphic rocks are classified principally according to texture.  Foliated textures are produced by the preferred orientation of platy minerals because of pressure.  Nonfoliated textures do not exhibit preferred orientation of minerals. The minerals are randomly oriented. Fig. 8.9a, p. 189

How are Metamorphic Rocks Classified?  Classification of Common Metamorphic Rocks Table 8.1, p. 190

How are Metamorphic Rocks Classified?  Foliated Metamorphic Rocks  Form a graded series of grain size and the development of foliation, from fine grained slate, to phyllite and coarser grained schist, to gneiss, with segregated bands of minerals Fig. 8.10, p. 190 Fig. 8.9b, p. 189

How are Metamorphic Rocks Classified?  Foliated Metamorphic Rocks  Schist and Gneiss Fig , p. 191

How are Metamorphic Rocks Classified?  Foliated Metamorphic Rocks  Migmatite: Actually a mixed igneous and metamorphic rock  Contains streaks of granite from partial melting Fig. 8.14, p. 192

How are Metamorphic Rocks Classified?  Foliated Metamorphic Rocks  Amphibolite is another fairly common coarse grained foliated metamorphic rock. Table 8.1, p. 190

How are Metamorphic Rocks Classified?  Nonfoliated Metamorphic Rocks  Nonfoliated textures do not exhibit preferred orientation of minerals.  Common nonfoliated metamorphic rocks are marble, quartzite, anthracite coal, greenstone, and hornfels. Fig. 8.16, p. 193 Fig. 8.17, p. 193

How are Metamorphic Rocks Classified?  Nonfoliated Metamorphic Rocks  Marble - metamorphosed limestones and dolostones  Quartzite - metamorphosed quartz sandstones  Greenstones - low grade metamorphic basalts  Hornfels - fine-grained contact metamorphic rocks  Anthracite - metamorphic coal Fig. 8.15, p. 192

Metamorphic Zones and Facies  Metamorphic zone – a belt of rocks showing roughly the same degree of metamorphism Fig. 8.18, p. 194

Metamorphic Zones and Facies  Metamorphic facies – refers to a group of rocks containing a distinctive mineral assemblage formed under similar conditions of temperature and pressure Fig. 8.19, p. 194

Metamorphic Zones and Facies  How do metamorphic zones and metamorphic facies differ?  Metamorphic zones show the gradational metamorphic change within a single rock composition.  Metamorphic facies are groups of many different rock compositions whose mineral contents all indicate common temperature and pressure conditions during metamorphism.

Plate Tectonics and Metamorphism  Metamorphism can occur along all types of plate boundaries, but is most common and extensive along convergent boundaries. Fig. 8.20, p. 195

Plate Tectonics and Metamorphism Fig. 8.19, p. 194  Subducting plates at oceanic-continental boundaries reach great depths (high pressures), but the plate may heat slowly (low temperatures), resulting in blueschist facies metamorphism.

Plate Tectonics and Metamorphism Fig. 8.20, p. 195  Higher temperatures and pressures in subduction zones produce higher grades of metamorphism in a subducting oceanic plate.

Fig. 8.20, p. 195

Metamorphism and Natural Resources Metamorphic rocks, including marble and slate, are resources. Fig. 8.10, p. 190 Fig. 8.21, p. 195 Slate

Metamorphism and Natural Resources Marble

Metamorphism and Natural Resources Metamorphic mineral resources include graphite, talc, asbestos, and garnet. Geo-Focus Fig. 1, p. 183 Fig a, p. 191

Metamorphism and Natural Resources  Metamorphic mineral resources  Graphite: Used in pencils and lubricants  Talc: Once used in talcum powder  Asbestos: Once used in insolation and fire-proofing Crocidolite variety caused lung cancer in miners  Garnets: Used in "sand" paper  Andulasite, sillimanite, kyanite: High-temperature porcelians  Metallic ores, such as: hematite, copper and zinc sulfides, tungsten ores in contact metamorphic zones

Metamorphism and Natural Resources  Metamorphic metallic mineral resources Table 8.2, p. 195

End of Chapter 8