Metamorphism and Metamorphic Rocks

Daily Question Correctly place the following Igneous Rocks Contact Metamorphic Regional rocks Correctly place the following statements in the Venn Diagram. Occurs at divergent boundaries Occurs at convergent boundaries Forms as a result of melting (magma) Temperature is an agent Pressure is an agent Generates foliated rocks Forms as a result of being near an intrusion of magma Found in mountain belts May have been originally been a metamorphic rock Form at temperatures above 200 oC May underlie several adjacent states

Definitions Metamorphism – mineralogical, chemical, and physical changes that occur in solid rocks. Occurs at depths greater than that of lithification (Diagenesis). Solid state recrystallization – changes that occur without the rock melting (rocks that melt are igneous).

Factors influencing Metamorphism Temperature Pressure Fluid Time Agents of Metamorphism

Factors influencing Metamorphism Temperature below about 150oC, most minerals are stable (little or no metamorphism) above 150oC, reaction rate increases as temperature increases, new minerals begin to form above 600oC, some minerals begin to melt (transition to igneous rocks) Temperature a function of depth within the Earth and the presence of a magma

Factors influencing Metamorphism Pressure as pressure increases, pore spaces reduced and density increases, pore fluids are expelled Minerals recrystallize and begin to realign Differential stress pressure is greater in one direction than in another

Temperature, Pressure and Rock Type

Effects of Stress Uniform Differential

Factors influencing Metamorphism Pore fluids (water and carbon dioxide) as pressure increases, pore fluids are expelled Pore fluids increase the rate of metamorphic reactions by: storing ions involved in reactions moving ions from one place to another Chemical Fluids from Magmas Carry ions from the melt Fluids increase the temperature in the rock

Veins Solutions are deposited in fractures and joints

Factors influencing Metamorphism Time solid state recrystallization is a slow process in general, size of minerals increases with time

Types of Metamorphism Burial Metamorphism 2. Contact Metamorphism 3. Regional Metamorphism

Metamorphism & Plate Tectonics

Types of Metamorphism Burial Metamorphism most common type, occurs where crust is greater than 5 km thick Relatively low temperature and pressure (low grade metamorphism) Maximum stress vertical, foliation parallel to ground surface

Types of Metamorphism Contact metamorphism Occurs in rocks around a magma body High temperature (heat from magma) “bakes” the rock around the magma creating an alternation High temperature & fluid primary agents, little to no pressure (occurs at shallow depths in the crust)

Non-foliated Metamorphic Rocks Granular texture Interlocking grains Composed primarily of one mineral Uniform grain size

Contact Metamorphism

Contact Metamorphism

Types of Metamorphism Regional metamorphism Large scale – large volume of rock is affected Associated with convergent plate margins and mountain building Folding and faulting increase thickness of the crust Occurs over a range of temperatures and pressures Fluids are also present Low grade to high grade metamorphism Will have zones of contact metamorphism

Regional Metamorphism

Metamorphic Grade

Metamorphic Grade As pressure and/or temperature increases, the grade of metamorphism increases

Low Grade Metamorphism Example: Slate Rocks become more dense and compact forms at low temperature and pressure microscopic crystals dull luster clay and mica minerals foliated

Low Grade - Slate

Metamorphic Grade

Intermediate Grade Metamorphism Example: Phyllite intermediate temperature and pressure small crystals shiny luster mostly mica minerals foliated

Intermediate Grade – Phyllite

Metamorphic Grade

High Grade Metamorphism Example: Schist high temperature and pressure large crystals mica-rich foliated

High Grade - Schist

Metamorphic Grade

High Grade Metamorphism Example: Gneiss high temperature and pressure large crystals mica-poor foliated

High Grade - Gneiss

Rock Cycle

Regional Revisited Slate Phyllite Schist, Gneiss Magma

Nonfoliated - Marble metamorphosed limestone (CaCO3) bedding and fossils obliterated

Nonfoliated - Quartzite metamorphosed quartz-rich sandstone Pore spaces filled with SiO2

Metamorphic Facies Different minerals form at different temperatures and pressures Group of stable minerals define a facies