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Metamorphism and Metamorphic Rocks Daily Question Correctly place the following statements in the Venn Diagram. Igneous Rocks 1. Occurs at divergent boundaries Contact 2. Occurs at convergent boundaries Metamorphic 3. Forms as a result of melting (magma) Rocks 4. Temperature is an agent 5. Pressure is an agent 6. Generates foliated rocks 7. Forms as a result of being near an intrusion of magma 8. Found in mountain belts 9. May have been originally been a metamorphic rock 10.Form at temperatures above 200 oC 11.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 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 1. 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) 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 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 Metamorphic Facies Different minerals form at different temperatures and pressures Group of stable minerals define a facies
