Transcript Chapter 7
Metamorphic Rocks Fig. 3-CO, p.44 Fig. 3-4, p.46 Metamorphism The transition of one rock into another by temperatures and/or pressures unlike those in which it formed. Metamorphic rocks are produced from • Igneous rocks • Sedimentary rocks • Other metamorphic rocks Metamorphism • Progresses incrementally from low to high-grade • Rock must remains essentially solid throughout process • Agents of metamorphism include heat, pressure and chemically-active fluids Metamorphic settings • Contact or thermal metamorphism – driven by a rise in temperature within the host rock • Hydrothermal metamorphism – chemical alterations from hot, ion-rich water • Regional metamorphism (mountain building) Agents of metamorphism Heat: The most important agent • recrystallization of mineral grains and chemical precipitates • Formation of new, stable minerals • Two sources of heat – Contact metamorphism – heat from magma – An increase in temperature with depth due to the geothermal gradient Geothermal Gradient Agents of metamorphism Pressure (stress) increases with depth • Confining pressure: applies forces equally in all directions – Example: depositional environment • Differential stress: applies unequal forces in different directions – Example: mountain-building environment Figure 7.3a Agents of metamorphism Pressure (stress) increases with depth • Confining pressure: applies forces equally in all directions – Example: depositional environment • Differential stress: applies unequal forces in different directions – Example: mountain-building environment Figure 7.2b Figure 7.4 Result of differential stress Role of Parent Rock • Mineral composition of parent rock determines mineral composition of metamorphic daughter rock (often the same or similar: – Sandstone and quartzite – Limestone and marble • Mineral composition of parent rock determines grade of metamorphism that takes place: Quartz sandstone, sedimentary parent rock of quartzite Quartzite – Metamorphosed Sandstone Limestone, sedimentary parent rock of marble Marble – Metamorphosed Limestone Low grade to high grade Metamorphism (left-right) Figure 7.17