Metamorphic Rocks Fig. 3-CO, p.44 Gneiss, a metamorphic Rock from Baffin Island, Canada. Fig. 3-CO, p.44

Figure 3. 4 The rock cycle shows that rocks change over geologic time Figure 3.4 The rock cycle shows that rocks change over geologic time. The arrows show paths that rocks can follow as they change. 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 Gneiss, a metamorphic Rock from Baffin Island, Canada.

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