Metamorphic rock grades and facies describe how rocks change when heat, pressure, and fluids act on them without melting. This cheat sheet helps students connect mineral changes to the conditions inside Earth. It is useful for identifying metamorphic rocks, reading geologic maps, and understanding mountain-building processes.
Grades and facies give geologists clues about a rock's pressure-temperature history.
Key Facts
- Metamorphic grade increases as temperature and pressure increase from low grade to intermediate grade to high grade.
- Low-grade metamorphism commonly forms slate and phyllite, intermediate-grade metamorphism commonly forms schist, and high-grade metamorphism commonly forms gneiss.
- The common shale index mineral sequence with increasing grade is chlorite, biotite, garnet, staurolite, kyanite, sillimanite.
- Metamorphic facies are groups of minerals that form under similar pressure-temperature conditions.
- Contact metamorphism usually has high temperature and low pressure, often producing hornfels near an igneous intrusion.
- Regional metamorphism usually has both increased pressure and temperature, often producing foliated rocks during mountain building.
- A geothermal gradient describes how temperature changes with depth and can be estimated as geothermal gradient = temperature increase / depth increase.
- Rocks do not melt during metamorphism, because melting produces magma and begins the igneous rock cycle.
Vocabulary
- Metamorphic grade
- The level of metamorphic change in a rock based mainly on the amount of heat and pressure it experienced.
- Index mineral
- A mineral that forms only under a limited range of temperature and pressure, helping identify metamorphic grade.
- Metamorphic facies
- A set of mineral assemblages that forms under a particular range of pressure and temperature conditions.
- Foliation
- A layered or banded texture caused by minerals aligning under directed pressure.
- Contact metamorphism
- Metamorphism caused mainly by heat from nearby magma, usually at relatively low pressure.
- Regional metamorphism
- Metamorphism over a large area caused by tectonic pressure and heat, commonly during mountain formation.
Common Mistakes to Avoid
- Calling every shiny metamorphic rock high grade, because luster alone does not measure grade. Mineral assemblage and texture provide better evidence.
- Confusing metamorphism with melting, because metamorphic rocks change while remaining solid. If the rock melts, it becomes magma and may form igneous rock.
- Using one index mineral as proof of exact temperature, because index minerals show a range of conditions. Pressure and the original rock composition also matter.
- Mixing up contact and regional metamorphism, because contact metamorphism is mainly heat near an intrusion while regional metamorphism involves large-scale pressure and heat.
- Assuming all metamorphic rocks are foliated, because some rocks such as marble and quartzite are nonfoliated. Foliation depends on mineral type and directed pressure.
Practice Questions
- 1 A shale contains chlorite and then changes to contain biotite and garnet closer to a mountain belt. Did metamorphic grade increase or decrease?
- 2 A rock is buried 12 km below Earth's surface. If the average geothermal gradient is 25 degrees C per km and the surface temperature is 10 degrees C, estimate the rock temperature.
- 3 Arrange these rocks from lowest to highest metamorphic grade: gneiss, slate, schist, phyllite.
- 4 A rock near a magma intrusion becomes hard and fine-grained but does not show foliation. Explain why this setting suggests contact metamorphism rather than regional metamorphism.