Metamorphic Rocks

“If something is gneiss, don’t take it for granite.”

Metamorphic Agents

• Pressure (>1,000 times atmospheric pressure)

• Temperature (>300 degrees C)

• Chemical Fluids (mineral saturated hot water)

Metamorphic Environments

• Contact (thermal) – intrusion by magma

• Hydrothermal – ion-rich hot water circulates through cracks

• Mountain Building – regional metamorphism

Metamorphic Changes -Rocks adjust to new conditions by:

• Changes in texture

• Changes in mineralogy

Changes in Texture

• Compaction

• Recrystallization - may cause – Foliation– Lineation

Changes in Minerology

• Recrystallization – small crystals reform into larger crystals of the same mineral

• Formation of new minerals – elements recombine to form new minerals– Garnet, Chlorite and others found only (or

mostly) in metamorphic rock

Grades of Metamorphosis

• Low Grade – Low temperature and pressure results in slate and phyllite

– “Rock cleavage” in slate– Foliated– Fine grained– Slate looks dull, phyllite is shiny

Grades of Metamorphosis (cont.)

• Intermediate Grade – higher temperatures and pressures than Low Grade– Schist

• Foliated

• Mica, garnet

Grades of Metamorphosis (cont.)

• High temperatures and pressures– Gneiss and Migmatite

• Dark & light banding of mica (dark), quartz & feldspar (light)

Foliated Metamorphic Rock Sequence

• Slate – parent rock is shale (sedimentary)

• Phyllite – parent rock is slate

• Schist – parent rock is phyllite

• Gneiss – parent rock is schist

• Migmatite – parent rock is gneiss

Non-Foliated Metamorphic Rocks & Their Parent Rocks

• Marble – Parent rock is limestone or dolostone (calcite, reacts with acid)

• Quartzite – Parent rock is sandstone

• Anthacite – Parent rock is bituminous coal

• Hornfels – Parent rock is any rock type

Metamorphic Rock Dichotomous Key

Proceed to Dichotomous Key Activity

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Dichotomous Metamorphic Key