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