Metamorphism and
Metamorphic Rocks
Daily Question
Correctly place the following
statements in the Venn Diagram.
Igneous
Rocks
1. Occurs at divergent boundaries
Contact
2. Occurs at convergent boundaries
Metamorphic
3. Forms as a result of melting (magma)
Rocks
4. Temperature is an agent
5. Pressure is an agent
6. Generates foliated rocks
7. Forms as a result of being near an intrusion of magma
8. Found in mountain belts
9. May have been originally been a metamorphic rock
10.Form at temperatures above 200 oC
11.May underlie several adjacent states
Definitions
Metamorphism – mineralogical, chemical,
and physical changes that occur in solid
rocks.
Occurs at depths greater than that of
lithification (Diagenesis).
Solid state recrystallization – changes that
occur without the rock melting (rocks that
melt are igneous).
Factors influencing
Metamorphism
•
•
•
•
Temperature
Pressure
Fluid
Time
Agents of
Metamorphism
Factors influencing
Metamorphism
Temperature
• below about 150oC, most minerals are stable
(little or no metamorphism)
• above 150oC, reaction rate increases as
temperature increases, new minerals begin to
form
• above 600oC, some minerals begin to melt
(transition to igneous rocks)
• Temperature a function of depth within the
Earth and the presence of a magma
Factors influencing
Metamorphism
Pressure
• as pressure increases, pore spaces reduced
and density increases, pore fluids are
expelled
• Minerals recrystallize and begin to realign
Differential stress
• pressure is greater in one direction
than in another
Temperature, Pressure and
Rock Type
Factors influencing Metamorphism
Pore fluids (water and carbon dioxide)
• as pressure increases, pore fluids are
expelled
• Pore fluids increase the rate of metamorphic
reactions by:
• storing ions involved in reactions
• moving ions from one place to another
Chemical Fluids from Magmas
• Carry ions from the melt
• Fluids increase the temperature in the rock
Veins
Solutions are deposited
in fractures and joints
Factors influencing
Metamorphism
Time
• solid state recrystallization is a slow
process
• in general, size of minerals increases
with time
Types of Metamorphism
1. Burial Metamorphism
2. Contact Metamorphism
3. Regional Metamorphism
Metamorphism & Plate Tectonics
Types of Metamorphism
Burial Metamorphism
• most common type, occurs where crust
is greater than 5 km thick
Relatively low temperature and pressure
(low grade metamorphism)
Maximum stress vertical, foliation parallel
to ground surface
Types of Metamorphism
Contact metamorphism
• Occurs in rocks around a magma body
• High temperature (heat from magma) “bakes”
the rock around the magma creating an
alternation
• High temperature & fluid primary agents, little
to no pressure (occurs at shallow depths in
the crust)
Contact Metamorphism
Contact Metamorphism
Types of Metamorphism
Regional metamorphism
• Large scale – large volume of rock is affected
• Associated with convergent plate margins and
mountain building
• Folding and faulting increase thickness of the
crust
• Occurs over a range of temperatures and
pressures
• Fluids are also present
• Low grade to high grade metamorphism
• Will have zones of contact metamorphism
Regional Metamorphism
Metamorphic Grade
Low Grade Metamorphism
Example: Slate
• Rocks become more dense and compact
• forms at low temperature and pressure
• microscopic crystals
• dull luster
• clay and mica minerals
• foliated
Low Grade - Slate
Metamorphic Grade
Intermediate Grade
Metamorphism
Example: Phyllite
• intermediate temperature and pressure
• small crystals
• shiny luster
• mostly mica minerals
• foliated
Intermediate Grade –
Phyllite
Metamorphic Grade
High Grade Metamorphism
Example: Schist
• high temperature and pressure
• large crystals
• mica-rich
• foliated
High Grade - Schist
Metamorphic Grade
High Grade Metamorphism
Example: Gneiss
• high temperature and pressure
• large crystals
• mica-poor
• foliated
High Grade - Gneiss
Rock Cycle
Regional Revisited
Slate
Phyllite
Schist,
Gneiss
Magma
Metamorphic Facies
Different
minerals
form at
different
temperatures
and
pressures
Group of stable
minerals
define a
facies