1. Metamorphic Rocks

2. Goals for mastering metamorphic rocks
Understand what it means to “metamorphose” a rock
Explain the differences between various types of
metamorphism processes – types of metamorphism
Describe the various common metamorphic textures
Associate the common textures with c0mmon
metamorphic rocks
Describe an index mineral – and how it relates to
metamorphism
Explain the association of metamorphic rocks with
various types of plate tectonic settings

3. What is “metamorphism” ?
meta (change) morphism (form)
transformation of pre-existing
rock from one to another
through pressure and
temperature relationships
all changes take place through
solid state transformation

4. takes place around 150oC (temperature)
Types of Metamorphism
takes place around 150oC (temperature)
typically at depths of 5 km below the
earth’s surface (Pressure)
At this temperature and pressure new minerals
are formed – changed from one mineral to
another

5. Low-grade Metamorphism
between 5-15 km below the earth’s
surface
temperatures up to 400oC
common low-grade minerals
quartz, chlorite, muscovite
Chlorite
Muscovite
Quartz

6. High-grade metamorphism 15 km below the surface to the point
melting (when melting occurs)
Temperatures greater than 400oC
Common high-grade minerals
garnet, kyanite, sillimanite
kyanite
Garnet
sillimanite

7. Migmatite How do fluids enhance metamorphism?
minerals dissolve – re-precipitated in the rock
fluids speed up chemical reactions – reduces
mineral melting points creating “migmatites”
Igneous rock material
Migmatite
Metamorphic rock
Melting occurs from lowered
melting points creating small pockets of igneous
rocks surrounded by
metamorphic rocks

8. When does metamorphism take place?
solid rock coming in contact with hot
magma
solid rock contacts hot fluids (hot
water)
during episodes of intense mountain
building
along earthquake fault zones

9. What are the various types of metamorphism processes?
contact metamorphism
hydrothermal metamorphism
regional metamorphism
Metasomatism

10. What is contact metamorphism
magma “contacts” solid rock –
heat is transferred into overlying
rocks through conduction – changing
the pre-existing rock it intruded
high heat / low pressure
localized metamorphism
“fast acting” like touching a
hot frying pan

11. Country rock (surrounding) is baked by conduction

12. What is regional metamorphism?
pre-existing rock is subjected to intense
stresses and strains (deformation) usually
from the forces of mountain building
high heat / high pressure
very wide spread geographically
like experiencing a slow sun burn

13. Mountain building
Wide spread belts

14. What is hydrothermal metamorphism
the chemical alteration of pre-existing
rock from the action of hot water –
usually hot water from a magma source
that is ion rich
low pressures / high heat
hot water alteration

15. Black smokers on the ocean floor
hydrothermal alteration – “spewing” hot
water concentrated in sulfide metals

16. Metasomatism fluids pass in / out of the rock changing the
overall chemical composition
fluids take away or add chemical compounds
to the rock
Chemical alteration from fluids --- moving and
out of the rock

17. Heat: What does each metamorphic process have in common?
provides the energy to cause
recrystallization of pre-existing
minerals into “new” type of minerals
Heat sources:
contact with hot magma – heat penetrates
overlying rocks (conduction)
geothermal gradient – heat increases with
depth – rocks are hotter when buried

18. Temperature increases as depth increases Large thermal gradient
Geothermal Gradient
Temperature increases
as depth increases
Large thermal gradient
at the surface
gradient decreases
after 1000 km depth
Steep gradient
1000
2000
3000
4000

19. Pressure – increases with depth
various types of pressure rearrange
minerals – compacts or elongates
Confining pressure
Differential pressure

20. Chemically active fluids
commonly hot water carries ions in
solution and ppt various solids (metals)
hot water dissolves minerals and deposits
minerals to other sites – concentrated
gold, silver, other earth metals
Gold in Quartz vein
Native Copper (Cu)

21. How does one know how much a rock has been metamorphosed?
the degree of metamorphism is
reflected in the rocks texture
and composition
Regional Metamorphism (HP/HT) – high grade
rock exhibits a foliation texture
rocks show a “preferred” mineral
orientation
parallel mineral alignments

22. How does one know how much a rock has been metamorphosed?
Contact Metamorphism (HT / LP) low-grade
rock textures decrease in
foliated textures – “sub” parallel type
textures
rocks composed of one type of
mineral (same chemistry) are called
non-foliated

23. How does one know the amount of
temperature / pressure the rock has been
subjected?
The Metamorphic facies
the set of metamorphic mineral assemblages
that form under various temperature/pressure
relationships
Associated with different plate boundaries
Blueschist / Eclogite facies occur in the
high pressure environment --- subduction
Greenschist, amphibolite, granulite facies
occur along convergent boundaries

24. Metamorphic facies – mineral assemblages that
represent various P / T relationships

25. What plate tectonic settings do metamorphic rocks form?
Divergent boundaries
hydrothermal metamorphism
with black smokers
Convergent plate boundary
metamorphism along plate
boundary
regional and contact
metamorphism
Contact
Regional

26. Metamorphism and Metamorphic Rocks
Define the following metamorphism terms:
contact metamorphism nonfoliated
foliation parent rock
hydrothermal regional metamorphism
metamorphism confining pressure
differential pressure index mineral