1. Metamorphic Rocks

2. Metamorphic Rocks
Chemical and physical alterations of pre-existing rocks brought about by heat and pressure
2 Types
Regional metamorphism
Contact Metamorphism
4 Main Factors
Temperature
Fluids
Pressure
Deviatoric (directional) Stress

3. Regional Metamorphism
Large scale deformation and alteration brought about by mountain building (orogeny) and related processes
Usually on a larger scale

4. Contact Metamorphism
Smaller scale alteration adjacent to intruding magma

5. Changes in Temperature and Pressure
The heat source is Earth’s internal heat.
Burial is inevitably accompanied by an increase in pressure due to the weight of the overlying rocks.
Both temperature and
pressure increase with depth
in the Earth resulting in
higher metamorphic “grades”
with depth.

6. Once we have melting, it is no longer a metamorphic rock, it is igneous!!

7. Differential Stress
Differential stress occurs if pressure is different in different directions.
Sometimes called Deviatoric stress

8. Differential Stress
Foliation vs Lineation

9. Foliation

10. Foliation--Slaty Cleavage
Alters orientation of existing minerals

11. Foliation--Schistocity
Alters orientation of existing minerals
Crystallization of new minerals

12. Foliation--Schistocity
Picture of schist
Parent rock is often a fine grained rock
Shale, mudstone, siltstone…

13. Foliation--Gneissic banding
Alters orientation of existing minerals
Crystallization of new minerals
Minerals become segregated into bands

14. Foliation--Gneissic banding
Parent rock is often a fine grained rock (as is both slate and schist)
Shale, mudstone, siltstone…

15. Original Rock Metamorphism Low Grade High Grade
Sedimentary rock with distinct bedding.
Original Rock
Metamorphism
Slate
Schist
Gneiss
Low Grade
High Grade
Foliation perpendicular to direction of maximum differential stress
Foliation and crystallization perpendicular to direction of maximum differential stress
Foliation, crystallization and differentiation perpendicular to direction of maximum differential stress

17. Index Mineral – a mineral in a metamorphic rock which indicates a certain metamorphic grade
Isograd – line marking the first appearance of an index mineral on a map or cross-section

18. Metamorphic Grade