Metamorphism & Metamorphic Rocks.pptx

1. Metamorphism &
Metamorphic
Rocks

2. Metamorphic rocks
Form when a rock's mineral composition
and texture change because of
metamorphism.
Metamorphism
The transformation of pre-exiting rocks
without melting, because of heat, pressure,
and/or fluid activity, produces metamorphic
rocks.

3. During metamorphism, The original
rock is subjected to temperatures
greater than 150 to 200 °C and, often,
elevated pressure of 100 megapascals
or more, causing profound physical or
chemical changes.
Metamorphism occurs below the
melting point in the solid state.

4. The effects of Metamorphism
1. Formation of new minerals
2. Changes in shape and size of
mineral grains
3. Development of new structures in
the rock.

5. Measurements of metamorphism intensity
1.Metamorphic Grade:
the degree of metamorphic change a rock has
undergone, usually listed as low, intermediate, or high.
2.Index Minerals:
Index minerals form only at certain temperatures and
pressures.
The presence of these minerals helps geologists
determine the metamorphic grade, which is a measure
of how intense the metamorphism was

7. Metamorphic Grades:
Low Grade: ~200–400°C; low pressure
Rocks may still show original features (like bedding)
Common minerals: chlorite, muscovite
Medium Grade: ~400–600°C
More new minerals grow; foliation becomes more
pronounced
Common minerals: biotite, garnet
High Grade: ~600–800+°C; high pressure
Original minerals often completely replaced
Common minerals: sillimanite, kyanite

8. Factors that Control
Metamorphism
1. Heat
2. Pressure
3. Fluids
4. Rock composition
5. Time

9. Effects of Heating on Metamorphism
Sources of heat include:
1. Lava
2. Magma
3. Deep burial
• Heating causes increase in bond length,
distortion of bonds, breaking of bonds, and
formation of new bonds.
Temperature increases
with depth. The
geothermal gradient
averages about
25oC/km.

10. Metamorphism and Plate Tectonics
• Metamorphism can occur at divergent and convergent
boundaries.
• Divergent boundaries are characterized by hydrothermal
metamorphism.
• Contact and regional metamorphism are common at
convergent boundaries.
Figs. 7.28b, 7.27b

11. Pressure and Metamorphism
Many metamorphic rocks form under
conditions of differential pressure, which
influences the development of metamorphic
structures and textures in significant ways.

12. Pressure and Metamorphism
1. Confining pressure acts on a rock equally in all directions.
2. Directed pressure acts more strongly in one direction than in
others.

13. Fluids and Rock Composition
Sources of metamorphic fluids are:
• Fluid trapped in sedimentary rock.
• Fluid introduced by tectonic and igneous
processes.
• Fluid expelled when minerals react.
Metamorphic fluids include water and carbon dioxide.
Kyanite schist (parent rock = mudstone)
Amphibolite (parent rock = basalt)

14. Types of Metamorphism
There are six main types of metamorphism:
1. Regional metamorphism
2. Contact metamorphism
3. Dynamic metamorphism
4. Shock metamorphism
5. Hydrothermal metamorphism
6. Burial metamorphism

15. Regional Metamorphism
Occurs when large regions of crustal rocks are
subjected to elevated temperatures and pressures;
takes place during mountain-building

16. Contact Metamorphism
• Occurs in rocks that are heated by nearby magma.
• Contact metamorphism affects rocks over a small
area.
• The degree of metamorphism decreases with
distance from the magma chamber.
Fig. 7.10

17. • Occurs along fractures in Earth’s crust where
significant movement has taken. It is associated
with faulting zones
Dynamic Metamorphism

18. Shock Metamorphism
• Produced by the high-velocity impact of a
meteorite or an asteroid on Earth’s surface.
The Barringer impact crater (Meteor Crater)
Arizona
Shock
metamorphism is
characterized by
distinctive, high-
pressure
minerals.

19. • Occurs when rocks
react TO hot, circulating
fluids.
• Hydrothermal
metamorphism
occurs primarily at mid-
ocean ridges.
Hydrothermal Metamorphism
Hydrothermal vent
Photo: NOAA

20. Occurs when sedimentary
rocks are buried deeply
and subjected to increased
heat and pressure from
overlying sediments.
Burial metamorphism is a
type of low-grade
metamorphism
Burial Metamorphism
During Moutain buildind
After Erosion

21. Classification of Metamorphic
Rocks
Metamorphic rocks are classified on the basis of:
1. Texture
2. Composition (mineralogical or chemical)

22. Metamorphic Textures
• The fabric of a metamorphic rock describes the geometric
arrangement of mineral grains.
• Metamorphic rocks textures are:
1. Foliated
2. Non-Foliated
.

23. Planar (layered) structure formed by the alignment
of minerals, usually due to differential pressure or
shear forces during metamorphism.
Foliation Texture
Foliated rocks are classified based on grain size
and foliation.

24. Foliated Metamorphic Rocks
• Slate, phyllite, schist, and gneiss are the four main types of
foliated metamorphic rock.
• As metamorphic grade increases, grain size increases.
Fig. 7.16

25. Nonfoliated textures do not exhibit preferred
orientation of minerals.
The minerals are randomly oriented.
Nonfoliated Metamorphic Rocks

26. Nonfoliated Metamorphic Rocks
• Nonfoliated rocks can be classified based on either origin or
composition.
• Hornfels, marble, quartzite, and amphibolite are common
examples of nonfoliated rocks.
Hornfels Marble Quartzite Amphibolite
Figs. 7.19, 7.20, 7.22, 7.23

27. Distribution of Metamorphic Rocks
1. Shields – oldest part of the continental crust
2. Cores of large mountain ranges

28. Comparing Metamorphic, Igneous,
and Sedimentary Rocks
• Metamorphic rocks form at higher temperatures
and pressures than sedimentary rocks.
• Metamorphic rocks form at lower temperatures
than igneous rocks.
• The rock does not melt.
• Metamorphic processes generally cannot be
observed directly.

29. Importance of Metamorphism
1. Metamorphic rocks, including marble and
slate, are resources.
2. Metamorphic mineral resources include
graphite, talc, asbestos, and garnet.

30. Metamorphic rocks, including marble and
slate.

31. Metamorphic mineral resources include
graphite, talc, asbestos, and garnet.
Graphite: Used in pencils
and lubricants
Talc: Once used in talcum
powder
Asbestos: Once used in
insolation and fire-proofing.
Garnets: Used in sand
paper.

32. Asbestos
Garnets