Metamorphism

1
Chapter 7

• Metamorphism
• and Metamorphic Rocks

2
Introduction

• Metamorphism - The transformation of rocks,
  usually beneath Earth's surface, as the result of
  heat, pressure, and/or fluid activity, produces
  metamorphic rocks
• During metamorphism, rocks are subjected to
  sufficient heat, pressure and fluid activity to
  change their mineral composition or texture, or
  both.
• All this occurs below the melting point in the
  solid state.

3
Introduction

• Metamorphism is also responsible for producing a
  number of economically valuable materials, like
  marble, a favorite of sculptors throughout
  history.
• Metamorphism is an important process that is
  closely related to plate tectonics, the growth of
  continents, and even climate change.

Geo-inSight, pgs. 180-181
4
Introduction

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

Fig. 7.1, p. 169
5
The Agents of Metamorphism

• The three principal agents of metamorphism are
  heat, pressure and fluid activity.
• Intrusive magmas or deep burial provide heat
  which causes metamorphism.
• Pressure is produced by overlying rocks
  (lithostatic) or is differential pressure
  produced by various stresses.
• Fluid activity increases the rate of
  metamorphism.

6
The Agents of Metamorphism

• Heat
• Heat is an important agent of metamorphism
• Heat Increases the rate of reactions
• Sources of heat include
• Extrusive lava
• Intrusive magma
• Deep burial
• Temperature increase with depth. The geothermal
  gradient averages about 25 degrees C/ km.

7
The Agents of Metamorphism

• Pressure
• What are lithostatic and differential pressures,
  and why are they important?

• Lithostatic pressure is a uniform field of
  pressure experienced by most rocks beneath
  Earths surface. Like the hydrostatic pressure
  experienced by divers underwater, the pressure
  acting on a rock embedded in the crust feels
  the same from all directions.

Fig. 7.2, p. 170
8
The Agents of Metamorphism

• Pressure
• What are lithostatic and differential pressures,
  and why are they important?

• Differential pressure is a nonuniform field of
  pressure the pressure acting on a rock in some
  directions is stronger than it is in others. Many
  metamorphic rocks form under conditions of
  differential pressure, which influences the
  development of metamorphic structures and
  textures in significant ways.

Fig. 7.2, p. 170
9
The Agents of Metamorphism

• Fluid Activity
• Fluids within sedimentary rocks or issuing from
  magmas can accelerate chemical changes which
  occur during metamorphism and can cause new
  minerals to form.

10
The Agents of Metamorphism

• The type of metamorphism that results largely
  depends on which of the three agents was
  dominant.

Fig. 7.4, p. 171
11
The Three Types of Metamorphism

• Contact
• Dynamic
• Regional metamorphism

12
The Three Types of Metamorphism

• Contact Metamorphism
• Contact metamorphic rocks form under conditions
  of high temperature and low pressure. They are
  arrayed in aureoles, or metamorphosed zones,
  around plutons and other intrusive igneous
  bodies.

Fig. 7.5, p. 171
Fig. 7.6, p. 174
13
The Three Types of Metamorphism

• Dynamic metamorphism
• Dynamic metamorphism is associated with faults
  and areas where high pressure builds up in the
  crust, but the temperature is low, such as in the
  accretionary wedges at convergent plate
  boundaries.

Fig. 7.7, p. 174
14
The Three Types of Metamorphism

• Regional metamorphism
• Regional metamorphism is the most common type of
  metamorphism.

• As the name implies, regional metamorphism has a
  broad range. Temperature and pressure both act as
  driving forces for metamorphic reactions in
  regional metamorphism.

Fig. 7.4, p. 171
15
The Three Types of Metamorphism

• Index Minerals and Metamorphic Grade
• Metamorphic grade the degree of metamorphic
  change a rock has undergone.
• Index minerals certain minerals are known to
  only form under specific temperatures and
  pressure.

Fig. 7.8 p. 175
16
How are Metamorphic Rocks Classified?

• Metamorphic rocks are classified principally
  according to texture.
• Foliated - Foliated texture is produced by the
  preferred orientation of platy minerals.
• Nonfoliated - Nonfoliated textures do not exhibit
  preferred orientation of minerals.

Fig. 7.9a, p. 177
17
How are Metamorphic Rocks Classified?

• Classification of Common Metamorphic Rocks

Table 7.1, p. 176
18
How are Metamorphic Rocks Classified?

• Foliated Metamorphic Rocks
• Foliated texture is produced by the preferred
  orientation of platy minerals.
• Foliated metamorphic rocks form a graded series
  of grain size and/or development of foliation,
  from fine grained slate, to phyllite and coarser
  grained schist, to gneiss, with segregated bands
  of minerals.
• Amphibolite is another fairly common coarse
  grained foliated metamorphic rock.

7.10a, p. 177
Fig. 7.9d, p. 177
19
How are Metamorphic Rocks Classified?

• Foliated Metamorphic Rocks
• Schist and Gneiss

Fig. 7.13, p. 178
20
How are Metamorphic Rocks Classified?

• Foliated Metamorphic Rocks
• Migmatite contain streaks of granite

Fig. 7.14, p. 179
21
How are Metamorphic Rocks Classified?

• Nonfoliated Metamorphic Rocks
• Nonfoliated textures do not exhibit preferred
  orientation of minerals.
• Common nonfoliated metamorphic rocks are marble,
  quartzite, greenstone, and hornfels.

Fig. 7.16, p. 179 Fig. 7.17, p. 182
22
Metamorphic Zones and Facies

• Metamorphic zone a belt of rocks showing
  roughly the same degree of metamorphism

Fig. 7.18, p. 182
23
Metamorphic Zones and Facies

• Metamorphic facies refers to a group of rocks
  containing a distinctive mineral assemblage
  formed under similar conditions of temperature
  and pressure.

Fig. 7.19, p. 183
24
Metamorphic Zones and Facies

• How do metamorphic zones and metamorphic facies
  differ?
• Metamorphic zones show the gradational
  metamorphic change within a single rock
  composition.
• Metamorphic facies are groups of many different
  rock compositions whose mineral contents all
  indicate common temperature and pressure
  conditions during metamorphism.

25
Plate Tectonics and Metamorphism

• Metamorphism can occur along all types of plate
  boundaries, but is most common and extensive
  along convergent boundaries.

Fig. 7.20, p. 183
26
Plate Tectonics and Metamorphism

• Metamorphic rocks formed near the surface and
  within an oceanic-continental convergent plate
  boundary zone result from low temperature and
  high pressure conditions.
• Blueschist facies

Fig. 7.20, p. 183
27
Plate Tectonics and Metamorphism

• Higher temperatures and pressures existing at
  depth within such plate boundaries produce higher
  grades of metamorphism in a subducting oceanic
  plate.

Fig. 7.20, p. 183
28
Metamorphism and Natural Resources

• Mineral resources which are metamorphic rocks
  include marble and slate.

Slate
Fig. 7.10c, p. 177
Fig. 7.21, p. 184
29
Metamorphism and Natural Resources

• Mineral resources which are metamorphic rocks
  include marble and slate.

Marble
Geo-inSight, pgs. 180-181
30
Metamorphism and Natural Resources

• Mineral resources which are metamorphic minerals
  include graphite, talc, asbestos and garnet.

Metamorphic Minerals
GeoFocus Fig. 1, p. 172 Fig. 7.13 a, p. 178
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End