Prepared by Iggy Isiorho for

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Chapter 7

• Prepared by Iggy Isiorho for
• Dr. Isiorho
• Metamorphism
• Index ?

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Metamorphism

• Metamorphism The transformation of preexisting
  rock new rock as a result of high temperature,
  high pressure, or both, but without the rock
  melting in the process.
• Metamorphic rock A rock produced by
  metamorphism.
• Parent rock Original rock before being
  metamorphosed.
• Ductile Capable of being molded and bent under
  stress.
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Factors of Metamorphic Rocks

• Composition of the Parent Rock
• Temperature Foliation
• Pressure Fluids
• Differential Stress Time
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Composition of the Parent Rock

• Usually no new elements or chemical compounds are
  added to the rock during metamorphism, except
  perhaps water. Therefore, the mineral content of
  the metamorphic rock is controlled by the
  chemical composition of the parent rock.
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Temperature

• Heat, necessary for metamorphic reactions, comes
  primarily from the outward flow of geothermal
  energy from the earths deep interior. The deeper
  a rock is beneath the surface, the hotter it will
  be. The particular temperature for rock at a
  given depth depends on the local geothermal
  gradient.
• A mineral is said to be stable if, given enough
  time, it does not react with another substance or
  convert to a new mineral or substance. Any
  mineral is stable only within a given temperature
  range.
• Minerals stable at higher temperatures tend to be
  less dense (or have a lower specific gravity)
  than chemically identical minerals stable at
  lower temperatures.
• The upper limit on temperature in metamorphism
  overlaps the temperature of partial melting of a
  rock. If partial melting takes place, the
  component that melts becomes magma the solid
  residue remains a metamorphic rock.
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Pressure

• Confining pressure Pressure applied equally on
  all surfaces of a body also called geostatic or
  lithostatic pressure.
• Any new mineral that has crystallized under
  high-pressure conditions tends to occupy less
  space than did the mineral or minerals from which
  it formed.
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Differential Stress

• Stress A force acting on a body, or rock unit,
  that tends to change the size or shape of that
  body, or rock unit. Force per unit area within a
  body.
• Differential stress When pressures on a body
  are not of equal strength in all directions.
• Compressive stress A stress due to a force
  pushing together on a body.
• Shearing Movement in which parts of a body
  slide relative to one another and parallel to the
  forces being exerted.
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Foliation

• Foliation Parallel alignment of textural and
  structural features.
• If the rock splits easily along nearly flat and
  parallel planes, indicating that preexisting,
  microscopic, platy minerals were pushed into
  alignment during metamorphism, we say the rock is
  slaty, or that it possesses slaty cleavage.
• If visible platy or needle-shaped minerals have
  grown essentially parallel to a plane due to
  differential stress, the rock is schistose If the
  rock became very ductile and the new minerals
  separated into distinct (light and dark) layers
  or lenses, the rock has a layered or gneissic
  texture.
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Fluids

• Hot water (as vapor) is the most important fluid
  involved in metamorphic processes, although other
  gases, such as carbon dioxide, sometimes play a
  role. The water may have been trapped in a parent
  sedimentary rock or given off by a cooling
  pluton.
• Water is thought to help trigger metamorphic
  chemical reactions. Water is a sort of intra-rock
  rapid transit for ions from one mineral, and then
  carries these ions elsewhere in the rock where
  they can react with the ions of a second mineral
  the new mineral that forms is stable under the
  existing conditions.
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Time

• The effect of time on metamorphism is hard to
  comprehend. Most metamorphic rocks are composed
  predominately of silicate minerals, and silicate
  compounds are notorious for their sluggish
  chemical reaction rates.
• Many laboratory attempts to duplicate metamorphic
  reactions believed to occur in nature have been
  frustrated by the time element. The several
  million years during which a particular
  combination of temperature and pressure may have
  prevailed in nature are impossible to duplicate.
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Classification of Metamorphic Rocks

• The kind of metamorphic rock that forms is
  determined by the metamorphic environment
  (primarily the particular combination of
  pressure, stress, and temperature) and by the
  chemical constituents of the parent rock. How do
  we classify the different types of metamorphic
  rocks?
• First consider the texture of a metamorphic rock.
  Is it foliated or nonfoliated If the rock is
  nonfoliated, it is named on the basis of its
  composition.
• If the rock is foliated, you must determine the
  type of foliation. For example, a schistose rock
  is called a schist. But this name tells us
  nothing about what minerals are in this rock so
  we add adjectives to describe the composition.
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Types of Metamorphism

• Contact Metamorphism
• Regional Metamorphism
• Shock Metamorphism
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Contact Metamorphism

• Contact metamorphism Metamorphism under
  conditions in which high temperature is the
  dominant factor.
• Confining pressure may influence which minerals
  crystallize. However, the confining pressure is
  usually relatively low.
• The zone of contact metamorphism (also called an
  aureole) is usually quite narrow generally from
  1 to 100 meters wide.
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Regional Metamorphism

• Regional Metamorphism Metamorphism that takes
  place at considerable depth underground.
• Depending on the pressure and temperature
  conditions during metamorphism, a particular
  parent rock may recrystallize into one of several
  metamorphic rock.
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Plate Tectonics and Metamorphism

• To demonstrate the relationship between regional
  metamorphism and plate tectonics, we will look at
  what is believed to take place at a convergent
  boundary in which oceanic lithosphere is
  subducted beneath continental lithosphere.
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Hydrothermal Processes

• Rocks that have precipitated from hot water or
  have been altered by hot water passing through
  are hard to classify. As described earlier, hot
  water is involved to some extent in most
  metamorphic processes. Beyond metamorphism, hot
  water also plays an important role creating new
  rocks and minerals. These form entirely by
  precipitation of ions derived from hydrothermal
  solutions. Hydrothermal minerals can form in void
  spaces or between the grains of a host rock. An
  aggregate of hydrothermal minerals, a
  hydrothermal rock, may crystallize within a
  preexisting fracture in a rock to form a
  hydrothermal vein.
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Hydrothermal Activity at Divergent Plate
Boundaries

• Hydrothermal processes are particularly important
  at mid-oceanic ridges (which are also divergent
  plate boundaries). As show in Fig. 7.17, cold
  seawater moves downward through cracks in the
  basaltic crust and is cycled upward by heat from
  magma beneath the ridge crest. Very hot water
  returns to the ocean at submarine hot springs.
  Hot water traveling through the bassalt, gabbro,
  and ultramafic rocks of the oceanic lithopshere
  helps metamorphose these rocks while they are
  close to the diverging boundary.
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Metasomatism

• Metasomatism Metamorphism coupled with th
  introduction of ions from an external source.
• If metasomatism is associated with contact
  metamorphism, the ions are introduced from a
  cooling magma. Some important commercially mined
  deposits of metals such as iron, tungsten,
  copper, lead, zinc, and silver are attributed to
  metasomatism.
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Sources of Water

• Where does the water come from? The following is
  a logical explanation. Ground water seeps
  downward from the earths surface through pores
  and fractures in rocks however, the depth to
  which surface-derived water can penetrate is
  quite limited.
• Plate tectonics can account for water at deeper
  levels in the lithosphere as seawater trapped in
  the oceanic crust can be carried to considerable
  depths through subduction
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