Transcript Metamorphism

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 earth’s
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.
Pressure
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.
 Confining
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Differential 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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 Stress
Foliation
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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
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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
 Shock
Metamorphism
Metamorphism
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Contact Metamorphism
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.
 Contact
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Regional Metamorphism
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.
 Regional
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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 midoceanic 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
– Metamorphism coupled with
th introduction of ions from an external source.
 Metasomatism
 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 earth’s 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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