08 Basic Petroleum Geology Chapter 8 Metamorphic Rocks
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Transcript of 08 Basic Petroleum Geology Chapter 8 Metamorphic Rocks
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Metamorphic Rocks
Metamorphic rocks can form from igneous rocks sedimentary
rocks or even from other metamorphic rocks
The agents of metamorphism are heat, pressure (stress), and
chemically active fluids
Perhaps the most important agent of metamorphism is heat,
because it provides the energy to drive chemical reactions.
Rocks found near the surface of Earth may be subjected to
intense heat when they are intruded by magma, a process
called contact metamorphism.
gneiss
slate
Mica schist
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Here the adjacent host rock is baked by the emplaced
magma.
Rocks may also be subjected to high temperatures if they are
carried deep within Earth.
Pressure, like temperature, also increases with depth.
Buried rocks are subjected to pressure, or stress, exerted by the
load above.
This confining pressure is applied equally in all directions and
causes a reduction in volume, thereby generating a more
compact (dense) rock.
Rocks are also subjected to forces during mountain building that
are unequal in different directions, differential stresses.
Chemically active fluids, most commonly hot water containing
ions in solution, also enhance the metamorphic process.
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For example, along oceanic ridges seawater circulates through
hot basaltic rocks, transforming the existing iron- rich minerals
into metamorphic minerals such as serpentine and talc.
Textural and Mineralogical Changes
The degree of metamorphism is reflected in the rocks texture
and/or mineral composition.
When rocks are subjected to low-grade metamorphism, they
become more compact and thus more dense.
Under more extreme pressures and temperatures, some
minerals recrystallize to form larger crystals.
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During metamorphism the crystals o some minerals, such as
micas which have a sheet structure, and hornblende which has
an elongated structure, will recrystallize with a preferred
orientation.
The new orientation will be essentially perpendicular to the
direction of stress.
The resulting mineral alignment usually gives the rock a layered
or banded appearance termed foliation.
Not all metamorphic rocks have a foliated texture. Metamorphic
rocks composed of only one mineral that forms
equidimensional crystals are generally non-foliated.
In some environments, new minerals are formed during the
metamorphic process. For example, rocks adjacent to a large
magma body would be altered by ion-rich hydrothermal
solutions released during the latter stages of crystallization.