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Engineering Geology
Engineering Geology is backbone of civil engineering
Rocks
6. Metamorphic Rocks
Eng. Iqbal Marie
Metamorphism
occurs when any previously existing rock, the parent rock ( protolith ) , is
buried in the earth under layers of other rock, at high temperature and
pressure to produce changes in texture and crystallization.
Metamorphic rock looks quite different from the original rock.
Metamorphic textures and minerals are most likely formed over 10 to 20
million years or longer
Metamorphic rocks are produced from
– Igneous rocks
– Sedimentary rocks
– Other metamorphic rocks
In most cases, the overall chemistry of the metamorphic rock is very
similar to that of the parent rock
Metamorphic Environments
There are three types of metamorphism
• Contact
• Regional
• Dynamic
• Contact or thermal metamorphism
• Result from a rise in temperature when magma invades a
host rock. Occurs at high temperature and (typically) low
pressure. Normally affects a small area.
• Regional metamorphism
• Affects a large area and produces the greatest quantity of
metamorphic rock. Associated with mountain building
• High temperature, high pressure and shear stress.
Fault zone metamorphic rocks
are formed as a consequence
of changes that happens
along fault lines within the
Earth's crust.
• Hydrothermal metamorphism
• Chemical alteration caused when hot, ion-rich
fluids, called hydrothermal solutions, circulate
through fissures and cracks that develop in rock
Factors Controlling Metamorphism
• Heat
• Most important agent
• Two sources of heat
– Contact metamorphism – heat from magma
– An increase in temperature with depth due to the geothermal gradient
15-30oC increase per km
•
Pressure and differential stress
• Confining pressure applies forces equally in all directions; increases with depth
• Rocks may also be subjected to differential stress which is unequal in different
directions
•
Chemically active fluids
• Mainly water with other volatile components
• Aids in re-crystallization of existing minerals
• Sources of fluids
– Pore spaces of sedimentary rocks
– Fractures in igneous rocks
– Hydrated minerals such as clays and micas
If the same protoliths experience different conditions of temperature and
pressure, they will yield different metamorphic rocks
Heat
Greenschist
Basalt
Pressure
Eclogite
Metamorphic Grade: intensity of Metamorphism
Metamorphism ends when melting begins
Melting begins at ~700oC
High-grade (extreme)
metamorphism:
radical changes in
texture and/or mineral
composition of the rock
High
above 550 OC
Low
Low-grade (mild)
metamorphism: small changes
in texture and/or mineralogy of
parent rock (200 to 350 OC )
Intermediate
350 to 550 OC
CHANGES DURING METAMORPHISM?
•
Rock texture changes
• Foliation: Parallel alignment of platy or elongate mineral
grains (mica/amphibole) in a rock caused by directed stress.
– slaty cleavage: parallel alignment of microscopic platy
minerals (mainly mica). LOW-GRADE METAMORPHISM
– phyllitic texture: parallel, but wavy, foliation of fine-grained
platy minerals (mainly mica and chlorite) exhibiting a shiny or
glossy luster. LOW-GRADE METAMORPHISM
– schistosity: parallel to sub-parallel foliation of medium to
coarse-grained platy minerals. INTERMEDIATE TO HIGHGRADE
METAMORPHISM
– gneissic layering: discontinuous light and dark layering due to
mineral segregation. INTERMEDIATE TO HIGH-GRADE
METAMORPHISM
•
Mineralogy changes
– New minerals form that are stable under the new metamorphic
conditions
Foliation. Due to compressive stress and differential stress and/or shearing
forces the mineral grains in a metamorphic rock form parallel layers or bands.
New metamorphic minerals crystallize along this foliation
Foliation usually formed planes of weakness in metamorphic rock because the
rock can be easily break along the foliation planes.
Kinds of foliation
Slaty cleavage
Schistosity
Gneissic banding
Some main points about metamorphic rocks
-The rocks which do not have planar patterns of strain or stretching
are termed as non-foliated metamorphic rocks. These rocks are most
often deduced from single mineral sedimentary rocks.
- High temperature and pressure erases out the fossils of the
metamorphic rocks.
- Metamorphic rocks do not have pores or openings, and may be
accompanied with visible layers of crystals.
Common metamorphic rocks
Foliated Metamorphic Rocks (Regional
Metamorphism)
•
Gneiss – highest grade of metamorphism,
coarse grained; generally banded
(segregation bands of light and darkcoloured minerals);
•
Schist – medium grade of metamorphism;
sand size, schistocity cleavage.
•
Phyllite – low grade of metamorphism; fine
grained (silt-sized);
•
Slate – lowest grade of metamorphism; very
fine-grained; mud-sized; smooth surface.
Phyllite
slate
shale
Schist
Gneiss
GNEISS
Medium- to coarse-grained
Often composed of white or lightcolored feldspar-rich layers with
bands of dark ferromagnesian
minerals
Non-Foliated Metamorphic Rocks
(Contact/Thermal Metamorphism)
•
Granulite (granite / acid igneous rocks
origin)
•
Amphibolite (basic igneous rocks origin)-
basalt ( Amphipole )
•
Quarzite (sandstone origin)
•
Marble (limestone origin)
•
Hornfels (mudstone/shale origin)
Recrystallization – minerals grow and develop an interlocking
texture
Marble :Coarse, crystalline
Quartzite
Formed from a parent rock of
quartz-rich sandstone Quartz
grains are fused together
•
•
•
Parent rock was limestone or dolostone
Used as a decorative and monument stone
Exhibits a variety of colors
Importance of Metamorphism
valuable mineral and rock resources
A. Building material:
– Marble which is used for ornamental building stone.
– Slate which is used for roofing, flooring, billiard/pool tables, and blackboards
B. Economic metamorphic minerals include:
– Graphite used in pencils and lubricants.
– Garnet and Corundum used as gemstones and abrasives.
– Asbestos formerly used as a heat insulator.
– Kyanite, Andalusite, Sillimanite (aluminum silicates) are used a raw material in
the ceramics industry.
C. Ore Deposits - result from contact metamorphism where hydrothermal
solutions precipitate ore minerals in surrounding rocks:
– Iron and Tin Oxides deposits (hematite, magnetite, and cassiterite)
– Precious metal deposits (gold)