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ERTH2404
Lecture 4: Metamorphic Rocks
Dr. Jason Mah
Photo:W.Carter,CarletonU.
Shattercones,Sudbury,ON
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Reading assignment
Please read Kehews book to complement the
material presented in this lecture:
Chap. 6;
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Review: Sedimentary rocks
Detrital and Chemical
Detrital rocks classified by particle size
Chemical can be inorganic or biochemical
Sedimentary rocks tell us about the
environment of which they are formed in
Sedimentary structures include bedding,
ripple marks, mud cracks, and fossils
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Lecture objectives
To learn the different types of metamorphism
To understand the relationship between
metamorphic rocks and other rock types
To be able to characterize and identify
metamorphic rocks
To be aware of how the characteristics ofmetamorphic rocks might impact engineering
work
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Remember the rock cycle
Where do
metamorphic
rocks fit in?
Formed from
any type of
pre-existing
rock
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Metamorphism
Metamorphism: transformation of one rock
into another through the action of
Heat
Pressure
Chemically active fluids (metamorphic agents)
Rocks retain their solid form
But they can flow in a plastic-manner
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Metamorphism
Metamorphism may cause many changes
to rock
Formation of larger grains
Reorientation of mineral grains
Texture
banded appearance, sheen
Increase in density
Transformation of existing minerals into minerals
stable in the new T-P conditions
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Metamorphism
Metamorphism occurs in the Earths crust over
hundreds of Ma
In the core of mountain belts
Within stable continental interiors (shields)
often in association with igneous intrusions
They become exposed at the surface when
overlying rocks have been removed by erosion Exposed outcrops are commonly of Precambrian age
due to the long time needed for erosion to occur
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Metamorphism
Metamorphic grade
Certain minerals and textures are diagnostic of the
degree of metamorphism experienced by rocks
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Low-grade
Low TLow P
Mineralogy and
texture changes High-grade
High THigh P
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Metamorphism: Temperature
Most important factor for metamorphism
Drives chemical changes
Recrystallization of existing minerals to new
minerals (Bowens reaction series)
Impacts:
Texture, crystal structure, crystal size
Source:
Magma intrusion, geothermal gradient
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Metamorphism: Temperature
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Geothermal gradient Bowens Reaction Series
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Metamorphism: Pressure
Approximate increase of 1 kbar per depth of 4.4
km
Confining pressure: pressure applied in all
directions
Causes decrease in rock size and an increase in density
Does not fold or deform rock
Differential stress:pressure applied from specificdirection
Creates deformed strata
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Metamorphism: Fluids
Water acts as catalyst during metamorphism
Conduit for ion exchange in growing of crystals
At low to medium T, water is removed from
minerals
At high T, water is driven from the rock
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Types Metamorphism
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Scale Type of metamorphismMetamorphic
agents
Change in
chemical
composition
Regional
Sedimentary rocks only: BurialContact Heat
Dynamic P
Impact P
Hydrothermal Fluids Yes
Regional
Local
Heat and P
No
Regional: large scale, plate tectonics
Local: smaller scale, faults, igneous bodies
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Types Metamorphism
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Contact
(Low P, High T)
Regional
(High P, High T)
Regional
(High P, Low T)
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Regional Metamorphism
Continental crust exposed to extreme
T and pressure at convergent plate boundaries
Enormous thickness of rocks involved
(9-12 km)
Greatest quantity of metamorphic rocks produced
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Regional Metamorphism
Crustal
thickening,
continental
collision
Leads to
foliation,
alignment ofminerals
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18Ref.: ERTH2404 Lab manual
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19Ref.: ERTH2404 Lab manual
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Regional Metamorphism
Low Grade: rock harder, more compact
(interlocking crystals), minor increase in grain
size
Medium Grade: grain size increases, new
mica crystals form
High grade: grains recrystallize with preferred
orientation (foliation)
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Regional Metamorphism
At subduction zones ofthe lithospheric plate
Low T, High P
Cold, slow to heat byconduction, butplunges quickly to greatdepth
Sediments, basalts formhigh-P minerals
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Burial Metamorphism
Subset of regional metamorphism
Associated with thick sedimentary strata
Metamorphism due to burial beneathother rocks
Certain mineral become unstable due to increased T and P,and change to new minerals
Required depth varies from one location to anotherdepending on the geothermal gradient
Clay becomes mica when buried several kms deep
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Continental geotherm
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Ref.: Kehew, A.E. 1995. Geology for Engineers & Environmental
Scientists. 2nd Edition. Fig. 5.2. Shown with permission.
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Local Metamorphism: Contact
Magma invades a hostrock leading to anincrease intemperature Increase in temperature
leads to re-crystallization
Aureole: zone of
alteration, forms in therock surrounding theigneous intrusion
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Local Metamorphism: Contact
Aureole
Extent depends on the size of the intrusion and
the host rock
Sills, dikes cmm wide aureole
Batholiths km wide aureole
Typically fine-grained, dense and tough
No directional alignment of minerals becausepressure is not a major factor
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Local Metamorphism: Dynamic
Pressure is the only metamorphic agent
Dynamic metamorphism is associated with
localized events along fault zones
Rocks are broken and pulverized as rock blocks alongfault zone grind past each other
Fault breccia: loose rock consisting of broken and
crushed rock fragments
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Local Metamorphism: Impact
Occurs when large space bodies strike the
Earths surface
Creates shattered and melted rocks adjacent to
the impact crater
Shatter cone: a conical fragment of host rock that
is formed from the high pressure of meteorite
impact and has striations radiating from the apex 2-30 GPa of pressure required
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Blast shatter cone
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Photo: W. Carter, Carleton U.
Rousell et al. 2003
http://dx.doi.org/10.1016/S0012-8252(02)00091-0
http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-0http://dx.doi.org/10.1016/S0012-8252(02)00091-07/28/2019 ERTH2404 L6 Metamorphic Upload
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Shocked quartz
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Chicxulub crater, Mexico
dia > 180km
Dislocation of crystal orientations due to
shock wave (~58 Gpa)
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Local Metamorphism: Hydrothermal
A form ofmetasomatism: change in thecomposition of a rock or mineral by theaddition or replacement of chemicals
Chemical alteration caused by hot, ion-rich fluids(hydrothermal solutions) that circulate throughcracks
Hydrothermal solutions are expelled from cooling
plutons Invade country rock
Add ions to rock to form new minerals
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Local Metamorphism: Hydrothermal
May generate metallic ore deposits
Copper, gold, molybdenum, tin, iron
Example:
Hydrothermal metamorphism is wide spread
along mid-ocean ridges
Fe, Mg silicates talc, serpentine
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Protolith
Protolith: original rock from which a givenmetamorphic rock was formed
Most metamorphic rocks have the same overallchemical composition as their protolith
Original mineral makeup determines, to large extent,the degree of which each metamorphic agent willcause change
Example: quartz sandstone is the protolith ofquartzite
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Protolith
Different metamorphic minerals can be
produced from the same original rock under
different temperature and pressure conditions
Example: shale slate schist gneiss
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Metamorphic textures
Three metamorphic textures:
Foliated textures
Pressure is the key metamorphic agent
Mostly associated with regional metamorphism Non-foliated texture
Temperature is the key metamorphic agent
Mostly associated with contact metamorphism
Porphyroblastic texture Large crystals growing in a finer matrix
Example: garnet
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Foliation
Foliation: parallel orientation of mineral grains
within a metamorphic rock
Foliation give metamorphic rocks a banded
appearance
Watch out! Foliation resembles the bedding
of sedimentary rocks
Generated by different mechanisms
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Foliation
Foliation can develop in different ways:
1. Rotation of platy minerals under pressure
2. Recrystallization of minerals under pressure
3. Flattening of rounded grains in the direction of
maximum stress
New mineral orientation is perpendicular tothe direction of maximum stress
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Foliation
Rotation of platy minerals
Flattening of rounded grains
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Direction of
maximum stress
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Foliation
Different expressions of foliation
Parallel alignment
Compositional banding
Zones of weakness where rocks can easily split
into thin sheets
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Foliated textures
Slaty cleavage
Closely spaced planar surfaces along which rocks
split
Schistose
Platy mineral grains grow larger when rock is
subjected to higher T and pressure Rock exhibit a scaly appearance
Platy minerals visible to unaided eye
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Foliated textures
Gneissic texture
During higher grades of metamorphism,
ion migration results in segregation of minerals
Distinct banded appearance due to segregation ofdark and light silicate minerals
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Foliated textures
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Metamorphic grade
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Non-Foliated textures
Develop in environments where deformationis minimal
Mostly associated with contact metamorphism
Develop from rocks composed primarily ofone mineral, exhibiting equi-dimensionalcrystals
Examples: Limestone marble
Quartz sandstone quartzite
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Porphyroblasts
Large mineral grains (porphyroblasts)
surrounded by a fine-grained matrix of other
minerals
Grows within the solid state
Examples: garnet, staurolite, andalusite
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Porphyroblasts
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Classification
Metamorphic rocks are classified according to:
Texture
Protolith
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Metamorphism of Shale
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Ref.: ERTH2404 Lab manual
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Slate
Generated from low-grade metamorphism of shale
Very fine grained (minute mica flakes)
Color depends on mineral constituent
Black: organic material
Red: iron oxide Green: chlorite minerals
Excellent cleavage
Tiles
Original bedding may or not be preserved Slaty cleavage may develop at an angle to original bedding
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Slate
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Source: Earth Science Australia
(http://earthsci.org)
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Slate
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Slaty cleavageShale
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Phyllite
Lies between slate and schist
Characteristic rock sheen
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Schist
Generated from intermediate-grademetamorphism of shale Often related to mountain building
Medium to coarse grained rock readily split into
thin scales Platy mica minerals dominate
Often contains accessory minerals unique tometamorphic rocks (e.g. garnet)
Term schist describes the texture To indicate composition, mineral names are used
(e.g. mica schist)
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Schist
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Source: Earth Science Australia
(http://earthsci.org)
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Schist
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Garnet schist
Naming convention applies porphyroblast(s) first If there are more than one, name most abundant first
Muscovite schist
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Gneiss
Generated from high grade metamorphism of
shale or granite
Medium to coarse grained rock with elongated
granular minerals (quartz, feldspar,
hornblende) and a lesser amount of platy
minerals (mica)
Banded appearance
Break in irregular fashion
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Gneiss
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Source: Earth Science Australia
(http://earthsci.org)
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Gneiss
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Marble
Common non-folitated
metamorphic rock
Protolith is limestone or
dolostone
Composed essentially of
calcite or dolomite
Decorative stone
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Quartzite
Common non-folitatedmetamorphic rock
Protolith is quartz
sandstone Quartz grains are fused
together
Very hard rock
Rock splits through quartzgrains, not in betweenoriginal grains
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Engineering considerations
Non-foliated metamorphic rocks have similar
properties as intrusive igneous rocks
Great strength
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Engineering considerations
Foliated metamorphic rocks are susceptible to
failure along specific planes
Orientation of foliation planes is a critical factor in
slope stability Avoid transferring load (from bridges, dams,
building foundations) onto foliated rock mass in a
direction parallel to the foliation
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Engineering considerations
Landslides associated with schist
Generally controlled by orientation of schistosity
Presence of slick minerals (e.g. talc, clay, graphite)
may increase sliding potential
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Failure of St. Francis Dam, CA
Built in 1926 to regulate the Los Angeles
Aqueduct
12 March 1928, the dam suffered from
catastrophic failure
Releasing 45 Billion liters
> 600 people killed from the resultant flood
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Failure of St. Francis Dam, CA
Geological context
Poorly consolidated conglomerate of gravels,
sands and muds held together by clay and
cemented by gypsum Schist with steeply dipping foliation planes
Load of the dam parallel to foliation
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Failure of St. Francis Dam, CA
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Ref.: Kehew, A.E. 1995. Geology for Engineers & Environmental
Scientists. 2nd Edition. Fig. 5.16. Shown with permission.
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Failure of St. Francis Dam, CA
Water seeping from reservoir weakened rocks
beneath the dam
Fault zone acts as a conduit for water
Degradation of weak bounds between mica grainsalong foliation planes in the schist
Water dissolved gypsum cement in the
conglomerate
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Next: Plate Tectonics!!