GEOL 1110 Lecture 9: Metamorphism and Metamorphic Rocks
3 Jul 2018
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Metamorphic Rocks: Chapter 7
Metamorphism- “change from”
oWhen a rock changed from original form due to temperature or pressure
oChemical or physical characteristics of the rock change
The degree of change in the “parent rock” to a metamorphic rock is dependant
upon the type and intensity of the metamorphic process (heat or pressure or
both)
While rocks may be different than the “parent” rock, they still hold attributes of the
“parent” rock (ex. Marble and Limestone are susceptible to acid erosion)
Composition of Parent Rock
The mineral composition of “parent rock” will determine the mineral composition
oMafic rocks = rich in iron and magnesium
oFelsic = rich in feldspar and quartz
oDifferent parent rocks will produce different rocks
In addition, volatiles (water, etc) can alter the chemical composition
oMay enter (leak) in or out
oWater lowers melting temperatures of minerals
Controlling Factors
The texture and specific mineral makeup (composition) of the rock is dependant
upon:
oParent rock composition
oHeat
oPressure
oChemically-active Fluids
Heat
oHeat drives basic chemical reactions
Causes minerals to transform
Influences mobility and reactive volatiles
oHeat sources:
Intrusive igneous rocks
Rocks brought to depth – Earth’s internal Heat
Pressure
oPressure- Primarily alters physical properties of parent rock
Also influences when some minerals are stable (chemical
composition)
oRocks are put under stress (pressure) which deforms the rock
oTwo types:
Directional
“differential Stress”
oUnequal pressure – deforms body
oAssociated with plate tectonics
find more resources at oneclass.com
find more resources at oneclass.com
o3 types:
Compressional
Tensional
Shear Stress
oBrittle vs. Ductile
Most rocks react in a ductile manner
oDirected pressure causes folding
oOn a microscopic scale
Increases pressure causes foliation
Platy (sheet like elongated mineral grains)
rotate to new orientation
Mineral grains change shape- elongate from
equidimensional shape, gain new alignment
Re-crystallization to form new mineral grains
with new orientation
Confining pressure
Uniform stress
oSqueezing the rock in all directions
oConfines material into smallest volume
oIncreases with depth
oMineral grains crowd together and crystal
arrangements increase in density and hardness
New crystal arrangement, but some similarity
to Parent rock
No internal deformation
Chemically Active Fluids
Water is released when magma solidifies
Clays, mica, amphiboles are hydrated
oUnder pressure dehydration occurs expels water (hot, cation laden)
oTransports ions
oFacilitates re-crystallization
oRocks in magma (pluton) will exchange fluids with surrounding rock –
change to chemical composition – “metasomatism”
Metamorphic Grade
The intensity of metamorphism decides the Grade
oIncreased grade is associated with minerals that remain stable at higher
temperatures and pressures
oSome minerals form and remain stable within a well defined range in
temperature and pressure
IF theres not a lot of pressure, but there is a lot of heat (magma came in contact
with it) Contact Metamorphism
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Metamorphism- change from : when a rock changed from original form due to temperature or pressure, chemical or physical characteristics of the rock change. The degree of change in the parent rock to a metamorphic rock is dependant upon the type and intensity of the metamorphic process (heat or pressure or both) While rocks may be different than the parent rock, they still hold attributes of the. Marble and limestone are susceptible to acid erosion) The mineral composition of parent rock will determine the mineral composition: mafic rocks = rich in iron and magnesium, felsic = rich in feldspar and quartz, different parent rocks will produce different rocks. In addition, volatiles (water, etc) can alter the chemical composition: may enter (leak) in or out, water lowers melting temperatures of minerals. The texture and specific mineral makeup (composition) of the rock is dependant upon: parent rock composition, heat, pressure, chemically-active fluids. Heat: heat drives basic chemical reactions.
