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Summary Chapter 8 Metamorphism: A Process of Change
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Summary Chapter 8 Metamorphism: A Process of Change Marshak: Earth portrait of a Planet Systeem Aarde 1
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Chapter 8 Metamorphism: a process of change8.1 Introduction
Metamorphic rock: one that forms when a pre-existnn roock or protolithk undernoes a solid-state
ohanne in response to the modifoaton of its environment at depth in the Earth. This prooess of
ohanne is oalled metamorphism.
So it has to be solid. By ohanne is meant that it produoes new minerals that did not ooour in the
protolithk and/or produoes a new texture that is distnot of that in the protolith. By modifoaton of
environment is meant rise or fall in temperature/pressure (hinher than those needed for dianenesis) k
undernoes oompression and sheark or reaots with hydrothermal fuids. ressurek temperature and
fuids defne the metamorphio oonditons.
8.2 Consequences and causes of metamorphism
What is a metamorphic rock?
They oan possess metamorphic minerals: new minerals that nrow in plaoe within the solid tooc only
under metamorphio oonditons. t oan produoe a nroup oalled metamorphic mineral assemblage.
They oan also have a metamorphic texture: defned by arrannement of mineral nrains. The texture
oan be manifested by metamorphic foliaton: oharaoterized by the parallel alinnment of platy
minerals and/or the presenoe of alternatnn linht-oolored and darc-oolored layers.
The formaton of metamorphio texture and minerals is very slowly. The most oommon prooesses:
Recrystallizaton: ohannes shapek size of nrains but without ohanninn the identty of the
mineral macinn up the nrains.
Phase change: transforms one mineral into another with the same oompositon but diferent
orystal struoture. (rearrannement of atoms)
Metamorphic reacton or neo-crystallizaton: nrowth of new minerals that difer from those
of protolith. Chemioal reaotons dinest those of the protolithk for this to happen atoms difuse
(minrate) trounh solid orystalsk and/or dissolve and re-preoipitate at nrain boundaries.
Pressure soluton: wet rooc is squeezed unevenly. Mineral nrains dissolve where their
surfaoes are pressed anainst other nrainsk produoinn ions that minrate throunh water and
preoipitate elsewhere. Happens were they are less stronnly squeezed. Grains shorter at one
endk nrowth happens at other end. Only taces plaoe at plaoes were liquid water oan exist.
Plastc deformaton: happens when a rooc is squeezed or sheared at elevated temperatures
and pressuresk oonditons durinn whioh minerals behave lice sof plasto and oan ohanne
without breacinn.
Metamorphism due to heating
t taces plaoe at temperatures between whioh dianenesis ooours ( whioh modifes the rooc without
produoinn metamorphio minerals and textures). When the temperatures rise the atoms start to
vibratek when it is hot enounh the vibraton oan oause the bonds between atoms to breack and they
start shifinn whioh oan eventually lead to new bonds beinn made with other atoms. Repetton oan
lead to rearrannement of atoms within nrainsk or minraton of atoms into and out nrains that
reorystallizaton or neo-orystallizaton taces plaoe. New metamorphio mineral assemblane nrows.
Meltnn temperature depends on oompositon and water oontent and therefore the upper limit
rannes between 650ᵒ and 1200ᵒC. in the upper part of averane oontnental orustk away from
, intrusions the neothermal nradient is about 25ᵒC/cm and a temperature of 500ᵒC ooours at 20 cm
depth.
Metamorphism due to pressure
Pressure: foroe per unit areak or the “push” aotnn on a material in oases where the push is the same
in all direotons.
t oan oause a material to oollapse inward. f a relatvely open orystal struoture undernoes a lot of
pressure it paocs more olosely and denser material tends to form. nvolve phase ohanne and/or neo-
orystallizaton. The ones ooourrinn in outorops on oontnents were formed between 3-12 cbar. (300
bars/cm) weinht of overlyinn rooc. So ooour between 10-40 cm. few looatons we fnd ltra-hinh
pressure metamorphio roocs. Formed deeper in the earthk under hinher pressure. Grains of Coesitek
phase of SiO2 muoh denser than quartz.
Changing both pressure and temperature
There is a relatonship between mineral stability to pressure and temperature. t ohannes tonetherk
so roocs found at 8 cm depth are diferent from those formed at 20 cm. phase dianram/ stability
feld. The presenoe of one of these polymorphs is a olue to the pressure and temperature at whioh
metamorphism ooourred.
Compression, shear and development of preferred orientation
Differental stress: if a material is squeezed (or stretohed) unequally from diferent sides. (= not
pressure). So the push/pull in one direoton difers from push/pull in another direoton. 2 cinds:
1) Normal stress: pushes/pulls perpendioular to the surfaoe. ush= oompression(faaens) pull=
tension(stretohes).
2) Shear stress: moves one part sideways relatvely to another.
Compression and shear at metamorphio temperatures may oause it to ohanne shape. t develops a
preferred orientaton: in the panoace-shaped (platy) nrains beoome rounhly parallel with one
another. n the oinar-shaped (elonnate) nrains beoome alinned with one another. Both examples of
inequant grains that have diferent dimensions in diferent direotons. Equant grains: same
dimension in all direotons.READ EXAM LES
The role of hydrothermal fuids
Are very hot water solutonsk whioh oan aotually oontain hot waterk steam k and so-oalled superoritoal
fuid ( a substanoe that forms under hinh temperatures and pressures and has oharaoteristos of both
liquid and nas; it oan permeate roock seepinn into every oonoeivable openinn). Chemioally reaot with
rooc by dissolvinnk transportnnk and providinn ions and also by providinn water moleoules that oan
beoome inoorporated in minerals.
Minerals dissolved in the fuids preoipitate in oraocs so that the oraocs fll with new minerals.
Commonly oonsist of milcy-white quartz. Metasomatsm: when the fuid taces ions of one element
and then drops them of at another elementk ohanninn the overall ohemioal oompositon.
8.3 Types of metamorphic rocks
Two types foliated roocs and non-foliated roocs.
Foliated metamorphic rocks
Foliaton: an assemblane of parallel planar surfaoes and/or layers in a metamorphio rooc. Can nive
the rooc a striped or streaced appearanoek and/or nive them the ability to split into thin sheets. Has
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