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Chapter 7- A Surface Veneer:Sediments, Soils, & Sedimentary Rocks

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Biochemical limestone can consist of:

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1. coral moundsand/or calciteshell fragments.

2. quartz sandgrains on abeach.

3. deposits of clay.4. deposits of

plankton shellscomposed of

silica.

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What is the difference between chertand quartz?

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1. Chert is always biologicalin origin, whereas quartzis inorganic.

2. They have differentchemical compositions

3. Quartz is a mineral butchert is not.

4. Quartz can be formedfrom the action of fluidsmoving through rocks, butchert cannot.

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h t t p : / / s t a f f . a i s t . g o .

j p / n o m u r a - k / c o m m o n / S

T R U

C I M A G E S / Q u a r t z . g i f

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Figure 1 A. Chalcedony. Photomicrograph (cross-polarized light; field of view heigth - ca . 4.8 mmacross) of banded chalcedony, from south-centralSaguache County, Colorado. As can be seen, thischalcedony consists of innumerable microscopicgrains, virtually all of which are quartz. As noted inthe discussion, this makeup, which is typical, is thebasis of calling chalcedony a rock -- i.e ., it is amicrogranular, monomineralic rock. (© photo by

Daniel E. Kile)

i

h t t p : / / s t a f f . a i s t . g o .

j p / n o m u r a - k / c o m m o n / S

T R U

C I M A G E S / Q u a r t z . g i f

http://mineralsciences.si.edu/staff/postdocs/gaillou.htm

http://www.cst.cmich.edu/USERS/DIETR1RV/introduction-group.jpg

www.quartzpage.de

If chert is not a mineral, then

what is it?C

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Which arrow indicates the direction of flow?

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1. A

2. B3. C4. D

A

B

C

D

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• Ripples are formed by water flowingover loose sediment

– Asymmetric ripples – Unidirectionalflow.

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Graded beds are bedding layers that “fine upward.”

7-128 Sedimentary structures, continued

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Graded beds are bedding layers that “fine upward.”

• Sediment added as a pulse of turbid water.• Water loses velocity and sediments settle.• Coarsest material settles first, mediumnext, then fine.

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• Repeated pulses ofhigh-energysediment transport

create multiplegraded-bedsequences calledturbidites .

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Bed-surface markings occur after deposition whilesediment is still soft.

• Mudcracks indicate alternatingwet and dry conditions (onland)

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http://www.depauw.edu/acad/geosciences/tcope/SedStruct/HiRes/Mudcracks2.jpg

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Bed-surface markings occur after deposition whilesediment is still soft.

• Scour marks are troughs eroded in soft mud by currentflow.

http://www.depauw.edu/acad/geosciences/tcope/SedStruct/HiRes/SoleMarks.jpg

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https://reader010.{domain}/reader010/html5/0622/5b2bfa36ea989/5b2bfa43b4ecc.jpghttp://fossilreproductions.com/images/DinosaurFootprints.jpg

http://static.guim.co.uk/sys-images/Guardian/Pix/pictures/2009/2/27/1235723159663/A-fossil-footprint-left-b-001.jpg

1.5 Ma

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R.Weller/Cochise College

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http://upload.wikimedia.org/wikipedia/ja/e/ee/Migmatite012.jpg

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Chapter 8Metamorphism: A Process of Change

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Introduction• Metamorphic – Changed

from an original “parent.” – Meta = Change. – Morph = Form or shape.

• Parent rocks are called“protoliths.”

• Metamorphism canoccur to any protolith.

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• Metamorphism occurs in the solid state.• Does not include weathering, diagenesis, melting.

• Metamorphic rocks often look totally unlikeprotoliths.

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Metamorphic rocks have distinctive properties.

• Texture – Intergrown andinterlocking grains.

• Some mineralsoccur only inmetamorphic rocks.

Fossiliferous limestone

Marble

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Metamorphic rocks have distinctive properties.• Foliation – Forces cause minerals to align.

Red mudstone

Garnet gneiss

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Metamorphic processesMetamorphic change is slow and in the solid state.

• Several metamorphic processes may operate atthe same time.

Kyanite

1. Recry s tal l izat ion –

Minerals change sizeand shape.

2. Phase ch ang e – Newminerals form with: – Same chemical

formula. – Different crystal

structure.

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3. Neoc ry st al l izat ion

– Pressure and temperature changes cause originalminerals to become unstable.

– Original minerals decompose in the protolith.

– Ions react to form new minerals.

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M hi i d b f8 10

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Metamorphism is caused by one or more of:• Heat (Temperature – T).• Pressure (P).• Differential stress.• Hydrothermal fluids.

Rocks may be overpr in ted by multiple events.

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Heat (Temperature)• Metamorphism occurs as the result of

heating between 200 oC and 850 oC. – The upper T limit is melting. – varies based upon rock mineral composition and

water content.• Heat energy breaks and reforms atomic

bonds.

• Sources of heat: – The geothermal gradient. – Magmatic intrusions.

– Compression.

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• Mineral stability is highly dependent upon T and P

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• Mineral stability is highly dependent upon T and P.

Andalusite

KyaniteSillimanite

Al2SiO 5

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• Often a result of tectonic forces.• Two kinds of differential stress: Normal and

shear.

Differential Stress: pressure that isnot uniform in all directions

1. Norm al s t ress – operates perpendicular to asurface. – Tension

– Compression

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Diff ti l St

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Differential Stress• Preferred platy mineral alignment is called foliation.

– Foliation imparts a layered or bandedappearance.

– Rocks commonly break parallel to foliationplanes.

• Foliation developsperpendicular tocompression.

How?

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http://earth.boisestate.edu/home/cjnorth/images/extension_fractures_cleavage2.jpg

…but how do grains align?

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Pl t i l llig t d diff ti l t

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Platy mineral allignment under differential stress8 18

Plat mineral allignment nder differential stress

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Platy mineral allignment under differential stress

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Hydrothermal fluids and8-23

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Hydrothermal fluids andmetamorphism

• Pore fluid: water, CO 2, dissolvedminerals

• Pore fluids affect metamorphism by: – Transporting dissolved material

– Acting as chemical reservoirs

– Speeding up chemical reactions

– Adding or subracting elementsPore space

Mineral crystals

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• If there is enough fluid flowingthrough a rock to substantiallychange the overall chemicalcomposition of a rock, the processis referred to as metasommatism .


Metamorphism vs.Metasomatism

Metamorphic Rock Types8-25

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Metamorphic Rock Types• Two major subdivisions: foliated and non-foliated• Foliated rocks have a through-going planar fabric.

– Subjected to differential stress. – Has a significant component of platy minerals. – Classified by composition, grain size, and foliation type.

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• Two major subdivisions: foliated and non-foliated• Non-foliated rocks have no planar fabric evident.

– Crystallized without differential stress. – Comprised of equant minerals only. – Classified by mineral composition.

N f li t d t hi k

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Non-foliated metamorphic rocks

1. Quartzite – Almost pure quartz in composition. – Protolith: quartz sandstone. – Sand grains in the protolith recrystallize and fuse.

Metamorphic Alteration

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Summary of metamorphic rocks8-30

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Summary of metamorphic rocksFoliated Non-foliated

QuartziteMarble

AmphiboliteHornfels

Sedimentary protolith

Igneous protolith

Grade is a measure of metamorphic intensity

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Foliated metamorphic rocks8-33

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Foliated metamorphic rocks2. Phyllite – Fine-grained mica-rich rock with satiny lustre.

– Protolith: slate (shale) – Grade: low – medium – Clay minerals neocrystallize into tiny micas.

slatephyllite


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3. Schist – Fine - coarse rock with larger micas.

– Protolith: phyllite (shale) or other mica-rich rock – Grade: Medium to high – Foliation: schistosity, from alignment of large mica

crystals grown at higher T.

– Schist often has other minerals due to neocrystallization(quartz, feldspars, kyanite, garnet, staurolite, sillimanite)

– Large non-mica minerals are called porphyroblasts .

Foliated metamorphic rocks


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Foliated metamorphic rocks4. Gneiss – distinct banded foliation (compositional banding).

– Light bands of felsic minerals (quartz and feldspars). – Dark bands of mafic minerals (biotite or amphibole).

Summary of metamorphic rocks8-36

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Summary of metamorphic rocksFoliated Non-foliated

QuartziteMarble

AmphiboliteHornfels

Sedimentary protolith

Igneous protolith

Slate

Phyllite

Schist

Gneiss

I n c r e

a s i n

gm

e t am

or ph i c

gr a

d e

I n c r e

a s i n

g gr ai n

s i z

e

Grains notvisible

Visiblegrains

How does compositional banding develop?

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1. Original layering in the protolith.

2. Extensive high T shearing.



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3. Compositional banding – Solid-state chemical

differentiation.

1

2 3

p g p

Migmatite8-39

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Migmatite• Migmatite is a partially melted gneiss.• It has features of igneous and metamorphic rocks.• Mineralogy controls behavior.

– Light-colored (felsic) minerals melt at lower T. – Dark-colored (mafic) minerals melt a higher T.

• Felsics melt first; mafics remain metamorphic.

quartzclay

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Fig. 8.19

muscovitebiotite

garnetKsp

claychlorite

Sillimanite

Staurolite


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• Specific minerals typify particular grades

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• Specific minerals typify particular grades.

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• Metamorphic facies – Mineral assemblage from

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a specific protolith at specific P-T conditions.

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Index

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Indexminerals have a limited

P-T range andrecordmetamorphicgrade.

Fig. 8.21

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Metamorphic types and environments8-47a

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1. Burial metamorphism

2. Regional metamorphism

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• Creates foliated rocks.

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3. Subduction metamorphism

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• Subduction creates theunique blueschist facies.

• Trenches and accretionaryprisms have low T, high P

• P-T condtions produceglaucophane , a blueamphibole mineral.

http://jm-derochette.be/images/Spectrometer/Glaucophane_base%20V_I.jpg

Glaucophane exhibits pleochroismunder polarized light

http://www.dvminerals.com/img2006/G-150.jpg

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4. Contact metamorphism8-50

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slate

andalusiteLow-gradehornfels

Int.-gradehornfels

andalusite &

sillimanite

5. Hydrothermal metamorphism

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• Alteration by hot, chemically aggressive water.• A dominant process near mid-ocean ridge magma.

5. Hydrothermal metamorphism8-52a

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• The hot water rises and isejected via black smokers.

oceanexplorer.noaa.gov

8-52b 5. Hydrothermal metamorphism

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http://z.about.com/d/geology/1/0/h/L/greenschist.jpg

1 43 Ga black smokers from a massive sulfide deposit in China

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http://highlyallochthonous.blogspot.com/2007/01/precambrian-black-smokers.html

1.43 Ga black smokers from a massive sulfide deposit in China

6. Dynamic metamorphism

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• Breakage of rock byshearing at a fault zone.

• Shallow crust (Upper 10-15 km): Brittledeformation forms faultbreccia

http://www.portervillecollege.edu/richardgoode/DeathValleyPicts/Fault%20Breccia.JPG

Fault breccia

6. Dynamic metamorphism

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• Deeper crust (Below 10-15

km.): ductile deformation formsmylonite

http://earth.boisestate.edu/home/cjnorth/images/mylonite.JPG

mylonite

7. Shock metamorphismWhen Earth is str ck b a comet or asteroid

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http://earth.boisestate.edu/home/cjnorth/images/mylonite.JPGhttp://www.portervillecollege.edu/richardgoode/DeathValleyPicts/Fault%20Breccia.JPG

• When Earth is struck by a comet or asteroid,impacts generate a compressional shock wave.

– Extremely high pressure. – Heat that vaporizes or melts large masses of rock.

• These conditions generate high-pressure mineralscoesite and stishovite.

http://www.chiemgau-impact.com/images/intro/2%20shattercone%20Steinheim.jpg

7. Shock metamorphism8-56

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http://gsc.nrcan.gc.ca/mindep/photolib/ni_cu_pge/sudbury/images/fig08.jpg

Shatter cones in quartzite near Sudbury

p

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