Geology of Plutonic Rocks

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Geology of Plutonic Rocks
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Igneous plutonic rocks

• Formed
• 900 degree C
• 50 km depth
• Uplift to earth surface
• Enormous decrease in confining pressure

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Extrusive
Intrusive or plutonic
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Shield regions

• Sweden is an example
• roots of former mountain ranges,
• stable interior,
• resembles granite but
• complex history
• often formed by extreme metamorphism rather than
  by solidification from a melt. Fig 6.1

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Mountains complex folding
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Mountains worn to flat land

• By the Precambrian

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Magma molten rock within the earth Lava on the
earth
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Geothermal gradient

• varies
• crust thicker in continental areas
• normal rise in temperature with depth of between
  10 to 50 C per km
• crust thinner in oceanic areas

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increased tempurature due to igneous intrusion
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normal rise in temperature with depth of between
10 to 50 C per km
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Question

• Where does magma form?
• In the crust and upper mantle NOT in the center
  of the earth

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Magma
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subduction relation

• crustal rocks subducted melt at a lower
  temperature than do oceanic rocks
• two magma producing events

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1. subduction - water rich ocean plate

• the rise of the moisture through the overlying
  rocks lowers their melting point and initiates
  melting

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2. subduction - heat increases with depth

• the crustal rocks begin to melt and mixes with
  the magma derived from the mantle

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Forms of igneous intrusions

• sheets layer of intrusion
• pluton irregular body
• dikes vertical sheet intrusions
• sills horizontal sheet intrusion
• laccoliths lens shaped
• ring dikes, cone sheets a cone shaped intrusion
• dike swarm several
• pipe of neck source of nourishment of a volcano
  
• batholiths largest body of an intrusion

• stocks smaller intrusive body
• xenoliths country rock mass surrounded by
  intrusive rocks
• roof pendants inliers of metamorphic rocks
• pegmatites coarse grained intrusions
• aplites fine grained intrusions
• stratiform complexes layered
• flow bedding segregation of layers
• lopolith and cone sill mineral deposits

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Forms of igneous intrusions

• pluton irregular body
• dikes vertical sheet intrusions
• sills horizontal sheet intrusion
• laccoliths lens shaped
• ring dikes, cone sheets a cone shaped intrusion
• dike swarm several
• pipe of neck source of nourishment of a volcano
  
• batholiths largest body of an intrusion

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Forms of igneous intrusions

• pluton irregular body
• dikes vertical sheet intrusions
• sills horizontal sheet intrusion
• ring dikes, cone sheets a cone shaped intrusion
• dike swarm several
• pipe of neck source of nourishment of a volcano
  
• batholiths largest body of an intrusion

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Forms of igneous intrusions

• xenoliths country rock mass surrounded by
  intrusive rocks

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Forms of igneous intrusions

• pegmatites coarse grained intrusions
• aplites fine grained intrusions

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Forms of igneous intrusions

• stratiform complexes layered
• flow bedding segregation of layersid
• lopolith and cone sill mineral deposits

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Classification of plutonic rocks Fig 6.6

• Few common minerals their abundance is the
  basis for classification
• Basic or Mafic rocks contain minerals with a
  high melting point and silica content of ca 43
  50
• Acidic or Felsic rocks contain minerals with
  low melting point and silica content of 65 72
• Intermediate have silica contents of 50 to 65

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Texture

• Textures normal slow cooling produces sand size
  interlocking crystalline grains
• Phenocrysts coarser grains
• Porphyry contains numerous coarse grains in an
  otherwise fine grained mass
• Coarse crystalline grains gt 2mm
• Medium crystalline grains 0.06-2mm
• Fine crystalline grains lt 0.06 mm
• Aphanitic crystals not visible
• Phaneritic visible grains

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Texture

• Phenocrysts coarser grains
• Porphyry contains numerous coarse grains
  (phenocrysts) in an otherwise fine grained mass

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Rock names Fig 6.6!!!
intrusive

• Granite
• Diorite
• Gabbro
• Peridotite (ultra basic)
• Dunite (untra basic)

extrusive

• Rhyolite
• Andesite
• Basalt

• Granodiorite

• Syenite

OTHERS?

• Diabas or dolerite

• Monzonite

• Anorthosite

• Porfyr

• Tonolite

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The three components, Q (quartz) A (alkali
(Na-K) feldspar) P (plagioclase)
Phaneritic visible grains
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Serpentinite

• an altered ultra basic, peridotite (olivine) has
  been replaced by the mineral serpentine
• this is a chemical weathering process which is
  associated with a 70 volume increase
• this increase in volume results often in the
  internal deformation of the rock fracturing and
  shearing

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jointing in granitic rocks

• arise from general crustal strain, cooling, and
  unloading

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Sheet joints

• typical for igneous rocks, called also
  exfoliation joints or lift joint
• no sheet joints below 60 m
• Sheet joints conform to the topography, fig
  6.12a, 6.10a
• slopes steeper than the angle of friction, ca 35
  degrees, tensile fractures develop and wall arch,
  an overhang
• sheet jointing is well developed in igneous
  rocks, but not exclusive, it also occurs in soils
  and other rocks to some extent

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Sheet weathering due to unconfinement

• Formed
• 900 degree C
• 50 km depth
• Uplift to earth surface
• Enormous decrease in confining pressure

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Joints due to relaxation
two to thee preferred directions of joints is
common, joint set
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Question

• ??Why is sheet jointing more prominent in igneous
  rocks than other rocks?
• Unloading is one of the main reasons.
• Igneous rocks are formed at up to 50 km depth.
  With 27Mpa/Km times 50 km 1350 MPa pressure at
  the time of formation uni directional!! Upon
  uplift this pressure is reduced and the rocks
  relax, with a vertical unload stress of 27 MPa.

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unloading

• unloading in tunnels different names for
  different rocks for igneous rocks it is called
• Popping rock - is a term used in underground
  operations where the rock pops off the rock face.
  This can be very violent and is due to the
  unloading due to the underground excavation

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weathering in plutonic rocks

• physical weathering mechanical breakdown of
  earth material at the earth surface. Ex.
  Heating/cooling, wetting/drying, plants and
  animals including man.
• chemical weathering chemical decomposition due
  to a chemical reaction changing the composition
  of the earth material, ex carbonic acid replacing
  silicate minerals, feldspar changing to kaolin,
  mica changing to limonite and kaolin.

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chemical weathering

• acts on igneous minerals in the order of
  solidification
• Bowens reaction series (fig 6.6)
• high temperature minerals are more rapidly
  affected
• low temperature minerals more stable

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chemical weathering

• Basic and ultrabasic form montmorillonite clays
• Grainitic rocks form kaolinites

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Weathering profiles

• form relative rapidly in granitic rocks
• a layer of clay minerals forms at the surface
• by the continuous downward percolation of water
  and carbon dioxide
• in the vadose zone above the water table

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Spheroidal weathering

• common in jointed igneous rocks where the
• percolation of water is concentrated to the
  joints
• the fresh rock delineated by the fractures is
  slowly effected but
• the corners are more rapidly effected thus
  spherical shapes are formed

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Spheroidal weathering

• common in jointed igneous rocks where the
• percolation of water is concentrated to the
  joints
• the fresh rock delineated by the fractures is
  slowly effected but
• the corners are more rapidly effected thus
  spherical shapes are formed

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Joints enhance weathering

• Paleozoic Sweden was near the equator
• Rounded rock mass due to weathering

Exfoliation is formed in the spheres by
chemical expansion in the weathering granite
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• Rounded blocks due to chemical weathering
• Open joints

It is clear that this is granite by the way it
weathers
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Saprolite

• decomposed granite, residual material formed from
  weathering resulting in a residual soil

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Description of a residual soil is fuzzy

• two variables
• I. the degree of weathering of the rock
• II. the abundance of altered minerals

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Classes of weathering of igneous rocks

• Several different classification systems
• Different authors

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All contain several classes

• in this case 6 classes
• I fresh (f)
• II slightly weathered (sw)
• III moderately weathered (mw)
• IV highly weathered (hw)
• V completely weathered (cw)
• VI residual soil (rs)

Hong Kong zones of weathering p. 225, zones A
(residual soil), B, C, D and Fresh rock Profile
development in Hong Kong figures 6.18 1-4, 6.19
a-f!
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All contain several classes

• in this case 6 classes
• I fresh (f)
• II slightly weathered (sw)
• III moderately weathered (mw)
• IV highly weathered (hw)
• V completely weathered (cw)
• VI residual soil (rs)

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All contain several classes

• in this case 6 classes
• I fresh (f)
• II slightly weathered (sw)
• III moderately weathered (mw)
• IV highly weathered (hw)
• V completely weathered (cw)
• VI residual soil (rs)

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Chemically weathered granite
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All contain several classes
Granite weathers to a sandy soil
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Rock Quality some tests

• Index tests give information about the rock
  fresh or weathered and to what degree
• Porosity
• Bulk density
• Compressibility
• Tensile strength
• Elastic constants
• Point load test

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Rock Quality some tests

• Fluid adsorption, classes 1-4
• Almost impermeable
• Slightly permeable
• Moderately permeable
• Highly permeable

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Rock Quality some tests

• Slake behavior - degree of disintegration of 40
  to 50 grams of specimen after 5-min immersion in
  water
• Class 1 no change
• Class 2 less than half
• Class 3 more than half
• Class 4 total disintegration

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Effect of climate and rock type on weathering

• Precipitation/evaporation ratio is important
• Weinert - N value is a weathering index
• Nlt5, chemical weathering is favored over
  mechanical decomposition is the predominate
  process
• Ngt5, mechanical weathering is favored over
  chemical decomposition is predominate

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Effect of climate and rock type on weathering

• Weathering of basic and ultrabasic rocks
• N gt 2, montmorillonite
• N between 1-2, kaolinite

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Effect of climate and rock type on weathering

• Extreme tropical climates laterite soils are
  produced
• where all silica is removed and
• some clay minerals replaced by iron, aluminum,
  and magnesioum oxided and hydroxides

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Engineering properties

• plutonic rocks

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exploration

• weather profile nature extent of rock and soil
  cover
• hazards of boulders
• hazard of soil flow
• slides of serpentine
• sheet slides
• rock falls

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excavation

• core stones
• size
• drilling can divert along joints

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foundations

• hardness and soundness
• core stones differential support
• driving piles difficult in weathered material
• collapsing residual soil
• disposal of water in weathered terrain, erosion
  susceptible

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dams

• earth fill dams can be placed on soil profiles of
  I-IV possible V
• concrete dams can be placed on sound rock and
  possible zones I and II
• Permeability a problem in weathered zones
• Permeability between sheets common
• Serpentine is not suitable for any dam
  construction

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underground works

• weathering down to 60 m (500 m)
• variable hardness difficult
• popping rock danger
• diabase dikes act often as subsurface dams
  water can be a problem upon penetration
• serpentine dangerous

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ground water

• fault zones
• weathered granite

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case histories
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mammoth pool dam sheeted granodiorite

• San Joaquin River, California

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mammoth pool dam sheeted granodiorite

• San Joaquin River, California
• biotite granodiorite
• weathering depth 30 m
• saprolite used as aggregate for a 100 m high dam
  without clay core

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mammoth pool dam sheeted granodiorite

• surface covered with core stones
• largest was a sheet of granite, 5000 m3,
• valley filled with alluvial sediments with
  maximum depth of 30 m

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mammoth pool dam sheeted granodiorite
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mammoth pool dam sheeted granodiorite

• bedrock contained numerous joints
• open or partly filled with alluvial sand and
  weathered debris
• bedrock grouted downward 5 m to reduce
  compressibility of the open fissures and joints
• grout curtain down to 15 m below the foundation
  and 12 m into the abutments

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mammoth pool dam sheeted granodiorite

• grouting
• must go slow
• at low pressures
• some sheets are bolted prior to grouting
• otherwise uplift of sheet joints

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mammoth pool dam sheeted granodiorite

• grouting
• estimated 5 000 sacks
• required 42 000 sacks
• why aperture of joints very large one as wide
  as 40 cm!
• NOTE apertures of 100 cm not uncommon in Sweden

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mammoth pool dam sheeted granodiorite

• rock bolts installed to stabilize sheets
• drainage holes were made to insure that low water
  pressures would be maintained between sheets
  after the dam was filled
• 15 m, 5º from horizontal, into the sheets to
  intercept all possible open sheet joints

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Malaysian granite hydroelectric project

• Porphyritic granite with 35 quartz and 5
  biotite
• hairline fractures
• occasional shear zone healed with calcite,
  chlorite or quartz

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Malaysian granite hydroelectric project

• Shear zones and mylonite and brecciated granite

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Malaysian granite hydroelectric project

• surface outcrops minimal due to jungle vegetation
• Lineaments visible on aerial photographs
  suggested faults and shear zones
• 67 drill holes

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Malaysian granite hydroelectric project

• Tunneling was the biggest problem with weathered
  zones and faults
• weathering average 30 m
• but also in the tunnel at 300 m
• residual soil was up to 6 m thick

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Malaysian granite hydroelectric project

• grade VI material in weathered profile had a clay
  content of 20
• grade V was sand with less than 10 clay
• Grade V1 material used to form a core
• Grade V formed the shells

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Malaysian granite hydroelectric project

• shear zones at 250 m depth contained 7 to 22 cm
  thick layers of grade IV and V weathered grainite
• at 450 m depth in the tunnel slabbing occurred in
  the walls
• erosion was a problem in weathered granite
• divert the tunnel to a different direction to
  avid problem zones and faults zones

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Question

• Can decomposed granite furnish satisfactory
  materials for concrete aggregate?

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Question

• How can it be determined that a borehole through
  soil and saprolite extending into unweathered
  rock has not actually bottomed in a core stone?

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Question

• A granitic pluton is not bedded in the sense that
  a sedimentary rock is bedded. How then could a
  conspicuous fracture be identified definitively
  as a fault?

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Question

• Granitic core stones are well developed in Hong
  Kong whereas granitic rocks of Korea generally
  lack then. How is this possible?