Geology of plutonic rocks
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Geology of Plutonic Rocks. Igneous plutonic rocks. Formed – 900 degree C 50 km depth Uplift to earth surface Enormous decrease in confining pressure . Extrusive. Intrusive or plutonic. Shield regions. Sweden is an example roots of former mountain ranges, stable interior,

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Geology of Plutonic Rocks

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Geology of Plutonic Rocks


Igneous plutonic rocks

  • Formed –

    • 900 degree C

    • 50 km depth

  • Uplift to earth surface

  • Enormous decrease in confining pressure


Extrusive

Intrusive or

plutonic


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


Mountains – complex folding


Mountains worn to flat land

  • By the Precambrian –


Magma molten rockwithin the earth Lava on the earth


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


increased tempurature due

to igneous intrusion


normal rise in temperature with depth of between 10 to 50 C per km


Question

  • Where does magma form?

  • In the crust and upper mantle NOT in the center of the earth


Magma


subduction relation

  • crustal rocks subducted melt at a lower temperature than do oceanic rocks

    • two magma producing events


1. subduction - water rich ocean plate

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


2. subduction - heat increases with depth

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


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

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

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

Forms of igneous intrusions


xenoliths – country rock mass surrounded by intrusive rocks

Forms of igneous intrusions


pegmatites – coarse grained intrusions

aplites – fine grained intrusions

Forms of igneous intrusions


stratiform complexes – layered

flow bedding – segregation of layersid

lopolith and cone sill – mineral deposits

Forms of igneous intrusions


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%


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 > 2mm

  • Medium crystalline – grains 0.06-2mm

  • Fine crystalline – grains < 0.06 mm

  • Aphanitic – crystals not visible

  • Phaneritic –visible grains


Texture

  • Phenocrysts – coarser grains

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


Granite

Diorite

Gabbro

Peridotite (ultra basic)

Dunite (untra basic)

Rock names Fig 6.6!!!

intrusive

extrusive

  • Rhyolite

  • Andesite

  • Basalt

  • Granodiorite

  • Syenite

OTHERS?

  • Diabas or dolerite

  • Monzonite

  • Porfyr

  • Anorthosite

  • Tonolite


The three components,

Q (quartz) +

A (alkali (Na-K) feldspar) +

P (plagioclase)

Phaneritic – visible grains


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


jointing in granitic rocks

  • arise from general crustal strain, cooling, and unloading


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


Sheet weathering due to unconfinement

  • Formed –

    • 900 degree C

    • 50 km depth

  • Uplift to earth surface

  • Enormous decrease in confining pressure


Joints due to relaxation

two to thee preferred directions of joints is common, joint set


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.


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


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.


chemical weathering –

  • acts on igneous minerals in the order of solidification

  • Bowen’s reaction series (fig 6.6)

  • high temperature minerals are more rapidly affected

  • low temperature minerals more stable


chemical weathering –

  • Basic and ultrabasic – form montmorillonite clays

  • Grainitic rocks – form kaolinites


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


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


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


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


  • Rounded blocks due to chemical weathering

  • Open joints

It is clear that this is “granite” by the way it weathers


Saprolite

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


Description of a residual soil is “fuzzy”

two variables

  • I.the degree of weathering of the rock

  • II. the abundance of altered minerals


Classes of weathering of igneous rocks

  • Several different classification systems

  • Different authors


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!


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)


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)


Chemically weathered granite


All contain several classes

Granite weathers to a sandy soil


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


Rock Quality – some tests

Fluid adsorption, classes 1-4

Almost impermeable

Slightly permeable

Moderately permeable

Highly permeable


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


Effect of climate and rock typeon weathering

Precipitation/evaporation ratio is important

  • Weinert - N value is a weathering index

    N<5, chemical weathering is favored over mechanical – decomposition is the predominate process

    N>5, mechanical weathering is favored over chemical – decomposition is predominate


Effect of climate and rock typeon weathering

Weathering of basic and ultrabasic rocks

  • N > 2, montmorillonite

  • N between 1-2, kaolinite


Effect of climate and rock typeon 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


Engineering properties

plutonic rocks


exploration

  • weather profile nature: extent of rock and soil cover

  • hazards of boulders

  • hazard of soil flow

  • slides of serpentine

  • sheet slides

  • rock falls


excavation

  • core stones

    • size

  • drilling can divert along joints


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


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


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


ground water

  • fault zones

  • weathered granite


case histories


mammoth pool dam – sheeted granodiorite

San Joaquin River, California


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


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


mammoth pool dam – sheeted granodiorite


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


mammoth pool dam – sheeted granodiorite

  • grouting

    • must go slow

    • at low pressures

    • some sheets are bolted prior to grouting

    • otherwise uplift of sheet joints


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


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


Malaysian granite hydroelectric project

  • Porphyritic granite with 35% quartz and 5% biotite

  • hairline fractures

  • occasional shear zone healed with calcite, chlorite or quartz


Malaysian granite hydroelectric project

  • Shear zones and mylonite and brecciated granite


Malaysian granite hydroelectric project

  • surface outcrops minimal due to jungle vegetation

  • Lineaments visible on aerial photographs suggested faults and shear zones

  • 67 drill holes


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


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


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


Question

  • Can decomposed granite furnish satisfactory materials for concrete aggregate?


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?


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?


Question

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


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