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(1)

IGNEOUS, SEDIMENTARY &

METAMORPHIC ROCKS

the appearance of a rock is determined by its mineralogy and its texture (Gefüge)

mineralogy – relative proportions of the different minerals texture – size and shape of crystals

crystals – coarse – seen with naked eye - fine – otherwise

shape – needle, flat, platy, equant

(2)

mineralogy and texture are determined by the rock’s origin – where & how it was made

all rocks that were formed by cooling of lava/molten rock are IGNEOUS

all rocks that were formed by the burial of

sediments (of crystals or rock fragments, or coral fragments) are SEDIMENTARY rock

when rocks are buried, the high temperatures and pressures experienced by the rock may cause

chemical reactions resulting in a change in mineralogy and texture

all rocks formed by transformation of pre-existing rocks in the solid state under the influence of high temperature and/or pressure are METAMORPHIC rocks

texture, mineralogy and chemical composition of a

rock reveal its origin

(3)
(4)

IGNEOUS ROCKS

a temperature of 700°C or more is needed to melt most rocks

y at depth in the earth, magma is both being

created – by melting; and being

destroyed – by crystallization

(5)

IGNEOUS ROCKS

intrusive and extrusive igneous rocks distinguished by the

size of the crystals

the assumption is that

y if magma erupts from a volcano y it cools rapidly and

y there is time only for small crystals to grow – small grains y extrusive rock - VULKANIT

y if the magma cools slowly within the earth,

y there is enough time for the crystals to continue to grow, y resulting in large grain size

y intrusive rock - PLUTONIT

(6)

plutonit

intrusive igneous rocks

- are formed by slowly crystallizing magmas that have intruded into rock masses deep within the interior of the earth.

- they can be recognized by their large interlocking crystals that grew slowly as the magma cooled - magmas cool slowly with the earth as they are

surrounded by rocks which conduct heat very slowly - e.g. GRANITE

(7)

vulkanit

extrusive igneous rocks

- e.g. BASALT

- formed from rapidly cooling magma - volcanism

most of the minerals in igneous rocks are silicates – as silicon is so abundant in ⊕;

– also because few oxides melt at the temperature and pressure conditions of the ⊕’s crust and mantle

(8)

SEDIMENTARY ROCKS

sediments – sand, silt, shells

y sand, silt and pebbles are formed when rocks weather – that is are broken up into fragments of various sizes y the fragments of rocks are then transported by erosion y erosion is the processes that loosen soil and rock and move

them downhill or downstream where they are laid down as layers of sediment

weathering produces CLASTIC and CHEMICAL & BIOCHEMICAL sediments

Clastic sediments - physically deposited sedimentary particles

- grains from weathered rocks clastic - broken

- laid down by water, wind & ice - to form sand, silt & gravel

Chemical sediments - new chemical substances

- formed when some of the components of a rock are dissolved during weathering and are carried away by rivers – e.g. halite NaCl, calcite CaCO3

CaCO3 – chemical & biochemical

(9)

SEDIMENTARY ROCKS

lithification is the process that converts sediments into solid rock

compaction

grains are squeezed together by the weight of

overlying sediment into a mass denser than the original cementation

minerals precipitate around deposited particles and glue them together

sediments are compacted and cemented after burial under additional layers of sediment

y sandstone forms by lithification of sand particles

y limestone forms by the lithification of shells and other particles of CaCO3

sediments and sedimentary rocks are characterised by bedding, the formation of parallel layers by the settling of particles

(10)

v

(11)

bedding may be due to y changes in mineralogy or

y differences in texture – e.g. grain size

settling of different grain-sizes of the same mineral can be due to the effects of wind or water

(12)

the same process occurs in igneous and

sedimentary rocks as crystals settle out of a liquid due to the (generally) lower density of the liquid crystals of smaller radius settle more slowly

(13)

SEDIMENTARY ROCKS

although most rocks found on the ⊕’s surface are

sedimentary, they form only a thin layer on the surface, with most of the crystal being metamorphic or igneous

< 2% of the Earth is sedimentary

the common minerals of clastic sedimentary rocks are of course silicates

- physically deposited sedimentary particles

y silicates are the dominant minerals in the rocks which weather to form sedimentary particles

in chemically or biochemically precipitated rocks the common minerals are carbonates.

calcite – CaCO3 – limestone

dolomite – CaMgCO3 in limestone formed by

precipitation during lithification gypsum CaSO4.2H2O & halite - NaCl

y form by chemical precipitation as seawater evaporites

(14)

METAMORPHIC ROCKS

meta – change

these rocks are formed when high temperatures and/or high pressures deep in ⊕ cause any kind of rock – igneous,

sedimentary or metamorphic – to change its mineralogy, texture or chemical composition while maintaining its solid state

- the temperatures are below the melting point of the rock (700°C)

- but high enough (250°C) for recrystallization and chemical reactions to occur

The temperature gradient in the upper 8 km of the crust has been measured in bore-holes. It is 2-3C per 100 m

(15)

METAMORPHIC ROCKS

REGIONAL AND CONTACT METAMORPHISM

metamorphism may take place over widespread or

limited area

regional metamorphism – accompanies plate collisions and the building of mountains and the folding and breaking of sedimentary layers that were once horizontal -

structural deformation

many regionally metamorphic rocks e.g. schists have foliation – wavy or flat planes produced when the rock was

structurally deformed into folds

quartz-mica schist

(16)

METAMORPHIC ROCKS

where high temperatures are restricted to smaller volumes, such as rocks near and in contact with intruding magma, rocks are transformed by contact metamorphism

granular textures are more typical of contact metamorphic

rocks, which contain minerals with equant crystals, y and of some regional metamorphic rocks formed by very

high pressure and temperature

typical minerals of metamorphic rocks are

quartz, SiO2 feldspar, (K,Na,Ca)(Al,Si)1-3O8 mica, (K,Na)2(Al,Mg,Fe2+,Fe3+)4-6(Si,Al)8(OH,F)4

pyroxene (Mg,Fe)SiO3 and

amphibole e.g. NaCa2(Mg,Fe,Al)5(Al,Si)8O22(OH)2

the same kinds of silicates which are characteristic of igneous rocks

several other silicates –

kyanite, Al2(SiO4)O staurolite, (Fe,Mg)2(Al,Fe)9O6-Si4O16,

and some varieties of garnet are characteristic of metamorphic rocks alone

(17)

CHEMICAL COMPOSITION OF

ROCKS

by convention the chemical composition of a whole rock is given in terms of the oxides

this convention is followed even though the elements exist in the form of silicates

also, by convention, everything is given in wt% of the oxide the 7 major elements – Si, Al, Fe, Ca, Mg, Na, K - along with oxygen make up the bulk of the rocks in ⊕

how does the chemical composition tell us about the origin of the basalt?

differences of ~0.2 wt% in the major elements tell us whether the basalt was formed at

a mid-ocean ridge (divergent plates) or at a subduction zone (convergent plates)

(18)

Data Calculation Data component MORB wt% OIB wt% molar weight

MORB OIB MORB

mol% OIB mol% SiO2 48,77 47,52 60,084 81,17 79,09 50,91 53,67 TiO2 1,33 3,29 79,898 1,66 4,12 1,04 2,80 Al2O3 15,90 15,95 101,961 15,59 15,64 9,78 10,61 Fe2O3 1,33 7,16 159,691 0,83 4,48 0,52 3,04 FeO 8,62 5,30 71,846 12,00 7,38 7,53 5,01 MnO 0,17 0,19 70,937 0,24 0,27 0,15 0,18 MgO 9,67 5,18 40,311 23,99 12,85 15,05 8,72 CaO 11,16 8,96 56,079 19,90 15,98 12,48 10,84 Na2O 2,43 3,56 61,979 3,92 5,74 2,46 3,89 K2O 0,08 1,29 94,203 0,08 1,37 0,05 0,93 P2O5 0,09 0,64 141,943 0,06 0,45 0,04 0,31 summe 99,55 99,04 159,44 147,37 moles = wt% molar weight moles Summe moles H 1,008 4,003 He Li 6,939 9,012 Be 10,811 B 12,011 C 14,007 N 15,999 O 18,998 F 20,183 Ne Na 22,990 24,312 Mg 26,982 Al 28,086 Si 30,974 P 32,064 S 35,453 Cl 39,948 Ar K 39,102 40,08 Ca 44,956 Sc 47,90 Ti 50,942 V 51,996 Cr 54,938 Mn 55,847 Fe 58,933 Co 58,71 Ni 63,54 Cu 65,37 Zn 69,72 Ga 72,59 Ge 74,922 As 78,96 Se 79,909 Br 83,80 Kr Rb 85,47 87,62 Sr 88,905 Y 91,22 Zr 92,906 Nb 95,94 Mo Tc 99 101,07 Ru 102,91 Rh 106,4 Pd 107,87 Ag 112,40 Cd 114,82 In 118,69 Sn 121,75 Sb 127,60 Te 126,90 I 131,3 Xe Cs 132,91 137,34 Ba 138,91 L 178,49 Hf 180,95 Ta 183,85 W 186,2 Re 190,2 Os 192,2 Ir 195,09 Pt 196,97 Au 200,59 Hg 204,37 Tl 207,19 Pb 208,98 Bi Po At Rn Fr Ra Ac Th Pa U

(19)

CHEMICAL COMPOSITION OF

ROCKS

the amount of water bound into the crystals in the rocks is also of importance

in igneous rocks this is about 1 wt%, whereas in sedimentary rocks this is 5%

- due to abundance of clay in the rock

rocks exposed on the surface are outcrops

where bedrock, the rock below the loose surface sediments is laid bare

(20)

there is large amount of deep drilling all over the world to look at rocks in the crust

but 12 – 15 km is as deep as it gets so mostly sedimentary

rocks are found and then igneous and metamorphic after about 6 km

(21)
(22)

THE ROCK CYCLE

starts with magma deep in the Earth

– all igneous intrusive rocks are plutonic, whereas igneous extrusive are volcanic

the 3 main sources of igneous rocks are at

Ž convergent and

(23)

Ž divergent plate margins and at Ž mantle plumes

(24)

plutonic rocks which formed at the convergent margin are uplifted in the mountain building process, and the loose material - sediments and metamorphic rocks - are eroded away leaving the igneous rock exposed

plate collision and mountain building is orogeny

the igneous rock then also weathers and chemical changes take place within it - based on the presence of water

the igneous rock then breaks up and the particles are transported by water and wind to streams and oceans where they are deposited as layers of sand and silt and other sediments formed from dissolved materials e.g. CaCO3 from shells

(25)

THE ROCK CYCLE

these sediments in the sea and on land are then covered by successive layers of sediment and gradually lithify into sedimentary rock

burial is accompanied by subsidence – a depression or sinking of the ⊕’s crust due to the added weight of the sediments

….and then more sediments are deposited on this

as the lithified material is buried more deeply in the crust, it gets hotter

at a depth of 10 km the temperature is 300°C, and metamorphism starts to occur with the solid minerals changing – in the solid state - to new minerals

 the new minerals are more stable at the high

temperatures and pressures

with more heating the rocks may melt to form magma which will then be the source of new igneous rocks

(26)

pressure must increase with depth because of the weight of material above;

P = ρgd N m

-2

P – pressure

g – acceleration due to gravity

d – depth of overlying material with average density ρ - density

(27)

 along with our igneous rock in the mountain, both

sedimentary and metamorphic rocks were uplifted and eroded and deposited as sediments, which may or may not have been buried deep enough in the crust to melt and become a magma

the rocks that make up the earth are recycled continuously

the oldest zircon found in a rock is 4.27 Ga

the oldest zircon in a meteorite is 4.56 Ga – this is the age of the solar system

(28)

PLATE TECTONICS

plutonism, volcanism, tectonic uplift, metamorphism,

weathering, sedimentation, transport, deposition, burial are all part of the rock forming process, and they are driven by plate tectonics

plutonism and volcanism are the result of the interior heat of the earth – occur in 3 tectonic settings

convergent boundaries – where oceanic plates descend into the mantle, where they melt at 50-100 km depth divergent boundaries at mid-ocean ridges where sea-floor

spreading allows basaltic magma to rise from the ⊕’s upper mantle to form new oceanic crust

mantle plumes or hot spots – where crystalline material rises through the mantle and pours out as magma at the

surface

(29)

sediment is carried away from mountains and deposited on continents and ocean floors

metamorphism and uplift occur as continental plates collide at

convergent boundaries - this uplifts mountains and creates the great pressures and temperatures that metamorphose rocks

mineralogy and texture define a rock

they are determined by the geological conditions and the chemistry of the rock that is formed

(30)

IGNEOUS ROCKS

How are igneous rocks classified?

all rocks were once molten,

and the whole rock forming process is driven by plate tectonics

classification of rocks by – y texture

y mineral and chemical composition

All igneous rocks can be divided into two broad textural classes:

(1) the coarsely crystalline rocks, which are intrusive and therefore cooled slowly,

plutonic

and

(2) the finely crystalline ones, which are extrusive and cooled rapidly.

volcanic

(31)

How are igneous rocks classified?

Within each of these broad categories, the rocks are classified on the chemical basis of their silica content or by the mineralogical equivalent, the proportions of lighter, felsic minerals and darker, mafic minerals.

Felsic rocks, such as granite and its corresponding extrusive, rhyolite, are rich in silica and dominated by quartz, potassium feldspar, and sodium-rich plagioclase feldspar.

Mafic rocks, such as gabbro and its corresponding extrusive, basalt, are poor in silica and consist primarily of pyroxene, olivine, and calcium-rich feldspar.

Intermediate rocks are granodiorite and diorite

and their corresponding extrusives, dacite and andesite.

(32)

TEXTURE

coarsely or finely crystalline

coarse grained rock – granite – has grains seen by eye fine grained rock – basalt – cannot be seen by eye or with

magnifying glass

photomicrographs of thin sections

hypersthene gabbro – plagioclase and hypersthene (orthopyroxene) dominate this rock.

by observing lava flows (lava on surface, magma in ⊕), we know:

y fast cooling results in fine grains or glass, and y slow cooling results in larger grains

crystals grow by atoms arranging themselves into repetitive structure

(33)

hypersthene gabbro – plagioclase and hypersthene (orthopyroxene) dominate this rock.

A diabase is a basaltic rock with grain size more or less transitional between gabbro (coarse) and basalt (fine). Notice the elongate

lath-shaped plagioclase and the colorful clinopyroxene in this rock.

(34)

y atoms must diffuse through the melt to come to the correct position to form a crystal and to grow on the surface of the small crystal

y it takes time for the atom to diffuse and the speed at which an atom can move depends upon temperature, the lower the temperature the slower the atom diffuses

1 2 0

exp

RT

E

m

s

D

D

=

R – gas constant – 8.314 J mol-1 K-1 E – activation energy (J mol-1)

T – absolute Temperature (K)

(35)

intrusive igneous rocks (plutonic rocks) are formed from magma that has forced its way into surrounding rock and then cooled to form large-grained igneous rock. The surrounding rock is called country rock

extrusive igneous rocks (volcanic rocks) are formed by the rapid cooling of a lava and produce fine-grained or glassy rocks. They form when lava or volcanic material is ejected from a volcano

lava – volcanic rocks formed from lava range from smooth and ropy to sharp spiky and jagged. These special textural qualities give information about how the rocks formed.

(36)
(37)

pyroclastic rocks – in violent eruptions pyroclasts are formed when broken pieces of lava and glass are thrown into the air.

pyroclasts include

crystals that started to form before the explosion, fragments of previously solidified lava and

pieces of glass that cooled and fractured during the eruption.

The finest pyroclasts are volcanic ash – extremely small fragments usually glass that forms when escaping gas forces a fine spray of magma from a volcano.

volcanic ash accumulates as layers of loose and uncemented material

volcanic rocks lithified from these pyroclastic materials are called tuff

(38)

Volcanic glass comes in a variety of forms when it is the only constituent of an igneous rock.

one common glassy rock type is pumice

a frothy mass with a great number of vesicles (air bubbles) - holes that remained in the glass after

trapped gas escaped from the solidifying melt - made the same as champagne

reticulite

Reticulite is basaltic pumice in which nearly all cell walls of gas bubbles have burst, leaving a honeycomb-like structure. Even though it is less dense than pumice,

reticulite does not float in water because of the open network of bubbles.

another glassy volcanic rock is obsidian which is pure glass y contains no trapped gases and so is solid with no bubbles y broken obsidian fragments are very sharp and can be

used as a knife or axe

(39)

A porphyry has a mixed texture in which large crystals float in a predominantly fine grained crystalline matrix

rhyolite porphyry

the large crystals are called phenocrysts

y they were formed while the magma was still below the ⊕'s surface

y and the volcanic eruption bought the magma to the surface before other crystals could grow

(40)

CHEMICAL & MINERAL

COMPOSITION

igneous rocks are sub-divided on the basis of their chemical & mineral composition as well as texture Silica (SiO2) is abundant in most igneous rocks and

accounts for 40 to 70 wt% of the rock

historically we refer to rock rich in silica - e.g. granites- as silicic

(41)

today, we group igneous rocks according to the relative proportions of silicate minerals

the silicate minerals quartz,

feldspar (orthoclase and feldspar),

muscovite and biotite micas,

the amphibole and pyroxene groups and olivine

form a systematic series

felsic minerals are high in silica – feldspar and silica

mafic minerals are low in silica – magnesium and ferric iron

the adjectives can be applied both to the rock and to the minerals in the rock

mafic minerals crystallize at higher temperatures

- that is earlier in the cooling history of the magma than those at which felsic minerals crystallize

(42)

some intrusive rocks have the same chemical composition

as extrusive rocks, but different texture this is the case for

coarse grained gabbro which is formed deep in the ⊕ and

fine grained basalt which cools as a lava on the ⊕'s surface

basalt gabbro and also

fine grained rhyolite and coarse grained granite

This crystal-rich rhyolite contains phenocrysts of quartz, K-feldspar (sanidine), plagioclase, and biotite in a fine-grained groundmass.

Minerals in this rock include quartz, plagioclase, biotite, and K-feldspar.

these rocks have the same mineral content, but different grain-sizes

(43)

CHEMICAL & MINERAL COMPOSITION

y most chemical and mineral compositions can appear either

as extrusive or intrusive rocks

y the sole exceptions to this possibility are the highly mafic rocks that rarely appear as extrusive igneous rocks – because the magma crystallizes at very high temperatures, and the liquid does not reach the surface before it has all solidified

(44)

FELSIC ROCKS

light coloured, poor in Fe and Mg

but rich in high silica content minerals –

quartz, orthoclase feldspar, and plagioclase feldspar (which contains both Ca and Na)

(45)

richer in Na near the felsic end richer in Ca at the mafic end

felsic minerals crystallise at temperatures lower than those at which mafic minerals crystallise

Ca-rich plag crystallises at higher temperatures than Na-rich plag

minerals and rocks are light in colour

Granite – the most abundant intrusive rock, felsic with ~70 wt% SiO2

it contains abundant quartz and orthoclase feldspar and a lesser amount of plagioclase feldspar. These felsic minerals give granite its pink or grey colour. It also contains small amounts of muscovite, biotite micas and amphibole.

Rhyolite – is the extrusive equivalent to granite. It is light brown to grey in colour, more finely grained and many rhyolites contain glass or are

completely volcanic glass

(46)

INTERMEDIATE IGNEOUS

ROCKS

midway between felsic and mafic

neither as rich in silica as the felsic rocks nor as poor in silica as the mafic rocks

Granodiorite – looks like granite and has

abundant quartz, but has more plagioclase than orthoclase

Diorite – less Si, dominated by plagioclase, little or no quartz. Contain a moderate amount of mafic minerals biotite, amphibole and

pyroxene. And tend to be darker in colour than granite or granodiorite.

the volcanic equivalents are dacite & andesite

(47)

MAFIC ROCKS

mafic rocks – high in pyroxenes and olivines which are relatively poor in silica but rich in Mg and Fe and therefore these rocks are dark in colour

Gabbro – a dark grey, coarsely grained intrusive rock with an abundance of mafic minerals, esp. pyroxene, no quartz and only moderate

amounts of Ca-rich plagioclase.

Basalt - is dark grey to black and is the fine grained extrusive equivalent to gabbro. It is the most abundant extrusive igneous rock on the

surface of the earth and on the ocean floors.

ULTRAMAFIC ROCKS

y primarily mafic minerals with less than 10% feldspar and about 45 wt% SiO2

peridotite coarsely grained dark green rock made up of

olivine with small amounts of pyroxene and amphibole - rarely found as extrusive rocks (Mg,Fe)2SiO4

y mafic rocks melt at high temperature and felsic at lower temperatures

(48)
(49)
(50)
(51)

VISCOSITY

Si – network-former increase viscosity Al – network-former increase viscosity Na, K – network-modifier decrease viscosity

H2O – network-modifier decrease viscosity

y felsic rocks have more Si and therefore when they melt their viscosity is higher than mafic composition melts viscosity – η - resistance to flow – increases with Si content

rate

strain

stress

ε

σ

η

=



igneous rocks 22

(52)

HOW

&

WHERE

DO

MAGMAS FORM?

Magmas form at places in the lower crust and mantle where temperatures and pressures are high enough for at least partial melting of

water-containing rock.

Basalt can partially melt in the upper mantle where convection currents bring hot rock upward at mid-ocean ridges.

Mixtures of basalt and other igneous rocks with sedimentary rocks, which contain significant quantities of water, have lower melting points than dry igneous rocks. These mixtures therefore melt when they are heated during subduction into the mantle.

(53)

HOW DO MAGMAS FORM?

seismic waves Î ⊕ is solid down to outer core volcanoes Î liquid regions within ⊕

(54)

TEMPERATURE & MELTING

a rock of varied mineralogy does not melt uniformly a partial melt forms first

- because the minerals that compose the rock melt at different temperatures

olivine melts @ 1890C

Ca-plagioclase melts @ 1550C

pyroxene melts @ 1400C

the ratio of liquid to solid depends upon

the composition and mineralogy of the rock and

the temperature and pressure conditions in the ⊕ which it experiences

partial melts of basaltic composition in the upper mantle have about 1% melt

granitic melt composition just before eruption would have about 10% crystals

(55)

TEMPERATURE & MELTING

as a rock melts, the composition of the rock changes as the different minerals add to the melt

therefore from the whole rock composition – melt + crystals in rock and the removed melt composition, one can estimate what minerals melted and at which

temperatures this occurred, and therefore the depth at which the rock partially melted

or basaltic magmas at the surface have different compositions due to the depth at which they formed

(56)

PRESSURE & MELTING

pressure increases in ⊕ due to the overlying rocks

the temperature required to melt a crystal increases with pressure

a rock that melts at 1000°C on the surface might need 1300°C at depth in the upper mantle

(57)

WATER & MELTING

there is water in most lavas – 1 wt% or more

melting temperature decreases in the presence of water which is bound in the crystal or simply in the rock between crystals

(58)

MAGMA CHAMBERS

density of melt is less than that of the rock (usually) Ö therefore melt rises within the ⊕

CRYSTAL

ρ

g cm-3 MELT

ρ

g cm-3

olivine 3,33 basalt 2,55-2,65

plagioclase 2,70 andesite 2,45

quartz 2,65 rhyolite 2,30

amphibole 3,20 Fe-melt 5,50

pyroxene 3,27 diopside melt 2,61

and forms magma chambers

– magma filled cavities in the lithosphere that form as melt pushes aside the surrounding rock – the melt is also at pressure

– may be many km3

– we know they exist because seismology shows them

beneath some volcanoes, but how they form is

questionable, and what happens as melt is forced out in volcanic eruptions is questionable

(59)

TEMPERATURE PROFILE IN THE

EARTH

y in tectonically & volcanically active regions the temperature at 40 km depth is already 1000°C

y this is almost high enough to melt basalt

y in stable regions the temperature at this depth may be as low as 500°C

(60)

TECTONIC ACTIVITY

2 types of plate boundaries are associated with magma formation –

mid-ocean ridges – where the divergence of two plates causes the seafloor to spread

new rock is formed

and

subduction zones where one plate dives beneath another old rocks are melted

(61)

magmas are labelled in terms of plutonic (intrusive) and volcanic (extrusive),

y and the magma is also named after the rock group y rocks can have identical compositions but different

textures common names: rhyolite (felsic) andesitic (intermediate) basaltic (mafic)

MID-OCEAN RIDGES

y heat in the form of rising convection currents in the mantle causes the formation of basaltic melt

y this magma forms in the hot upper mantle below mid-ocean ridges and rises to collect in narrow wedge shaped magma chambers near the crest of the ridge

y large quantities of this fluid magma flows out of the rifts and fissures at the mid-ocean ridges producing abundant basaltic lavas on the seafloor

(62)
(63)

SUBDUCTION ZONES

y the magma forms from a mixture of seafloor sediments and

basaltic and felsic crust

y the sediments contain water both in the pore space between crystals and bound into the crystal structure of the clays which are present

y sediments become deeply buried as the subducting lithospheric plate moves into the lower crust

y at about 5 km and 150°C most of the water is released by chemical reactions

y the rest of the water is released at 15 – 25 km depth

(64)

y as the water moves up from the top of the subducting slab it reacts with the minerals in the mantle wedge and promotes melting in the plate overlying the subducting plate (water lowers melting

temperature of minerals – chemical effect)

y the composition of the sedimentary, basaltic and felsic magmas that combine in this process determine the type of igneous rock formed from the melt

y the igneous rocks formed at subduction zones are generally more silicic than the basalts of mid ocean ridges, with some andesite and lesser amounts of felsic rocks

y deep in the crust, beneath the volcanoes, intrusive rocks of

intermediate to silicic compositions - from diorite to granite - are formed at the same time as the magmas erupt at the earths surface y these intrusives are added to the base of the crust thickening it by

the process called underplating

MANTLE PLUMES

y basalts similar to those at mid-ocean ridges are sometimes found distant from plate boundaries

- Columbia & Snake River Basalts

- the Hawaiian Islands are volcanic islands that are not near a plate boundary

y in such places "plumes" of hot crystalline material rise from deep in the mantle – perhaps as deep as the

mantle/core boundary

(65)

y mantle plumes are hot spots - and responsible for huge outpourings of basaltic melt

summary

y basaltic magmas form in the upper mantle below mid-ocean ridges and in the lower mantle beneath intraplate hot spots

y magmas of varying composition form at subduction zones – depending upon how much felsic material and water the rocks overlying the subduction zone contribute to the melt

(66)

How does magmatic differentiation account

for the variety of igneous rocks?

Minerals crystallize from magmas along two paths: (1) a continuous reaction series of the plagioclase

feldspars and

(2) a discontinuous reaction series of the mafic minerals.

PHASE DIAGRAMS

(67)

How does magmatic differentiation account

for the variety of igneous rocks?

NO REMOVAL OF MATERIAL EQUILIBRIUM REACTIONS

In these series, crystals continuously react with the melt through successive stages of crystallization and magma composition until they solidify

completely, at which point the final product (rock) has the same composition as the original magma.

REMOVAL OF MATERIAL

NON-EQUILIBRIUM REACTIONS

If there is fractional crystallization, so that the crystals do not react with the melt, either because they grow very rapidly or because they are

separated from the liquid, the final product (rock) may be more silicic than the earlier, more mafic crystals.

mafic minerals (Si-poor) crystallize at higher temperatures than Si-rich felsic minerals

(68)

How does magmatic differentiation account

for the variety of igneous rocks?

Bowen's continuous and discontinuous reaction series explain how fractional crystallization can produce mafic igneous rocks from earlier stages of crystallization and differentiation; and felsic rocks from later stages, but Bowen's theory does not adequately explain the abundance of granite.

Magmatic differentiation of basalt does not explain the composition and abundance of igneous rocks.

Different kinds of igneous rocks may be produced by variations in the compositions of magmas caused by the melting of different mixtures of sedimentary and other rocks and by mixing of magmas.

(69)

MAGMATIC DIFFERENTIATION

a homogenous parent melt may produce rocks of differing composition

y this is because, as the magma cools and minerals form the

composition of the remaining melt changes - it is depleted in the chemicals that have been used to form crystals

y the first minerals to crystallise in a cooling melt are those that were the last to melt in a partial melt

liquidus – marks the temperature above which all is molten solidus - marks the temperature below which all is solid

(70)

continuous and gradual change

the composition of the successively formed plagioclase feldspars changes continuously and gradually

abrupt and discrete change

mafic minerals e.g. olivine & pyroxene, the composition of the minerals changes discontinuously

y with one mineral abruptly changing to another at a particular temperature

(71)

CONTINUOUS REACTION

when a melt of plagioclase feldspar composition is cooled

y the first crystals to form are richer in Ca than the melt is y this depletes the melt in Ca, and makes it become richer in Na

y the Ca-rich crystals then begin to react with the Na-rich melt and exchange Ca Ö melt and Na Ö crystal

y as this process continues both melt and crystals become richer in Na and poorer in Ca

y end up with first crystals being rich in Ca and last rich in Na

(72)

DISCONTINUOUS REACTION

mafic minerals such as olivine, pyroxene, amphibole and biotite micas display a different process

when a mafic composition melt is cooled y olivine crystallises first

y however, below 1557°C pyroxene begins to crystallize and the olivines convert to pyroxene

y at 1543°C, cristobalite begins to crystallise along with the pyroxene

y with different composition melts, amphibole and then biotite crystallise at temperatures lower than the olivine-pyroxene series

Mg2SiO4 MgSiO3 SiO2

(73)
(74)

MAGMATIC DIFFERENTIATION

in the continuous process,

y the crystal structure remains constant but the y composition changes with decreasing temperature in the discontinuous series,

y the crystal structure changes

at high temperatures, simple structures crystallise

– olivine has isolated SiO4 tetrahedra,

– pyroxene has single chains of SiO4 tetrahedra, – while amphiboles have double chains,

– micas have sheets of tetrahedra

The end stages of both reaction types are quartz and feldspars with 3D frameworks of tetrahedra

(75)

MAGMATIC DIFFERENTIATION

in the cooling of a natural magma, both patterns occur simultaneously

with the olivine Ö pyroxene change occurring alongside the continuous crystallization of

plagioclase feldspar

if you follow this scenario which was derived from laboratory experiments - each igneous rock should have

only a single plagioclase feldspar corresponding to the composition of the original melt and a pyroxene – there

should be no olivine and no Ca-rich plagioclase

(76)

FRACTIONAL CRYSTALLIZATION

………… the theory of magmatic differentiation needs to account for the preservation of minerals formed

earlier…………

Bowen (a Canadian) in the 1920’s, looked at plagioclase feldspars that failed to change composition by reacting with the remaining melt

proposed that –

if a melt cooled quickly, the Ca-rich crystals would have time to grow, but only the outer surfaces of existing crystals would have time to react with the melt – diffusion of atoms take time

as a result only the outer layer of each crystal would change composition

∴ as the temperature decreased and

crystallization continued each outer layer would become more rich in Na

(77)

the end product would be a ZONED CRYSTAL.

– a single crystal of one mineral that has a different composition in its inner and outer parts

(78)

simple theory of magmatic differentiation (Bowen, 1920)

– ystal settling

feldspars should settle to the –

fractional crystallization is the term applied melt

so you expect olivine at the bottom, pyroxene plus Ca-– first formed crystals become segregated from the

melt e.g. cr

– therefore the Ca-rich

bottom of the magma chamber and be removed from the chemical reaction with the melt,

which would become more Na-rich

to separation and removal of successive fractions of crystals formed in a cooling

y

rich plagioclase and then Na-rich plagioclase at the top of a cooled magma chamber

however, reality is not so simple –

(79)
(80)

theories of fractional crystallization and magmatic differentiation have difficulties in explaining the apparently contradictory facts:

(a) the widespread abundance of granites – intrusive silicic rocks, with Na-rich plag and other low

melting-temperature minerals (felsic)

(b) equally abundant basalt – extrusive mafic rocks, low silica content Ca-rich feldspars and other high melting temperature minerals

Bowen’s idea was that basaltic magma would cool and differentiate by fractional crystallization and erupt (as the magma evolved) to produce lavas ranging from basaltic to andesitic to rhyolitic – to produce granite in the late stages

(81)

BOWEN’S REACTION SERIES

However, back in the laboratory

Áexperimentalists always add reality checks to ravings of geologistsÁ

– olivine crystals take too long to settle to the bottom of a viscous magma chamber

(

xl m

)

T

r

η

g

ρ

- ρ

S

2

9

2

=

m s-1

density (g cm-3) viscosity (Pa s)

basalt melt 2.33 2×104

olivine 3,33

1 mm diameter: time to fall 1 m

in basalt melt olivine 1.16 years

– convection within the magma chamber usually stirs the sinking crystals and destroys this simple process

(82)

BOWEN’S REACTION SERIES

to produce a granite intrusion, 10 times as much basalt melt is needed to start with

therefore, you expect to see huge quantities of basalt underlying granite intrusions – but do not

starting point is the problem

–Bowen said all granitic rocks form by differentiation of a starting melt of basaltic composition; but

in reality-

the melting of various source rocks in the crust and upper mantle is responsible for the variation in magma

compositions

1. rocks in the upper mantle might partially melt to produce basaltic magmas

2. a mixture of sedimentary and oceanic basaltic rocks (subduction zone) might melt to form an andesitic melt

3. a melt of sedimentary, igneous, and metamorphic

continental crustal rock might produce granitic magma

(83)

MAGMATIC DIFFERENTATION

magmatic differentiation does operate but is much more complex than Bowen’s original proposal

partial melting

– basaltic melt can be formed by 10-15% partial melting of upper mantle rocks at 100 km depth

– andesitic melt can be formed by partial melting of water-rich basaltic oceanic crust that heats up as it descends along a subduction zone

- rhyolitic magma can be formed by partial melting in the lower crust of a mixture of continental crustal rocks or andesite

in all of these one can apply Bowen’s reaction series in reverse to predict the composition of the magma as it is formed from partial melting

magmas do not cool uniformly, there may exist a wide range of temperatures within the magma chamber the differences in temperature may produce d

in chemical composition

ifferences

(84)

MAGMATIC DIFFERENTATION

some melt compositions are immiscible – they do not mix with each other

magma at different temperatures in different parts of the magma chamber may flow turbulently, crystallizing as it circulates; crystals may settle and then be caught up in the flow again

the margins of a magma chamber are usually thought to be mushy - that is a mixture of crystal and melt

(85)

What are the forms of

intrusive igneous rocks?

Igneous bodies of large size are plutons. The largest

plutons are batholiths, which are thick tabular masses with a central funnel. Stocks are smaller plutons.

Less massive than plutons are sills, which are concordant, with the intruded rock, following its layering, and dikes, which are discordant with the layering, cutting across it. Hydrothermal veins are formed where water is abundant, either in the magma or in surrounding country rock.

(86)

FORMS OF MAGMATIC

INTRUSIONS

field work looks at old intrusions – solid rock that

has been deformed and uplifted

seismic waves through current magma bodies – but resolution is ca. km, so fine detail is lost deep drill hole – temperature changes in crust –

indicate presence of melt?

plutons, sills, dykes, veins

PLUTONS

PLUTONS – large igneous bodies that formed deep in the ⊕'s crust

y from 1 km3 to 100 km3

y seen after uplift and erosion, or in mines or drill holes y variety of shapes, sizes and compositions

(87)

most magmas intrude at 8-10 km depth

– pressure 300 MPa – 3000 times that at the surface – this is more than enough pressure to close the cracks

between crystals

– but the upwelling melt is coming from a higher pressure source and so can force its way between rocks

MAGMA RISING THROUGH THE CRUST MAKES SPACE BY:

1. wedging open overlying rock – as magma forces the rock up, the rock crack horizontally and the melt flows into the horizontal layer, the rock above may bulge up in response – rock is brittle but also plastic

2. breaking off large rock blocks – the block then sinks through the magma, and may melt to change the composition of the upwelling melt

3. melting the surrounding rock – melts the walls of the country rock as the magma chamber rises

(88)

BATHOLITH

- the largest plutons

y coarse grained igneous rock that by definition must be at least 100 km3

smaller plutons are called stocks

both are discordant intrusions – they cut across the layers

of the country rock that they intrude

batholiths are found in the cores of tectonically deformed mountain belts

batholith sources may extend 10 to 15 km into the crust, the coarse grain size indicates slow cooling at depth

(89)

SILLS & DYKES

are smaller than plutons and have a different relationship to the country rock

SILL

is a sheet of magma that was injected between parallel layers of bedded country rock – concordant intrusion –

y boundary of sill lies parallel to the layers of country rock – independent of whether the layers are horizontal

y 1 cm to 100 m thick.

they can be differentiated from a lava flow in that they lack the ropey structure of a lava flow, and there are no vesicles

they are more coarsely grained than lava flow rocks

rocks above and below the sill show the effects of being heated by the magma – contact metamorphism

sills do not overly older flows, or soils

(90)

DYKE

- major route of magma transport in the crust

- they are layers – like sills - but cut across bedded country rock

- they usually form by cracking open the country rock due to the pressure of the melt - magmatic injection

- width cm to m

in some dykes you can see xenoliths – fragments of the broken country rock that float in the magma

rarely occur alone – dyke swarms – 100 or more in region that has been deformed by large igneous intrusion

textures of dykes and sills vary as a function of whether they invaded country rock near the Earth's surface (fine grained) or deep in the crust (coarse grained)

(91)

VEINS

y deposits of minerals found within a rock fracture that are foreign to the host rock

y tubes or sheet shaped veins branch off the sides and tops of many intrusive bodies

y mm to m in width and m to km long e.g. gold veins veins of very coarse grained granite cutting across fine grained country rock are pegmatites y they crystallized from a water-rich magma in the

late stage of solidification

y pegmatites provide ores of many rare elements – Li, Be

(92)

VEINS

some veins contain minerals with water in the crystal structure

– crystallised from hot water solution – crystallize at 250 – 350 °C –

– hydrothermal veins –

a lot of water was present – some from the magma itself - some from underground water in the cracks and pore

spaces of the intruded country rock

groundwater is due to rainwater seeping into soil and surface rocks

hydrothermal veins are common along mid-ocean ridges as the sea water infiltrates cracks in the basalt and

circulates into hotter regions of the basalt ridge emerging at the hot vent on the sea floor in the rift valley between the spreading plates

(93)
(94)

How are igneous rocks related to

plate tectonics?

The two major sites of magmatic activity are mid-ocean ridges, where basalt wells up from the upper mantle, and subduction zones, where a series of differentiated

magmas produces both extrusives and intrusives in island or continental volcanic arcs as the subducting oceanic lithosphere moves down into the deep crust and upper mantle. Large volumes of basalt are produced at oceanic islands and on landmasses that overlie mantle plumes.

batholiths are found in the cores on many mountain ranges that were formed by tectonic process – the convergence

of 2 plates ∴ there is a connection between plutonism,

mountain building and plate tectonics

(95)

IGNEOUS ACTIVITY & PLATE

TECTONICS

the major sites of IGNEOUS ROCK FORMATION are divergent zones – mid-ocean ridges

- at these sites, basaltic magma derived from partial melting of the mantle wells up along rising

convection currents

igneous rocks 66

- magma is extruded as lava, fed from the magma chambers below the ridge axis - at the same time gabbroic intrusions are emplaced at depth

(96)

subduction zones - where one plate dives below another - are MAJOR SITES OF ROCK MELTING

y the top of the subducting lithospheric plate includes oceanic crust which is largely basalt originally formed at a mid-ocean ridge

y this plate also carries water and soft ocean sediment which it accumulated in its trip form the mid-ocean ridge to the

subduction zone

y as the plate move downwards, the increasing temperature and pressure converts the sediments to sedimentary rocks and then to metamorphic rocks, and then to magma - releasing water as this all happens

y the presence of water lowers the melting temperature

y the magma and water then rise from the top of the subducting slab – they may melt some of the rock in the overlying wedge of mantle material and change their composition

y at the same time the magmas may differentiate by fractional

crystallization

y the result is a range of igneous rocks both extrusive and intrusive

y volcanoes over the deeper parts of the subduction zone where

melting is occurring produce basaltic, andesitic and rhyolitic lavas with pyroclasts – a wide range of different melt compositions

(97)

y formation of islands oceanic volcanic arcs – island arc

– Aleutian Islands of Alaska, Japan

y when subduction takes place beneath a continent, the volcanoes join together to form a mountainous arc on land

y subduction of an off shore plate has formed such an arc – Mount St. Helens

looking at the magmas above the subducting zone, we may try to estimate the composition of the parent magma and the depth of the descending slab from which it came - to figure out what happened millions of years ago

magmatic differentiation

the process by which a uniform composition parent magma forms rocks of different compositions

Different minerals crystallize at different temperatures. During such crystallization, the composition of the magma changes as it is depleted of the chemical elements taken away to form the crystallizing minerals.

(98)

fractional crystallization

the crystals formed in a cooling magma are separated from the magma

HOW CAN WE CREATE MELT

IN THE UPPER MANTLE?

(@ ~ 100 km depth)

add heat

convection currents at mid-ocean ridge & hot-spot plumes

decrease

pressure

adiabatic rise of material at

mid-ocean ridges & hot-spots

add water

melt mantle wedge at subduction

zone

(99)

70 igneous rocks

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(100)

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(102)

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(103)
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References

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