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
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
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
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
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
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
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
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
v
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
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
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
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
METAMORPHIC ROCKS
REGIONAL AND CONTACT METAMORPHISM
metamorphism may take place over widespread orlimited 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
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
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)
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
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
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
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
convergent and
divergent plate margins and at mantle plumes
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
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
pressure must increase with depth because of the weight of material above;
P = ρgd N m
-2P – pressure
g – acceleration due to gravity
d – depth of overlying material with average density ρ - density
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
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
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
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
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.
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
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.
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)
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.
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
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
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
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
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
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
CHEMICAL & MINERAL COMPOSITION
y most chemical and mineral compositions can appear eitheras 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
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)
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
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
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
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 22HOW
&
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.
HOW DO MAGMAS FORM?
seismic waves Î ⊕ is solid down to outer core volcanoes Î liquid regions within ⊕
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
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
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
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
MAGMA CHAMBERS
density of melt is less than that of the rock (usually) Ö therefore melt rises within the ⊕
CRYSTAL
ρ
g cm-3 MELTρ
g cm-3olivine 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
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
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
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
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
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
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
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
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
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.
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
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
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
DISCONTINUOUS REACTION
mafic minerals such as olivine, pyroxene, amphibole and biotite micas display a different processwhen 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
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
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
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
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
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 –
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
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)
Tr
η
g
ρ
- ρ
S
29
2
=
m s-1density (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
BOWEN’S REACTION SERIES
to produce a granite intrusion, 10 times as much basalt melt is needed to start withtherefore, 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
MAGMATIC DIFFERENTATION
magmatic differentiation does operate but is much more complex than Bowen’s original proposalpartial 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
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
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.
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
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
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
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
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)
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
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
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
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
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
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.
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 plumesdecrease
pressure
adiabatic rise of material at
mid-ocean ridges & hot-spots
add water
melt mantle wedge at subduction
zone
70 igneous rocks