IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS

Transcription

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 1

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 2

3 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 3

4 IGNEOUS ROCKS a temperature of 700 C or more is needed to melt most rocks at depth in the earth, magma is both being created by melting; and being destroyed by crystallization IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 4

5 IGNEOUS ROCKS intrusive and extrusive igneous rocks distinguished by the size of the crystals the assumption is that if magma erupts from a volcano it cools rapidly and there is time only for small crystals to grow small grains extrusive rock - VULKANIT if the magma cools slowly within the earth, there is enough time for the crystals to continue to grow, resulting in large grain size intrusive rock - PLUTONIT IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 5

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 6

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 7

8 SEDIMENTARY ROCKS sediments sand, silt, shells sand, silt and pebbles are formed when rocks weather that is are broken up into fragments of various sizes the fragments of rocks are then transported by erosion 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 CaCO 3 CaCO 3 chemical & biochemical IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 8

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 sandstone forms by lithification of sand particles limestone forms by the lithification of shells and other particles of CaCO 3 sediments and sedimentary rocks are characterised by bedding, the formation of parallel layers by the settling of particles IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 9

10 v IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 10

11 bedding may be due to changes in mineralogy or 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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 11

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 12

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 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 CaCO 3 limestone dolomite CaMgCO 3 in limestone formed by precipitation during lithification gypsum CaSO 4.2H 2 O & halite - NaCl form by chemical precipitation as seawater evaporites IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 13

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 14

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 15

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, and of some regional metamorphic rocks formed by very high pressure and temperature typical minerals of metamorphic rocks are quartz, SiO 2 feldspar, (K,Na,Ca)(Al,Si) 1-3 O 8 mica, (K,Na) 2 (Al,Mg,Fe 2+,Fe 3+ ) 4-6 (Si,Al) 8 (OH,F) 4 pyroxene (Mg,Fe)SiO 3 and amphibole e.g. NaCa 2 (Mg,Fe,Al) 5 (Al,Si) 8 O 22 (OH) 2 the same kinds of silicates which are characteristic of igneous rocks several other silicates kyanite, Al 2 (SiO 4 )O staurolite, (Fe,Mg) 2 (Al,Fe) 9 O 6 -Si 4 O 16, and some varieties of garnet are characteristic of metamorphic rocks alone IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 16

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) IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 17

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

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 19

20 there is large amount of deep drilling all over the world to look at rocks in the crust but km is as deep as it gets so mostly sedimentary rocks are found and then igneous and metamorphic after about 6 km IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 20

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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 convergent and IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 22

23 divergent plate margins and at mantle plumes IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 23

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. CaCO 3 from shells IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 24

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 25

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 26

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 27

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 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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 28

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 IGNEOUS, SEDIMENTARY & METAMORPHIC ROCKS 29

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 texture 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 igneous rocks 1

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. igneous rocks 2

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: fast cooling results in fine grains or glass, and slow cooling results in larger grains crystals grow by atoms arranging themselves into repetitive structure igneous rocks 3

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 lathshaped plagioclase and the colorful clinopyroxene in this rock. igneous rocks 4

34 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 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 D = D E RT exp m s R gas constant J mol -1 K -1 E activation energy (J mol -1 ) T absolute Temperature (K) igneous rocks 5

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. igneous rocks 6

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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 igneous rocks 8

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 contains no trapped gases and so is solid with no bubbles broken obsidian fragments are very sharp and can be used as a knife or axe igneous rocks 9

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 they were formed while the magma was still below the 's surface and the volcanic eruption bought the magma to the surface before other crystals could grow igneous rocks 10

40 CHEMICAL & MINERAL COMPOSITION igneous rocks are sub-divided on the basis of their chemical & mineral composition as well as texture Silica (SiO 2 ) 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. granitesas silicic igneous rocks 11

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 igneous rocks 12

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 igneous rocks 13

43 CHEMICAL & MINERAL COMPOSITION most chemical and mineral compositions can appear either as extrusive or intrusive rocks 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 igneous rocks 14

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) igneous rocks 15

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% SiO 2 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 igneous rocks 16

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 igneous rocks 17

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 primarily mafic minerals with less than 10% feldspar and about 45 wt% SiO 2 peridotite coarsely grained dark green rock made up of olivine with small amounts of pyroxene and amphibole - rarely found as extrusive rocks (Mg,Fe) 2 SiO 4 mafic rocks melt at high temperature and felsic at lower temperatures igneous rocks 18

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51 VISCOSITY Si network-former Al network-former Na, K network-modifier H 2 O network-modifier increase viscosity increase viscosity decrease viscosity decrease viscosity 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 η σ = ε stress strain rate 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 watercontaining 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. igneous rocks 23

53 HOW DO MAGMAS FORM? seismic waves is solid down to outer core volcanoes liquid regions within igneous rocks 24

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 Ca-plagioclase pyroxene 1890C 1550C 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 igneous rocks 25

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 igneous rocks 26

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 igneous rocks 27

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 igneous rocks 28

58 we MAGMA CHAMBERS density of melt is less than that of the rock (usually) 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 km 3 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 igneous rocks 29

59 TEMPERATURE PROFILE IN THE EARTH in tectonically & volcanically active regions the temperature at 40 km depth is already 1000 C this is almost high enough to melt basalt in stable regions the temperature at this depth may be as low as 500 C igneous rocks 30

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 igneous rocks 31

61 magmas are labelled in terms of plutonic (intrusive) and volcanic (extrusive), and the magma is also named after the rock group rocks can have identical compositions but different textures common names: rhyolite (felsic) andesitic (intermediate) basaltic (mafic) MID-OCEAN RIDGES heat in the form of rising convection currents in the mantle causes the formation of basaltic melt this magma forms in the hot upper mantle below midocean ridges and rises to collect in narrow wedge shaped magma chambers near the crest of the ridge 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 igneous rocks 32

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63 SUBDUCTION ZONES the magma forms from a mixture of seafloor sediments and basaltic and felsic crust the sediments contain water both in the pore space between crystals and bound into the crystal structure of the clays which are present sediments become deeply buried as the subducting lithospheric plate moves into the lower crust at about 5 km and 150 C most of the water is released by chemical reactions the rest of the water is released at km depth igneous rocks 34

64 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) the composition of the sedimentary, basaltic and felsic magmas that combine in this process determine the type of igneous rock formed from the melt 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 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 these intrusives are added to the base of the crust thickening it by the process called underplating MANTLE PLUMES 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 in such places "plumes" of hot crystalline material rise from deep in the mantle perhaps as deep as the mantle/core boundary igneous rocks 35

65 mantle plumes are hot spots - and responsible for huge outpourings of basaltic melt summary basaltic magmas form in the upper mantle below midocean ridges and in the lower mantle beneath intraplate hot spots 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 igneous rocks 36

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 igneous rocks 37

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 igneous rocks 38

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. igneous rocks 39

69 MAGMATIC DIFFERENTIATION a homogenous parent melt may produce rocks of differing composition 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 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 igneous rocks 40

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 with one mineral abruptly changing to another at a particular temperature igneous rocks 41

71 CONTINUOUS REACTION when a melt of plagioclase feldspar composition is cooled the first crystals to form are richer in Ca than the melt is this depletes the melt in Ca, and makes it become richer in Na the Ca-rich crystals then begin to react with the Na-rich melt and exchange Ca melt and Na crystal as this process continues both melt and crystals become richer in Na and poorer in Ca end up with first crystals being rich in Ca and last rich in Na igneous rocks 42

72 DISCONTINUOUS REACTION mafic minerals such as olivine, pyroxene, amphibole and biotite micas display a different process when a mafic composition melt is cooled olivine crystallises first however, below 1557 C pyroxene begins to crystallize and the olivines convert to pyroxene at 1543 C, cristobalite begins to crystallise along with the pyroxene with different composition melts, amphibole and then biotite crystallise at temperatures lower than the olivine-pyroxene series Mg 2 SiO 4 MgSiO 3 SiO 2 igneous rocks 43

73 igneous rocks 44

74 MAGMATIC DIFFERENTIATION in the continuous process, the crystal structure remains constant but the composition changes with decreasing temperature in the discontinuous series, the crystal structure changes at high temperatures, simple structures crystallise olivine has isolated SiO 4 tetrahedra, pyroxene has single chains of SiO 4 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 igneous rocks 45

75 MAGMATIC DIFFERENTIATION in the cooling of a natural magma, both patterns occur simultaneously with the olivine 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 igneous rocks 46

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 igneous rocks 47

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 igneous rocks 48

78 simple theory of magmatic differentiation (Bowen, 1920) first formed crystals become segregated from the melt e.g. crystal settling therefore the Ca-rich feldspars should settle to the bottom of the magma chamber and be removed from the chemical reaction with the melt, which would become more Na-rich fractional crystallization is the term applied to separation and removal of successive fractions of crystals formed in a cooling melt so you expect olivine at the bottom, pyroxene plus Carich plagioclase and then Na-rich plagioclase at the top of a cooled magma chamber however, reality is not so simple igneous rocks 49

79 igneous rocks 50

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 igneous rocks 51

81 BOWEN S REACTION SERIES However, back in the laboratory experimentalists always add reality checks to ravings of geologists olivine crystals take too long to settle to the bottom of a viscous magma chamber 2 2 r g ST = ( ρxl - ρm ) m s -1 9 η density (g cm -3 ) viscosity (Pa s) basalt melt 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 igneous rocks 52

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 realitythe 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 igneous rocks 53

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 differences in chemical composition igneous rocks 54

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 igneous rocks 55

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. igneous rocks 56

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 from 1 km 3 to 100 km 3 seen after uplift and erosion, or in mines or drill holes variety of shapes, sizes and compositions igneous rocks 57

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 igneous rocks 58

88 BATHOLITH - the largest plutons coarse grained igneous rock that by definition must be at least 100 km 3 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 igneous rocks 59

89 SILLS & DYKES SILL are smaller than plutons and have a different relationship to the country rock is a sheet of magma that was injected between parallel layers of bedded country rock concordant intrusion boundary of sill lies parallel to the layers of country rock independent of whether the layers are horizontal 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 igneous rocks 60

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) igneous rocks 61

91 VEINS deposits of minerals found within a rock fracture that are foreign to the host rock tubes or sheet shaped veins branch off the sides and tops of many intrusive bodies 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 they crystallized from a water-rich magma in the late stage of solidification pegmatites provide ores of many rare elements Li, Be igneous rocks 62

92 VEINS some veins contain minerals with water in the crystal structure crystallised from hot water solution crystallize at 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 igneous rocks 63

93 igneous rocks 64

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 igneous rocks 65

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 - magma is extruded as lava, fed from the magma chambers below the ridge axis - at the same time gabbroic intrusions are emplaced at depth igneous rocks 66

96 subduction zones - where one plate dives below another - are MAJOR SITES OF ROCK MELTING the top of the subducting lithospheric plate includes oceanic crust which is largely basalt originally formed at a mid-ocean ridge this plate also carries water and soft ocean sediment which it accumulated in its trip form the mid-ocean ridge to the subduction zone 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 the presence of water lowers the melting temperature 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 at the same time the magmas may differentiate by fractional crystallization the result is a range of igneous rocks both extrusive and intrusive 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 igneous rocks 67

97 formation of islands oceanic volcanic arcs island arc Aleutian Islands of Alaska, Japan when subduction takes place beneath a continent, the volcanoes join together to form a mountainous arc on land 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. igneous rocks 68

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 midocean ridges & hot-spots add water melt mantle wedge at subduction zone igneous rocks 69

99 Phase diagram for aluminous 4-phase lherzolite: Al-phase = λ Plagioclase Φ shallow (< 50 km) λ Spinel Φ km λ Garnet Φ km λ Si VI coord. Φ > 400 km igneous rocks 70 Figure 10-2 After Wyllie, P. J. (1981). Geol. Rundsch. 70,

100 How does the mantle melt?? 1) Increase the temperature Figure 10-3 igneous rocks 71

101 2) Lower the pressure Φ Adiabatic rise of mantle with no conductive heat loss Φ Decompression melting could melt at least 30% igneous rocks 72

102 3) Add volatiles (especially H 2 O) igneous rocks 73

103 igneous rocks 74

104 igneous rocks 75