Igneous Rocks. By: Abboud Suliman Ahmed. With Some Graphics from Press et al., Understanding Earth, 4th Ed.

Transcription

1 Igneous Rocks With Some Graphics from Press et al., Understanding Earth, 4th Ed. (Copyright 2004 by W. H. Freeman & Company) By: Abboud Suliman Ahmed

2 Igneous Rocks Igneous rocks form from molten rock (magma) crystallizing below earth's surface or from volcanic activity. They commonly form at plate boundaries and are commonly exposed in mountainous areas. Igneous rocks form from molten rock (magma) crystallizing below earth's surface or from volcanic activity. They commonly form at plate boundaries and are commonly exposed in mountainous areas. Igneous rocks form from crystallization of magma at depth (within the earth's crust) or at the surface (from volcanic eruptions) There are two (2) basic types or forms of igneous rocks: 1. Plutonic rocks = intrusive igneous rocks = igneous rocks that form from cooling magma at depth 2. Extrusive igneous rocks = igneous rocks that form from volcanic activity (at or near surface)

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4 Key Terminology Plutonic Intrusive Extrusive Volcanic Texture Phaneritic Aphanitic Porphyritic Glassy Vesicular Pyroclastic Magma Lava Bowen s Reaction Series Assimilation Partial melting Fractional crystallization Discordant Concordant Dike Stock Batholith Sill Laccolith Lopolith

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7 Table. 5.2 Felsic Intermediate Mafic Granite Granodiorite Diorite Gabbro Rhyolite Dacite Andesite Basalt Viscosity Melting Temperature

8 Bowen's Reaction Series- Olivine Plagioclase (Ca-feldspar) Pyroxene Amphibole Biotite Orthoclase (K-feldspar) Muscovite Quartz Plagioclase (Na-feldspar)

9 Common Minerals

10 General characteristics of magma Igneous rocks form as molten rock cools and solidifies General characteristics of magma: Parent material of igneous rocks Forms from partial melting of rocks Magma at surface is called lava

11 General characteristics of magma General characteristic of magma Rocks formed from lava = extrusive, or volcanic rocks Rocks formed from magma at depth = intrusive, or plutonic rocks

12 The Rock Cycle- Minerals form rocks All rocks can be transformed into other rock types Rocks are divided into 3 categories Igneous- crystalline- forms as liquid cools Metamorphic- crystalline-forms as rocks are heated and squeezed Sedimentary- non-crystalline- smaller pieces or chemicals from other rocks

13 Magma molten rock below Earth's surface. L ava Igneous formed from Magma and Lava magma on the Earth's surface. Pyroclastic material ( pyro = fire, clastic = debris) Airborne lava cools as it falls

14 Igneous Rock Chemistry Major elements: Approximately 99% of Igneous Rocks are comprised of only eight elements. Oxygen Silicon Aluminum Iron Calcium Sodium Potassium Magnesium The amount of silica (SiO 2 ) determines the mineral content and general color of igneous rocks.

15 Rare Elements: Trace elements are those which occur in very low concentrations in common rocks (usually < 0.1 % by weight). Their concentrations are therefore commonly expressed in parts per million (ppm; 1 ppm = 10-4 weight%). Unlike major elements, trace elements tend to concentrate in fewer minerals, and are therefore more useful in formulating models for magmatic differentiation, and in some cases, in predicting the source of a particular magma. Trace elements most commonly used for the interpretation of the petrogenesis of igneous rocks include: Ni, Cr, Sc, V, Rb, Ba, Sr, Zr, Y, Nb and the rare earth elements (La to Lu).

16 عندما تتبلور المعادن االساسية من الصهير تسمح بعض المعادن المكونة للعناصر النادرة بدخول بنيتها البلورية حيث وضع العلم Goldschmidt عام 1937 قاعدة توضح سلوك العناصر النادرة تسمى قاعدة )Goldschmidt( و هى كاالتى: 1 -عناصر مستترة :)Camouflage( ويسمى االحالل بالتخفى - ايون اساسى )A( و ايون نادر )B( A ++ = B ++ A R = B R مثال mg++ : R=0.80 A و co++ R=0.83 A

17 )Capture( : حالتين A R = B R A ++ B ++++ K + Ba ++ R=1.33 A R=1.34 A A R > B R A +++ = B االسر

18 3 -القبول )Admission( : حالتين A R = B R A +++ = B + mg++ Li+ مثال : R=0.68 A R=0.66A A R < B R A +++ = B +++

19 Sources of Heat for Melting Heat from below a. : Heat moves upward (by conduction and convection) from the very hot (>5000 C) core through the mantle and crust. Minerals start to crystallize from a cooling magma between 1200 C C.

20 b. : i. 3 o C /100 m (30 o C/km) ii. iii. Geothermal Gradient At great depth temperature alone would melt rock BUT high pressure may cause it to remain solid. Not the same everywhere (i.e., It s higher in volcanic regions).

21 c) Radioactive Decay Heat byproduct during decay. High concentration may cause temperature to increase with depth at a rate greater than the geothermal gradient.

22 d) Friction Rock grinding past rock Active Mountain building regions. Friction of moving and shifting rock masses in regions of mountain building may combine with heat from other sources to melt rock.

23 Melting due to the Addition of Volatiles

24 Viscosity of Magma/ Lava Viscosity- important for volcanic activity the resistance of a liquid to flow high viscosity = thick and stiff low viscosity = thin and "runny".

25 Convergent Margins- flux melting

26 Divergent Boundary

27 Hot Spots (e.g. Hawaii)

28 Silica Tetrahedron Silicon Oxygen Silicates are classified on the basis of Si-O polymerism

29 The Silicate structures are known from X-rays. The complexity of igneous rocks are attributed to complexity of silicate structures. Decompositions of mineral depends on silicate structures

30 [SiO 4 ] 4- Independent tetrahedra Nesosilicates Si : O 1 : 4 Example: Olivine group (Fe,mg) 2 SiO 4

31 Garnet A 3 B 2 Si 3 O 12 Usually B is Aluminum, A divalent (Mg,Fe,Mn,Ca) 3 (Fe 3+,Cr,Al) 2 Si 3 O 12 characteristic colors: Pyrope Mg 3 Al 2 Si 3 O 12 deep red to black Almandine Fe 3 Al 2 Si 3 O 12 deep brownish red Spessartine Mn 3 Al 2 Si 3 O 12 brownish red to black Grossular Ca 3 Al 2 Si 3 O 12 yellow-green to brown Andradite Ca 3 Fe 2 Si 3 O 12 variable-yellow, green, brown, black Uvarovite Ca 3 Cr 2 Si 3 O 12 emerald green

32 Common Nesosilicates: The Aluminosilicates Kyanite, Sillimanite, Andalusite Al 2 SiO 5 = Al 2 (SiO 4 )O Topaz Al 2 SiO 4 (F,OH) 2,

33 Si 2 O 7 ] 6- Double tetrahedra Sorosilicates Si : O 2 : 7 Double silicon tetrahedra linked by one bridging oxygen Sorosilicates commonly also contain independent silica tetrahedra (SiO 4 ) -4 Typically monoclinic symmetry

34 Epidote Group Zoisite/Clinozoisite CaAl 3 O(SiO 4 )(Si 2 O 7 )(OH) Epidote Ca 2 (Fe,Al)Al 2 O(SiO 4 )(Si 2 O 7 )(OH)

35 Cyclosilicates n[sio 3 ] 2- n = 3, 4, 6 Si : O 1 : 3 N=3, [Si 3 O 9 ] 6- N=4, [Si 4 O 12 ] 8- N=6, [Si 6 O 18 ] 12- Beryl, Be 3 Al 2 (Si 6 O 18 )

36 Inosilicates single chains [SiO 3 ] 2- Si : O 1 : 3 Example: Pyroxine group (Fe,mg) SiO 3 Diopside, camg si 2 o 6 Augite, ca (mg, Fe) si 2 o 6

37 Inosilicates Double- chains [Si 4 O 11 ] 6- Si : O 4 : 11 Example: amphibole minerals Actinolite, ca 2 (mg,fe) 5 Si 8 O 22 (OH) 2

38 Phyllosilicates Sheets of tetrahedra [Si 2 O 5 ] 2- Si : O 2 : 5 Mica group: Muscovite, KAl 2 (AlSi 3 )O 10 (OH) 2

39 Tectosilicates [SiO 2 ] Si : O 1 : 2 3-D frameworks of tetrahedral: fully polymerized quartz and the silica minerals feldspars feldspathoids zeolites Examples: Quartz, SiO 2. Orthoclase, KAlSi 3 O 8. Plagioclase, CaAl 2 Si 2 O 8

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41 Plate Tectonic - Igneous Genesis 1. Mid-ocean Ridges 2. Intracontinental Rifts 3. Island Arcs 4. Active Continental Margins 5. Back-arc Basins 6. Ocean Island Basalts 7. Miscellaneous Intra- Continental Activity kimberlites, carbonatites, anorthosites...

42 Binary phase diagram for a solid solution of Olivine Fayallite (Fa) % Fo (Mg 2 SiO 4 ) Forsterite (Fo)

43 quartz hornblende feldspar

44 Olivine (Mg,Fe) 2 SiO 4 Pyroxene (Mg,Fe)SiO 3 Amphibole Ca 2 (Mg,Fe) 5 Si (OH) 2 Biotite Mica K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2 Single tetrahedra No cleavage Single chains Double chains 2 cleavages (90 o ) 2 cleavages (60 o and 120 o ) Sheets 1 cleavage

45 Olivine Found in mafic and ultramafic igneous rocks, can be found as beach sand from volcanic islands, used in manufacturing of basic refractories, or as magnesium ore, gem quality peridot Micas - Found in igneous (pegmatities and granites) and metamorphic rocks (schists and amphibolites), used for elect and heat insulation, paper products, fireproof paint and dry lubricants Hematite Found in extrusive igneous rocks and secondary mineral of sed deposits (oxidizing environment), most important iron ore, red ochre pigments and polishing powders

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47 Hornblende a member of the amphibole group

48 Potassium feldspar

49 Plagioclase feldspar

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