Igneous textures and structures

1. Textures of Igneous Rocks Made By Anchit Gupta Badal Dutt Mathur Apurv Singh Deepak Rawat Dhruv Gaur

2. IGNEOUS ROCK TEXTURES - PRINCIPLE The fundamental principle behind igneous rock textures is that grain size is controlled by cooling rate. Thus, rapid cooling at the Earth’s surface of extrusive molten material, or lava, results in the growth of smaller crystals, or prevents crystal growth altogether. Conversely, slow cooling within the Earth’s crust of intrusive molten material, called magma, results in the growth of fewer but larger crystals, because atoms are able to migrate through the liquid to attach themselves to crystals that have already begun to form. The many igneous rock textures are simply variations on or modifications of this principle.

3. Igneous Textures Texture: Individual grains relate to grains immediately surrounding them. I)Textures are useful indicators of cooling and crystallization rates and of phase relations between minerals and magma at the time of crystallization. ii)Texture deals with small-scale features seen in hand specimen or under the microscope, such as the degree of crystallinity. grain size. grain shape, grain orientation, grain boundary relations crystal intergrowths.

4. 1. Degree of crystallinity .

5. 1. Holocrystalline : Consisting entirely of crystal.   2 . Holohyaline Consisting both crystal and glass. 3. Hypocrystalline   Degree of crystallinity Hornblendite Consisting entirely of glass.

6. Phaneritic texture Coarse crystals cooled slowly at great depth

7. Phaneritic – With Evident Crystals Igneous intrusive rocks have evident crystals [the Greek word phaneros means visible or evident] that can be discerned without the aid of microscope .

8. Phaneritic – With smaller crystals Rock : Gabbro Crystals are small in size but easily distinguishable from each other

9. Phaneritic – Economic importance Used as grave markers and facing stone for buildings owing to the coarse size of crystals. Granite

10. A Spectacular Pegmatite Vein of Feldspar and Quartz Phenocryst

11. Pegmatite Extremely coarse-grained igneous intrusive rocks, usually of a felsic composition. C rystal size > 5 cm. Usually formed by concentration of volatiles in magma lowering its viscosity in the late stages of cooling. Attractive and economically significant.

12. Porphyritic texture Granite

13. Porphyritic Phenocrysts – coarser grains Porphyry – contains numerous coarse grains (phenocrysts) in an otherwise fine grained mass

14. Porphyritic Large, evident crystals called phenocrysts are surrounded by an aphanitic matrix or groundmass. Granite Granodiorite Granite

15. Porphyritic 2 stage cooling process: I)Slow cooling of magma underground for growth of phenocrysts ii)Eruption of magma as lava which solidifies quickly allowing growth of only small crystals Cathedral Peak Granodiorite in which K-feldspar crystals are the phenocrysts

16. Granite Porphyry

17. 2. Grain size

18. PHANERIC TEXTURE Is characterized by LARGE SIZE MINERALS which can be easily seen by Naked eye (size at least 2mm or greater) Coarse-grained Phaneric - > 5mm Medium-grained Phaneric - 1 mm - 5mm Fine-grained Phaneric <1 mm

19. A. Equigranular : Rocks with equigranular texture have mineral grains that are generally the same size.  Diameters of component minerals are comparable.  

20. Equigranular granite

21. B. Inequigranular: Not of uniform size Porphyritic texture: One or more mineral species or a generation of one or more mineral species that are conspicuously greater in size than those minerals constituting the rest of the rock. There are number of larger grains called phenocrysts , surrounded by a population of grains of significantly smaller size, the groundmass .

22. 3. Grain shape Anhedral-allotriomorphic Subhedral-hypidiomorphic Euhedral -idiomorphic

23. Allotriomorphic: All the component mineral grains are anhedral .  

24. Hypidiomorphic: Some mineral species are anhedral , those of others subhedral , and those of some may even be euhedral . *Granitic rocks: Quartz and orthoclase- anhedral. *Plagioclase and biotite-subhedral to euhedral.  

25. 3. Idiomorphic Texture All mineral grains euhedral

26. Figure 3.7. Euhedral early pyroxene with late interstitial plagioclase (horizontal twins). Stillwater complex, Montana. Field width 5 mm. © John Winter and Prentice Hall.

27. 4. Grain orientation

28. Trachytic texture - a texture wherein plagioclase grains show a preferred orientation due to flowage, and the interstices between plagioclase grains are occupied by glass or cryptocrystalline material. Trachytic texture in which microphenocrysts of plagioclase are aligned due to flow. Note flow around phenocryst (P).

29. Photomicrograph showing strain bands in trachytic texture in Unit 3b (Sample 197-1205A-10R-2, 73-75 cm) (cross-polarized light; field of view = 5 mm; photomicrograph 1205A-202).

30. Photomicrographs illustrating mineral grains present within the sands and sandstones of Woodlark rift. 5. Hornblende and feldspar phyric colorless vitric volcanic lithic fragment displaying an internal trachytic texture (Sample 180-1115C-12R-4, 144-148 cm [394.34 mbsf ]) (plane-polarized light).

31. Figure 3.12a. Trachytic texture in which microphenocrysts of plagioclase are aligned due to flow. Note flow around phenocryst (P). Trachyte , Germany. Width 1 mm. From MacKenzie et al . (1982). © John Winter and Prentice Hall .

32. Trachytoidal texture: The texture of a phaneritic extrusive igneous rock in which the microlites of a mineral, not necessarily feldspar, in the groundmass have a subparallel or randomly divergent alignment.  

33. cr ystal intergrowths .

34. Sieve textured crystals Are those which contain abundant, small, interconnected, box shaped glass inclusions, giving the crystals a spongy or porous appearance.  

35. Figure 3.11a . Sieve texture in a cumulophyric cluster of plagioclase phenocrysts . Note the later non-sieve rim on the cluster. Andesite , Mt. McLoughlin , OR. Width 1 mm. © John Winter and Prentice Hall.

36. Glomeroporphyritic texture Phenocrysts of the same or different minerals occur in cluster and grow together form a  glomeroporphyritic  texture. Large crystals that are surrounded by finer-grained matrix are referred to as phenocrysts

37. Poikilitic texture  - Refers to small, typically euhedral crystals ( chadacrysts ), that are enclosed (included) within a much larger mineral of different composition. Unlike the porphyritic texture, the large crystals known as oikocrysts , are devoid of crystal faces. Chadacryst also refers to a grain that is foreign to the rest of the rock a.k.a. xenocryst .   Poikilitic texture. Orthopyroxene oikocryst that encloses rounded chadacrysts of olivine

38. Ophitic Textures An igneous texture in which plagioclase grains are completely surrounded by pyroxene grains. Refers to a dense network of lath-shaped plagioclase microphenocryst included in larger pyroxene with little or no associated glass. A single pyroxene envelops several well-developed plagioclase laths.

39. This refers to a common igneous texture found in gabbroic rocks, consisting of plagioclase laths which are partly surrounded by pyroxene grains, and that are partly in contact with other plagioclase grains. Sub-Ophitic

40. A . Photomicrograph of subophitic texture with plagioclase partially enclosed by clinopyroxene B . Photomicrograph of subophitic texture with plagioclase enclosed by olivine

41. Intergranular texture In this texture angular interstices between plagioclase grains are occupied by grains of ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides.                                                                                      Tiny, equant clinopyroxene grains interstitial to plagioclase laths.

42. Compositionally Zoned Plagioclase is abundant, almost completely homogeneous in composition, and is virtually pure anorthite.　No evidence of zoning .　Large olivine grain (bottom center) shows compositional zoning from Mg-rich core to Fe and Ca-rich rims. Angrite in XPL.

43. Angrite in PPL.

44. Compositionally zoned a. hornblende phenocryst with pronounced color variation visible in plane-polarized light. Field width 1 mm. b. Zoned plagioclase twinned on the carlsbad law. Andesite, Crater Lake, OR. Field width 0.3 mm.

45. Graphic Texture Exsolved or devitrified minerals form angular wedge like shapes which look like reminiscent of writing. Graphic texture. The feldspar is white and roughly 10 x 10 centimeters. Quartz are the little gray ones

46. Graphic texture: a single crystal of cuneiform quartz (darker) intergrown with alkali feldspar (lighter).

47. Granophyric Texture Intergrowth of quartz and alkali feldspar the granophyric texture radiates out from large plagioclase grains (lower left-gray, lower right-gray/black). View is under crossed polarizers.

48. Granophyric quartz-alkali feldspar intergrowth at the margin of a 1-cm Golden Horn granite, WA.

49. 5. Grain boundary relations

50. Seriate texture R efers to a situation where there is a continuous range in grain size of one or more mineral species from that of phenocryst to groundmass size, and in which crystals of progressively smaller sizes are increasingly numerous. This texture is commonly shown by plagioclase in some andesite porphyrites .                                               

51. Plagioclase and clinopyroxene phenocrysts in a groundmass of plagioclase, clinopyroxene, and Fe-Ti oxide minerals Medium-grained diabase with interlocking grains of plagioclase, clinopyroxene, and Fe-Ti oxide minerals

52. Myrmekitic texture An intergrowth of plagioclase feldspar (commonly oligoclase) and vermicular quartz, generally replacing potassium feldspar; formed during the later stages of consolidation in an igneous rock or during a subsequent period of plutonic activity. The quartz occurs as blobs. A related term is vermicular quartz. .                                                                                 Myrmekitic texture defined by wormy intergrowths of quartz and K-feldspar in plagioclase which is adjacent to K-feldspar.

53. Perthite is very common in igneous rocks and consists of quantitatively minor lamellae, shreds, patches and rims of an albite component within and around host orthoclase or microcline. Whatever the orientation in thin section, the albite component always has the higher birefringence and appears brighter under crossed nicols , a useful feature in identification, as the exsolution lamellae are generally far too small to show any diagnostic multiple twinning.     Perthitic texture

54. IGNEOUS STRUCTURES 55

55. IGNEOUS STRUCTURES The structures of igneous rocks are large scale features, which are dependent on several factors like: (a) Composition of magma. (b) Viscosity of magma. (c) Temperature and pressure at which cooling and consolidation takes place. (d) Presence of gases and other volatiles. 56

56. Structures developed in igneous rocks are of two types- INTRUSIVE - which form by the crystallization of  magma  at a depth within the Earth. Intrusive rocks are characterized by large crystal sizes, i.e., their visual appearance shows individual crystals interlocked together to form the rock mass. The cooling of magma deep in the Earth is typically much slower than the cooling process at the surface, so larger crystals can grow.  57

57. EXTRUSIVE- which form by the crystallization of  magma  at the surface of the Earth. They are characterized by fine-grained textures because their rapid cooling at or near the surface did not provide enough time for large crystals to grow. Rocks with this fine-grained texture are called  aphanitic rocks . The most common extrusive rock is  basalt . 58

58. Intrusive and Extrusive Igneous Rock Structures 59 Basalt dikes hosted in a granitoid pluton , with metasediment roof pendant; Wallowa Mts , Oregon

59. Igneous Structures Intrusive (Plutonic) Magma cools slowly at depth Characteristic rock texture Characteristic structures 60 Extrusive (Volcanic) Magma cools quickly at surface Characteristic rock textures Characteristic structures

60. Igneous Structures Intrusive Batholith Stock Lopolith Laccolith Volcanic neck Sill Dike Extrusive Lava flow or plateau Volcano (many types) Crater Caldera Fissure 61

61. Intrusive Igneous Structures Contacts (boundary between two rock bodies) can be: Concordant Does not cross cut country rock (surrounding rock) structure, bedding, or metamorphic fabric Ex: laccolith, sill Discordant Cross cuts country rock structure Ex: dike, batholith, stock 62

62. Intrusive Igneous Structures Categorized by depth of emplacement Epizonal Mesozonal Catazonal Depth Shallow <6-10 km Intermediate ~8-14 km Deep >~12 km Contacts Discordant Variable Concordant Size Small to moderate Small to large Small to large Contact metamorphism Very common Uncommon Absent Age Cenozoic Mesozoic-Paleozoic Paleozoic or older 63

63. Intrusive Igneous Structures: Large Scale Major scale intrusive bodies: Plutons Batholith: >100 km 2 in map area (usually discordant) Stock: <100 km 2 in map area Lopolith: dish-shaped layered intrusive rocks (concordant) 64

64. Intrusive Igneous Structures: Intermediate Scale Concordant intrusives Sill: tabular shape Laccolith: mushroom-shaped Roof pendant (remaining country rock) Discordant intrusives Dike: tabular shape Volcanic neck: cylindrical 65

65. Intrusive Igneous Structures: Small Scale Apophyses: Irregular dikes extending from pluton Veins: Tabular body filling a fracture (filled with 1-2 minerals) Xenoliths: Unrelated material in an igneous body Autoliths: Genetically related inclusions (related igneous material) 66

66. Extrusive Igneous Structures Volcanism Directly observable petrologic process Redistributes heat and matter (rocks) from the interior to the exterior of the earth’s surface Occurs in oceanic & continental settings Volcano: Anywhere material reaches earth’s surface 67

67. Extrusive Igneous Structures: Scale Large scale structures Lava plateau (LIP; flood basalt) Ignimbrite (ash flow tuff; pyroclastic sheet) Intermediate scale structures Shield volcano Composite volcano (stratovolcano) Caldera, crater Lava flow or dome Small scale structures Tephra (pyroclastic material) Lava flow features Cinder cone 68

68. Extrusive Igneous Structures: Eruption Styles Effusive Eruptions Lava flows and domes Erupted from localized fissures or vents Generally low silica content (basalt, “primitive” magma) Explosive Eruptions Tephra (fragmental material) Pyroclastic falls or flows Erupted from vents Generally high silica content (felsic, “recycled” magma) Photo glossary of volcano terms 69

69. Extrusive Igneous Structures: Eruption Controls Two main controls on eruption style: VISCOSITY A fluid’s resistance to flow Determined largely by fluid composition DISSOLVED GAS CONTENT Main magmatic gasses: H 2 O, CO 2 , SO 2 (or H 2 S) At high pressure, gasses are dissolved in the magma At low pressure (near surface), gasses form a vapor, expand, and rise = “boiling” Interaction controls eruption style: Gas bubbles rise and escape from low viscosity magma = EFFUSIVE ERUPTION Gas bubbles are trapped in high viscosity magma; increase of pressure = EXPLOSIVE ERUPTION 70

70. Extrusive Igneous Structures: Eruption Controls Two main controls on eruption style: VISCOSITY and DISSOLVED GAS CONTENT 71 In general, both viscosity and gas content are related to magma composition High silica content –> higher viscosity, more dissolved gas Low silica content –> lower viscosity, less dissolved gas

71. Types of Volcanic Products: Effusive Lava Flow Dominantly basalt (low viscosity and gas) Thin and laterally extensive sheets Pahoehoe flows: smooth, ropey flows Aa or block flows: rough and irregular flows Baked zones: oxidized zones due to contact with high temperature lava flow 72 Lava Dome Dacite or rhyolite (high viscosity, low gas content) Thick, steep- sided flows

72. Types of Volcanic Products: Explosive Pyroclastic particles Fragmental volcanic material (TEPHRA) Vitric (glass shards) Crystals Lithic (volcanic rock fragments) Broken during eruption of magma Typically higher silica, high gas content Categorized by size: Ash (< 2.0 mm) Lapilli (2-64 mm) Blocks and bombs (>64 mm) 73 Ash Tephra Bombs

73. Types of Volcanic Products: Explosive Pyroclastic fall (mainly Ash fall ) Material ejected directly from volcano (fallout, “air fall”) Ash, lapilli (pumice, scoria), blocks, and bombs Sorted (small particles carried further) Laterally extensive, mantles topography Pyroclastic flow (nue é ardante or ignimbrite) Fast moving, high density flow of hot ash, crystals, blocks, and/or pumice Follow topographic lows Can be hot enough after deposition to weld, fuse vitric fragments 74

74. Hydroclastic Products Water-magma interaction (phreatomagmatic) causes explosive fragmentation Typically basaltic lavas Any water-magma interaction (sea floor, caldera lake, groundwater) 75 Types of Volcanic Products: Explosive Great volumes of hydroclastics on the sea floor and in the edifice of submarine volcanoes Highly subject to alteration –> clay minerals, microcrystalline silica, and zeolite

75. Styles of Volcanic Eruption : Effusive Lava Plateaus and Flood Basalts (LIPs) Generally low viscosity, low gas content effusive lava flows (basalt) Hot spot and continental rift settings Great areal extent and enormous individual flows Erupted from fissures Examples (no modern): Columbia River Basalt Group Deccan Traps 76

76. Styles of Volcanic Eruption : Effusive Shield volcanoes Generally low viscosity, low gas content effusive lava flows (basalt) Hot spot and continental rift settings Central vent and surrounding broad, gentle sloping volcanic edifice 77 Mauna Loa, Hawaii Repeated eruption of mainly thin, laterally extensive lava flows Modern examples: Mauna Loa, Kiluaea (Hawaii) Krafla (Iceland) Erta Ale (Ethiopia)

77. Styles of Volcanic Eruption : Effusive Submarine eruptions and pillow lava Generally low viscosity, low gas content effusive lava flows (basalt) Divergent margin (mid-ocean ridge) settings Produces rounded “pillows” of lava with glassy outer rind Can produce abundant hydroclastic material (shallow) Modern examples: Loihi , Hawaii 78

78. Styles of Volcanic Eruption : Explosive Cinder cone Generally low viscosity, high gas content (basalt) Subduction zone settings (also continental rifts and continental hot spots) 79 SP Crater, Arizona Small, steep sided pile of loose tephra (mainly lapilli, blocks, and bombs) Scoria or cinder Often form on larger volcanoes (shield or stratovolcano) Modern example: Par í cutin, Mexico

79. Styles of Volcanic Eruption : Explosive Composite cones and Stratovolcanoes Generally higher viscosity, high gas content (andesites) Dominantly subduction zone settings 80 Mayon Volcano Philippines Composed of layers of loose pyroclastic material (fallout and flows) and minor lava flows, some shallow intrusions Form from multiple eruptions over hundreds to thousands of years Examples: Mt. St. Helens, Mt. Rainier (USA) Pinatubo (Indonesia)

80. Styles of Volcanic Eruption : Explosive Calderas and pyroclastic sheet (ignimbrite) deposits Generally high viscosity, high gas content (rhyolite) Subduction zone and continental hot spots 81 Crater Lake, Oregon Form by collapse of volcano following evacuation of the magma chamber Often produce widespread ash, ignimbrite (pyroclastic flow) Examples: Krakatoa, Indonesia (modern example) Crater Lake, Yellowstone (USA)

81. Thank You

82. References http://publications.iodp.org/proceedings/304_305/102/102_2.htm http://www-odp.tamu.edu/publications/180_IR/chap_04/ch4_htm4.htm