IGNEOUS ROCK TEXTURES - PRINCIPLEThe 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.
Igneous TexturesTexture: 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.
Porphyritic2 stage coolingprocess:I)Slow cooling ofmagmaunderground forgrowth ofphenocrystsii)Eruption ofmagma as lavawhich solidifiesquickly allowinggrowth of only Cathedral Peak Granodiorite in which K-feldspar crystalssmall crystals are the phenocrysts
PHANERIC TEXTURE Is characterized by LARGE SIZE MINERALS which can be easily seen by Naked eye (size at least 2mm or greater)Coarse-grained Medium-grained Fine-grainedPhaneric Phaneric Phaneric- > 5mm - 1 mm - 5mm <1 mm
A. Equigranular: Rocks with equigranular texture have mineral grains that aregenerally the same size. Diameters of component minerals arecomparable.
B. Inequigranular: Not of uniform sizePorphyritic texture: One or more mineral species or a generation of one or moremineral species that are conspicuously greater in size than those mineralsconstituting the rest of the rock. There are number of larger grains calledphenocrysts, surrounded by a population of grains of significantlysmaller size, the groundmass.
Allotriomorphic:All the componentmineral grains areanhedral.
Hypidiomorphic: Some mineral species are anhedral, thoseof others subhedral, and those of some may even beeuhedral.*Granitic rocks: Quartz and orthoclase- anhedral.*Plagioclase and biotite-subhedral to euhedral.
3. Idiomorphic TextureAll mineral grains euhedral
Trachytic texture - a texture wherein plagioclase grains show a preferred orientationdue to flowage, and the interstices between plagioclase grains are occupied by glassor cryptocrystalline material. Trachytic texture in which microphenocrysts of plagioclase are aligned due to flow. Note flow around phenocryst (P).
Photomicrograph showing strain bands in trachytic texturein Unit 3b (Sample 197-1205A-10R-2, 73-75 cm) (cross-polarized light; field of view = 5 mm; photomicrograph1205A-202).
Photomicrographs illustrating mineral grains present withinthe sands and sandstones of Woodlark rift. 5. Hornblendeand feldspar phyric colorless vitric volcanic lithic fragmentdisplaying an internal trachytic texture (Sample 180-1115C-12R-4, 144-148 cm [394.34 mbsf]) (plane-polarized light).
Trachytoidal texture:The texture of a phaneritic extrusive igneous rock in which themicrolites of a mineral, not necessarily feldspar, in thegroundmass have a subparallel or randomly divergentalignment.
Glomeroporphyritic texturePhenocrysts of the same or different minerals occur in cluster and grow together forma glomeroporphyritic texture.Large crystals that are surrounded by finer-grained matrix are referred to asphenocrysts
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
Ophitic TexturesAn igneous texture in whichplagioclase grains are completelysurrounded by pyroxene grains.Refers to a dense network oflath-shaped plagioclasemicrophenocryst included inlarger pyroxene with little or noassociated glass. A single pyroxene envelops several well- developed plagioclase laths.
Sub-Ophitic This refers to a common igneous texture found ingabbroic rocks, consisting of plagioclase laths which arepartly surrounded by pyroxene grains, and that are partlyin contact with other plagioclase grains.
A . Photomicrograph of subophitictexture with plagioclase partiallyenclosed by clinopyroxeneB. Photomicrograph ofsubophitic texture withplagioclase enclosed by olivine
Intergranular textureIn 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.
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.
Compositionally zoneda.hornblende phenocryst withpronounced color variationvisible in plane-polarized light.Field width 1 mm.b. Zoned plagioclase twinned onthe carlsbad law. Andesite, CraterLake, OR. Field width 0.3 mm.
Graphic Texture Exsolved or devitrified minerals form angular wedge like shapes which look like reminiscent of writing.Graphic texture. The feldspar iswhite and roughly 10 x 10centimeters. Quartz are the littlegray ones
Graphic texture: a singlecrystal of cuneiform quartz(darker) intergrown withalkali feldspar (lighter).
Granophyric TextureIntergrowth of quartz andalkali feldsparthe granophyric textureradiates out from largeplagioclase grains (lowerleft-gray, lower right-gray/black). View is undercrossed polarizers.
Granophyric quartz-alkalifeldspar intergrowth at themargin of a 1-cm GoldenHorn granite, WA.
Seriate texture Refers to a situation where there is a continuousrange in grain size of one or more mineral speciesfrom that of phenocryst to groundmass size, and inwhich crystals of progressively smaller sizes areincreasingly numerous. This texture is commonlyshown by plagioclase in some andesiteporphyrites.
Plagioclase and clinopyroxenephenocrysts in a groundmass ofplagioclase, clinopyroxene, and Fe-Tioxide minerals Medium-grained diabase with interlocking grains of plagioclase, clinopyroxene, and Fe- Ti oxide minerals
Myrmekitic textureAn intergrowth of plagioclase feldspar (commonlyoligoclase) and vermicular quartz, generally replacingpotassium feldspar; formed during the later stages ofconsolidation in an igneous rock or during asubsequent period of plutonic activity. The quartzoccurs 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.
Perthitic texturePerthite is very common in igneous rocks and consists of quantitatively minorlamellae, shreds, patches and rims of an albite component within and aroundhost orthoclase or microcline. Whatever the orientation in thin section, the albitecomponent always has the higher birefringence and appears brighter undercrossed nicols, a useful feature in identification, as the exsolution lamellae aregenerally far too small to show any diagnostic multiple twinning.
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
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
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 aphaniticrocks. The most common extrusive rock is basalt. 58
Basalt dikes hosted in a granitoid pluton, withmetasediment roof pendant; Wallowa Mts, Oregon 59
Igneous Structures Intrusive (Plutonic) • Extrusive (Volcanic) Magma cools slowly at – Magma cools quickly at depth surface Characteristic rock texture – Characteristic rock textures Characteristic structures – Characteristic structures 60
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
Intrusive Igneous Structures Categorized by depth of emplacement Epizonal Mesozonal CatazonalDepth Shallow Intermediate Deep <6-10 km ~8-14 km >~12 kmContacts Discordant Variable ConcordantSize Small to Small to large Small to large moderateContact Very common Uncommon AbsentmetamorphismAge Cenozoic Mesozoic- Paleozoic or Paleozoic older 63
Intrusive Igneous Structures:Large Scale Major scale intrusive bodies: Plutons Batholith: >100 km2 in map area (usually discordant) Stock: <100 km2 in map area Lopolith: dish-shaped layered intrusive rocks (concordant) 64
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
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
Extrusive Igneous Structures: EruptionStyles 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
Extrusive Igneous Structures: EruptionControls Two main controls on eruption style: VISCOSITY ○ A fluid’s resistance to flow ○ Determined largely by fluid composition DISSOLVED GAS CONTENT ○ Main magmatic gasses: H2O, CO2, SO2 (or H2S) ○ 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
Extrusive Igneous Structures: EruptionControls Two main controls on eruption style: VISCOSITY and DISSOLVED GAS CONTENT – 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• 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 Bombs Tephra ○ 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) Ash 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 (nueé 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
Types of Volcanic Products: Explosive Hydroclastic Products Water-magma interaction (phreatomagmatic) causes explosive fragmentation Typically basaltic lavas Any water-magma interaction (sea floor, caldera lake, groundwater) – 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
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 – Repeated eruption of mainly thin, laterally extensive lava flows – Modern examples: • Mauna Loa, Kiluaea (Hawaii) • Krafla (Iceland) • Erta Ale (Ethiopia) Mauna Loa, Hawaii 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
Styles of Volcanic Eruption: Explosive Cinder cone Generally low viscosity, high gas content (basalt) Subduction zone settings (also continental rifts and continental hot spots)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: • Parícutin, Mexico 79
Styles of Volcanic Eruption: Explosive Composite cones and Mayon Volcano Stratovolcanoes Philippines Generally higher viscosity, high gas content (andesites) Dominantly subduction zone settings – 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 Crater Lake, continental hot spots 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