Textures ofIgneous Rocks

Made By Anchit GuptaBadal Dutt MathurApurv Singh Deepak Rawat Dhruv Gaur

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.

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.

1. Degree of crystallinity.

1. Holocrystalline: Consisting entirely of crystal.  

2. HolohyalineConsisting both crystal and glass.

3. Hypocrystalline 

Degree of crystallinity

Hornblendite

Consisting entirely of glass.

Phaneritic texture

Coarse crystals cooled slowly at great depth

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.

Phaneritic – With smaller crystals

• Rock : Gabbro

• Crystals are small in size but easily distinguishable from each other

Phaneritic – Economic importance

Used as grave markers and facing stone for buildings owing to the coarse size of crystals.

Granite

A Spectacular Pegmatite Vein of Feldspar and Quartz

Phenocryst

• PegmatiteExtremely coarse-grained igneous intrusive rocks, usually of a felsic composition.

Crystal size > 5 cm. Usually formed by concentration of volatiles in magma lowering its viscosity in the late stages of cooling.Attractive and economically significant.

Porphyritic texture

Granite

Porphyritic

Phenocrysts – coarser grains Porphyry – contains numerous coarse grains

(phenocrysts) in an otherwise fine grained mass

Porphyritic

Large, evident crystals called phenocrysts are surrounded by an aphanitic matrix or groundmass.

Granite

Granodiorite

Granite

Porphyritic2 stage cooling process:I)Slow cooling of magma underground for growth of phenocrystsii)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

Granite Porphyry

2. Grain size

PHANERIC TEXTURE

Is characterized by LARGE SIZE MINERALS which can be easily seen by Naked eye (size at least 2mm or greater)

Coarse-grainedPhaneric- > 5mm

Medium-grainedPhaneric- 1 mm - 5mm

Fine-grainedPhaneric<1 mm

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

Equigranular granite

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.

3. Grain shapeA.Anhedral-allotriomorphic

B. Subhedral-hypidiomorphic

C. Euhedral-idiomorphic

Allotriomorphic: All the component mineral grains are anhedral.  

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.  

3. Idiomorphic TextureAll mineral grains euhedral

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

4. Grain orientation

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).

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).

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).

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.

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. 

crystal intergrowths.

Sieve textured crystals Are those which contain abundant, small,

interconnected, box shaped glass inclusions, giving the crystals a spongy or

porous appearance.  

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.

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

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 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.

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

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

B. Photomicrograph of subophitic texture with plagioclase enclosed by olivine

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.

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.

Angrite in PPL.

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.

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

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

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.

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

5. Grain boundary relations

Seriate texture Refers 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.                                              

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

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.

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

IGNEOUS STRUCTURES

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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.

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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. 

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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.

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INTRUSIVE AND EXTRUSIVE IGNEOUS ROCK STRUCTURES

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Basalt dikes hosted in a granitoid pluton, with metasediment

roof pendant; Wallowa Mts,

Oregon

Igneous Structures

Intrusive (Plutonic) Magma cools slowly at

depth Characteristic rock texture Characteristic structures

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• Extrusive (Volcanic)– Magma cools quickly at

surface– Characteristic rock textures– Characteristic structures

Igneous Structures Intrusive

Batholith Stock Lopolith Laccolith Volcanic

neck Sill Dike

Extrusive Lava flow or

plateau Volcano

(many types)

Crater Caldera Fissure

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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, sillDiscordant

○ Cross cuts country rock structure

○ Ex: dike, batholith, stock

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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

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Intrusive Igneous Structures: Large Scale

Major scale intrusive bodies: PlutonsBatholith: >100 km2 in map area (usually discordant)Stock: <100 km2 in map area Lopolith: dish-shaped layered intrusive

rocks (concordant)

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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

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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)

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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

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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

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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

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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: 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

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Extrusive Igneous Structures: Eruption Controls Two main controls on eruption style:

VISCOSITY and DISSOLVED GAS CONTENT

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– 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

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

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• Lava Dome– Dacite or rhyolite (high viscosity, low gas

content)– Thick,

steep-sidedflows

Types of Volcanic Products: Explosive Pyroclastic particles

Fragmental volcanic material (TEPHRA)○ Vitric (glass shards)○ Crystals○ Lithic (volcanic rock

fragments)Broken during

eruption of magmaTypically higher silica,

high gas contentCategorized by size:

○ Ash (< 2.0 mm)○ Lapilli (2-64 mm)○ Blocks and bombs (>64

mm)

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TephraBombs

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 pumiceFollow topographic lows Can be hot enough after deposition to

weld, fuse vitric fragments74

Hydroclastic ProductsWater-magma interaction (phreatomagmatic) causes

explosive fragmentationTypically basaltic lavasAny water-magma interaction (sea floor, caldera lake,

groundwater)

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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

Styles of Volcanic Eruption: Effusive Lava Plateaus and Floo

d 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 fissuresExamples (no modern):

○ Columbia River Basalt Group

○ Deccan Traps

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Styles of Volcanic Eruption: Effusive Shield volcanoes

Generally low viscosity, low gas content effusive lava flows (basalt)

Hot spot and continental rift settingsCentral vent and surrounding broad, gentle sloping

volcanic edifice

77Mauna Loa, Hawaii

– Repeated eruption of mainly thin, laterally extensive lava flows

– Modern examples:• Mauna Loa, Kiluaea

(Hawaii)• Krafla (Iceland)• Erta Ale (Ethiopia)

Styles of Volcanic Eruption: Effusive

Submarine eruptions and pillow lavaGenerally low viscosity, low gas

content effusive lava flows (basalt)Divergent margin (mid-ocean

ridge) settingsProduces rounded “pillows” of lava

with glassy outer rindCan produce

abundant hydroclastic material (shallow)

Modern examples:○ Loihi, Hawaii

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Styles of Volcanic Eruption: Explosive

Cinder coneGenerally low viscosity, high gas content (basalt)Subduction zone settings (also continental rifts and

continental hot spots)

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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

Styles of Volcanic Eruption: Explosive Composite cones and

StratovolcanoesGenerally higher viscosity,

high gas content (andesites)

Dominantly subduction zone settings

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Mayon VolcanoPhilippines

– 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)

Styles of Volcanic Eruption: Explosive Calderas and pyroclastic

sheet (ignimbrite) deposits Generally high viscosity,

high gas content (rhyolite)Subduction zone and

continental hot spots

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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)

References

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