Igneous rock textures are controlled by cooling rate, with rapid cooling resulting in smaller crystals and slower cooling allowing larger crystals to form. Textures provide information about cooling/crystallization rates and phase relations during crystallization. Textures describe grain features like size, shape, orientation, and boundaries, seen in hand samples or microscopically. Common textures include phaneritic (with evident crystals), porphyritic (with larger phenocrysts in fine-grained groundmass), and graphic (with exsolved minerals forming angular shapes). Compositionally zoned crystals also occur.

1. Textures of
Igneous Rocks

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. Degree of
crystallinit
1. Holocrystalline:
y
Consisting entirely of
crystal.
2. Holohyaline
Consisting both crystal and
glass.
3. Hypocrystalline 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. Phenocryst
A Spectacular Pegmatite Vein

11. • Pegmatite
Extremely 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.

12. Porphyritic texture
Granite

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

14. Porphyriti
c
Large, evident
crystals called
phenocrysts are
surrounded by
an aphanitic Granodiorite
matrix or
groundmass.
Granite
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 Cathedral Peak Granodiorite in which K-feldspar crystals
small 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 Medium-grained Fine-grained
Phaneric Phaneric Phaneric
- > 5mm - 1 mm - 5mm <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
A.Anhedral-allotriomorphic
B. Subhedral-
hypidiomorphic
C. 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. crystal 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. Sub-Ophitic
 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.

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

42. 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.

43. 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.

44. Angrite in PPL.

45. 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.

46. 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

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

48. 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.

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

50. 5. Grain
boundary
relations

51. 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.

52. 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

53. 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.

54. Perthitic texture
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.

55. IGNEOUS
STRUCTURES
55

56. 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

57.  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

58.  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

59. Basalt dikes hosted
in a granitoid
pluton, with
metasediment roof
pendant; Wallowa
Mts, Oregon
59

60. 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

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

62. 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

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

64. 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

65. 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

66. 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

67. 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

68. 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

69. 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

70. 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
70

71. Extrusive Igneous Structures: Eruption
Controls
 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

72. 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

73. 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

74. 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

75. 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

76. 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

77. 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

78. 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

79. 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

80. 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

81. 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

83. 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