Igneous rocks form from molten rock (magma) crystallizing below Earth's surface or from volcanic activity. There are two basic types: plutonic rocks, which form at depth and have a coarse-grained texture; and extrusive rocks, which form at or near the surface and have a fine-grained texture. Igneous structures include intrusive bodies like batholiths, stocks, dikes and sills; and extrusive features like lava flows, domes, and volcanic constructs like cinder cones, shield volcanoes, and composite volcanoes. Igneous rocks exhibit a variety of textures that provide clues to their cooling history, such as phaneritic, porphyritic,

1. IGNEOUS ROCKSIGNEOUS ROCKS
Dr. P. Sarathbabu M.Sc. B.Ed. Ph.D.
Department of Geology
Acharya Nagarjuna University

2. 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.
magma
volcano
Magma
cools and
solidifies
forming
igneous
rocks

3. What is Igneous Rock?

4. There are two (2) basic types or forms of igneous
rocks:
1.1. PlutonicPlutonic rocksrocks = intrusive igneous rocks = igneous
rocks that form from cooling magma at depth
2. ExtrusiveExtrusive igneous rocksigneous rocks = igneous rocks that form
from volcanic activity (at or near surface)
*Plutonic rocks are usually coarse-grained
*Extrusive rocks are usually fine-grained

5. Intrusive igneous rock
Extrusive igneous rock

6. Formation of Igneous Rocks
Pyroclasts
Extrusive
Intrusive
Porphyry:
partially
crystallineFig. 5.3

7. Igneous Rocks
• Intrusive igneous rocks are coarse-grained
and cool slowly beneath Earth’s surface.

8. Igneous Rocks
• Extrusive igneous rocks are fine-grained
rocks that cool quickly on Earth’s surface.

9. 9
Igneous Structures
• Intrusive
• Batholith
• Stock
• Lopolith
• Laccolith
• Volcanic
neck
• Sill
• Dike
• Extrusive
• Lava flow
or plateau
• Volcano
(many
types)
• Crater
• Caldera
• Fissure

11. BatholithsBatholiths are huge igneous intrusions made of many
stocks. Their size is on the scale of an entire
mountain range (100’s of miles).
StocksStocks are fairly large (10’s of miles) igneous
intrusions that cut across pre-existing rock layers.
In size, they are on the order of an individual
mountain peak.
LopolithsLopoliths are also large, inverted mushroom-shaped
(spoon-shaped) intrusions that “sag down” in the
middle because of dense rocks.
LaccolithsLaccoliths are rather large, mushroom-shaped
intrusions that “puff up” in the center due to gases.

12. SillsSills are also small igneous intrusions. They are sheets
of rock that, unlike dikes, are parallel to pre-existing
rocks. Think of magma invading sedimentary rocks by
spreading out between rock layers. That magma would
cool to form a sill.
Dykes These are discordant igneous bodies exhibit a
cross cutting relationship with country rock

13. 13
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

14. 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)

15. 15
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

16. 16
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)

17. 17
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

18. 18
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

19. 19
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)

20. 20
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

22. 22
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

23. 23
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)
Ash
TephraBombs

24. 24
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

25. Direct measurements of
pyroclastic flows are extremely
dangerous!!!

26. Pompeii (79AD) The cities remained
buried and
undiscovered for almost
1700 years until
excavation began in
1748. These
excavations continue
today and provide
insight into life during
the Roman Empire.

27. Pyroclastic Flow - burial

28. Pompeii (79AD)
Pyroclastic flows of poisonous
gas and hot volcanic debris
engulfed the cities of Pompeii,
Herculaneum and Stabiae
suffocating the inhabitants
and burying the buildings.

29. Mount VesuviusMount Vesuvius
• Images of victims in
eruption of Vesuvius in
79 AD. Most died as a
result of suffocation.

30. Effusive Eruptions
• Effusive eruptions are characterised by
outpourings of lava on to the ground.
Hawaii
Courtesy of www.swisseduc.ch

31. • An eruption of Mt Peleé in 1902 produced a pyroclastic flow that
destroyed the city of St. Pierre.
before after
Mt Peleé, Martinique (1902)

32. Types of VolcanoesTypes of Volcanoes
• Different types of volcanic eruptions form
different types of volcanoes.
• Cinder cones
• Shield volcanoes
• Composite volcanoes

33. Cinder ConesCinder Cones
• Volcanoes made mostly of cinders and other
rock particles that have been blown into the
air are called cinder cones.
• Cinder cones form from explosive eruptions.
Because the material is loosely arranged, the
cones are not high. They have a narrow base
and steep sides such as Paricutin in Mexico.

34. Cinder Cone VolcanoCinder Cone Volcano

35. Shield VolcanoesShield Volcanoes
• Volcanoes composed of quiet flows are
called shield volcanoes. Because it is
rummy, the lava flows over a large area.
After several eruptions, a dome-shaped
mountain is formed such as Mauna Loa (4km
over sea level) in the Hawaiian Islands.

36. Composite VolcanoesComposite Volcanoes
• Volcanoes built up of alternating layers of rock particles
and lava are called composite volcanoes.
• During the formation of a composite volcano, a violent
eruption first occurs, hurling volcanic bombs, cinder and
ash out of the vent.
• Then a quiet eruption, produces lava flow that covers the
rock particles. After alternating eruptions, a cone-shaped
mountain forms such as Mount Vesuvius.

37. Mount VesuviusMount Vesuvius

38. CraterCrater
• There is often a funnel-shaped pit or depression at
the top of a volcanic cone. This pit is called a crater.
If the crater becomes very large as a result of the
collapse of its walls, it is called a caldera. A caldera
may also form when the top of a volcano explodes or
collapses.

39. Caldera : Some times because of violent
volcanic explosion the entire central portion
of the volcano is destroyed and only a great
central depression, named a caldera.
The calderas may be also be formed due to
erosion and enlargement of the cratar.

40. Mount VesuviusMount Vesuvius
• Images of victims in
eruption of Vesuvius in
79 AD. Most died as a
result of suffocation.

41. Textural classification of
igneous rocks
Phaneritic: crystals visible with naked eye
Plutonic or intrusive rocks
Aphanitic: crystal too small for naked eye
Volcanic or extrusive rocks
Porphyritic: two different, dominant grain sizes
Large xtals = phenocrysts; small xtals = groundmass
Fragmental: composed of disagregated igneous material
Pyroclastic rocks

42. Igneous rock texturestextures depend on cooling historycooling history
Intrusive textures:
1.1. Fine-grained texture (Fine-grained texture (AphaniticAphanitic)) -- due to fast cooling
(at or near surface)
2. Coarse-grained texture (Coarse-grained texture (PhaneriticPhaneritic)) -- due to slow
cooling at depth
3. PorphyriticPorphyritic texturetexture -- coarse crystals (phenocrysts)
surrounded by fine-grained matrix (groundmass)
forms due to initial slow cooling, then magma rising to
(or close to) surface and the remaining magma cooling
quickly

43. Textural classification of
igneous rocks
Pegmatitic: very large xtals (cm to 10s of cm); i.e., slowly cooled
Forms veins or layers within plutonic body
Glassy: non-crystalline; cools very fast (e.g., obsidian)
Volcanic rocks
Vesicular: vesicles (holes, pores, cavities) form as gases expand
Volcanic rocks

44. Extrusive textures:
4. GlassyGlassy texturetexture -- due to very rapid cooling -- magma
cools so fast crystals don't have time to form. Obsidian
(volcanic glass) forms this way.
5. VesicularVesicular texturetexture -- full of rounded holes (vesicles) --
forms due to escape of gas bubbles during cooling of
lava. Pumice is a light-colored rock with this vesicular
texture.
6. PyroclasticPyroclastic texturetexture- chunks of molten material that fuse
together

45. Coarse-grained Fine-grained PorphyriticCoarse-grained Fine-grained Porphyritic
Glassy Vesicular PyroclasticGlassy Vesicular Pyroclastic
Igneous Rock TexturesIgneous Rock Textures

46. Igneous rocks
There are 5 main kinds of igneous rocks, depending on
the mix of minerals in the rocks.
1.GRANITE
Grain size : Coarse grained
Usual Colour : Light (Leucocratic)
Structure : Holocrystalline
Texture : Fine – coarse, Equigranular
Composition : feldspar and quartz with minor
mica, biotite, hornblende
Occurrence: Its occurs as major intrusive bodies,
such as batholiths and stocks.

48. BASALT

49. Igneous rocks
1.BASALT
Grain size : Fine graine to very fine grained
Usual Colour : Dark colour (melanocratic)
Structure : vesicular and amygdaloidal, columnar,
Texture : Aphanitic texture
Composition : augite, iron oxides, biotite, magnetite,
olivine, quartz
Occurrence: Its most abundant among volcanic
rocks.
Uses: Its used as foundation rock, building stones,
road metal etc.

50. Dolerite

51. Igneous rocks
1.DOLERITE
Grain size : Fine graine
Usual Colour : Dark colour (melanocratic)
Structure : It is very dense, massive and compact
Texture : Phaneric (equigranular) texture
Composition : contains feldspars, hornblende (dark)
is chief mineral, ilmenite,magnatite, qtz and
biotite minerals are accessories
Occurrence: Its occurs as dyke (intrusive)
Uses: It is used as railway ballast, concrete, road
metal

52. 5. Pegmatite
Grain size :
Structure :
Texture : Composition :

53. Igneous rocks
1. PEGMATITE
Grain size : Coarse grained, beautiful crystal outlines
Usual Colour : Usual Colour : White (Leucocratic)
Structure : Holo-crystalline (large crystals)
Texture : inequigranular (phaneric)
Composition : contains Feldspar, Quartz, Mica and
Biotite.
Occurrence: Its occurs as final magmatic process,
occurs as viens in intrusive rocks
Uses: It is used as economic minerals.

54. Igneous rocks
1. CHARNOCKITE
Grain size : Coarse grained, beautiful crystal outlines
UsualColour:BlackColour(Melanocratic)
Structure : massive and dense
Texture : Coarse grained, foliation (phaneric)
Composition : contains Feldspar, Quartz, Mica and
Biotite.
Occurrence: Its occurs as igneous rocks but shows
metamorphic character
Uses: These are hard, strong and durable rocks with hi
load bearing capacity. It is suitable engineering work.