1. Igneous Rocks
I.G.Kenyon

2. Definition of Igneous
Derived from the latin
‘ignis’ meaning fire
Formed by the cooling
and solidification of molten
lava or magma
Comprise an interlocking
mosaic of crystals

3. Extrusive Igneous Rocks
Molten rock reaching the
earth’s surface via volcanoes
(lava) is termed extrusive

4. Intrusive Igneous rocks
Molten rock (magma) that solidifies at
depth within the lithosphere is intrusive
Intrusive rocks may eventually be
exposed at the earth’s surface following
a long period of uplift and erosion

5. Crystal Size and Cooling Rates
Crystal size is determined
by the rate of cooling of
the magma or lava
Instantaneous cooling of lava
erupted under water as pillow
lavas results in a glassy texture,
devoid of any crystalline form

6. Crystal Size and Cooling Rates
Rapid cooling in lava flows at the
earth’s surface over a few months
results in crystals of <0.5mm in
diameter forming (Volcanic)

7. Crystal Size and Cooling Rates
Slower cooling in dykes
and sills over hundreds to
thousands of years results in
crystals 0.5mm to 2mm in
diameter (Hypabyssal)

8. Crystal Size and Cooling Rates
Slow cooling in magma chambers
deep underground over millions of
years results in larger crystals
>2mm in diameter (Plutonic)

9. Crystal Shape 1- Euhedral
Well formed crystals with a
regular and recognisable shape.
They form when a crystals can
grow freely in a melt and are not
impeded by the presence of any
surrounding pre-existing crystals

10. Euhedral Olivine
Six-sided shape
3mm
Olivine basalt from Ubekendt Ejland, West Greenland

11. Crystal Shape 2 - Subhedral
Partially formed crystals with
some recognisable shape. They
have been partially impeded as
they grew by the surrounding
pre-existing crystals

12. Subhedral Olivine
Some faces flat and planar
1mm
Some faces curved
and embayed
Picritic basalt, Ubekendt Ejland, West Greenland

13. Crystal Shape 3 - Anhedral
Anhedral – no regular
crystalline shape visible.
The shape of the growing
crystal is controlled by the
arrangement and orientation
of the surrounding pre-
existing crystals

14. Anhedral Olivine
Olivine basalt from Mauritius, Indian Ocean
1mm
Irregular outline with no
planar faces evident

15. Phenocrysts
Large well formed (euhedral)
crystals in an igneous rock
In Shap granite the flesh
coloured orthoclase phenocrysts
are up to 3cm in diameter

16. Groundmass
The remainder of the igneous
rock made up of smaller crystals
In the case of Shap granite, the
groundmass is mainly crystals
of biotite mica and quartz

17. Phenocrysts and Groundmass
Orthoclase phenocrysts
up to 6cm in diameter
Phenocrysts are euhedral
and rectangular
Implies 2 stage
cooling history
1cm
Finer groundmass
0.5-1.0 mm in
diameter

18. Equigranular Texture
All the crystals in the rock
are roughly the same size
Produced by a steady or
constant cooling rate

19. Equigranular Texture
2cm
Microgranite – even cooling, all crystals 0.5 – 1.0mm

20. Porphyritic Texture
Large crystals (phenocrysts)
set in a finer grained
groundmass
Produced by two-stage cooling

21. Porphyritic Texture-Giant Feldspar Porphyry
Phenocrysts up
to 5cm long
Long axes of phenocrysts
aligned parallel implies
flow of magma
Groundmass 0.5-1.0mm

22. Vesicular Texture
Small spherical or ellipsoidal
cavities found in lavas
Formed by gas bubbles being
trapped during solidification
of the rock. Eg Pumice

23. Vesicular Texture
Vesicles represent trapped gas bubbles within a lava flow
Vesicles range from 2mm
to 1.5cm in diameter
Vesicles are stretched and
curved indicating flow of the lava
Car key for scale

24. Glassy Texture
No crystals visible, rocks are
often dark green or black in
colour and show conchoidal
fracture (like glass)
Eg Obsidian formed by the
instantaneous cooling of acid lava

25. Glassy Texture - Obsidian
1cm
Shows Conchoidal Fracture

26. Amygdaloidal Texture
The vesicles in a lava are later
infilled by secondary minerals
precipitated from solution
Commonly quartz and calcite
Amygdale means ‘almond-shaped’

27. Amygdaloidal Texture
Former vesicles Basalt, volcanic,
infilled by quartz crystals <0.5mm
Euro coin
for scale

28. Mineral Content
Igneous rocks are classified
chemically as Acidic or Basic
according to the main
constituent minerals present

29. Felsic Igneous Rocks
Quartz, Orthoclase Feldspar,
Plagioclase Feldspar, Biotite
Mica and Muscovite Mica.
Rich in silica >66%

30. Mafic Igneous Rocks
Plagioclase Feldspar,
Augite and Olivine
Contain less silica 45 – 55%

31. Igneous Rock Classification
Felsic Mafic
Quartz, feldspar Plagioclase feldspar,
and mica augite and olivine
Volcanic
Crystal size
<0.5mm in diameter
Rhyolite Basalt
Hypabyssal
Crystal size
0.5-2mm in diameter
Microgranite Dolerite
Plutonic
Crystal size
>2mm in diameter
Granite Gabbro

32. Cornish Granite
All crystals over 2mm
Glassy,
in diameter-Plutonic
colourless quartz
1cm
Black biotite mica
with pearly lustre
Subhedral
crystal form
White/creamy
plagioclase feldspar

33. Shap Granite (Ademallite)
Porphyritic texture, large
phenocrysts and finer groundmass
Finer groundmass of
quartz and biotite mica Feldspar phenocrysts
2-3mm in diameter are euhedral
1cm
Flesh-coloured orthoclase feldspar
phenocrysts up to 3cm long

34. Kaolinised Granite
Iron oxide staining due to release
of Fe ions from biotite mica
Biotite mica
breaking down
Orthoclase feldspar to form chlorite
altered to kaolinite
by hydrolysis
Unaltered grey, glassy quartz
Granite is very crumbly and
is described as Growan

35. Micro-Granite
Formed within the crust
Mineralogy: quartz,
in a sill or dyke
feldspar and mica
Subhedral
crystals
Equigranular texture,
all crystals 0.5 – 1.5mm
in diameter
Formed by an even
cooling rate over 2 cm
thousands of years

36. Vesicular Rhyolite
1 cm
Formed by rapid cooling
at the earth’s surface
Spherical vesicles up
to 3mm in diameter
Fine grained < 1mm, no
crystals visible, volcanic
Mineralogy: quartz,
Vesicles represent trapped feldspar and mica
gas bubbles in a lava flow

37. Gabbro
Greenish-black augite
Equigranular texture, all
crystals roughly similar in size
Formed deep
underground by very
slow cooling over
millions of years
2cm
Coarse grained, crystals
over 2mm in diameter,
suggesting slow cooling Grey/creamy plagioclase
feldspar, variety calcium
rich anorthite

38. Porphyritic Dolerite (Micro-gabbro)
Hypabyssal, crystal Mineralogy: plagioclase
size mainly 1-2mm feldspar, augite and olivine
Subhedral phenocrysts
of plagioclase feldspar
up to 3mm in diameter
Groundmass
constitutes over
75% of the rock
1 cm
Two-stage cooling, finally
forming an intrusive dyke or sill

39. Basalt
1 cm
Chilled margin,
very fine grained
almost glassy
Formed by rapid
cooling at the earth’s Mineralogy: plagioclase
surface over a few feldspar, augite and olivine
weeks or months
Crystal size well under
0.25mm, volcanic

40. Pyroclastic Rocks
Consist of fragmental volcanic
material blown into the atmosphere
by explosive activity
Mainly associated with andesitic
and acidic volcanoes

41. Pyroclastic Rocks – 2 Main Groups
Material ejected from the volcano as
liquid globules which solidifies in the
air and is deposited as solid particles
Material ejected from the volcano
as solid fragments, this solid
material has been fractured
by the explosive activity

42. Materials Ejected in a Liquid State
Pelées Hair
Volcanic Bombs
Pumice
Scoriae

43. Pelées Hair
A fine mass of
hair-like glass
Formed by lava being
exuded through a small
orifice and blown
about by the wind
Resembles candy
floss in 1cm
appearance

44. Volcanic Bombs
Larger masses of liquid
lava thrown into the air
They rotate and take on
characteristic shapes
Spindle-bombs and breadcrust bombs
are most common-usually vesicular
Vary in size from small droplets to
several cubic metres

45. Volcanic Bombs
Some bombs have a Volcanic bombs are large
characteristic breadcrust fragments of molten lava up
surface, others resemble to 1m in diameter expelled
cauliflowers or cowpats during an eruption.
depending on the way
they land and solidify.
Bombs develop a rounded or almond
shape as they are twirled through the air.

46. Section through a Volcanic Bomb
Highly vesicular interior
Breadcrust exterior, finer
5cm grained and less vesicular
due to more rapid cooling

47. Pumice
Highly vesicular material
derived from acid lavas
Very high porosity
and low density
So light that it may
float on water

48. Pumice
Specimen from
Mt. Teide,
Tenerife Mineralogy: quartz,
feldspar and mica
Volcanic,
felsic igneous
rock
Low density, high Vesicles up to
porosity, floats on water 3cm in diameter
Microscopic grain size, very rapid
cooling at the earth’s surface 2cm

49. Scoriae
Associated with basic lavas
Vesicular but denser than pumice
Globules of lava are ejected and
the exterior chills and solidifies
Interior is still hot and molten
Upon landing they are still soft and
are flattened into pancake shapes

50. Section through Strombolian Scoriae Cone
Bedding dips 32°SE
The cone has been half excavated for
use in the construction industry
Some layers rich in
volcanic bombs
2m Strombolian refers to the style of
pyroclastic eruption in which
fragments of incandescent,
vesiculating basaltic magma are
ejected to a moderate height, landing
as solid scoria to form a cone

51. Strombolian Scoriae Cone
Volcanic bombs occur
up to 50cm in diameter
The structure is very friable
and has an unstable surface
Scoriae clasts range
in size from 3 to 15cm

52. Material Ejected in a Solid State
Agglomerate-fragments >64mm in diameter
Lapilli – fragments 64mm - 2mm in diameter
Ash, Tuff & Dust – fragments <2mm in diameter

53. Agglomerate – Volcanic Breccia
Derived from agglomero
meaning ‘gather into a heap’
Formed of volcanic or country rock
in the vent or as part of the cone
Produced by explosive activity which
often shatters the top of the cone
Coarse material is ejected a relatively short
distance before settling back to earth
Comprises angular fragments >32mm
surrounded by finer tuff and lapilli

54. Agglomerate – Volcanic Breccia
Large angular fragments
up to 10cm in diameter
5cm
Vent
Agglomerate
Large fragments surrounded
by material of ash and lapilli size

55. Agglomerate–Volcanic Breccia, Arico, Tenerife
Chaotic mixture of boulders over
2m to ash <2mm in diameter

56. Lapilli – Particles 2 – 64mm
Derived from lapillus
meaning ‘a little stone’
Most commonly small
pea to walnut sized

57. Tuff – Particles <2mm
The lithified equivalent of volcanic ash
Classified according to the nature
of the pyroclastic fragments
Crystal Tuffs – composed of mainly crystals
Lithic Tuffs – composed of fragments of rock
Vitric Tuffs – composed of glassy fragments
Welded Tuffs (Ignimbrites) – hot fragments
welded together in Nuées Ardentes eruptions

58. The Formation of Ignimbrites
Associated with Pyroclastic Flows
Nueés Ardentes style eruptions
Glowing fireclouds 300-1000°C
Particles weld together on settling
Activity on Augustine, Alaska, photograph by M.Krafft

59. The Chimiche Ignimbrite, Arico, Tenerife
It is unwelded as the
particles were cool by the
15m
time they had fallen 10-15km
through the atmosphere
back to earth
This deposit covers more than 150 km2 of the Chimiche-
Arico part of the island. It is thought to represent the
collapse of a 10-15 km high plinian eruptive column

60. Volcanic Ash – Unconsolidated
material <2mm in diameter
Road cutting in the Guimar Valley, Tenerife

61. The End