Summary
1. The Rock Cycle
2. Formation of Igneous Rocks
3. Classification of Igneous Rocks
Rock Cycle
 All rock types can be
converted into any other rock
type
 Formation of Sedimentary Rocks
(weathering, erosion, deposition,
cementation)
 Formation of Igneous Rocks
(subduction, remelting,
crystallization)
 Formation of Metamorphic Rocks
(burial, heat and pressure)
Igneous Rocks
 Formed from magma (liquid rock)
 Classified in two common ways:
1)
Crystal or grain size (relates to crystallization)
Coarse grained
2)
porphyry
Fine grained
Chemical Composition (related to colour)
Ultramafic
Mafic
Intermediate
Felsic
Grain Size of Igneous Rocks
 Extrusive (Aphanitic) – fine grained
Magma is erupted at the surface. (Volcanic)
Minerals have little or no time to crystallize
Mineral grains are very small (often invisible)
 Intrusive (Phaneritic) – coarse grained
Magma cools below the surface.
Minerals have time to grow slowly.
Mineral grains are larger and are easily visible.
In some cases, crystals can grow very large (Pegmatite).
Extrusive (Aphanitic) Rocks
 Fine grained rock that cooled
quickly on the Earth’s surface.
 Eruption at the surface causes
rapid cooling of lava which does
not allow mineral crystals much
time to grow.
 Basalt, the most common
extrusive rock, often erupts from
the bottom of oceans
Surface Textures of Hawaiian
Basalts (Extrusive, Mafic Rocks)
 Aa (Rubbley)
 Mostly solid when
flowing
 Pahoehoe (Ropey)
 mostly liquid when
flowing)
Hawaiian Volcano eruption
http://www.youtube.com/watch?
v=WwBVG0Si7rs
Intrusive (Phaneritic) Rocks
 Course grained rock that cooled
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slowly under the surface.
More time is allowed for crystal
growth. Crystals are clearly visible to
the eye.
This is the most common type of
igneous rock.
Also described as Plutonic
Diapirs – a body of magma that rises
through another rock (country rock)
and then eventually crystallizes as a
pluton
See Kehew, Fig 3-6
Igneous Rock Textures
Igneous Rock Terminology
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Intrusive
Extrusive
Pluton
Dike
Laccolith
Magma
Batholith
Sill
Xenolith
Ash
Lava
volcano
Igneous Intrusions
Igneous Rock Terminology
 Magma – liquid rock that has not solidified or been
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erupted.
Lava – recently erupted magma that has yet to solidify
Ash – fine (often glassy) igneous rock that solidifies
rapidly (in the air) during an eruption and is deposited
on the surface.
Intrusive – coarse grained rock not erupted at surface
Extrusive - fine grained rock erupted at surface
(Volcanic)
Pluton – A large body of intrusive igneous rock – they
often form the basis of continental crust.
Igneous Rock Terminology
 Dike – a roughly vertical intrusion of igneous rock that
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cuts through country rock
Laccolith – A large igneous body that is intruded
horizontally between layers of rock often uplifting the
rocks above
Batholith – A large body of igneous rock
Sill – A large horizontal intrusion of igneous rock
Xenolith – a large chunk of country rock that has fallen
into a body of magma (often the xenolith undergoes
contact metamorphism)
Volcano – a structure where magma is erupted
Formation of Magma
 How are rocks melted?

 1. Heating
Rocks melted in the crust. Rocks carried
back into the mantle by subduction or
heated from below by a Mantle Plume (Hot
Spot)
 2. Depressurization
Magma requires 10% more volume than the 
rock. Often rocks remain solid only due to
pressure – a release in pressure allows rocks
to liquify into magma.
 3. Partial Melting
Only a portion of a pluton is melted,
resulting in a magma with a different
chemical composition than its parent rock
(see Bowen’s Reaction Series)
Types of Volcanism
 Hawaiian
 Stratovolcano
 Intermediate
Types of magma?
 Ultramafic
 Mafic
 Intermediate
 Silicic or Felsic
Chemical Composition of Magma
 Intermediate

Ultramafic
Upper Mantle
Less than 45% Silica (SiO2)
Very high Mg, Fe (20-30%)
olivine and pyroxenes, no Qtz
Dark minerals (green, black)
Peridotite

Mafic
Oceanic Crust
45-55% Silica (SiO2)
High Mg and Fe (10 - 20%)
Dark minerals, no Qtz
Basalt, Gabbro
Cordilleran Mountains (ex
Andes, Rockies)
55-63% Silica (SiO2)
5-10% Mg + Fe
Mix of light and dark minerals,
some Qtz
Andesite, Diorite
 Silicic or Felsic
Continental crust
Greater than 63% silica
0-5% Mg + Fe
High in Group 1 metals (K, Na,
Light coloured minerals
Orthoclase and abundant Qtz
Rhyolite, Granite
Classification of Igneous Rocks
 Remember that grain size and chemistry are the two
main factors when classifying igneous rocks.
 Grain Size reflects the rate at which magma cools –
there are two major classifications – intrusive and
extrusive
 Chemistry reflects the colour of the minerals and is
ultimately related to the geological source of the rocks
(oceanic, continental, mix (at subduction zones))
Igneous Rock Classification
Igneous Rock Classification Exercise
 Materials:
1) Pencil crayons – 2 shades of green, white, gray
(regular pencil), black, red or pink
2) Igneous rock diagram sheet.
Igneous Rock Classification
Mineral Percentage
Intrusive (Plutonic)
Extrusive (Volcanic)
Continental Crust
Oceanic Mantle
Crust
Igneous Rock Classification
Intermediate
Mafic
Intrusive
Silicic
Diorite
Andesite
Extrusive
Granite
Rhyolite
(Porphyritic)
Gabbro
Basalt
Felsic Rocks
 Granite – most common
intrusive, course-grained
 Rhyolite – rare
extrusive, fine-grained rocks
Rhyolite Volcanoes very explosive
- Yellowstone Park (no cone – only a
massive 75 km wide caldera)
 Felsic (a.k.a, silicic) Magmas
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Cool (<700oC)
Viscous (sticky, doesn’t flow easily)
Gaseous (steam of H2O and C02)
Explosive Volcanism (rare)
Intrusive Felsic Igneous Rock
Granite (Silicic, Phaneritic):
Poor in: Fe, Mg, Ca
Rich in: Silica (>63%)
Rich in: Al, K, Na
Light Coloured
Coarse Grained
Pegmatites show coarse
grained crystals
http://www.youtube.com/watc
h?v=nWNE_f6HqAk
Orthoclase, Quartz,
Amphibole, Micas
Extrusive Felsic Igneous Rock
Rhyolite (Silicic, Aphanitic):
Poor in: Fe, Mg, Ca
Rich in: Silica (>63%)
Rich in: Al, K, Na
Light Coloured – usually grey
Fine Grained
Often Ash (Welded Tuff) may
contain obsidian
Orthoclase, Quartz, Amphibole,
http://www.youtube.com/watc Micas
h?v=nWNE_f6HqAk
Felsic Volcanism
 Mantle plumes intrude through the
continental crust
 Magma is very silicic, highly gaseous,
low temperature and pressure builds for
millenia
 The volcanism is highly explosive
 The Yellowstone caldera is 75 km across
Intermediate Rocks
 Andesite (common)
Extrusive, Fine-grained,
intermediate rocks are found in
subduction zones
 Diorite
Intrusive, Course-grained.
If intrusive, Dikes and Sills more
common
 Intermediate Magmas
 Vary (700 - 1000oC)
 Produce lavas and ash and
therefore can be unpredictable.
Lower temperature magmas are
gassy and explosive (ex. Mt. St.
Helens and Vesuvius)
Extrusive Intermediate Igneous Rock
Andesite (Intermediate, Aphanitic):
High in: Ca, Al
Rich in: Silica (55-65%)
Median amounts of: Al, K, Na,
Mg, Fe
Colour – mixture of light and
dark
Fine Grained and Porphyritic
Orthoclase and Plagioclase, low
Quartz, low pyroxene,
amphibole, Micas
Intrusive Intermediate Igneous Rock
Diorite (Intermediate, Phaneritic):
High in: Ca, Al
Rich in: Silica (55-65%)
Median amounts of: Al, K, Na,
Mg, Fe
Colour – mixture of light and
dark
Fine Grained - Orthoclase and
Plagioclase, low Quartz,
Amphibole, low pyroxene,
micas
Types of Volcanoes - Intermediate
 Composite Volcanoes (Stratovolcanoes) - interayered lava
flows and pyroclastic deposits - ranges from intermediate
to felsic in composition (andesite-dacite-rhyolite) - in
volcanic arcs above subduction zones
The more felsic these
volcanoes, the more
explosive. They can be
subdivided into:
Strombolian
Vulcanian
Plinian
Peleean
Intermediate Volcanism
Stratovolcanoes
 Pyroclastic flows – A mixture of ash, small lava bombs and
hot gases that can race down the side of a volcano at speeds
of up to 200 km an hour – this the biggest killer in most
volcanic eruptions.
 Lahars – pyroclastic material melts the glaciers on the sides
of mountains forming a rapidly moving mudflow – this was
the greatest killer in the Cerro Arenal eruption in
Columbia.
 Lava - Slow moving andesitic lava flows.
Mafic Rocks
 Basalt (very common)
Extrusive, Fine-grained,
Mafic rock forms oceanic crust,
Shield Volcanoes and Basalt Floods
 Gabbro
Intrusive, Course-grained.
If intrusive, Dikes and Sills more
common
 Mafic Magmas
 Hot (>1000oC)
 Non-Viscous (runny, flows easily)
 “Dry” (no H2O or C02)
See Kehew,
Fig 3-40
Extrusive Mafic Igneous Rock
E.g., Basalt (Mafic, Aphanitic):
Poor in: Si(45-55%)
Poor in: K, Na
Rich in: Mg, Fe, some Ca
Rich in: Al, K, Na
Dark Coloured
Fine Grained
Plagioclase, Amphibole or
Pyroxene, maybe Olivine, no
Quartz, dark mica
Pillow Lava
Intrusive Mafic Igneous Rock
E.g., Gabbro (Mafic, Phaneritic):
Poor in: Si(45-55%)
Poor in: K, Na
Rich in: Mg, Fe, some Ca
Rich in: Al, K, Na
Dark Colured
Coarse Grained
Plagioclase, Amphibole or
Pyroxene, maybe Olivine, no
Quartz, dark mica
Types of Volcanism
 Mafic (Basalt)
 Shield – Hawaiian, Iceland
 Cinder Cones
 Intermediate (Andesite)
 Composite Volcanoes (Stratovolcanoes) – Vesuvius,
Mt. St. Helens, Mt. Fuji
 Felsic (Rhyolite)
 Mid-Continent Volcanism - Yellowstone
Types of Volcanism
Types of Volcanoes - Mafic
1) Shield Volcanoes - low viscosity basaltic
magmas - lava lakes and flows common - not
explosive - flood basalts and submarine
pillow lavas are basaltic, too
2) Cinder Cones - variable composition, often
basaltic - usually a single batch of magma steep cones formed by cinders piled around
vent at angle of repose – associated with
shield volcanoes
Mafic Volcanism, Hawaii
 Mantle Hot Spot
Volcano

See Kehew, 3-8
Hawaii
A long chain of inactive volcanoes
 Island ages, millions of years
59.6
55.2
43.4
42.4
56.2
48.1
Midway
27.2 19.9
20.6
12.0
10.3
Hawaii
0-5
Shield Volcanoes
 Mafic Magma
 Low-viscosity
 Non-explosive eruptions
 Gentle slopes
 Covering large areas
Shield Volcanoes
Another method of classifying volcanos
Types of Eruptions (Mafic Volcanism)
 Types of Eruptions
 Lava floods
 Lava fountains
 Fissure eruptions
 Rock Textures (Table 3-1)
 Aphanitic
 Porphyritic
 Vesicular
 Glassy (Obsidian)
Mafic Sill: Intruded between layers
 Mafic magma is less
viscous and hotter so
 Does not form plutons
but
 Cuts along layers (Sills)
or even
across layers (Dikes)
 Also Baked Zones
of
adjacent country rock
and Chill Zones within
the intrusion
Mafic Volcanic Features
Lava flows reaching the
ocean, Kilauea, Hawaii
Pumice – basalt with
gas bubbles
Columnar Basalt – The Giant’s
Causeway (N. Ireland)
Types of Volcanoes - Felsic
 Dome Volcanoes - similar in composition and often related
to composite volcanoes - usually more viscous (more
silicic) magma type (dacite/rhyolite) - pyroclastic flows
abundant - explosive eruptions common
 Continental Calderas - in continental regions the result of
the largest types of volcanic eruptions - large, shallow,
silicic magma chambers empty catastrophically with
unparalleled violence; roof of magma chamber collapses
into emptied portion of magma chamber . The
Yellowstone caldera is over 100 km across.
Types of eruptions – Felsic (Silicic)
 Uncommon – usually form
 Yellowstone Supervolcano
at continental hot spots
such as Yellowstone.
 Extraordinarily violent due
to the high gas content and
low vicosity of the lava.
 See the videos
video
 http://www.youtube.com/
watch?v=Ap_YUwdiy8I
 http://www.youtube.com/
watch?feature=endscreen&
v=aVUx1JtT-5I&NR=1
Types of Eruptions (Intermediate)
 Higher gas content due to
water content in
subducted crust and
higher gas content of
continental crust
 Formation of Composite
Volcanos (Mt. Fuji, Mt. St.
Helens, Etna, Vesuvius)
Mt. St. Helens eruption, 1980
http://www.youtube.com/wat
ch?v=-H_HZVY1tT4
http://www.youtube.com/wat
ch?v=gmwylbF3-CA
10 most active volcanos and dangers
related to volcanos
10 most active volcanos:
http://www.youtube.com/watch?v=4aYQixhdWY4
Lake Nyos Videos (volcanic gas release):
http://www.youtube.com/watch?v=eHvPI_pYZBs&list=LPEB
BnueOle7g&index=6&feature=plcp
http://www.youtube.com/watch?v=pVMcSvG5Mpg
Nevada del Ruiz, Columbia (Lahar – volcanic mudflow)
http://www.youtube.com/watch?v=3XMS-quxdGg
Pyroclastic Flow
http://www.youtube.com/watch?v=yvG_N7eqMWk
Location of Igneous Rock Production
 Sea-Floor Spreading Zones
 Mid-Ocean Ridges
 Iceland
 Subduction Zones (Crustal Melting)
 Cordilleran Mountains (Andes Mtns., Vesuvius)
 Island Arcs (Japan, Aleutian Islands)
 Mid-Plate Hot Spots
 Hawaii (mid-ocean)
 Yellowstone (mid-continent)
Sea Floor Spreading (Mid-Ocean Ridge)
 Magma is constantly
erupted from the sea floor
at mid-ocean ridges. (ex.
Mid Atlantic ridge)
 Mostly intrusive – basalt
 Dark coloured lavas
 Forms the bulk of oceanic
crust which is denser and
thinner than continental
crust
Subduction Zones
 Ocean Crust is forced under
the continental crust and
melts
 Oceanic crust melts due to
heat and release of H2O
 Mafic magma mixes with felsic
continental crust to make
intermediate rocks
 Often massive stratovolcanoes
are formed
Mantle Plumes or Hot Spots
 Due to convection in the
upper Mantle, plumes of
hot mantle rise through
the crust and produce a
variety of volcanic features:
 Island Arcs (Hawaiian
Islands) with shield
volcanoes
 Continental Flood Basalts
(Deccan Traps)
 Yellowstone style
continental Hot Spots
Bowen’s Reaction Series
Two series of minerals formed during crystallization of magma
Low Silica Magma
1200oC
1000o
Intrus. Extrus.
Gabbro Basalt
Diorite Andesite
750o
Framework
Sheet
Double
Chain
Single
Chain
Isolated
Temperature of
Crystallization
Granite Rhyolite
High Silica Magma
Bowen’s Reaction Series
 Illustrates the relationship between the cooling
magma and the crystallization of the minerals
contained in the rock.
 Rocks on the right side of this chart are rich in calcium
and sodium
 Rocks on the left side represents iron-rich minerals.
They cool and create quartz.
Bowen’s Dilemma
 Problem – The Mantle is the source of most igneous
rocks.
 So, if the mantle is made of ultramafic rocks – how come
continental crust is felsic?
 And how can felsic rocks be produced from an ultramafic
source?
 ANSWER – Think of the Bowen’s Reaction Series
Partial Melting
 Different minerals melt at different temperatures.
 As the temperature of a rock rises, minerals melt in the
reverse order in which they cooled – the Bowen’s
reaction series in reverse!
 Even if the rock was mafic or ultra mafic, the magma
produced can be felsic if the temperatures only rise to
700-800°C
 This explains why continental rocks are felsic – they are
partially melted oceanic crust
Terminology
 Felsic or Silicic – silicate minerals, magmas and rocks
enriched in lighter elements such as oxygen,
aluminum, sodium, and silicon.
Terminology
 Mafic – iron and magnesium enriched minerals are
found in these rocks. They are dark in colour.
 Phaneritic – Grains in the rock can be seen by the
naked eye.
Peridotite
 Ultrabasic
 Rare and part of the
mantle
 Course grained
 Green in colour
Obsidian
 Glassy (volcanic glass)
 Glass is produced when
any rock is cooled
immediately
 Can be green or black
 Extrusive
 Often used by native
cultures to produce
cutting tools or arrows
Pumice or Scoria
 Glassy (Frothy)
 Light (Pumice) to dark
(Scoria) in colour
depending on its
impurities.
 Solidified foam
 Extrusive
 Can float on water
Homework
 P. 106 #1-3
 P. 113 #1-4
Rocks in the Collection
1.
2.
3.
4.
5.
6.
7.
8.
9.
Grantie
Rhyolite
Andesite
Obsidian
Pumice
Basalt
Gabbro
Anorthosite
Diorite
10. Scoria
11. Syenite
12. Peridotite