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Igneous Rocks
Summary
1. The Rock Cycle
2. Formation of Igneous Rocks
3. Classification of Igneous Rocks
The
Rock Cycle
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Geological Materials
Transformation Processes
Igneous Rock
Solidification

Magma
Partial Melting

Mantle Rock
Fig 3.1
Formation of Magma

How are rocks melted?

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1. Heating
2. Depressurization
3. Partial Melting
Where do rocks melt?

Subduction zones (Silicic
and Intermediate)
Mantle Plumes or Hot Spots

Partial Melting

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Different rocks melt
at different
temperatures.
Depending on the
temperature, some
rocks in the mixture
may be liquidified
and others might
not.
Types of Igneous Rock

Extrusive

Fine grained rock
that cooled quickly
on the Earth’s
surface.
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Intrusive
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Course grained
rock that cooled
slowly under the
surface.
This is the most
common type of
rock.
Also describe as
Plutonic
See Kehew, Fig 3-6
Classification of Igneous Rocks


These classes of rock are based on the
type of magma that cooled to create the
rock.
These are describe in terms of
temperature, composition, viscosity, and
pH level.
Silicic Rocks

Silicic (a.k.a, felsic) Magmas
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Cool (<700oC)
Viscous (sticky, doesn’t flow easily)
Gaseous (steam of H2O and C02)
Silicic Rocks
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Usually intrusive, course-grained,
Silicic (Granite) to Intermediate
(Diorite).
If extrusive, fine-grained rocks
formed by explosive volcanoes
Rhyolite or Andesite Volcanoes
Intrusive Silicic Igneous Rock
E.g., Granite (Silicic, Phaneritic):
Quartz
Biotite
Na Plagioclase
Crystallized (Solidified)
Silicic Magma
Poor in: Fe, Mg, Ca,
(<20%)
Rich in: Silica (>70%)
Mafic Rocks

Mafic Magmas
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Hot (>1000oC)
Non-Viscous (runny, flows easily)
“Dry” (no H2O or C02)
Mafic Rocks
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
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Usually Extrusive, Fine-grained,
Mafic (Basalt) rock forms oceanic
crust, Shield Volcanoes and
Basalt Floods
If Intrusive, course-grained mafic
rocks are formed Gabbro.
If intrusive, Dikes and Sills more
common
See Kehew,
Fig 3-40
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
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Mafic Magma
Low-viscosity
Non-explosive eruptions
Gentle slopes
Covering large areas
Types of Eruptions (Mafic Volc.)

Types of Eruptions

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Lava floods
Lava fountains
Fissure eruptions
Rock Textures (Table 3-1)

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Aphanitic
Porphyritic
Vesicular
Glassy (Obsidian)
Surface Textures
(Extrusive, Mafic Rocks)

Ahah (Rubbley)


Mostly solid when
flowing
Pahoehoe (Ropey)

mostly liquid when
flowing)
Mafic Sill: Intruded between layers

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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
Igneous Rock Classification
Mineral Percentage
Intrusive (Plutonic)
Extrusive (Volcanic)
Continental Crust
Oceanic Mantle
Crust
Igneous Rock Classification
Intermediate
Mafic
Granite
Rhyolite
Diorite
Andesite
Gabbro
Basalt
Extrusive
Intrusive
Silicic
(Porphyritic)
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


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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.
Terminology


Felsic – silicate minerals, magmas and
rocks enriched in lighter elements for
oxygen, aluminum, sodium, and silicon.
Aphanitic – dark-coloured rock where the
grains are so fine they cannot be seen by
the naked eye.
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.
Types of Igneous Rocks

Granite
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Acidic
Very pale
Coarse-grained
Intrusive, felsic
Can be pink to dark
gray.
Rhyolite

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Extrusive form
equivalent to Granite
Fine grained.
Make up Mt Fuji.
Andersite
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Intermediate rock
Less quartz that
granite, thus has a
darker colour.
Both sizes of grains.
Not as acidic as
Granite.
Found in the Andes
Extrusive
Diorite
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Coarse grained
Grey to dark gray in
colour
Intrusive.
Basalt
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Black to gray in
colour (dark)
Fine-grained
Mafic
Found in Ireland
(Giants Causeway)
and Hawaii.
Basic
Extrusive
Gabbro

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Dark
Basic
Intrusive
Coarse-grained
Part of the oceanic
crust
Peridotite
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Ultrabasic
Rare and part of the
mantle
Course grained
Green in colour
Obsidian
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Glassy (volcanic
glass)
Can be green or
black
Extrusive
Pumice
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Glassy (Frothy)
Light to dark in colour
depending on its
impurities.
Solidified foam
Extrusive
Homework
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P. 106 #1-3
P. 113 #1-4
Rocks in the Collection
1.
2.
3.
4.
5.
6.
7.
Grantie
Rhyolite
Andesite
Obsidian
Pumice
Basalt
Gabbro
Anorthosite
9. Diorite
10. Scoria
11. Syenite
12. Peridotite
8.