Chapter 5
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Explain the principle of uniformitarianism and
relate it to the formation of igneous,
sedimentary, and metamorphic rocks.
Discuss the differences between plutonic and
volcanic igneous rock.
Define rock texture and list some factors that
control the texture of igneous rock.
Name and describe members of the granite,
diorite, and gabbro igneous rock families
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Modern geology began in 1795
This is when Scottish geologist James Hutton
described a new concept
called uniformitarianism
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Before Hutton goelogists thought
 Earth was formed by spectacular events and
catastrophes
 They believed these catastrophies formed
mountains, canyons, waterfalls and almost all
land forms
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James Hutton’s ideas were different.
He came to the conclusion, “the present is
the key to the past”
 This statement has 2 key concepts
1. The geological process now at work were also active in
the past
2. The present physical features of Earth were formed by
these same processes, at work over long periods of
time
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For example: a river valley
 Otheres would describe this as a sudden split of
the Earth’s crust
 Hutton explained it as the slow steady wearing
away of the land by the river over thousands of
years
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A rock is defined as a group of minerals
bound together in some way.
Using Hutton’s principle, and studying
different rocks around the Earth goelogists
have concluded all rock from the earths crust
is formed in one of 3 ways
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Igneous Rock – formed by the cooling and
hardening of magma from inside the earth.
 Magma is hot molten rock (under surface).
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Sedimentary rock –formed by hardening and
cementing of layers of sediments.
 Sediments may consist of rock fragments, plant
and animal remains, or chemicals that form on
lake and ocean bottoms.
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Metamorphic Rocks – formed when rocks
that already exist are changed by heat and
pressure into new rocks.
 Garnet is an example
Plutonic, or intrusive, igneous rock
 Rocks that form underground from cooled
magma
 These rocks are seen on the surface only after
the rocks that cover them are cleared away.
 Example: Granite
Volcanic, or extrusive, igneous rock
 Magma that pours onto
the earths crust during a
volcanic eruption is called
lava
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These rocks are formed when the lava cools
They can also be formed out of the dust and
ash of a volcanic eruption
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Volcanic rock example:
 Scoria – formed from lava slowly cooling
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Plutonic and volcanic rocks are simular in
mineral composition, but volcanic rocks lack
mineral grains
Example: obsidian
– volcanic rock
formed from lava
cooling rapidly
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There are 2 general kinds that are most
common
1. Felsic
2. Mafic
Felsic
 when hardens forms rocks with mainly light
coloured minerals
 high-silica
 Does not have much clacium, iron or
magnesium
 Thick and slow flowing magma
Felsic
 Granite is a common felsic rock
 It contains light coloured minerals like quartz and
orthoclase
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Most plutonic rocks are felsic
Mafic
 Low percentage of silica
 Higher percentage of calcuim, iron, and
magnesium
 Magma is hotter, thinner, and more fluid than
felsic
 When solid forms rocks with dark
ferromagnesian minerals
Mafic
 An example is basalt
 It contains a number
of dark minerals like
hornblende, augite,
and biotite
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Most volcanic rock are mafic.
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Texture is another way to group rocks.
A rock’s texture depends on the size, shape,
and arrangement of it’s mineral crystals.
Igneous rock textures range from glassysmooth (obsidian), to course grained
(granite).
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Crystal size is most important for texture.
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This depends on
 how fast the magma hardens.
 The amount of gas dissolved in the magma
COARSE-GRAINED TEXTURE
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Magma deep within the
crust cools slowly
Creates large mineral
grains of fairly uniform size
Plutonic rock has this
texture
Granite is an example
FINE-GRAINED AND SMOOTH
TEXTURE
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Magma that reaches the
Earth’s crust as lava
hardens quickly
This rock has tiny crystals
The crystals are too small
to see without a
microscope
Volcanic rock like basalt is
fine-grained; obsidian is
glassy
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Porphyry (POR fur ee) rock has 2 distinctly
different textures.
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How are these rocks formed?
 There are 2 stages of cooling
1. Magma is at a large depth, and here is cools slowly so
large crystals can form of one mineral, the rest
remains liquid
2. As the magma gets closer to the surface the rest of the
magma cools into fine grained rock around the larger
crystals
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Igneous rock is grouped into families
according to mineral composition
Granite Family
Gabbro Family
Diorite Family
And some igneous rocks don’t fit into any of
these families
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Granite Family
 Forms from felsic magmas
 Consist mainly of orthoclase feldspar and quartz
▪ These are light in colour, so the rocks in this family are
usually light-coloured
 other minerals likely present are plagioclase
feldspar, mica, and hornblende
 Rocks in this family will have different textures,
but simular chemical compositions
GRANITE FAMILY
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Course grained granite
GRANITE FAMILY
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Fine-grained rhyolite
GRANITE FAMILY
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Glassy obsidian
GRANITE FAMILY
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Gabbro Family
 Forms from mafic magma
 Made mainly of dark plagioclase feldspar and
augite
 Other likely minerals are olivine, hormblend, and
biotite.
 These are generally darker and more dense than
those found in the granite family
GABBRO FAMILY
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Course-grained gabbro
GABBRO FAMILY
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Fine-grained basalt
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Gabbro Family
 Glassy example:
basalt glass
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Diorite Family
 Composition and colour between the granite and
gabbro families
DIORITE FAMILY
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Diorite: Course-grained
 Has less quartz than granite
 Has less dark plagioclase than
gabbro
DIORITE FAMILY
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Andesite: fine-grained
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Some igneous rock do not fit into any of
these 3 families.
DON’T FIT
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Granodiorite
DON’T FIT
 Course grained rock
Pyroxenite – nearly all
pyroxene
 Composition between granite
 Coarse, dark and heavy
and diorite
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 May be like rock in Earth’s
mantle
DON’T FIT
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Dunite – almost all olivine
DON’T FIT
 Coarse, dark and heavy
Peridotite – a mix of
pyroxene and olivine
 May be like rock in Earth’s
 Coarse, dark and heavy
mantle
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 May be like rock in Earth’s
mantle
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Granite
 Made of:
▪ quartz – looks like tiny chips of cloudy or grayish glass
▪ orthoclase feldspar – often has smooth cleavage
surfaces and is white, gray, pink or orange
▪ and one other mineral like mica (shiny black flakes) or
hornblende (tiny dull black chunks or sticks)
 Ranges in colour from light to medium gray and
pinks
 Have course-grained texture
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Granite
 The most common continental igneous rock
 Occurs in thr Rockies, the Aairondacks of New
York State, the Black Hills in South Dakota, the
White Mountains of New Hampshire, and in many
other mountain areas
 It is plutonic, and when it appears on the surface it
shows errosion removed thousands of meters or
rock since the magma cooled
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Granite
Felsite is the general name
for light-coloured,
fine grained granite
Rhyolite is an example of a
fine-grained, light gray to
pink rock
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Obsidian
 Volcanic glass of the granite family
▪ Contains many of the same minerals and other light
coloured rocks like granite
 Usually dark brown or black
▪ This is due totiny amounts of dark coloured minerals
scattered throughout the rock
 Is hard and brittle
 It has a conchoidal, or shell-like, fracture
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Pumice
 Formed from felsic lava that hardened while
steam and other gases were still bubbling out of it
 It looks like a sponge with many small holes in it
 Because of all the air holes, it is sometimes light
enough to float on water
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Basalt
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The most common rock in the gabbro family
A fine-grained rock
Ranges in colour from dark green to black
Basalt is the igneous rock on the ocean floor
On land it is the most common rock formed from
flows of lava
 Large areas occur in the lava flows of: Iceland,
Hawaiian Islands, and Columbia and Snake River in
western United States
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Gabbro
 Has about the same composition as basalt
 It is cooled slowly deep underground, so it is
course-grained
 Very dark in colour
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Diabase
 Has a composition simular to gabbro
 It has a finer texturethan gabbro, but courser than
basalt
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Basalt Glass
 Is like obsidian, but has a mafic composition
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Scoria
 Is full of holes like pumice
 Is made of denser material than pumice, so not all
scoria floats
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