How Earth’s Rocks Were Formed
Three Groups of Rocks
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A rock is a group of minerals bound
together in some way
Uniformitarianism explains the origin of
rocks: all rocks of the crust form in one of
three general ways.
Igneous rocks are formed by the cooling
and hardening of hot molten rock (magma)
from inside Earth
Sedimentary rocks are formed by the
hardening and cementing of layers of
sediments
Metamorphic rocks are formed when
rocks that already exist are changed by
heat and pressure into new kinds of rocks
Igneous Rocks
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Plutonic or intrusive rocks form
underground from cooled magma eg.
Granite
Volcanic or extrusive rocks forms when lava
cools on Earth’s surface
Volcanic rocks lack distinct mineral grains
Granite
Basalt
Magma
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Two general kinds of magma, both are
silicates
Temperatures range from 600°C to 1200°C
One has a high % of silica (SiO2), and not
much Ca, Fe, or Mg
This high silica magma is thick and slow
flowing, and forms light coloured minerals,
called felsic rocks  granite
The second kind of magma has a lower %
of silica, and a higher % of Ca, Fe, and Mg.
This low silica magma is hotter,thinner, and
more fluid, it forms rocks that are dark
ferromagnesian minerals, called mafic 
basalt
Texture of Igneous Rocks
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Texture depends on the size, shape, and
arrangement of its mineral crystals
Igneous rock range from glassy smooth to coarse
grained
Crystal size is the most important factor affecting
texture, and it depends upon how fast the magma
hardens
The longer the magma stays liquid, the larger the
crystals become
A high % of dissolved gases helps crystals grow
faster
Magmas trapped in the crust grow slowly, such
plutonic rocks have large mineral grains, and a
coarse grained texture
Granite
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Magmas that reach Earth’s surface as lava
harden rapidly
Volcanic rock that forms has tiny crystals
Basalt is a good example
Some rocks harden so rapidly that no
crystals form at all and are glassy eg.
obsidian
Porphyritic Texture
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Some igneous rocks have two
distinctly different textures
Large crystals are surrounded
by a fine-grained mass of
rock
Such a rock is called a
porphyry
Two stages of cooling can
form a porphyry
In the first stage the magma
cools slowly at great depth,
and large crystals can form,
the rest of the magma
remains liquid
Then the magma moves up,
and the rest of the magma
cools into fine-grained rock
around the first-stage
crystals
Families of Igneous Rocks:
Granite
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Forms from felsic magmas
Mainly orthoclase feldspar and
quartz
Likely minerals are plagioclase
feldspar, mica, and some
horneblende
Usually light coloured
Granite: coarse grained
Rhyolite: fine grained
Obsidian & pumice: glassy
Granite
Rhyolite
Obsidian
Families of Igneous Rocks:
Gabbro
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Forms from mafic magmas
Mainly dark plagioclase
feldspar and augite
Other likely minerals are
olivine, hornblende, and
biotite
Generally dark in colour and
dense
Gabbro is coarse grained
Basalt is fine grained
Gabbro
Basalt
Families of Igneous Rocks:
Diorite
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Composition and
colour between
those of granite and
gabbro
Diorite has less
quartz than granite,
and less plagioclase
than gabbro
Andesite is the finegrained member
Diorite
Andesite
Some Igneous Rocks do not have a
Family…
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Granodiorite is a coarse-grained rock, with
a composition between granite and diorite
Pyroxenite is nearly all pyroxene
Dunite is almost all olivine
Peridotite is a mixture of pyroxene and
olivine.
Granodiorite
Pyroxenite
Dunite
Common Igneous Rocks
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Specific rocks can be
identified by their minerals
and texture
Granite is made of quartz,
orthoclase feldspar, and at
least on other mineral, such
as mica or hornblende, and is
coarse-grained
Quartz: little chips of cloudy
or grayish glass
Feldspar: smooth cleavage
surfaces, is gray, pink or
orange
Mica: black biotite, is shiny
little flakes
Horneblende: tiny dull black
chunks or sticks
Obsidian
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Pumice
Granite is the most common continental
igneous rock.
Felsite is the general name for any lightcoloured, fine-grained rock in the granite
family.
Rhyolite is an example
Obsidian is a volcanic glass of the granite
family. It is usually dark brown or black,
hard and brittle, with conchoidal fracture.
Pumice is formed from felsic lava that
hardened while steam and other gases
were still bubbling out of it.
Basalt
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Gabbro
Scoria
The most common rock of the gabbro
family is basalt
Basalt ranges from dark green to black
Gabbro has about the same composition
as basalt, but is coarse grained
Basalt glass is like obsidian but has a
mafic composition
Scoria is full of holes like pumice