Igneous Rocks
2006 Prentice Hall Science Explorer- Earth Science
Liz LaRosa http://www.middleschoolscience.com 2009
Images from Geology.com unless otherwise noted
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The first rocks to form on Earth probably looked like the
igneous rocks that can be seen today.
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Igneous rock begins as magma.
Magma can form:
▪ When rock is heated
▪ When pressure is released
▪ When rock changes composition
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Magma freezes between
700 °C and 1,250 °C
Magma is a mixture of
many minerals
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http://www.fi.edu/fellows/payton/rocks/create/igneous.
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Igneous rock is any rock that forms from
magma or lava.
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The name igneous comes from the Latin
word ignis, meaning “fire”.
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Igneous rocks are classified according to their
origin, texture, and mineral composition.
Origin of Igneous Rock
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Intrusive Igneous Rocks: forms when
magma pushes into surrounding rock
beneath Earth’s surface and hardens. The
most abundant intrusive rock on Earth’s
continents is granite. Granite forms the
core of many mountains ranges.
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Extrusive Rocks: forms when magma
erupts onto the Earth’s surface (lava),
cools quickly with very small or no crystals
formed. Basalt is one of the most common
rocks on Earth. A layer of basalt forms
much of Earth’s ocean floors.
http://www.windows.ucar.edu/tour/link=/earth/geology/ig_intrusive.html&edu=high&fr=
Classification of Igneous Rocks
by
Color and Texture
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Felsic: light colored rocks that are rich in
elements such as aluminum, potassium,
silicon, and sodium
Mafic: dark colored rocks that are rich in
calcium, iron, and magnesium, poor in silicon
Coarse-grained: takes longer to cool, giving
mineral crystals more time to grow
Fine-grained: cools quickly with little to no
crystals
Coarse-Grained
Fine-Grained
Felsic
Granite
Rhyolite
Mafic
Gabbro
Basalt
Obsidian is a dark-colored volcanic glass that forms from the very rapid
cooling of molten rock material. It cools so rapidly that crystals do not form.
Is this rock Felsic or
Mafic?
Is it fine-grained or
coarse-grained?
Is this rock Intrusive or
Extrusive?
Mafic, fine grained, extrusive
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Most of Earth’s minerals contain silica, a
material formed from oxygen and silicon.
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The silica content of magma and lava affects
the types of rock the form.
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Lava that is low in silica usually forms darkcolored rocks such as basalt.
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Magma that is high in silica usually forms
light-colored rocks such as granite.
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People throughout history have used igneous rock for tools and
building materials.
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Granite has a long history of use in building statues, bridges, public
buildings and paving streets with cobblestones.
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Granite today is used in curbstones, floors, kitchen counters and is
crushed to make gravel that is used in construction.
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Pumice and obsidian are good abrasives used for cleaning and
polishing. Obsidian is used to make sharp tools for cutting and
scraping.
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Perlite, formed from the heating of obsidian, is often mixed with soil
for starting vegetable seeds.