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Igneous Rocks
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• Rocks are records of events
that took place at the time they
formed. They are books. They
have a different vocabulary, a
different alphabet, but you
learn how to read them.
•
John McPhee
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Rock – any naturally
occurring solid mass of
mineral or mineral-like
matter
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Rocks are Aggregates of Minerals
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Minerals: Building Blocks of Rocks
• Definition of a mineral
–Natural
–Inorganic
–Solid
–Possess an orderly internal
structure of atoms
–Have a definite chemical
composition
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Calcite Crystal
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Mineral Groups
• Rock-forming minerals
–Only a few dozen
–Most abundant atoms in Earth’s
crust are oxygen (46.6% by
weight) and silicon (27.7% by
weight)
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Mineral Groups
• Eight elements compose most rock-forming
minerals:
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oxygen (O),
silicon (Si),
aluminum (Al)
iron (Fe)
calcium (Ca)
sodium (Na)
potassium (K)
magnesium (Mg)
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Composition of Continental Crust
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Mineral Groups
• Rock-forming silicates
– Most common mineral group
– Contain the silicon-oxygen
tetrahedron
• Four oxygen atoms
surrounding a much smaller
silicon atom
• Combines with other atoms
to form the various silicate
structures
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Potassium feldspar
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Igneous Rocks
Form as magma cools and
crystallizes
Rocks formed inside Earth
are called plutonic or
intrusive rocks
Rocks formed on the surface
Formed from lava (a
material similar to
magma, but without gas
Called volcanic or
extrusive rocks
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Igneous Rocks
• Crystallization of magma
– Ions are arranged into orderly patterns
– Crystal size is determined by the rate of cooling
• Slow rate forms large crystals
• Fast rate forms microscopic crystals
• Very fast rate forms glass
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Igneous Rocks
• Classification is based on the rock’s texture and
composition
– Texture: Size and arrangement of crystals:
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Fine-grained – fast rate of cooling
Coarse-grained – slow rate of cooling
Porphyritic (two crystal sizes) – two rates of cooling
Glassy – very fast rate of cooling
Vesicular – contains hole left by gas bubbles
Pyroclastic – fragmented; produced by consolidation of
volcanic fragments
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Fine-Grained Igneous Texture
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Coarse-Grained Igneous Texture
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Porphyritic Igneous Texture
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Vesicular Igneous Texture
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Pyroclastic Igneous Texture
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Igneous Rocks
• Classification is based on the rock’s texture and
mineral constituents
– Mineral composition
• Explained by Bowen’s reaction series which shows the order
of mineral crystallization
• Influenced by crystal settling in the magma
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Bowen’s Reaction Series
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Igneous Rocks
• Naming igneous rocks
– Granitic rocks
• Composed almost entirely of light-colored silicates – quartz
and feldspar
• Also referred to as felsic: feldspar and silica (quartz)
• High silica content (about 70 percent)
• Common rock is granite
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Granite and Granitic Rock
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Igneous Rocks
• Naming igneous rocks
– Basaltic rocks
• Contain substantial dark silicate minerals and calcium-rich
plagioclase feldspar
• Also referred to as mafic: magnesium and ferrum (iron)
• Common rock is basalt
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Basaltic Lava
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Igneous Rocks
• Naming igneous rocks
– Other compositional groups
• Andesitic (or intermediate)
• Ultramafic
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Palisades Sill
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Geologic Dike
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