Igneous Rocks
Igneous Rocks
Why Study Igneous Rocks?
Why study igneous rocks?
• All igneous rocks form by cooling and crystallization of
molten rock. Molten rock forms by melting of rocks in
Earth’s crust and mantle. So – the study of igneous
rocks helps us understand:
– The mineralogical/chemical composition of Earth’s
interior
– Processes involved in the formation of magmas
– How magmas change as they rise toward Earth’s
surface
– Internal processes of volcanoes from which lavas are
erupted
Important Definitions
• Magma – molten rock beneath Earth’s
surface
• Lava – molten rock above Earth’s surface
• Extrusive (Volcanic) Rock – forms when
lava solidifies on Earth’s surface
– Pyroclastic Rock – extrusive rock made of
material explosively ejected from a volcano
• Intrusive (Plutonic) Rock – forms when
magma solidifies beneath Earth’s surface
How do we know if a rock is
intrusive or extrusive?
• Rock Texture – a description of the size, shape
and arrangement of the mineral grains making
up the rock
– Intrusive Rocks – are typically coarse grained
– Extrusive Rocks – are typically fine grained
• Pyroclastic Rocks – are typically made of volcanic
glass and/or pieces of pre-existing rocks
• So – grain size is an excellent clue!
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Phaneritic Texture
Terms Describing Rock Texture
• Phaneritic – all grains can be seen with
the naked eye
– Results from slow cooling, slow crystallization
(intrusive rocks)
• Aphanitic - all grains cannot be seen with
the naked eye (need hand lens,
microscope, etc.)
– Results from fast cooling, fast crystallization
(extrusive rocks)
Aphanitic Texture
Terms Describing Rock Texture
• Porphyritic – two distinctly different crystal sizes
are present (reflecting two distinctly different
cooling/crystallization rates)
– Phenocrysts – large crystals (slow
cooling/crystallization)
– Groundmass – microscopic crystals (fast
cooling/crystallization)
• Question: What can be learned from an igneous
rock with an aphanitic, porphyritic texture?
Porphyritic Texture
Terms Describing Rock Texture
• Glassy – no crystals form due to very rapid
cooling (forms volcanic glass)
• Vesicular – abundant holes (vesicles),
formed by expanding gas bubbles in lava
• Fragmental – consists of fragments of preexisting rock (typical of pyroclastic rocks)
and volcanic ash (microscopic pieces of
volcanic glass)
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Glassy Texture
Vesicular Texture
Glassy, Vesicular Texture
Fragmental Texture
Classification of Igneous Rocks
Igneous Rock Classification
• The classification of igneous rocks is
largely based upon:
– Whether the rock is intrusive or
extrusive
• If extrusive, is the rock crystallized
lava or a pyroclastic rock?
–Chemical and mineralogical
composition
High Si, K, Na <<<<<<<<<<<<<<<<<<<< Low Si, K, Na
Low Fe, Mg, Ca >>>>>>>>>>>>>>>>High Fe, Mg, Ca
Low Temperature >>>>>>>>>>>> High Temperature
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Classification of Igneous Rocks
Mafic Rocks
Extrusive
Intrusive
High Si, K, Na <<<<<<<<<<<<<<<<<<<< Low Si, K, Na
Low Fe, Mg, Ca >>>>>>>>>>>>>>>>High Fe, Mg, Ca
Low Temperature >>>>>>>>>>>> High Temperature
Classification of Igneous Rocks
Intermediate Rocks
Andesite
Diorite
Extrusive
Intrusive
High Si, K, Na <<<<<<<<<<<<<<<<<<<< Low Si, K, Na
Low Fe, Mg, Ca >>>>>>>>>>>>>>>>High Fe, Mg, Ca
Low Temperature >>>>>>>>>>>> High Temperature
Classification of Igneous Rocks
Felsic Rocks
Rhyolite
Granite
Extrusive
Intrusive
High Si, K, Na <<<<<<<<<<<<<<<<<<<< Low Si, K, Na
Low Fe, Mg, Ca >>>>>>>>>>>>>>>>High Fe, Mg, Ca
Low Temperature >>>>>>>>>>>> High Temperature
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Obsidian
Pumice
Important Question
Bowen’s Reaction Series
• If all igneous rocks form by cooling and
crystallization of magma/lava, then:
– Why do igneous rocks exhibit such a
wide range of mineralogical/chemical
compositions?
– Why do igneous rocks exhibit such a
wide range of colors?
• Norman Bowen (early 20th century) –
performed laboratory experiments to
simulate crystallization of a mafic magma
over a range of temperatures.
• Bowen’s objective: to reproduce the
naturally occurring results of the cooling
and crystallization of mafic magma.
• Results are summarized by ‘Bowen’s
Reaction Series’
Bowen’s Reaction Series
As mafic magmas undergo progressive cooling
and crystallization, more Si-rich magmas are
produced
– There is a regular sequence of silicate mineral
crystallization
• Highest temperatures – crystallization of
minerals common to mafic rocks
• Lowest temperatures – crystallization of
minerals common to felsic rocks
– Mineral crystals undergo chemical reactions
with the surrounding melt to produce the next
lower temperature mineral in the
crystallization sequence
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Discontinuous vs. Continuous ?
• Discontinuous Series – minerals have
differing arrangement of Si-O tetrahedra
– Degree of O sharing among tetrahedra
increases with decreasing temperature
• Highest temperature – independent tetrahedra (no
O sharing)
• Lowest temperature – framework silicates (all 4 O
shared)
Discontinuous vs. Continuous ?
• Continuous Series – All minerals are
members of the plagioclase (Ca, Na)
feldspar group
– All framework silicates
– Highest temperatures – Ca-rich plagioclase
(anorthite)
– Lowest temperatures – Na-rich plagioclase
(albite)
Importance of Bowen’s Reaction
Series
• Explains how rocks with a variety of
mineralogical/chemical compositions can
evolve from a single magma.
– Fractionational Crystalization – the
progressive change in the
chemical/mineralogical composition of a
magma as the crystallization series proceeds
– Crystal settling and volcanic eruption may
stop the crystallization sequence before it
‘completes’
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Importance of Bowen’s Reaction
Series
Classification of Igneous Rocks
• Allows interpretation of an igneous rock’s
crystallization temperature based upon its
mineralogical composition
– Highest temperature crystallization produces
rocks ultramafic in composition
– Lowest temperature crstyallization produces
rocks felsic in composition
High Si, K, Na <<<<<<<<<<<<<<<<<<<< Low Si, K, Na
Low Fe, Mg, Ca >>>>>>>>>>>>>>>>High Fe, Mg, Ca
Low Temperature >>>>>>>>>>>> High Temperature
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