Igneous Rocks and
Classifying Igneous Rocks
Chapter 5
Igneous Rocks
• Igneous rocks are
formed from the
crystallization of
magma.
Igneous Rocks
• Extrusive igneous rocks are fine-grained
rocks that cool quickly on Earth’s surface.
Igneous rocks
• Basalt is an extrusive igneous rock that is
very dark in color. It is the most common
type of rock in the Earth's crust and it
makes up most of the ocean floor.
Igneous Rocks
• Intrusive igneous rocks are coarse-grained
and cool slowly beneath Earth’s surface.
Igneous Rocks
• Granite
– The most common
intrusive igneous rock
– Many granite deposits
cross-cut into other
rock formations
– This cross-cutting is
evidence that granite
was intruded into
existing rocks
Igneous Rocks
• Review of magma:
– A slushy mix of molten rock, gases, and
mineral crystals.
– Silica (SiO2) is the most abundant compound
in magma and has the greatest effect on its
characteristics.
– Basaltic: 50% silica, Andesitic: 60% silica,
Rhyolitic: 70% silica
– Silica affects melting temp. and viscosity
Igneous Rocks
• Factors that affect magma formation
– Temperature
• Temperature generally increases with depth in
Earth’s crust.
– Pressure
• Pressure also increases with depth
• As the pressure on a rock increases, its melting
point also increases
Igneous Rocks
– Water content
• As water content increases, melting pt. decreases
– Mineral content
• Different minerals have different melting points
• In general, oceanic crust melts at higher
temperatures than continental crust
– Rocks melt only under certain conditions – the
right combination of temperature, pressure,
and composition
Igneous Rocks
• How rocks melt
– Different parts of a rock may melt at different
temperatures due to the different minerals
present in the rock
– Partial melting: the process whereby some
minerals melt at low temperatures while other
minerals remain solid
Igneous Rocks
• How rocks melt
– As each group of minerals melts, different
elements are added to the magma “stew,”
thereby changing its composition
– If temperatures are not great enough to melt
the entire rock, the resulting magma will have
a different chemistry from that of the original
rock.
Igneous Rocks
• How rocks melt
– Fractional crystallization
• The process wherein different minerals form at
different temperatures
• When magma cools, it crystallizes in the reverse
order of partial melting (the first minerals to
crystallize from magma are the last minerals to
melt during partial melting)
Igneous Rocks
• Feldspars
– Feldspar minerals undergo a continuous
change of composition
– As magma cools, the first feldspars to form
are rich in calcium
– As cooling continues, these feldspars react
with magma, and their calcium-rich
compositions change to sodium-rich
compositions
Igneous Rocks
• Feldspars
– In come instances, as when magma cools
rapidly, the calcium-rich cores are unable to
react completely with the magma.
– The result is a zoned crystal that has sodium-
rich outer layers and calcium-rich cores
Igneous Rocks
• Iron-rich minerals
– These minerals undergo abrupt changes
during fractional crystallization.
– As minerals form, elements are removed from
the remaining magma
– Silica and oxygen are left over
– When the remaining magma finally
crystallizes, quartz is formed.
Igneous Rocks
• Crystal separation
– Crystal separation can occur when:
• Crystals settle to the bottom of the magma body
• Liquid magma is squeezed from the crystal mush
to form two distinct bodies with different
compositions.
• Layered intrusions
– Formed when minerals form into distinct
bands
Intermission – Part II next class
Classifying Igneous Rocks
• Mineral composition
– Felsic
• Light-colored, have high silica contents
• Contain quartz and feldspars orthoclase, and
plagioclase
• Example: Granite
Classifying Igneous Rocks
• Mineral composition
– Mafic
• Dark-colored, have lower silica contents, rich in
iron and magnesium
• Contain plagioclase, biotite, amphibole, pyroxene,
and olivine.
• Example: Diorite
Classifying Igneous Rocks
– Ultramafic
• Low silica content and very rich in iron and
magnesium
• Theory: formed by the fractional crystallization of
olivine and pyroxene
• The minerals may have separated from magma
and did not convert to another mineral upon
reaching a particular temperature
Classifying Igneous Rocks
• Grain size
– Fine-grained vs. coarse-grained
• Cooling rates
– When lava cools on Earth’s surface, there is not
enough time for large crystals to form.
– Thus, extrusive igneous rocks have no visible mineral
grains
– When magma cools beneath the surface, large
crystals form.
– Thus, intrusive igneous rocks may have crystals
larger than 1cm.
Classifying Igneous Rocks
• Texture
– Porphyritic texture: when a rock has grains of
two different sizes.
– Large, well-formed crystals surrounded by
finer-grained crystals of the same mineral or
different minerals.
– Porphyritic textures indicate a complex
cooling history wherein a slowly cooling
magma suddenly began cooling rapidly.
Classifying Igneous Rocks
• Ore deposits
– Veins
• The fluid left over during fractional crystallization
contains any leftover elements that were not
incorporated into the common igneous minerals
• They include: gold, silver, lead, and copper.
• These elements are released at the end of magma
crystallization in a hot, mineral-rich fluid that fills
cracks and voids in the surrounding rock
• This fluid solidifies to form metal-rich quartz veins.
Classifying Igneous Rocks
• Pegmatites
– Veins of extremely large-grained minerals are
called pegmatites.
– Ores of rare elements such as lithium and
beryllium are found in pegmatites
– Pegmatites can also produce beautiful
crystals
– Because these veins fill cavities and fractures
in rock, minerals grow into voids and retain
their shapes.
Classifying Igneous Rocks
• Kimberlites
– Rare, ultramafic rocks where minerals such
as diamonds are found
– Kimberlites are a variety of periodite
– They likely form deep in the crust at depths of
150-300km or in the mantle because diamond
and other minerals found in kimberlites can
form only under very high pressures.