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Chapter 5
IGNEOUS ROCKS
SECTION 5.1
What Are Igneous Rocks?
 Rocks that formed from the crystallization of
magma.
Looking At Magma and Lava Flow
 http://www.youtube.com/watch?v=NIWlA7ndAlw
Types of Igneous Rocks
 Extrusive igneous rocks
are fined grained rocks
that cool quickly on
Earth’s surface.
 (Rhyolite, Obsidian)
 Intrusive igneous rocks
are coarse grained
igneous rocks that cool
slowly beneath the
Earth’s surface.
 (Pegmatite, Granite)
Intrusive Igneous Rock (Pegmatite)
Extrusive Igneous Rocks (Basalt)
Composition of Magma
 Magma mostly consist of these elements: Oxygen
(O), Silicon (Si), Aluminum (Al), Iron (Fe),
Magnesium (Mg), Calcium (Ca), Potassium (K), and
Sodium (Na). The most abundant composition is
SiO2.
 In table (5-1), magmas are classified by rhyolitic,
andesitic, and basaltic.
Origins of Magma
 Most rocks must be heated to 800 Celsius and 1200
Celsius in order to melt. In nature, these
temperatures are found in the upper mantle and
lower crust.
 The main factors that affect magma formation are
temperature, pressure, water content, and mineral
composition.
Factors of Magma Formation
 Temperature: Generally increases with depth in the Earth’s
crust. For example, the geothermal gradient explains why
temperatures in deep mines are quite high.
 Pressure: Pressure also increases with depth. This is a result
of the weight of the overlying rock. One thing to take note off,
as the pressure increases, melting point increases.
 Water Content: The third factor that affects the formation of
magma is water content. As water content increases, the
melting point decreases.
 Mineral Content: Olivine, calcium feldspar, etc melt and
higher temperatures than rocks containing quartz and
potassium feldspar. Oceanic crust (high in iron/magnesium)
melt at higher temperatures than continental crust
silicon/aluminum)
Things To Remember
-Temperature generally increases with depth in the Earth’s
crust.
-Pressure increases with depth, and increase in pressure
raises the rocks melting point.
-Rocks and minerals often contain small percentages of
water, water lowers the melting point.
-Mineral composition also affects the melting point.
Different minerals have different melting points.
Rocks only melt at certain conditions, the right
combination of temperature, pressure, and composition
must be present.
How Do Rocks Melt and Crystallize
 Partial Melting: Rocks have different melting points
which means not all rocks melt at the same time.
This explains why magma is often a slushy mix of
crystals and molten rock. Some minerals melt at low
temperatures while others remain solid. (Figure 5-4
is a great example from your textbook).
 Fractional Crystallization: When magma cools, it
crystallizes in the reverse order of partial melting the
first minerals to crystallize are the last ones to melt.
That means minerals with high melting points will
form first.
Quick Overview of Bowen’s Reaction Series
 “As magma cools, minerals form in predictable
patterns.”
 The first pattern is characterized by a continuous,
gradual change of mineral composition. The second
part is characterized by abrupt change of mineral
type in the iron-magnesium group.
Bowens Reaction Series Chart (Figure 5-6)
Feldspars
 In Bowen’s reaction series the right branch
represents the
Conundrum?
 If Olivine converts to pyroxene during cooling, why
is olivine found in rock?
-Geologists hypothesize that under certain conditions,
newly formed crystals are separated from magma, and
the chemical reactions between the magma and the
minerals stop.
End of Section 5.1
Classifying Igneous Rocks
Mineral Composition
There are three main groups of igneous rocks, felsic,
mafic, and intermediate.
Felsic Rocks
 Felsic rocks, such as granite, are light-colored, and
have high silica contents.
 Minerals: Contain quartz, orthoclase, and
plagioclase.
Minerals: Feldspar
Orthoclase, Quartz,
possible mica and other
amphiboles.
Granite
Mafic Rocks
 Mafic Rocks, such as Gabbro, are dark-colored have
lower silica contents, and are rich in ron and
magnesium.
 Mineral contents (most common): biotite,
plagioclase, pyroxene, and olivine.
Minerals: Plagioclase,
Olivine,
Gabbro
Grain Size
 -What might account for the lack of risible crystals in
obsidian and large crystals of gabbro?
 -Cooling Rates: When lava flows on Earth’s surface,
it is exposed to air and moisture. This means lava
cools quickly and the crystals will be smaller.
Intrusive rocks are cool slowly beneath the earth’s
surface. Which means intrusive rocks will have larger
crystals.
Going Over A Special Case
 Porphyritic texture indicates a
complex cooling history.
These rocks have both large
and small crystals.
 For example, a rock could
have been “cooling” slowly
beneath the earth’s surface.
But then a rock suddenly
intruded higher in the crust
and remaining magma would
cool quickly and form smaller
crystals.
 (Won’t be a main question on
the test, but possibly an extra
credit question.)
Igneous Rocks As Resources
Diamonds
 Diamond is found in a rare,
ultramafic rock known as
kimberlites. Likely form
deep in the crust at depths
of 150 km to 300 km.
Diamonds can only form
under very high pressures.
 Most likely intruded
rapidly upwards to the
Earth’s surface, where it
forms long, narrow,
pipelike structures.
Summary
 http://www.youtube.com/watch?v=deC5af9AW6w