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Chapter 5: Igneous Rocks
5.1 What are igneous rocks?
* There are 3 basic rock types: igneous,
sedimentary, and metamorphic.
Main Idea: Igneous rocks are the rocks that form when molten material cools and crystallize/solidifies.
Formation of Igneous Rock - Video
Lava: magma that flows out onto Earth’s surface.
 Igneous rocks form when lava or magma cools and minerals crystallize.
Video to Watch:
One & Two
Composition of Magma

The type of igneous rock that forms depends on the composition of the magma, which is often a slushy mix
of molten rock, dissolved gases, and mineral crystals.

Magma is classified as basaltic (42-52% silica content), andesitic (52-66% silica), or rhyolitic (more than 66%
silica), based on the amount of silica in it. Silica affects melting temperature and how quickly magma flows.
Magma Formation

Magma can be formed either by melting of Earth’s crust or by
melting within the mantle. The 4 main factors involved include:
temperature, pressure, water content, and mineral content.

Temperature generally increases with depth in Earth’s crust. This
temperature increase is known as the geothermal gradient (right).

Pressure also increases with depth as a result of the weight of
overlying rock, and this increases the rock’s melting point.

Rocks and minerals often contain small percentages of water, which
changes the melting point of the rocks. As water content increases,
the melting point decreases.

Rocks melt at lower or higher temperatures because their
component minerals have lower or higher melting points. In general, rocks rich in iron and magnesium melt at
higher temperatures than rocks that contain higher levels of silicon.
Partial Melting: when some minerals melt at relatively low temperatures while other minerals remain solid.

If temperatures are not high enough to melt the entire
rock, the resulting magma will have a different
composition than that of the original rock.
Bowen’s Reaction Series - Video

N. L. Bowen demonstrated that as magma cools and
crystallizes, minerals form in predictable patterns in a
process now known as the Bowen’s reaction series. There
are two main patterns, or branches, of crystallization.

The left branch of Bowen’s reaction series represents the
iron-rich minerals, which undergo abrupt discontinuous
changes as magma cools and crystallizes.

The right branch denotes plagioclase feldspars, which is a continuous change of composition as magma cools.
Fractional Crystallization: When magma cools, the first minerals that crystallize are the last minerals that melted
during partial melting.
5.2 Classification of Igneous Rocks
Main Idea: Classification of igneous rocks is based on mineral composition, crystal size, and texture.
Review - Fractional crystallization: a sequential process during which early formed crystals are removed from the
melt and do not react with the remaining magma.
Mineral Composition of Igneous Rocks
•
When magma cools and crystallizes below Earth’s surface, intrusive rocks form.
•
Lava that cools and crystallizes on Earth’s surface forms extrusive rocks.
•
Igneous rocks are classified by their mineral compositions and there are four basic types.
Basaltic (mafic) rocks: dark-colored, lower silica contents, and contain mostly plagioclase and pyroxene.
Granitic (felsic) Rocks: light-colored, high silica content, and contain mostly quartz, potassium feldspar, and
plagioclase feldspar.
Intermediate Rocks: composition of minerals that is somewhere between basaltic and granitic
Ultrabasic Rocks: contains the rock peridotite. Ultrabasic rocks contain only iron-rich minerals such as olivine
and pyroxene and are always dark.
Texture - Video
Texture: Refers to the size, shape, and distribution of the crystals or grains that make up a rock.
•
When lava flows on Earth’s surface, it cools
quickly and there is not enough time for
large crystals to form. The resulting
extrusive igneous rocks have crystals so
small they cannot be seen without
magnification.
•
When lava cools so quickly that crystals do
not form, volcanic glass is the result.
•
When magma cools slowly below Earth’s
surface, there is sufficient time for large
crystals to form. Thus, intrusive rocks can
have crystals larger than 1 cm.
Porphyritic Texture: Characterized by large,
well-formed crystals surrounded by finergrained crystals of the same mineral or different minerals.
Vesicular Texture: Spongy appearance that results from gas bubbles trapped in lava.
Igneous Rocks as Resources
•
Scientists use a special microscope, called a petrographic microscope, to help them identify minerals by
examining the mineral grains in sections of rock thin enough for light to pass through.
•
Valuable ore deposits often occur within igneous intrusions. At other times, they may occur as veins in the
rocks surrounding intrusions. Veins of extremely large-grained minerals that can contain ores are called
pegmatites.
•
Diamonds and other minerals that can form only under very high pressure are found in rare, ultrabasic rocks
known as kimberlites.
•
The interlocking grain textures and the resistance to weathering of many of the minerals present in igneous
rocks make them especially useful as building materials.