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Chapter 6 – Igneous rock
Magma and igneous rocks.
Igneous rocks are the most
abundant rocks on earth and
make up all of the mantle, the
oceanic crust, and much of the
continental crust.
Melt that solidifies beneath the
earth’s crust is called magma.
Melt that erupts at the surface is
called lava.
Intrusive igneous rock forms
Extrusive igneous rock forms
above-ground. Extrusive
igneous rock can form from lava
and pyroclastic debris,
particles from lava that
exploded into the atmosphere.
• Intrusive igneous rocks (magma that hardens belowground) include such
common, coarse-grained rocks as granite, diorite, and gabbro.
• When large blobs of magma harden underground, it can create different
structures such as plutons (i.e. Enchanted Rock) and tabular intrusions.
• Extrusive igneous rocks spill out on the surface as lava, which hardens to
produce rocks such as rhyolite, andesite, basalt, obsidian, pumice, and different
types of pyroclastic rocks.
• Extrusive lava produce volcanoes and lava flows.
• The Earth’s interior is not composed mostly of molten rock. Molten rock only
occurs in special areas within the lithosphere and crust under special conditions.
Decreased pressure
Addition of volatiles
Heat transfer
melting results
when hot mantle
rock rises to
shallower depths in
the earth. Normally,
even though it is
very hot at depths
in the upper mantle,
the tremendous
pressure of the
overlying rock
keeps the atoms
together and
prevents melting. If
that pressure is
released, melting
The geothermal gradient
is the temperature change
with depth. The rate (i.e.
slope of line) of change
decreases with depth.
Decompression and heat transfer melting
Heat transfer melting caused
by the hot basaltic magma
melting the surrounding rock.
An example of melting as a result of
the addition of volatiles. For
example, when water and/or carbon
dioxide percolate into a solid hot
rock. The addition of volatile
substances trigger melting because
it decreases the melting
• The major types of magma are:
• silicic magmas (> 70% silica with little Mg or Fe);
• intermediate magmas (55% silica);
• mafic magmas (< 50% silica and rich in Mg and Fe;
• ultramafic magmas (< 40% silica and lots of Mg and Fe). Mafic magma
is hotter than silicic magma.
• Magmas and rocks are frequently defined by the amount of silica they
contain. Remember though, all magma’s usually contain some silica.
• Magma’s are different
because of:
• source rock composition;
• partial melting of the
source rock;
• contamination; and
• fractional crystallization
• When rocks melt, only
some of the minerals that
make up the rock melt
because each mineral has a
different melting
temperature. Minerals with
lower melting points melt
first, minerals with higher
melting points remain
• Because of differential melting, magma’s do not have the same
composition as the source rock.
• Silicic minerals tend to have a lower melting point that mafic rocks
so magma’s tend to be enriched in silica compared to their source
• Contamination occurs when magma beneath the crust starts to melt
the surrounding rock.
• Similarly, the composition of the magma can change when blocks of
unmelted crust fall into the magma and dissolve, a process called
called assimilation.
• Fractional crystallization. When magma starts to cool and, just as
different minerals have different melting points, different minerals
have different “freezing” points. For example, mafic minerals
solidify at a higher temperature than silicic minerals, leaving the
magma progressively more silicic as it cools.
• Bowen’s Reaction Series was developed to show the different
crystallization rates of different common minerals.
•Magma is less dense than surrounding rock and heated rock expands as it melts
so magma tends to move upward due to buoyancy forces.
•The pressure of surrounding rocks squeeze magma in an upward direction.
•Magma containing volatiles can move upward faster because the presence of
gas bubbles decrease its density.
•Vesicles form when magma is extruded and freezes before the gas bubbles can
• Viscosity is the resistance to flow.
Low viscosity magmas flow more
easily than high viscosity magmas.
• Viscous lava is silica-rich and tends
to form a mound-like dome on top of
the volcano
• Mafic lava is thinner, can fountain
upward, and can travel far from the
vent. How fast magma travels
depends on the viscosity.
• Viscosity is also affected by
presence or absence of volatiles and
also temperature. The presence of
volatiles make lava less viscous. Hot
lavas are less viscous than cooler
Summary of the two major kinds of magma:
(1) A chemical composition high in silica, low in iron and
magnesium (i.e. silicic); light-colored, light-weight rocks, lowtemperature, high-viscosity melts
(2) A chemical composition high in iron and magnesium, low in
silica (mafic), dark-colored, heavy rocks, high-temperature, lowviscosity melts.
• When extruded lava freezes, it forms a sheet of igneous rock known as a lava
flow. Volatile-rich lavas tend to erupt explosively.
• Volcanic ash can form an ash flow, ash cloud or ash fall. Volcanic ash consists of
tiny, glass shards that form during an explosive eruption.
A layer of lava buried by
subsequent layers of ash fall
that has then eroded away. The
lava flow has broken into
fractures called columnar
• When magma intrudes
belowground into country or wall
rock, the boundary between the two
is called an intrusive boundary.
• Stoping is the process of magma
breaking off blocks of wall rock
• Sometimes these broken off blocks
don’t melt. Then they are called
• Picture at right shows a xenolith in
• However, most
intruded magma
forms tabular
intrusions called
dikes and sills.
• Dikes cut across
existing rock
• Sills are
A basalt dike within
granite. The dike extends,
wall-like into the granite
An example of a basalt sill in
sandstone country or wall rock.
Some sills dome upward and
are called laccoliths. Plutons
are blob-shaped intrusions
and, if large and numerous,
can create a batholiths.
At time 2, the magma
chamber itself freezes into a
At time 3, most of the
volanic rocks have eroded
exposing the pluton or, if
there are large and numerous
plutons, a batholith.
Mt. Rushmore is a granite pluton.
• Magma cools at different
rates depending upon
depth, shape and size of a
magma body and the
presence or absence of
circulating groundwater.
The large interlocking crystals of granite.
A thin section of fine-grained, glassy
volcanic rock
• Coarse-grained igneous rocks are called phaneritic.
• Fine-grained igneous rocks are aphanitic.
Igneous rocks can be either coarse
or fine-grained, dependent on
cooling rate. Horizontal lines
represent proportions of different
minerals in a particular rock.
Low density rocks
High density rocks
• Crystalline igneous rocks are classified based on silica content and texture and
composition. Mafic rocks tend to be black or dark gray, rocks with an intermediate
silica content tend towards lighter gray or greenish gray, and rocks with the greatest
amount of silica tend to be light tan to pink or maroon.
• Cooling rate affects grain size and therefore rock types: silicic magma that cools
quickly at the surface (lava) can become rhyolite or, if cooled slowly underground,
granite. Mafic lavas can form basalt if cooled quickly but gabbro if a mafic magma
is cooled slowly underground.
• Glassy (non-crystalline) igneous rocks can include obsidian, pumice (many open
pores), scoria (air-filled pores < 50% of the rock) and tuff (a pyroclastic rock
formed from composed of ash and/or pieces of lava and pumice).
• Igneous activity occurs along volcanic arcs, in isolated hot spots, within continental
rifts, and along mid-ocean ridges.
Molten rock or melt develops when a plume of hot rock rises to the base of the
lithosphere; in the asthenosphere above subducting plates; in the asthenosphere above a
mid-ocean ridge; along a continental rift.
Ex: Hawaii
Ex: Japan
Ex: Iceland
Ex: East African
Rift zone
The world-wide distribution of volcanos.
Flood basalts occur when huge amounts of basalt (mafic lava) erupt at the surface
and spread out into sheets covering tens to hundreds of kilometers across the
End of Chapter 6
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