Igneous rocks
Geology 101
Igneous rocks’ origin
• Ignis (Latin for fire) – these rocks were
crystallized from a molten state. They
are not formed by sediment
accumulation.
The diversity of volcanic materials
• Vesicular basalt
• Banded rhyolite
Igneous rock classification
• The names of different igneous rocks
depends first on if they have a
distinctive texture, and second on
composition and grain size.
Volcanic rock names are either due
to composition or to texture
• Basalt, because the
rock is mafic
• Obsidian, because
the rock is glassy
Igneous rocks have textures
• Mostly erupted rocks – these textures
are all due to extremely rapid cooling,
which tends to trap gas bubbles (which
results in vesicles), to fuse falling hot
rocks into already-fallen rocks
(“welding”) and to create glassy
surfaces (a vitreous luster).
Vesicles
Vesicles represent areas of trapped gas frozen in place
when the surrounding rock crystallized. Scoria (left) and
pumice may differ in composition but they principally differ
in density: scoria will sink in water, pumice floats.
Welding
Micrograph of sample on left shows a welded tuff; note
the melted, elongated look to many of the crystals.
On the right is a sample of a tuff; note that many of the
larger crystals look well-shaped and not melted.
Welding hardens the rock.
Vitreous – as it forms
As a runny (basaltic) lava erupts, trails of molten rock
are thrown into the air and freeze in glassy threads
(Pele’s hair) and glassy teardrop shapes (Pele’s
tears).
Vitreous – on a large scale
Obsidian is a glassy volcanic rock; shown below is snowflake
obsidian (the snowflakes are crystals of the mineral
cristobalite, a polymorph of quartz). In fact, obsidian is mostly
amorphous silica with the same chemical composition as
quartz. There can be thick flows of obsidian, as seen in the
Big Obsidian Flow at Newberry Caldera in Oregon.
Grain size allows sorting of
igneous rocks
• Terminology:
• Volcanic rocks = extrusive =
fine-grained (cannot see
mineral grains within rocks)
• Plutonic rocks = intrusive =
coarse-grained (easily seen
mineral grains)
Why the different grain sizes?
• Grain size is dependent on the rate of
magma crystallization, which is in turn
dependent on the depth at which
crystallization occurs
• Deep = slow cooling = coarse grains
Plutonic rocks
• The general structure in
which plutonic rocks exist
is called, not surprisingly,
a pluton
• Types of plutons:
batholiths (large), stocks
(small), laccoliths,
lopoliths, etc.
Volcanoes are complex structures
When the volcano erodes away, ring
dikes may remain; example below is
near Odessa
The diversity of volcanic eruptions
• Kilauea (Hawaii)
• quiescent
• Vesuvius (Italy)
• explosive
More terminology
• Magma = molten
rock underground
• Lava = molten rock
above ground
• New tectonic
setting: the “hotspot”
-- small, intense
mantle convection
cell
Connections
• Tectonic setting leads to type of magma
generated leads to type of volcano
made leads to behavior of volcano
• Thus, understanding magma generation
is the key
Magma generation at subduction
• At a subduction
zone, the subducted
slab loses volatiles
(water, principally)
and causes the
melting temperature
of the rocks above it
to lower
Magma generation at rift
• At a rift, as the
pressure of
overlying layers
lessens (crust thins),
melting temperature
of rock lowers.
But why different rocks?
• Start with mantle
rocks, which are
very rich in iron and
other metals (like
aluminum and
magnesium)
• Magnesium,
Aluminum, iron
(Fe) = mafic
Bowen’s reaction series shows
mineral crystallization temperatures
• NL Bowen (Carnegie Institute)
in 1915-1922 measured the
melting (crystallizing)
temperatures of various
minerals and also found that
there were two distinct
progressions of mineral
changes as the temperature
changed: the continuous and
the discontinuous series.
Which is why we can have non-mafic
igneous rocks
• Using Bowen’s series,
then, the temperature of
magma sets its mineral
composition, and thus the
rock’s eventual identity.
• Mafic minerals form at high
temperatures, but are
replaced by more and
more felsic (“feldspar” +
“silica” meaning rich in
silicon, oxygen, aluminum,
potassium and sodium)
minerals at lower temps.
Modification of magma
• At subduction zone, the
descending slab loses
volatiles and partially
melts the mafic or
ultramafic rock above,
some of the slab
material (or continental
material) can melt and
mix with the mafic
magma
Modification of magma
• Fractional crystallization causes higher
melting point minerals to crystallize and
settle, removing mafic components of magma
Tectonic settings and magma
Magma composition leads to rock name
leads to lava behavior
• Mafic
intermediate
felsic
Lava viscosity leads to volcano types
• Low viscosity lava yields low angle slope
volcanoes -- shield volcanoes
Higher viscosity = steeper slopes
• Composite volcano
or stratovolcano
Many different volcano types