Igneous Rocks
Pamela J. W. Gore
Georgia Perimeter
College
Objectives
1. Contrast magma and lava.
2. Describe how the rate of cooling
influences the size of crystals in
igneous rocks.
3. List the different igneous rock
textures (aphanitic, phaneritic,
porphyritic, pegmatitic,
vesicular, glass, and pyroclastic)
and explain their origins.
4. Discuss the contributions of N. L.
Bowen to the understanding of
igneous rocks. Be able to list the
minerals of Bowen's Reaction
series in order. Explain the
significance of Bowen's Reaction
Series to melting and
crystallization.
5. Discuss the various types of
magma and how they relate to
igneous rocks.
6. List the various kinds of intrusive
igneous bodies (dike, sill,
laccolith, stock, batholith) and
describe each in terms of the
criteria used to classify plutons.
Lava cools more quickly because it is
on the surface.
Cooling Rates
Cooling rates influence the texture if
the igneous rock:


Quick cooling = fine grains
Slow cooling = coarse grains
Igneous rocks are classified on their
texture and their composition.
Igneous textures:

Glassy - instantaneous cooling
o Obsidian = volcanic glass
Obsidian

Aphanitic - fine grain size (< 1
mm); result of quick cooling
Origin
"fire-formed rocks"
Crystallize from molten material:


Magma - below the Earth's
surface
Lava - erupts onto the Earth's
surface through a volcano or
crack (fissure)
Rhyolite
Basalt
Rhyolite
Andesite
Phaneritic - coarse grain size;
visible grains (1-10 mm); result
of slow cooling
o
o
o

o
Granite - polished
Granite
Diorite
Gabbro
Pegmatitic - very large crystals
(many over 2 cm)
o Granite pegmatite or
pegmatitic granite
Porphyritic- Mixture of grain
sizes caused by mixed cooling
history; slow cooling first,
followed by a period of
somewhat faster cooling.
o Terms for the textural
components:
 Phenocrysts - the
large crystals
 Groundmass or
matrix - the finer
crystals surrounding
the large crystals.
The groundmass
may be either
aphanitic or
phaneritic.
o Types of porphyritic
textures:
 Porphyriticaphanitic
 Porphyriticphaneritic
o Origin: mixed grain sizes
and hence cooling rates,
imply upward movement
of magma from a deeper
o
o
o


(hotter) location of
extremely slow cooling, to
either:
 a much shallower
(cooler) location with
fast cooling
(porphyriticaphanitic), or
 a somewhat
shallower (slightly
cooler) location with
continued fairly slow
cooling (porphyriticphaneritic).
Rock name = porphyry
 Granite porphyry or
porphyritic granite
(porphyriticphaneritic) phenocrysts usually
potassium feldspar
Granite porphyry

Andesite porphyry or
porphyritic andesite
(porphyriticaphanitic) phenocrysts usually
hornblende
(amphibole)


Rhyolite porphyry or
porphyritic rhyolite
(porphyriticaphanitic)
Rhyolite Porphyry
Vesicular - contains tiny holes
called vesicles which formed
due to gas bubbles in the lava or
magma. Very porous. May
resemble a sponge. Commonly
low density; may float on water.
o Vesicular basalt - basalt
with a vesicles, which may
be quite large. Sometimes
lined with crystals to form
geodes.
Vesicular basalt with
olivine phenocrysts,
building stone at Hawaiian
Volcano Observatory, Big
Island of Hawaii
o
Pumice - light in color;
white to gray; may be
glassy or dull. Fully riddled
with holes. Very spongelike. Floats. Used as an
abrasive. (Pumice stone,
Lava Soap).
Pumice
Vesicular basalt
Pumice
o
Scoria - dark in color;
brown, black, or dark red;
similar to vesicular basalt
but is fully riddled with
holes to form a spongy
mass. (May find in
barbecue grills as lava
rock).
Composition of
Igneous Rocks
Igneous rocks can be placed into four
groups based on their chemical
compositions:
1. Sialic (or granitic or felsic)
1. Dominated by silicon and
aluminum (SiAl)
2. Usually light in color
3. Characteristic of
continental crust
4. Forms a stiff (viscous) lava
or magma
5. Rock types include:
1. Granite
Scoria

Pyroclastic or Fragmental pieces of rock and ash come out
of a volcano and get welded
together by heat. May resemble
rhyolite or andesite, but close
examination shows pieces of
fine-grained rock fragments in it.
May also resemble a
sedimentary conglomerate or
breccia, except that rock
fragments are all fine-grained
igneous or vesicular.
o Tuff - made of volcanic ash
o Volcanic breccia - contains
fragments of fine-grained
igneous rocks that are
larger than ash.
Granite
2. Rhyolite
Rhyolite
Pyroclastic rock
6. Minerals commonly
present include:
1. potassium feldspar
(generally pink or
white)
2. Na-plagioclase
feldspar (generally
white)
3. quartz (generally
gray or colorless)
4. biotite
5. amphibole?
6. muscovite?
2. Intermediate (or andesitic)
1. Intermediate in
composition between sialic
and mafic
2. Rock types include:
1. Andesite (aphanitic)
2. Diorite (phaneritic)
Basalt
2. Gabbro (phaneritic)
Gabbro
Diorite
3. Minerals commonly
present include:
1. plagioclase feldspar
2. amphibole
3. pyroxene
4. biotite
5. quartz
3. Mafic (or basaltic)
1. Contains abundant
ferromagnesian minerals
(magnesium and iron
silicates)
2. Usually dark in color (dark
gray to black)
3. Characteristic of Earth's
oceanic crust, Hawaiian
volcanoes
4. Forms a runny (low
viscosity) lava
5. Also found on the Moon,
Mars, and Venus
6. Rock types include:
1. Basalt (aphanitic)
3. Diabase - texture
intermediate
between basalt and
gabbro;
characteristic of
Early Mesozoic dikes
in eastern North
America.
7. Minerals commonly
present include:
1. Ca-plagioclase
feldspar
2. pyroxene
3. olivine
4. amphibole
4. Ultramafic
1. Almost entirely magnesium
and iron silicates
(ferromagnesian minerals)
2. Rarely observed on the
Earth's surface
3. Believed to be major
constituent of Earth's
mantle
4. Commonly found as
xenoliths in basaltic lavas
5. Rock types include:
1. Peridotite
(phaneritic)
1. dominated by
olivine - the
birthstone is
Peridot, which
gives its name
to Peridotite
A polished syenite called larvikite with
centimeter- to inch-scale gray to blue
plagioclase crystals. The industrial
name for the rocks is "blue pearl".
Photographed in an above-ground
cemetery in New Orleans, LA
Peridotite
6. Minerals commonly
present include:
1. Olivine is dominant.
(Olivine is olive
green).
2. may have minor
amounts of pyroxene
and Ca-plagioclase
Other types of igneous rock:
Syenite
Bowen's Reaction
Series
Bowen's Reaction Series is a listing of
the most common silicate minerals in
igneous rocks, in the order in which
they crystallize from a magma, as the
temperature decreases and the
magma cools.
Bowen's Reaction Series also lists the
minerals in the order in which they
melt with increasing temperatures.
Minerals higher on the reaction series
crystallize before minerals lower on
the reaction series. The earlier-formed
minerals react with the magma to
form minerals lower on the series. The
process ends when the magma has
completely crystallized. The
composition of the resulting igneous
rock depends on the composition of
the magma.
Visit this interactive web site to see
the minerals of Bowen's Reaction
Series (in order)
http://www.cat.cc.md.us/courses/eas1
01/unit1/brs.html.
Bowen's Reaction Series has two
branches. They are:
1. Discontinuous reaction
series, from olivine to biotite,.
2. Continuous reaction series,
from Ca plagioclase to Na
plagioclase.
The discontinuous reaction series
involves the dark-colored
ferromagnesian minerals:
1.
2.
3.
4.
olivine
pyroxene
amphibole
biotite.
As a magma cools, olivine crystallizes
first. The olivine crystals react with the
remaining magma to form pyroxene.
Pyroxene reacts with the magma to
form amphibole. Amphibole reacts
with the magma to form biotite. Each
successive mineral, from olivine to
biotite, has a different composition
and a different silicate crystal
structure. As crystallization proceeds,
the crystal structures become more
complex (olivine has an isolated
tetrahedral structure, pyroxene has a
single chain structure, amphibole has
a double chain structure, and biotite
has a sheet structure). The series of
minerals is called discontinuous
because a series of different minerals
is formed, each with a different crystal
structure.
The continuous reaction series
involves the plagioclase feldspars.
Plagioclase feldspars are an example
of a "solid solution series", exhibiting
gradations in chemical and physical
properties. Chemically, this series
consists of two "end members":
1. albite or Na plagioclase
(NaAlSi3O8), the sodium "end
member", and
2. anorthite or Ca plagioclase
(CaAlSi2O8), the calcium "end
member".
There is a continuous chemical and
physical gradation between the two
end members. (Various plagioclase
mineral names are given, based on the
percentages of calcium and sodium
present, including anorthite,
bytownite, labradorite, andesine,
oligoclase, and albite).
Ca-plagioclase is the first to
crystallize. It reacts with the melt to
become more sodium rich. (If reaction
is not complete, a zoned plagioclase
crystal results which has a calciumrich center and sodium-rich edges).
This series of plagioclase minerals is
called continuous because all of the
plagioclase minerals have the
same crystal structure. The
minerals differ primarily in the
proportions of calcium and sodium
present.
During the last stages of
crystallization, potassium feldspar
(KAlSi308) crystallizes. Muscovite may
also form. If the remaining melt
contains excess silica, quartz will
crystallize.
Bowen's Reaction Series helps us to
understand why certain minerals
tend to occur together in igneous
rocks. For example, the mafic rocks,
basalt and gabbro tend to contain
olivine, pyroxene, and calcium-rich
plagioclase feldspar. These are all
minerals which crystallize at high
temperatures. As another example,
felsic or sialic rocks such as granite
and rhyolite tend to contain quartz,
potassium feldspar, sodium-rich
plagioclase feldspar, and sometimes
muscovite. These are minerals which
crystallize at lower temperatures. The
minerals that ultimately form are
controlled by the initial composition of
the magma.
Bowen's Reaction Series also helps us
to understand why certain minerals
do NOT occur together in igneous
rocks. For example, olivine and quartz
are unlikely to occur in the same
igneous rock, because olivine is a high
temperature mineral, and quartz is a
low temperature mineral.
Bowen's Reaction Series also shows us
that the range of igneous rocks,
from ultramafic to sialic (or felsic),
can be produced by the same
original mafic magma. The magma
changes as it cools. As a magma cools,
the early-formed crystals may settle to
the bottom of the magma chamber.
This would produce a rock type at the
bottom of the magma chamber that is
dominated by early-formed minerals
such as olivine, pyroxene, and calcic
plagioclase (a mafic or ultramafic
rock). The remaining melt would be
enriched in silica (relative to its
original composition), and may
continue moving upward toward the
earth's surface. Crystal settling may
occur again, producing an
intermediate rock. As the last
remaining melt moves toward the
Earth's surface, it will crystallize to
produce a sialic or felsic rock. The
removal of crystals from the magma
by settling (or other processes) is
known as fractional crystallization.
The formation of several different rock
types from one initial magma, through
separation of earlier-formed crystals,
causing the magma to evolve to
become more silica-rich, is known as
magmatic differentiation. Magmatic
differentiation can produce a variety of
types of igneous rocks through
evolution of the original parent
magma.
Memorizing Bowen's Reaction
Series
You need to memorize Bowen's
Reaction Series. It helps to use a
mnemonic device (or phrase using the
initial letters of each mineral) to help
remember the names of the minerals
in order. Remember the Y-like shape.
For the discontinuous reaction series
and the minerals in the "trunk" of the
Y, you would have the following first
letters of mineral names: O P A B K M
Q. (K is for potassium feldspar, which
is abbreviated with the chemical
symbol, K.) Try to make up your own
mnemonic device or phrase, but here
is one that one of my students made
up. I don't like it, but I remember it.
Old People Are Bad; Keep Mother
Quiet. Hopefully you can make up a
better one.
You have to know the series AND
understand the concepts of how
Bowen's Reaction Series relates to
melting and to crystallization, and to
the origin of igneous rocks of various
composition.
Plutons
Subsurface igneous bodies
Named for Pluto, Roman god of the
underworld


Concordant Plutons
Oriented parallel to surrounding
layered rocks. Formed from
magma that is injected between
the layers of the surrounding
rock.
o sill
o laccolith
Discordant Plutons
Oriented at an angle to
surrounding layered rocks.
Magmas cut through the existing
layering.
o dike - less than 1 cm (less
than 0.5 inch) to 1 km
(0.6 mi)
o stock - 1 km2 (less than 1
mi2) to 100 km2 (40 mi2)
o batholith - more than 100
km2 (40 mi2)
Xenoliths
When magma moves upward through
pre-existing rocks, some of the
surrounding rocks are melted and
become assimilated into the magma.
In other cases, blocks of the
surrounding rocks are broken off
and carried along by the magma,
unmelted. These blocks are called
xenoliths, a word meaning "foreign
rocks". The rocks that make up the
xenoliths would be carried along
without melting if the temperature of
the magma is not high enough to melt
the rock (for example, if the minerals
in the rock were higher on Bowen's
reaction series).
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Copyright 2000 Pamela Gore
This page created by Pamela J. W.
Gore
Georgia Perimeter College, Clarkston
Campus, Clarkston, GA
October 2, 1995
Modified January 24, 1997
Modified September 21, 1998
Modified July 17, 1999
Last modified August 21, 2000