1121 (Written by T. Weiland)
rocks were originally derived from igneous rocks. Elevated
planetary temperatures during earth formation produced widespread
melting. The study of igneous rocks not only provides important
information about earths origin and evolution, but also
records important data on more recent global volcanism and
I. Igneous Rock
Definition - rocks formed by the cooling and solidification
of magma. (Magma - naturally-occurring molten-rock material
generated within the earth. Magma is made primarily of the
elements found in the silicate minerals plus some gases.)
Rocks (Extrusive Rocks) - igneous rocks that
form by the extrusion and cooling of magma on the earth's
surface. They can be recognized by the fine-grained or
glassy nature of the portion of the rock that cooled
rapidly as it was extruded. Lava - another name
for fluid magma which reaches the surface before it is
Rocks (Intrusive Rocks) - igneous rocks which form by
the solidification of magma beneath the surface (or
within the earth). Named after Pluton, god of the
underworld. These are only seen at the surface today
(like the granite at Stone Mountain) because of extensive
erosion and/or uplift. Plutonic rocks are recognized by
the presence of visible crystal throughout the rock.
II. Origin of
Magma - Scientists originally believed that the interior of
the earth was totally molten; however, an abundance of evidence
now indicates that earth is almost totally solid down to the
outer core. It appears that magma occurs as liquid segregations
in localized areas (vein-like) throughout the upper part of the
mantle and parts of the lower crust.
Factors Which Cause Melting (Melting - the change
from solid to liquid). Melting is most commonly caused by
heating which increases ion vibration until the chemical
bonds are broken.
Changes - higher temperatures cause melting.
Rocks are poor conductors of thermal energy so
much of that original heat remains. Temperature
could be increased by moving material deeper
within the earth or by increasing the temperature
of existing levels. The geothermal gradient
is the increase in temperature with increasing
depth. The geothermal gradient in the upper crust
is approximately 25° C/Km.
- Residual heat remaining
from earths formation
- Heat given off from
naturally-occurring radioactive elements
- Frictional heat from
colliding plates (minimal)
- Solar heat(minimal)
- Pressure Changes - lower
pressures generally decrease melting
temperatures, such that rocks which are stable
deep within the earth at high pressures become
unstable and melt when moved upward in areas of
lower pressure. Source of Pressure
weight of the overlying rocks
in Water Pressure - the addition of water
generally lowers the melting temperature of
rocks. The release of water from hydrated oceanic
crust which is subducted along active margins is
believed to cause melting of overlying rocks
which were originally stable.Source of
Water subducted oceanic crust
Areas of Magma Generation
Ridges - hot mantle rock material is upwelled
along elongate ridges forming new oceanic crust.
Melting is largely due to decreased pressure in
these areas of divergent (pull-apart) oceanic
Zones - oceanic crust is pushed beneath other
oceanic or continental crust. Melting is largely
due to water that is derived
subducted oceanic crust.
areas - where continental crust is being
pulled apart, molten mantle rock is often
erupted. Melting is a result of elevated crustal
temperatures and decreased pressures associated
with the rifting.
Crystallization - change from liquid to a solid. Generally
long-term (lava flows take years to hundreds of years, while
intrusions take thousands of years). Different minerals form and
react during cooling because they have different crystallization
temperatures and stability ranges.
that Control Crystallization - Crystallization
involves the arrangement of ions into an orderly atomic
pattern. Requires slow cooling and low to moderate
viscosity (magma thickness). Vibrational energy of the
atoms is much higher at higher temperatures, therefore
crystallization can't occur until some of this energy is
released. If the energy is released too fast, the magma
is solidified into a glass, a material without an ordered
atomic arrangement. Melting is the opposite of
crystallization - energy is added until the atomic bonds
Rate - most important - rapid cooling leads
to fine-grained or glass-rich rocks (glass -
amorphous solid w/out an ordered atomic
arrangement). Slow rates of cooling are required
for coarse-grained crystalline rocks to form (ex.
Stone Mt. probably took thousands of years to
crystallize beneath the surface). Crystallization
requires a slow decrease in thermal energy for
extensive ion migration (movement of charged
atoms) and nucleation (initial growth of seed
of the Magma - Viscosity is the resistance to
flow. If the magma is too viscous, internal
friction prevents ion migration.
Reactions During Magma Cooling
Reaction Series Several minerals will
display solid solution, a process where solid
crystalline material will change chemical
composition during cooling or heating like a
liquid solution can change by adding or removing
elements. Solid solution mineral series are
define by end members that crystallize over a
range of temperatures and which continuously
change in composition as the temperature
decreases (continuous reaction with the melt).
Example - Anorthite (1553° C.) and Albite
(1118° C.) Na exchanges for Ca in the
crystalline structure. This involves a continuous
reaction with the melt.
Reaction Series Several early-formed
minerals will become unstable and react with the
melt to form more stable lower-temperature
minerals as the magma cools. Example - Olivine
reacting to form pyroxene: Mg2SiO4(olivine)
+ SiO2(in melt) ----® 2MgSiO3
Reaction Some minerals are stable over
a large temperature range and will not chemically
react with the magma unless they melt. An example
Reaction Series - simplified sequence in
which minerals crystallize in a magma as
temperature drops. See your notes and textbook
for a better description and diagram. *The series
is not true in all magmas because temperature
might not reach the lower range and
compositional, pressure and viscosity differences
also control mineral stability.
Differentiation - Basalt magma appears to be the
parental composition of most igneous rocks. There are
several important processes which subsequently change the
magma composition and result in the great variation seen
in the composition of igneous rocks.
- melting of the wall rock which surrounds the
magma (wall rock must be a different
Settling - gravitational settling of the
early-formed denser crystals that changes the
composition of the residual magma.
Mixing - magma is mixed with other magma that
enters the magma chamber.
Classification of Igneous Rocks -based on texture,
composition and stratigraphic (field) relationships.
- the general appearance or character of the rock. This
includes the grain size, shapes, and the arrangement in
the rock. **Records the cooling and crystallization
history of the rock.
- crystals are too small to be seen with the
unaided eye. These rocks must be studied with the
microscope or by geochemical methods. Volcanic
rocks (extrusive) are at least partially
aphanitic due to rapid cooling. Glassy -
lava which cools so rapidly that the crystals
don't have time to grow. (unordered atomic
arrangement) Example: obsidian
- mineral grains can be seen with the unaided
eye. Plutonic (intrusive) rocks are totally
phaneritic due to slow rates of cooling. Pegmatite
- very coarse-grained rocks which crystallize
from very water-rich magmas. The water in these
magmas lowers the viscosity allow excessive ion
migration and crystallization.
- two distinct crystal sizes (can be either
porphyritic or aphanitic). Larger crystals
(phenocrysts)are surrounded by finer-grained
crystals or glass (groundmass). This texture
records two distinct cooling histories.
Volcanic Textures - important only in
volcanic rocks - Most magmas contain a few
percent of dissolved gases (mostly water and
carbon dioxide). Before eruption, this gas is in
solution; however as the magma nears the surface,
the gas is released from solution (as pressure is
lowered) - resulting in violent eruptions.
Example - Mt. St. Helen
- Pyroclastic Rocks -
result from subaerial volcanic eruptions
which expel magma so quickly that it
cools forming dust-size glass fragments
that is shattered and highly broken. Tuff
- pyroclastic rock composed of compacted
ash and rock fragments.
- Vesicular Rocks -
rocks formed from the cooling of a froth
of magma and gas. Gas is released more
passively, especially in less viscous
magmas. Pumice - light-colored,
low density vesicular rock. Scoria
- dark-colored, more-dense vesicular
- mineralogy and chemical composition. Most magma
consists of predominantly 8 elements (Si, O, Al, Mg, Na,
K, Ca, Fe). Composition provides insight into the nature
and origin of the magma. Quartz, feldspar and the
ferromagnesian silicates are the most common minerals in
igneous rocks; therefore, these minerals form the basis
for the classification of igneous rocks.
Mineralogic, Chemical and Textural Subdivisions
- (Fel - feldspar, si - silica) applied to
igneous rocks which have abundant light-colored
minerals such as quartz and feldspar
- granular, coarse-grained, silica-rich igneous
rock which is predominantly composed of
K-feldspar, Na-plagioclase and quartz with minor
biotite, iron oxides and/or amphibole. It is an
abundant rock type on continents (example
Stone Mt. And Elberton granites).
- aphanitic equivalent of granite. Commonly
occurs as volcanic flows and tuffs along active
Rocks - (Ma - magnesium, fic - iron) - igneous
rocks composed predominantly of dark ferromagnesium
minerals such as amphiboles, olivine and pyroxene and
occasionally Ca-rich plagioclase. *Most abundant rock
types - 75% of all igneous rocks.
- granular, intrusive, mafic igneous rock
composed of Ca-plagioclase, pyroxene and some
olivine and iron oxides. Commonly found at lower
crustal and/or mantle levels of both oceanic and
- aphanitic equivalent of gabbro. It is the
dominant component of oceanic crust and oceanic
islands. It is commonly erupted along fissures as
extensive thick flows. Basalt rarely occurs as a
tuff due to the low viscosity of basaltic magma.
Rocks - Because there is a complete gradation
between the mafic and silicic igneous rocks, this
group includes many of the compositions between the
two extremes. These rocks generally contain a
substantial amount of both ferromagnesian and
- coarse-grained, granular rock which is
generally darker than a granite but lighter than
a gabbro (salt and pepper appearance). It usually
contains abundant intermediate (Na-Ca)
plagioclase, amphibole and/or pyroxene and iron
oxides. It is differentiated from granite by the
general absence of quartz.
- extrusive equivalent of diorite. It is commonly
found along subduction zones(active continental
margins as both flows and tuffs.
Rocks - composed almost entirely of
ferromagnesian minerals with only minor plagioclase
and iron oxides. These include dunite (mostly
olivine) and peridotite (olivine and
pyroxene). These rock types are the dominant
components of the mantle, but are much less common
than the other three types on the surface.
(Field) Relationships - the nature of the contacts
between the magmatic body and the surrounding rocks.
- any large intrusive body
- tabular plutons that have formed by the
injection of magma between beds of layered rocks.
- tabular bodies that cut across the layering of
the country rock.
- the largest of the plutons (>100 kms.2). Ex.
Sierra Nevada Batholith
- magma that is extruded passively onto the
earth's surface (mostly basalt or andesite).
Lavas usually follow the pre-existing topography.
and Pyroclastic Rocks - more silicic in
composition. Tend to mantle the countryside.