Classification of igneous rocks

Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

Considerable evidence indicates that most magma originates in the
uppermost mantle. It is also clear that plate tectonics plays a major role in
generating most magma. The greatest quantities are produced at divergent
plate boundaries in association with seafloor spreading, whereas lesser
amounts form at subduction zones, where oceanic lithosphere descends into
the mantle. Some igneous activity occurs far from plate boundaries, indicating
that not all magma is produced in these relatively narrow zones.
Origin of Magma

Origin of Magma

Where do Magmas Form?
➢ Mid Oceanic Ridges at Divergent plate boundary
➢ Subduction zones at Convergence plate boundary
➢ Mantle Plume or Hot Spot

Most of the magmas originate when essentially solid rock, located in the crust and
upper mantle, melts. The most obvious way to generate magma from solid rock is to
raise the temperature above the rock’s melting point.
Keep in mind:
❑ Temperature of hottest erupting basaltic lavas
between 1000°C and 1350°C.
❑ Felsic lavas are about 650-800°C.
❑ Melting point of basalt is about 1250°C
❑ Melting point of granite is about 700°C

1. Role of heat
Most of the heat that contributes to the generation of magma comes from the very hot
Earth’s core (where temperatures are estimated to be greater than 5,000°C). Heat is
conducted toward the Earth’s surface through the mantle and crust. Heat is also brought
from the lower mantle when part of the mantle flows upward, either through convection or
by hot mantle plumes.
Factors controlled the melting of
Solid Rocks

2. Role of Pressure
Melting, which is accompanied by an increase in volume, occurs at higher temperatures at
depth because of greater confining pressure. Consequently, an increase in confining pressure
causes an increase in the rock’s melting temperature. Conversely, reducing confining
pressure lowers a rock’s melting temperature.

3. Role of Volatiles
Another important factor affecting the melting temperature of rock is its water content. Water and
other volatiles act as salt does to melt ice. That is, volatiles cause rock to melt at lower
temperatures. Further, the effect of volatiles is magnified by increased pressure.

Crystallization of Magma
➢ Formation of crystals from a melt.
➢ Liquids have lots of entropy (disorder) due to temperature → kinetic energy (movement)
of atoms and molecules.
➢ Solids, especially, crystals have less entropy because of their ordered, fixed internal
structures.
➢ Entropy lost as temperature is decreased.
➢ Crystallization begins when atoms, by chance, bond together to form nuclei. Other atoms
will follow…

➢ Why igneous rocks are so varied in composition?
➢ On a global scale, magma composition is clearly
controlled by geologic setting. But why?
➢ On a local scale, igneous bodies often show
considerable variation in rock type. Why?

Bowens Reaction Series

Discontinuous Reaction Series:
❑ The upper left branch of Bowen’s reaction series
indicates that as a magma cools, olivine is the first
mineral to crystallize. Once formed, olivine will
chemically react with the remaining melt to form the
mineral pyroxene.
❑ In this reaction, olivine, which is composed of
individual silicon–oxygen tetrahedra, incorporates more
silica into its structure, thereby linking its tetrahedra
into single chain structures of the mineral pyroxene.
(Note: pyroxene has a lower crystallization
temperature than olivine and is more stable at lower
temperatures.)

❑ As the magma body cools further, the pyroxene crystals will in turn react with the melt to generate the double-
chain structure of amphibole. This reaction will continue until the last mineral in this series, biotite mica,
crystallizes.
❑ This branch of Bowen’s reaction series is called a discontinuous reaction series because at each step a different
silicate structure emerges. Olivine, the first mineral in the sequence, is composed of isolated tetrahedra, whereas
pyroxene is composed of single chains, amphibole of double chains, and biotite of sheet structures.

Continuous Reaction Series:
➢ The right branch of the reaction series is
called continuous reaction series. The
continuous branch contains only
plagioclase feldspar.
➢ Plagioclase is a solid solution mineral in
which either sodium or calcium atoms can
be accommodated in its crystal structure,
along with aluminum, silicon, and oxygen.

➢ The composition of plagioclase changes as magma is cooled and earlier formed crystals react with the
melt. The first plagioclase crystals to form as a hot melt cools contain calcium but little or no sodium.
➢ As cooling continues, the early formed crystals grow and incorporate progressively more sodium into their
crystal structures.

Final Stage:
✓ Any magma left after the
crystallization is completed along the
two branches is richer in silicon than
the original magma and also contains
abundant potassium and aluminum.
✓ The potassium and aluminum
combine with silicon to form potassium
feldspar. (If the water pressure is high,
muscovite may also form at this stage.)
✓ Finally, if the remaining melt has
excess silica, the mineral quartz will
form.

❖ Magmatic Differentiation
❖ Assimilation
❖ Magma Mixing
❖ Partial Melting

Magmatic Differentiation:
The process by which different ingredients separate from an originally homogenous mixture is
differentiation. Differentiation in magmas takes place mainly through crystal settling, the
downward movement of minerals that are denser (heavier) than the magma from which they
crystallized.
When the remaining melt solidifies—either in place or in another location if it migrates into fractures
in the surrounding rocks—it will form a rock with a chemical composition much different from the
parent magma.

Once a magma body forms, its composition can
change through the incorporation of foreign
material. For example, as magma migrates
upward, it may incorporate some of the
surrounding host rock, a process called
assimilation.
This process may operate in a near-surface
environment where rocks are brittle. As the
magma pushes upward, stress causes
numerous cracks in the overlying rock. The force
of the injected magma is often sufficient to
dislodge blocks of “foreign” rock and incorporate
them into the magma body.
In deeper environments, the magma may be hot
enough to simply melt and assimilate some of
the surrounding host rock, which is near its
melting temperature.
❖ Assimilation

Some of our igneous rocks may be
“cocktails” of different magmas. The
concept is quite simple. If two
magmas meet and merge within the
crust, the combined magma should be
compositionally intermediate.
Magma Mixing:

Partial Melting:
Recall that the crystallization of a magma
occurs over a temperature range of at least
200°C. As you might expect, melting, the
reverse process, spans a similar
temperature range. As rock begins to melt,
those minerals with the lowest melting
temperatures are the first to melt.
Should melting continue, minerals with
higher melting points begin to melt and the
composition of the magma steadily
approaches the overall composition of the
rock from which it was derived.
Most often, however, melting is not
complete. The incomplete melting of rocks
is known as partial melting, a process that
produces most, if not all, magma.

Classification of igneous rocks

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