A ROCK is defined as an aggregate of minerals. IGNEOUS ROCKS are those rocks (aggregates of minerals) crystallized or SOLIDIFIED from magma.
Igneous rocks are classified according to
their TEXTURE and MINERAL COMPOSITION.
Igneous rock texture refers to the general
SIZE OF MINERAL CRYSTALS which compose the rock.
Rock texture (that is the SIZE
OF MINERAL CRYSTALS) provides us with
a clue to the COOLING HISTORY of the rock.
An igneous rock which cools from magma at
a relatively slow rate allows time for the mineral crystals to grow large.
Therefore, rocks which cool slowly will have minerals VISIBLE TO THE UNAIDED EYE.
Igneous rocks with visible minerals cool
from magma slowly beneath the Earth’s surface. As such, these rocks
are considered to be PLUTONIC (or INTRUSIVE) igneous rocks.
In contrast, an igneous rock which cools
at a relatively rapid rate allows only a short time for mineral crystals
to grow. Thus, rocks which cool rapidly will have minerals TOO SMALL TO BE SEEN WITH THE UNAIDED EYE.
Igneous rocks with minerals too small
to be seen cool from magma rapidly at or very near the Earth’s surface.
These rocks are called VOLCANIC (or EXTRUSIVE) igneous rocks.
Igneous rocks with PORPHYRITIC TEXTURE have minerals with at least TWO DISTINCT SIZE RANGES. The different mineral size indicates MULTIPLE
STAGES OF COOLING of the magma.
The mineral composition of igneous rocks
varies according to the CHEMICAL COMPOSITION and TEMPERATURE of the magma from which the rock formed.
In the 1920’s, N.L. Bowen examined
the processes of crystallization of minerals from magmas and derived BOWEN’S REACTION SERIES to describe the results of his observations.
 |
| The diagram above is a schematic representation
of Bowen's Reaction Series. Bowen found that mineral crystallization
within a magma began at about 1200oC with the formation
of OLIVINE and a CALCIUM-RICH FELDSPAR. |
In general, Bowen found that different minerals
formed in magma as the TEMPERATURE and CHEMICAL COMPOSITION of the magma changed.
There are two branches of Bowen’s Reaction
Series. The DISCONTINUOUS SERIES and the CONTINUOUS SERIES.
Along the Discontinuous Series of Bowen’s
Reaction Series, minerals of somewhat UNIFORM
CHEMICAL COMPOSITION are reorganized
into increasingly complex SILICATE STRUCTURES
as the temperature of the magma declines.
Point to Ponder:
Why might this occur?
NOTE: The crystallization pathway described
for the discontinuous series starts with a silicate mineral composed of
ISOLATED SILICA TETRAHEDRA (OLIVINE), then, as the temperature of the magma falls,
yields SINGLE CHAIN SILICATES (PYROXENES), DOUBLE CHAIN SILICATES (AMPHIBOLES), SHEET SILICATES (BIOTITE & MUSCOVITE), and finally FRAMEWORK SILICATES.
Point to Ponder:
Could this be part of the reason we make you learn these?
Along the Continuous Series of Bowen’s
Reaction Series, minerals of UNIFORM SILICATE STRUCTURE (FELDSPARS)
are reorganized as the CHEMICAL COMPOSITION
of the magma changes with falling temperature and mineral formation. Thus,
we observe that the FELDSPAR structure is stable over a broad temperature
range, but the CHEMICAL COMPOSITION of the feldspars changes with falling temperature.
Understanding of Bowen’s Reaction Series
helps geologists to understand the great diversity of igneous rocks found
on Earth. Indeed, understanding Bowen’s Reaction Series helps us to
interpret the geological history of a given igneous rock.
Point to Ponder:
How does Bowen's Reaction Series aid our interpretation of igneous rocks?
For example, we often find that MINERAL
ASSEMBLAGES (minerals which often
occur together) are common in igneous rocks. Rocks composed of the minerals
OLIVINE, CALCIUM FELDSPAR and PYROXENE are quite common. If this rock has a PHANERITIC texture (implying that it cooled slowly beneath
the Earth’s surface and is, therefore, plutonic or intrusive), it is
called GABBRO. If this rock has an APHANITIC texture (implying that it cooled rapidly at
or near the Earth’s surface and is, therefore, volcanic or extrusive),
it is called BASALT.
A rock containing the minerals AMPHIBOLE,
SODIUM PLAGIOCLASE and BIOTITE is called DIORITE if it has an intrusive (PHANERITIC) texture and ANDESITE if it has an extrusive (APHANITIC) texture. Note that the word andesite is derived
from the Andes Mountains of South America, where this rock type is the dominant
volcanic rock of stratovolcanoes.
A rock containing the minerals MUSCOVITE (or, commonly, BIOTITE), POTASSIUM FELDSPAR and QUARTZ is called GRANITE if it has an intrusive (PHANERITIC) texture and RHYOLITE (APHANITIC) if it has an
extrusive texture.
