Igneous Rock
Igneous rocks are formed by the solidification of molten magma ejected from deep within
the earth’s mantle. After ejection by either fissure eruption or volcanic eruption, some of
the molten magma cools on the surface of the earth. Sometimes magma ceases its mobility
below the earth’s surface and cools to form intrusive igneous rocks that are called plutons.
Intrusive rocks formed in the past may be exposed at the surface as a result of the
continuous process of erosion of the materials that once covered them.
The types of igneous rock formed by the cooling of magma depend on factors such
as the composition of the magma and the rate of cooling associated with it. After conducting
several laboratory tests, Bowen (1922) was able to explain the relation of the rate
of magma cooling to the formation of different types of rock. This explanation—known
as Bowen’s reaction principle—describes the sequence by which new minerals are formed
as magma cools. The mineral crystals grow larger and some of them settle. The crystals
that remain suspended in the liquid react with the remaining melt to form a new mineralat a lower temperature. This process continues until the entire body of melt is solidified.
Bowen classified these reactions into two groups: (1) discontinuous ferromagnesian reaction
series, in which the minerals formed are different in their chemical composition and
crystalline structure, and (2) continuous plagioclase feldspar reaction series, in which the
minerals formed have different chemical compositions with similar crystalline structures.
Figure 2.2 shows Bowen’s reaction series. The chemical compositions of the minerals are
given in Table 2.1. Figure 2.3 is a scanning electron micrograph of a fractured surface of
quartz showing glass-like fractures with no discrete planar cleavage. Figure 2.4 is a scanning
electron micrograph that shows basal cleavage of individual mica grains.
Thus, depending on the proportions of minerals available, different types of igneous
rock are formed. Granite, gabbro, and basalt are some of the common types of igneous
rock generally encountered in the field. Table 2.2 shows the general composition of some
igneous rocks.