The origin of igneous rocks starts from the cooling and solidification of magma. The mineral composition of igneous rocks varies from rock to rock. We can find the mineral composition of igneous rocks by means of studying the chemical composition of the magma from which the rock has been crystallized. Because there is a large variety of igneous rocks, thus it was originally thought an equally large variety of magma also existed. But the later research found out that there are only two types of magma existed and all types of igneous rocks were derived from them. These two types are; acidic magma and basic magma.
- Acidic Magma: The magma, which is enriched with Si, Na, and K, and contains a limited quantity of CA, Mg, and Fe is known as “Acidic Magma”. It results from the melting of the earth’s crust itself as a part of the rock cycle. Acid magma produces “acid rocks”, such as granites and rhyolites.
- Basic Magma: The magam, which contains a large amount of Ca, Mg, and Fe, while the quantity of Si, N, and Potassium in it is very small. This type of magma originates as a result of the partial melting of rocks lying beneath the earth’s crust. Basic magma gives rise to “basic rocks” such as gabbro and basalt.
The two magma theory was also discarded soon and it was suggested that a single magma of basaltic composition could produce rocks of varying minerals composition. The process by which the primary basaltic magma splits up into fractions that give rise to rocks of various types is called “differentiation”.
Differentiation of magma (Magmatic Differentiation):
The process by which magma is divided into parts that have different compositions is known as “differentiation”. These parts on cooling and solidification produce rocks of different types. The rocks are differentiated from one another due to the presence of different types of chemicals in them. Differentiation frequently takes place during the cooling of magma. Various processes like liquid immiscibility, fractional crystallization, gravity settling, filter pressing, and gaseous transfer takes place during the process of “differentiation”. Processes involved in “Magmatic Differentiation”
- Physical Geology