Origin and formation of the Igneous Rocks

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 later research found 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.

1. 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.
2. Basic Magma: The magam, 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”.

Composition of Magma

Magma is a complex mixture of molten rock, dissolved gases, and solid mineral crystals. The composition of magma can vary greatly depending on the geological setting and the source of the magma.

The primary components of magma are:

1. Silicate minerals: These are the most abundant components of magma, including minerals such as feldspar, quartz, mica, and pyroxene. Silicate minerals are responsible for the viscosity and flow of magma.
2. Volatiles: These are gases that are dissolved in magma, including water vapour, carbon dioxide, and sulfur dioxide. Volatiles affect the explosiveness and viscosity of magma.
3. Metals: These are elements such as iron, magnesium, and calcium that are found in the magma as dissolved ions. They affect the physical and chemical properties of the magma.

The overall composition of magma is typically categorized into two main types: mafic and felsic. Mafic magma is rich in iron and magnesium, and has a low viscosity, while felsic magma is rich in silica and has a higher viscosity. Intermediate magmas, with a composition between mafic and felsic, also exist.

The composition of magma is determined by the source of the magma, which can be either the mantle or the crust. Magma that originates from the mantle tends to be mafic, while magma that originates from the crust tends to be felsic. The composition of magma can also be affected by factors such as the pressure and temperature of the magma chamber, and the presence of other minerals and gases.

Differentiation of Magma

Magmatic differentiation means the split of magma into various parts due to the presence of different types of chemicals in it. These parts of magma on cooling and solidification produce rocks of different types. As the chemical composition of magma differs in its different parts and layers, the newly born rocks also differ from one another. Differentiation frequently takes place during the cooling of magma. Differentiation of magma while cooling and solidification is a complex process, which involves several other processes in it. Various processes that operate during differentiation are (i) liquid immiscibility, (ii) fractional crystallization, (iii) gravity settling, (iv) gaseous transfer, and (v) filter pressing.

(1). Liquid Immiscibility: 

A magma may split up into two immiscible liquid fractions of different compositions like oil and water, as a result of cooling. Subsequent crystallization of these immiscible parts gives rise to different types of rocks. Liquid immiscibility, however, does not occur between fluid rock-forming silicate, and therefore its role in causing differentiation of magma is insignificant. The only well-known case of liquid immiscibility is between sulfides and silicates.

(2). Fractional Crystallization: 

As soon as magma starts crystallizing, differentiation begins. Differentiation is brought about in two ways: (i) through the location of crystallization, and (ii) through the relative movement of crystals and liquids. Crystallization may be localized at the cooling margin. As crystallization proceeds, the peripheral parts of the mass get impoverished in the crystallizing substance. The supply of this substance is believed to be maintained by ionic diffusion or convection currents from all parts of the magma. Thus in an igneous mass concentration of basic minerals may occur near the margins and acidic minerals may segregate in the central part.

It was, however, found that ionic diffusion or convection currents play very little role in causing fractional crystallization.

(3). Gravity Settling: 

Heavy crystals formed during the early phase of crystallization of magma have a tendency to sink down. This process of sinking is controlled by the viscosity of the magma and by the size, shape, and density of the crystals. Settling of crystals under gravity is an effective process of differentiation. This process acts simultaneously with fractional crystallization. Many cases are known where the segregation of olivine, pyroxene, magnetite, and other heavy minerals are found near the base of an intrusive body.

(4). Gaseous Transfer: 

With the release of pressure, the magmatic gases and volatile compounds tend to migrate upward in the direction of lessened pressure. Their movement generates a sort of convection, which causes the separation of some of the magmatic constituents. Deposits of sulfide metallic ores that occur near the apices of some igneous bodies are believed to have formed in this way.

(5). Filter Pressing: 

Filter pressing is one more important process, which influences the process of magmatic differentiation. Filter pressing generally occurs in mountains building regions, where lateral pressure prevails. When the lateral pressure acts on a crystallizing magma, it drives out the residual fluid from the crystalline mesh. This results in the separation of the residual fluid from the solid face. In the new location, the residual fluid crystallizes and forms a rock much different from the first. This process of differentiation is called “filter pressing”.

Formation of Igneous Rocks

The igneous rocks are formed from the cooling and solidification of the Magma/Lava. The minerals and the chemical makeup of igneous rocks are important in order to understand their formation. Experimental studies of the crystallization sequence of minerals from a melt have helped greatly in understanding the origin of igneous rocks. The temperature-composition diagrams involving a liquid phase are called the “liquidus diagram”. Most of the igneous rocks are multicomponent. Generally, they consist of two, three, four, and five principal mineral constituents. To study the solidification of magma, they are taken in the order of increasing complexity.

Crystallization of binary magmas

Crystallization of binary magmas is the process of solidification of molten rock (magma) that contains only two chemical components. Binary magmas are typically composed of a silicate (SiO2) and a second oxide, such as FeO, MgO, CaO, or Al2O3.

During the cooling and solidification of a binary magma, crystals of the mineral(s) that have the highest melting temperature(s) will precipitate first. These crystals will be composed primarily of the lower-melting-temperature component, with lesser amounts of the higher-melting-temperature component.

As cooling continues, the remaining liquid will become enriched in the higher-melting-temperature component. As a result, crystals of the mineral(s) that have the higher melting temperature(s) will begin to precipitate and will be composed primarily of the higher-melting-temperature component, with lesser amounts of the lower-melting-temperature component.

The sequence of mineral precipitation during the cooling and solidification of a binary magma is called the crystallization sequence. The exact sequence of minerals that crystallize is dependent on the chemical composition of the magma and the cooling rate.

Crystallization of Ternary Magmas