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Differentiation of Magma while cooling and solidification

Differentiation of Magma while cooling and solidification:

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.

Differentiation of Magma while cooling/ solidification
Different parts of the magma cool at different levels of time

(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 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”.

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