Rocks are naturally occurring aggregate of minerals . They are of two types i.e. Monomineralic rock and polymineralic rock. Monomineralic rocks are the ones that contains only one mineral. While polymineralic rocks are the ones that contains more than one minerals.
Igneous rock is one of the three main rock types, other than the sedimentary and metamorphic. It is formed through the cooling and solidification of magma or lave. Igneous rock may form with or without crystallization, either below the surface as intrusive(plutonic) rocks or on the surface as extrusive(volcanic) rocks.
This magma can be derived from partial melts of pre-existing rocks in either a mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition.
Process of Formation of Igneous Rocks
There are different views regarding how igneous rocks are formed. In this post we will talk about the recent world-wide accepted view of process of igneous rock formation.
Old view:
Here’s what the old view of igneous rock formation was like.
- Acidic magma: composition as silica, alumina and alkali E.g. Granite
- Basic magma: composition as iron, magnesia and lime E.g. Basalt
- Intermediate magma: admixture of acidic and basic magma E.g. Diorite
Recent worldwide-accepted view(Bowmen’s Reaction Series)
This is the theory of single parental magma(homogenous magma) of basaltic composition, put forwarded by N.L Bowen(1928). According to this theory, the basaltic magma contains diverse type of minerals. The resulting rock formed due to this magma depends upon reaction series.
Here, the early-formed minerals react with melt to form new minerals that go further down in the series. Reaction series is represented as continuous and discontinuous series.
Continuous series is one in which there are no abrupt phase changes during the reaction of early-formed crystals with later melts. The series is of feldspar minerals i.e. Plagioclase.
Discontinuous series is one in which reaction of early-formed crystals with later melts represents the sharp phase changes. E.g. early-formed Olivine minerals react with melt to form Pyroxene.
This reaction series is known as Bowmen’s reaction series.
Processes Responsible For Changing Magma’s Composition
Magmatic differentiation: The internal diversification processes that separate an initially homogenous closed magma body into two or more daughter magmas of different chemical composition, thus forming more than a single type of igneous rock from a common magma. A variety of mechanisms have been suggested, of which principal ones are:
- Liquid immiscibility: involves the separation of two compositionally different liquid phases from an initially homogenous melt. E.g. immiscibility between Sulphide and Silicate liquids for the formation of magmatic Nickel- Sulphide ore deposits.
- Fractional crystallization: It is an effective differentiation process. Crystals that form at high temperatures are prevented from equilibrating with parent magma, either by mantling with an overgrowth of another mineral or by separation from magma. Or the
separation of first formed minerals of different melting points from parent magma due to precipitation or gravitational means. The mineral crystallized out in a magma mass due to the formation of solid and liquid phases. This produces a series of residual magmas with composition that are more and more depleted in the components of the early formed minerals and very different from that of the original magma. - Gravity settling: The crystallized mineral is of a specific gravity higher than that of the fluid magma, the former would naturally have a tendency to sink down towards the floor of the magma chamber i.e. from settling of crystals due to gravity.
- Filter pressing: The driven out of the fluid magma from the solid crystals due to severe lateral pressure set up within mountain building movement or operation.
- Gaseous transfer: During crystallization of magma, the volatile matters tend to move towards the regions of comparatively less pressure and their transfer from one place to another may be accompanied by the removal of some of the magmatic constituents from the original magma. This process of separation of some of the magmatic constituents due to movement of volatile matters may be called gaseous transfer.
- Assimilation: The process of incorporation of the country- rock within the erupted magma- mass is known as assimilation. The above mention mechanisms of origin of magma, they reflect the following rock types.
– When Olivine mineral is completely separated out from the magma, the resulting rock is Dunite i.e. ultramafic rock.
– If Pyroxene and Calcium rich Plagioclase separated out together, forming basic rocks i.e. Gabbro, Basalt etc.
– If minerals like Quartz, Orthoclase, Biotite and Sodic plagioclase as a principal constituent, forming acidic rock E.g. Granite.
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