The structures of igneous rocks are large scale features, which are dependent on several factors like

The structures of igneous rocks are large scale features, which are dependent on several factors like:

(a) Composition of magma.

(b) Viscosity of magma.

(c) Temperature and pressure at which cooling and consolidation takes place.

(d) Presence of gases and other volatiles.

Igneous structures are mostly classified into three major groups, as follows:

1. Mega-structures.

2. Minor structures.

3. Micro-structures.

1. Mega-structures:

These are usually formed in the flow stage of the magma (i.e., in the extrusive rocks), and include:

(i) Vesicular and amygdaloidal structures:

When lavas heavily charged with gases and other volatiles are erupted on the surface, the gaseous constituent’s escapes from the magma as there is a decrease in the pressure. Thus, near the top of flows, empty cavities of variable dimensions are formed. The individual openings are known as vesicles and the structure as a whole is known as vesicular structure.

If, however, the vesicles thus formed are subsequently filled in with some low-temperature secondary minerals, such as calcite’ -zeolite, chalcedony etc., these infillings are called ‘amygdales’

(ii) Cellular or scoriaceous structure:

By the bubbling out of the gases, from lava heavily charged with volatile and gaseous constituents, numerous cavities are formed with the solidification of the lava. When the cavities are very much abundant, the term ‘pumice’ or ‘rock-froth’ is applied. Such structures are known as cellular or scoriaceous structures and are characteristic of highly siliceous lavas.

(iii) Lava-drain tunnels:

Sometimes while the upper surface of the lava consolidates, the interior may still remain fluid. When the enclosed fluid lava drains out through some weak-spots lying at the periphery of the flow, the resulting structure is known as lava- drain tunnel.

(iv) Block-lava:

Since lavas of acidic composition, due to their high viscosity, do not flow to greater distances, they after solidi­fication are found to offer a very rough surface. Such lava flows are known as block lava. It is also known as ‘aa’ structure.

(v) Ropy lava:

Lavas of basic composition are quite mobile because of their low viscosity and they can flow to greater distances and after solidification offers very smooth surface. Such lava flows are known as ropy lava and are also known as ‘pahoehoe’ structure.

(vi) Pillow structure:

It consists of isolated pillow shaped masses piled one upon another… These are produced by extrusion of lava into rain-soaked air, beneath ice-sheets, under water logged sediments or in sea water. Spilite, a lava rich in albite (i.e., sodium rich) characteristically exhibits pillow structure.

(vii) Sheet structure:

The development of one set of well defined joints sometimes brings about a slicing effect on the massive igneous rock body. If all such slices are horizontal, the structure is said to be sheet structure.

(viii) Platy structure:

This is also due to the development of different sets of joints, which gives rise to only plates of the rock mass, on striking the rock. Such a feature is known as platy structure.

(ix) Columnar structure:

As a consequence of contraction due to cooling, a few sets of vertical joints develop. Such joints bring about the formation of columns, which may be square, rectangular, rhombic or hexagonal in outline.

(x) Flow structure:

Subsequent to eruption of lava upon the surface the viscous, varieties flow from one place to the other with great difficulty and in their attempt to do so, the dissimilar patches within the lava are drawn out in the form of elongated lenticels. Sometimes the already ciystallised particles within the magma are: arranged parallel to the direction of flow pf the lava. They naturally indicate the direction of flowing of the mass, prior to its consolida­tion. These are also known as directional structure or more commonly flow structure.

(xi) Rift and grain:

These are due to jointing. In granites, three mutually perpendicular, equally spaced joints, which are taken into advantage while producing cubical blocks, are known as ‘mural jointing. But for processing of the blocks down to smaller dimen­sions, the mutually perpendicular closely spaced joints (one horizontal and the other vertical) are taken into advantages. These joints are known as rift and grains.

2. Minor structure:

These structures are formed in the fluid stage of the magma (i.e., in the intrusive rocks) and include the following:

(i) Primary foliation:

Sometimes many plutonic rocks are characterised by foliation resulting from the parallel arrangement of platy and ellipsoidal mineral grains.

(ii) Banding in rocks:

These are also known as layered rocks consisting of alternating bands of different composition. It may result from lamellar flow, from settling of minerals from a crystallized magma or from successive injections.

(iii) Schlieren:

These are somewhat wavy, streaky, irregular sheets, usually lacking sharp contact with the surrounding igneous rocks. They may be altered inclusions, segregation or may represent concentration of residual fluids into layers in a rock that had otherwise crystallised.

3. Micro structures:

These are formed due to reaction between already solidified crystals and the rest of the magma and include the following:

(i) Reaction rims:

When the reaction between an already crystallized mineral and the rest of the magma is incomplete, the corroded crystals are found surrounded by the products of reaction i.e., some new mineral. Such zones are known as reaction-rim. When the reaction rims are produced by primary magmatic reaction, they are known as ‘corona structures’ and ‘kelyphitic borders’ when secondary.

(ii) Myrmekite structure:

It is produced by an intergrowth of quartz and plagioclase feldspar where quartz occurs as blebs or drops in plagioclase.

(iii) Graphic structure:

It results from an intergrowth of quartz and orthoclase feldspar.

(iv) Xenolithic structure:

Occurrence of foreign rock fragments within an igneous rock gives rise to xenolithic structure. The xenoliths are said to be ‘cognate’ when they are genetically related to erclosing rocks and ‘accidental’, when they are fragments of country- rocks without having any genetic relation with the enclosing rock.

(v) Orbicular structure:

These are spherical segregations consisting of concentric shells of different mineral composition and texture, which occasionally occurs in granitic rocks.

(vi) Spherulitic structure:

Its essential feature is simultaneous crystallization of fibres with radiating arrangement about a common centre. The large spherulites are known as ‘Lithophyse’. In basic lavas and intrusions, they are called ‘vorioles’ and the rocks contain­ing them variolites.

(vii) Perlitic-cracks:

These are curved, concentric lines of fracture, often seen in volcanic glass. These are simply due to contraction of the glassy mass on cooling.