Structures of Igneous Rocks are the features that are of large scale and are developed in extrusive or intrusive igneous body. They may be well developed as to be recognized easily on visual inspection or they may be become appeared only when their thin section are studied under microscope. Some of the major structures are as followed:-
- Flow Structures: Sometimes an igneous rock shows parallel or sub parallel bands or streaks which are caused by the flow of magma or lava during cooling and crystallization. Such structures are called the ‘flow structures’.
- Pillow Structure: This is characterized by the development of bulbous, overlapping, pillow like structure in the body of igneous mass. It is typical structure of rock formed from mobile basaltic lava. In such a lava flow its upper surface gets solidify while the lava beneath remains hot and capable of moving. But further flow is possible only when the crust formed at the top ruptures. The rupture occur at the margins of previously congealed over mass, from such ruptured lava flow out and cools down again forming fresh bulbous tops. The process is repeated producing overlapping bulbous masses.
- Ropy and Blocky Lava: These are structural variation developed in the volcanic rocks due to different mobility. Highly viscous “dry” lava undergo very little movement after their eruption and before cooling. Their surface show broken and fragmented appearance. These are called the blocky lava. On the other hand very mobile lava flow for considerable distance and cooling during flow process so that its upper surface is smooth wrinkled rather than that actually broken. The surface is referred as ropy lava.
- Vesicular Structure: Most lava contains large amounts of gas and volatiles. These gas and volatiles escape into the atmosphere when they solidify on the earth’s surface. As a result of this, numerous gas cavities are formed near tops of lava flows. These gas cavities are called the “vesicles” and the volcanic rock contains vesicles is said to have a “vesicular structure”.
- Amygdaloidal Structure: The vesicles of volcanic rocks may subsequently be filled by the secondary minerals such as calcite and zeolites. Such filled vesicles are called the “amygdales” and the rock is said to have an “amygdaloidal structure”.
- Pegmatitic Structure: If the constituent mineral grains exceed several centimeter in size, the rock is said to have a “pegmatitic structure”. The pegmatitic structure shows a course and very irregular type of crystallization.
- Xenothilic Structures: When these foreign rock fragments are included into the magma when it rises up towards the Earth’s Surface. If they are not digested, they remain entrapped within the mass of the igneous rock and produce heterogeneity in the texture. Such entrapped fragments of foreign rocks are called the ‘xenoliths’ and the structure is called the “Xenolith structure”.