Once the molten magma extrudes out, it is affected by several factors. These factors provide varied structures to the rocks on cooling. Some important factors are as under;
(1). Flow Structures:
Sometimes an igneous rock shows parallel or subparallel bands or streaks which are caused by the flow of magma or lava during cooling and crystallization. Such structures are known as the ‘’flow structures’’.
(2). Reaction Rims:
During the formation of the igneous rocks, the reaction often takes place between early formed minerals and magma. If this reaction is complete, the early formed minerals disappear altogether. On the other hand, if the reaction is incomplete, reaction products frequently occur around the corroded mineral grains. This zone of reaction products, which occurs near the boundary of the mineral grains is known as the “reaction rim’’. The reaction rims are of two types (i) Coronas, (ii) Kelyphitic borders. “Coronas’’ is the reaction rims formed by primary magmatic reaction, while “Kelyphitic borders’’ are those which develop during secondary processes such as metamorphism.
(3). Xenolithic Structures:
Foreign rock fragments are included in 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 the foreign rocks are called the “xenoliths’’ and the structure is called the ‘’xenolithic structure’’.
(4). Vesicular structure:
Most lavas contain large amounts of gas and volatiles. These gases and volatile escape into the atmosphere when they solidify on the earth’s surface. As a result of this, numerous gas cavities are formed near the tops of lava flows. These gas cavities are called the “vesicles’’, and the volcanic rock which contains vesicles is said to have a “vesicle structure”.
(5). Amygdaloidal Structure:
The vesicles of volcanic rock may subsequently be filled by secondary minerals such as calcite and zeolites. Such filled vesicles are called the “amygdales” and the rock is said to have an “amygdaloidal structure’’.
(6). Pegmatitic structure:
If the constituent minerals grains exceed several centimeters in size, the rock is said to have a “pegmatitic structure”. The pegmatitic structure shows a coarse and very irregular type of crystallization.
In summary, it may be said that the texture and structure of an igneous rock provide numerous clues that suggest the circumstances of the formation of the rocks.
- Introduction to rocks
- Igneous Rocks and their types
- Chemical Composition of Igneous Rocks
- Occurrence of Igneous Rocks
- Texture of Igneous Rocks