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In this article we will discuss about the seven main characteristics of igneous rocks.
(1) In all, the igneous rocks are roughly hard rocks and water percolates with great difficulty along the joints. Sometimes the rocks become so soft, due to their exposure to environmental conditions for longer duration, that they can be easily dug out by a spade (e.g., basalt).
(2) Igneous rocks are granular or crystalline rocks but there are much variations in the size, form and texture of grains because these properties largely depend upon the rate and place of cooling and solidification of magmas or lavas. For example, when the lavas are quickly cooled down and solidified at the surface of the earth, there is no sufficient time for the development of grains/crystals.
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Consequently, either there are no crystals in the resultant basaltic rocks or if there are some crystals at all, they are so minute that they cannot be seen without the help of a microscope. Contrary to this, if magmas are cooled and solidified at a very slow rate inside the earth, there is sufficient time for the full development of grains, and thus the resultant igneous rocks are characterized by coarse grains.
(3) Igneous rocks do not have strata like sedimentary rocks. When lava flows in a region occur in several phases, layers after layers of lavas are deposited and solidified one upon another and thus there is some sort of confusion about the layers or strata but actually these are not strata rather these are layers of lavas.
Such examples may be seen anywhere in the Western Ghats where several lava flows during Cretaceous period resulted into the formation of thick basaltic cover having numerous layers of lavas of varying compositions. One can see such lava layers near Khandala or along the deeply entrenched valleys of Koyna River, Krishna River, Saraswati river etc.in and around Mahabaleshwar plateau.
(4) Since water does not penetrate the rocks easily and hence igneous rocks are less affected by chemical weathering but basalts are very easily weathered and eroded away when they come in constant touch with water. Coarse grained igneous rocks are affected by mechanical or physical weathering and thus the rocks are easily disintegrated and decomposed.
(5) Igneous rocks do not contain fossils because:
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(i) when the ancient igneous rocks were formed due to cooling and solidification of molten rock materials at the time of the origin of the earth, there was no life on newly born earth and
(ii) since the igneous rocks are formed due to cooling and solidification of very hot and molten materials and hence any remains of plants or animals (fossils) are destroyed because of very high temperature.
(6) The number of joints increases upward in any igneous rock.
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The joints are formed due to:
(i) Cooling and contraction,
(ii) Expansion and contraction during mechanical weathering,
(iii) Decrease in superincumbent load due to removal of materials through denudational processes and
(iv) Earth movement caused by isostatic disturbances.
Whenever these joints are plugged by minerals, the rocks become quite hard and resistant to weathering and erosion.
(7) Igneous rocks are mostly associated with the volcanic activities and thus they are also called as volcanic rocks. Igneous rocks are generally found in the volcanic zones.