Explanation:

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. 

(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.

(4)  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:

                   (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.

(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.