Theory:

The rocks formed due to the cooling, solidification and crystallization of molten magma (molten rock within the earth surface) and lava (molten rock when erupted) are called igneous rocks.  These are the first rocks to have originated during the formation of the upper crust of the earth. Henceforth, they are known as ‘Primary or Parent Rocks’.
Characteristics:
  1. They are found in volcanic zones.
  2. The igneous rocks are hard, crystalline  and  impermeable in nature.
  3. They do not have strata and fossils.
Based on the mode of occurrence, the igneous rocks are further classified into 2 types as: 
  1. Intrusive Igneous rocks
  2. Extrusive Igneous rocks
types-of-igneous-rocks.png
 
Intrusive Igneous Rocks are formed when the molten magmas do not reach the earth surface during the volcanic activity instead they get cooled and solidified below the earth’s surface.
 
When the cooling and solidification of magma happens at greater depths from the earth’s surface Plutonic igneous rocks are formed. The process of cooling is very slow and the grains are formed.   And crystals are also developed because of the slow cooling process, and they are also called ‘ Crystalline Rocks’. When the molten magma gets cooled and solidified near the earth’s surface, Hypabyssal igneous rocks are formed. Granite, Gabbro and Pegmatite are the examples of Intrusive Igneous rock type.
 
granite-coarse-grained.jpgpegmatite.jpg
 
Extrusive Igneous rocks are formed when the molten magma cooled and solidified at the earth’s surface. These rocks do not form crystals since the rate of cooling is rapid and they are fine grained and glassy textured. Pumice, Obsidian  and basalt are the representative examples of this type.
 
obsidian.jpgbasalt.jpg
Reference:
https://eschooltoday.com/rocks/what-are-igneous-rocks.html
https://geology.com/rocks/igneous-rocks.shtml