The term igneous comes from the Latin word for fire. Underneath the Earth’s crust are extremely hot molten materials called magma. They find their way to the Earth’s surface through crustal openings such as volcano craters and fissures. When this magma crystallizes and solidifies, an igneous rock is formed. There are several different kinds of igneous rocks depending on their texture, mineral component, and occurrence.
When magma cools beneath the surface of the Earth, it forms a type of rock that is called intrusive or plutonic rock. Its slow cooling allows the growth of individual mineral grains giving it its coarse-grained texture. This means that we can still visibly tell one particle from the other. This then allows us to identify the individual mineral components without using any complex equipment. Such kind of igneous rocks are typically formed when magma penetrates or intrudes a pre-existing rock and solidifies below the surface. Examples of intrusive rocks are diorites, gabbro, and peridotites.
When magma cools above or near the surface of the Earth, it forms the extrusive or volcanic type of igneous rock. Being at or near the surface of the Earth, the molten rock rapidly cools leaving not much time for the individual mineral grains to grow. They are commonly formed when magma is strongly released to the surface. This usually happens after a violent forceful event such as explosive volcanic eruptions. Examples of these fine-grained rocks include basalt, andesite, rhyolite.