Igneous rocks are formed from the cooling and solidification of molten rock, called magma or lava.

Here's a breakdown of the process:

* Magma: Molten rock found beneath the Earth's surface.

* Lava: Magma that has erupted onto the Earth's surface.

Formation:

1. Melting: Rocks deep within the Earth's crust or upper mantle melt due to increased temperature and/or pressure. This creates magma.

2. Crystallization: As magma cools, the minerals within it start to crystallize. The order and rate of crystallization influence the texture and mineral composition of the resulting igneous rock.

3. Solidification: The magma or lava eventually cools and solidifies into solid igneous rock.

Two main types of igneous rocks:

* Intrusive igneous rocks: Formed from magma that cools and solidifies beneath the Earth's surface. They tend to have larger crystals due to slower cooling. Examples: granite, gabbro.

* Extrusive igneous rocks: Formed from lava that cools and solidifies on the Earth's surface. They tend to have smaller crystals or even a glassy texture due to faster cooling. Examples: basalt, rhyolite.

Key factors influencing igneous rock formation:

* Composition of the original magma/lava: The chemical makeup of the magma determines the types of minerals that crystallize.

* Cooling rate: The rate at which the magma or lava cools affects the size and arrangement of the crystals.

* Presence of gases: Gases dissolved in magma can escape during cooling, creating holes or vesicles in the rock.

The formation of igneous rocks is a fundamental process in the rock cycle, constantly shaping and reshaping the Earth's crust.