MAGMA INTRUSION Definition/Introduction: Magma intrusion, also known as igneous intrusion, refers to the process by which molten rock (magma) from beneath the Earth’s surface forces its way into existing rock layers (the crust) but does not reach the surface. Instead, the magma cools and solidifies underground, forming intrusive igneous bodies. These intrusions are important in shaping the Earth's crust and are commonly studied in geology to understand subsurface processes and mountain building.