Intrusive igneous rock forms when molten rock, or magma, cools and solidifies beneath the Earth's surface. This slow cooling process allows large crystals to develop, giving these rocks a coarse-grained texture. Common examples include granite and diorite. These rocks are typically found in large formations called plutons, which can be exposed at the surface through erosion over time. Intrusive igneous rocks are important for understanding the Earth's geology and are often used in construction due to their durability and aesthetic appeal.