Final answer:
Intrusive igneous rocks are left behind when surrounding rocks are worn away due to their slow cooling process beneath the Earth's surface, which allows for the formation of large, durable crystals.
Step-by-step explanation:
Intrusive igneous rocks, also known as plutonic rocks, form from magma that cools slowly beneath the Earth's surface, allowing time for large crystals to grow. This slow cooling process results in rocks that are coarse-grained, such as granite.
Because these crystals are large and robust, the resulting intrusive igneous rock is very durable and resistant to weathering and erosion. Conversely, extrusive igneous rocks are formed from lava that cools quickly on the Earth's surface, leading to fine-grained or glassy textures due to the lack of time for crystal growth.
Over geological time frames, the softer and less resistant surrounding rocks wear away through processes like wind, water erosion, and ice, while the harder intrusive igneous rocks can withstand these forces better.
This is why intrusive igneous rocks like the sills of granite in the Catalina Mountains often become exposed at the surface, forming prominent landforms as the surrounding, softer rock is eroded away.