When an intrusive igneous rock forms, the surrounding rock hardens and cools beneath the surface.

Here's why:

* Intrusive igneous rocks form when magma (molten rock) cools and solidifies below the Earth's surface.

* The hot magma heats up the surrounding rocks, causing them to metamorphose, which means they change their mineral composition and texture.

* This metamorphic process often involves hardening the surrounding rocks.

* As the magma cools, it transfers heat to the surrounding rocks, eventually causing them to cool down as well.

So, while the intrusive igneous rock itself hardens and cools, the surrounding rock also undergoes these changes due to the heat and pressure from the magma intrusion.