* Thermal Expansion: When materials heat up, their particles move further apart, increasing the volume. Cooling causes the particles to move closer together, decreasing the volume.

* Density Relationship: Density is calculated by dividing mass by volume (Density = Mass/Volume). Since the mass of the rock remains constant, a decrease in volume due to cooling results in an increase in density.

Exceptions:

* Phase Changes: If the rock undergoes a phase change (e.g., from liquid magma to solid rock), the density can change in a more complex way. While cooling generally increases density, the transition from liquid to solid can involve a decrease in density.

* Mineral Composition: Different minerals have varying thermal expansion coefficients. The overall density change in a rock due to cooling can be influenced by the proportions of different minerals present.

In summary: Cooling generally causes an increase in the density of rock due to the decrease in volume. However, phase changes and mineral composition can influence this trend.