Stone Mountain is primarily formed by granite, a type of igneous rock. While the initial formation of the granite was through igneous processes (cooling and solidifying of magma), the distinctive shape of Stone Mountain is largely due to exfoliation weathering.

Here's how it works:

1. Intrusive cooling: The granite formed deep underground, where it cooled slowly. This slow cooling resulted in large crystals and a relatively uniform structure.

2. Uplift and erosion: Over millions of years, the landmass containing the granite was uplifted and exposed to the elements.

3. Exfoliation weathering: The granite was subjected to cycles of heating and cooling (due to the sun and the weather). This caused the outer layers of the granite to expand and contract. The expansion and contraction create stress on the rock, causing it to crack and peel away in thin layers, like an onion. This process is called exfoliation.

4. Erosion by water and wind: After exfoliation, the exposed surfaces of the granite were further eroded by the actions of water and wind, shaping the mountain into its distinctive dome-like form.

In summary: While the granite of Stone Mountain formed through igneous processes, its distinctive shape is primarily attributed to exfoliation weathering, aided by the erosive forces of water and wind.