Mineral and rock types are created in world/fortress gen. This simulates what has occurred until that point. The earth shouldn't stop changing since the end of world gen but should instead continue. Geologic events such as volcanic eruptions, earthquakes, floods, landslides, tidal waves/tsunami, etc should occur and alter both the surface and underground landscapes. These event could then give opportunity for other formations to occur/change.
Volcanic events, either at your fortress or nearby, can either spell doom or a boom for your fortress. Volcanic eruptions would be historic events that would affect wide areas. This would be due to pyroclastic flows, lahars, poisonous gas and ash (cause syndromes),and the magma itself. If a fortress were to be far enough away to survive and/or just lucky, they can utilize the output resources from the volcano. Volcanic ash could be used for glass/soap/fertilizer, lava flows can be pumped to cisterns and other structures for later FUN, and an eventual large supply of igneous rocks once/if it cools . Volcanism could also cause hydrothermal and metamorphic events. Hydrothermal activity may cause underground, and aquifer water to begin to steam and rise to higher elevations as hot springs. This can be utilized to increase your dwarves happiness and increase value of the fortress. Also the hydrothermal changes could begin the precipitation of gemstones and metals into rocks as veins.
Metamorphic events could just be the turn existing rock into other rocks, such a limestone into marble.
Earthquakes could, just like real life, vary in intensity. Weak earthquakes may just cause a dwarf to get unhappy thoughts while strong earthquakes would cause faults; along which rocks/minerals would change z levels and/or rip apart creating fissures. Structural damage should be a factor in that a poorly built fortress may just bury its inhabitants alive. Earthquakes should also be able to trigger landslides and tsunamis which could bury/wash away entire villages/fortresses/regions.
The fissures created may be a bonus if hydrothermal activity begins in that mineral veins may form, more cave space can be etched from weak rocks, hot spring of clean water will rise to the surface, and it could create a natural buffer from non-flying enemies.
More subtle events could involve rivers and stream eroding/depositing over time leading to deeper and or shallower sediments. Small cave may form and collapse in areas of high precipitation/ground water retention. Collapses could result in sinkholes proportional to the overall size of the cave.
Just a few ideas but overall there is a lot that can be done to create a more kinetic geologic world.