This adverb means “when you look through geology” — through the rocks, the soil, and the Earth's long change since it was formed. “Geologically speaking, this mountain is young” means: in rock-time it has not been there long, even if to us it has always been there. “Geologically active” means the ground still moves: volcanoes, earthquakes, land still being pushed up. You add -ly when you want to say “in the way geology sees things.” Though actually, students sometimes pick this word because it sounds serious, then use it for any “Earth” idea. If you mean “this city is in the south,” you want geographically. If you mean “this land was under the sea millions of years ago,” you want geologically.