cultural memory of lava coming from the ground? 

 Could it be linked with and perpetuated by religion? Anyway, probably just humans being scared of the dark because they can’t see. 

 well, i think some of them resemble insects, but mostly humanoid

every culture has stories about them and the modern format is the "aliens"

the bible calls them Watchers and Fallen Angels, the irish call them Aes Dana, every culture on the planet has stories... when you look at south america, the "food of the gods" - theobroma - cacao - according to the stories of those people, they had big, somewhat elongated heads, pale skin, and demanded human sacrifices, there is traces of them everywhere, they built pyramids etc