system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: We see all those layers from millions of years ago... but how is new dirt constantly and consistently added to the surface? Where does it come from?
gpt
Dirt is mainly decomposed organic matter and eroded sediment from rocks.

So, it kinda comes from everywhere.

In an environment that only has eroded material and no organic material, you get a sandy desert.

In an environment lush with life and with few exposed rock areas, you get rich dark soil.