Sometimes lava will turn into obsidian in the caves and cliffs experiment. I activated the experiment on a world generated in 1.17 and in some places the lava pools turned to obsidian at the bottom of caves.

It appears to be on chunk boarders and It goes all the way down to the bottom of the lava lake. When I first loaded the chunks in the experiment there was no obsidian but after I left the area and came back the obsidian was there.

The video is from the first time I visited the area. Minecraft 2021-10-19 7_53_11 PM is a picture of two of the obsidian formations, one I cleared the lava away so you can see that it extends to the bottom of the lake. Minecraft 2021-10-19 8_02_20 PM is a picture of another obsidian formation in the same cave.