I awakened thinking that on top of being insane,

[ProfessorGAC]

75. Lava As A Skin Drying Agent

Jumping in as surfactant chemistry was one of my specialties.
Lava is a true soap. Most of what we use are synthetic detergents or a mix of the two.
The phase behavior of soap differs from detergents as does the full solubility.
The micellization and phase structures differ a bit as it's easier at the surface, due to the reduced solubility, to form water inverted micellar tubes.
The emollients, very water soluble, get trapped inside those "tubes" and far less is deposited on the substrate. (Skin)
Add to that the abrasiveness of the pumice, and we now have exfoliation, reduced emollient absorption & a higher pH.
It's the reason why "syndet" bars became so popular. And, the growth of liquid & gel hand & body wash products has been profound because of the lower pH, better emollient distribution and general mildness.
I know a bit off topic. I couldn't resist.