The type specimen was collected from the Kuyuwini Landing (Rupununi) at an elevation of 200 m (660 ft); here, in a savannah forest, it was found growing on the smooth bark of trees.

The lichen has a smooth, somewhat shiny thallus surrounded by a thin black prothallus, and covering areas up to 25 cm (10 in) in diameter.

Its ascospores are spindle-shaped (fusiform) with rounded edges, with three septa and dimensions of 20–25 by 6.5–7.5 μm.

Astrothelium vulcanum contains lichexanthone, a lichen product that causes the thallus to fluoresce when lit with a long-wavelength UV light.

[2] Astrothelium infravulcanum, found in Brazil and described as a new species in 2022, was named for its similar appearance to A. vulcanum, but is distinguished by its smaller ascospores, which measure 14–17 by 5–6 μm.