Kalaphurka soup of Potosi

by Monisha Das
(Towson, MD, USA)

I'd like to make this soup which I had in Potosi. Can you tell me what is the rock that is dropped in the soup to keep it sizzling and bubbling? Is it igneous, metamorphic or sedimentary? Is it andesite, obsidian or what? How is it prepared for the soup?