What I "want":
My fantasy world has lots of volcanos. Deep under the surface there is a root-like form of life predominant. It grows in big bundles and searches for important minerals, gases, fluids, etc. to feed a big network of itself. When an attempt of finding resources has failed, all the roots that dug in that direction just die instantly and get eaten after by small rat-like creatures. Some smaller roots around will stay, stabilizing the structure a bit. Some of these caves will just cave in, others will probably hold. Now what I want are sulphurous rivers flowing through some of these caves. When a part of one of the caves collapses, what's left in the part where the water is cut off is a yellow riverbed. This riverbed may be inflammable. Wouldn't that be cool? A blue flamestorm blasting through a system of big yellow caves?
Is it likely that my caves can be "generated" in the described way?
Will the sulfur crystallize in the riverbed while it's mixed with flowing water and how much of it would have to be in there e.g. per gallon? (in case of it not being likely to happen)
Is there anything important about sulfur I don't seem to know?