A blue flamestorm blasting through a system of big yellow caves?

Question

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?

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Question

1. Is it likely that my caves can be "generated" in the described way?

2. 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)

3. Is there anything important about sulfur I don't seem to know?

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Answer 1

Cave formation

This setup reminds me of rhizomes, which are essentially underground root systems that can spread out underground, sending up new seedlings at various intervals. Rhizomatic root systems make colonies of aspen possible, and colonies can be both massive and long-lived. If one tree dies, another one can start growing in its place, or somewhere else in the colony.

This seems, in a sense, similar to what you're asking. You've got a giant network of roots extending through the soil searching for nutrients, and rhizomes do basically the same thing - they just usually have plants grow up through the soil, rather than further down.

Sulfur formation

A commonly-occuring yellow sulfur crystal is octasulfur, or $\text{S}_8$. $\text{S}_8$ is often produced by volcanoes, but can also be made by humans. Volcanic elemental sulfur often contains a large percentage of $\text{S}_8$ by mass, and this is a good thing, because there may be a natural way to seed your riverbeds.

There are lava tubes and subterranean caves on Mars, some of which formed through mineral processes (e.g. involving limestone) and some of which formed through the cooling of lava. Now, it's possible that your world could have pockets of lava tubes deep below the surface, and it's also possible that those tubes could have $\text{S}_8$. Your rhizomatic roots therefore don't even have to form new tunnels; they could already exist, and might simply be expanded by the invading plants.

$\text{S}_8$ melts at 392 K (119$^\circ$C, 246$^\circ$F). This means that to have actual rivers of sulfur, you'd need to raise temperatures to this point. However, the boiling point of water is 100$^\circ$C, meaning that water should be gaseous at temperatures where sulfur is liquid. We therefore have a problem; we can't really have a river of liquid sulfur and water except under extreme conditions.

Therefore, it seems much more likely that the already-existing crystallized sulfur will remain crystallized, or at least in a solid state. Another possibility is that we change the ambient pressure, as boiling and melting points are pressure-dependent.

Answer 2

You are not looking for sulfur, you are looking for sulfate and sulfide.

Let's start at the beginning. Sulfur (the yellow stuff) is elemental sulfur. It is not soluble in water, and pretty hard to make it burn, unless you ignite it. However, you can oxidise it to sulfate, or reduce it to sulfide, where it is much more reactive and can participate in interesting chemical reactions, some of which are rather spectacular. This is not hand waving - on Earth, most of the (inorganic) sulfur is in one of those two states. Elemental or native sulfur is rather rare.

You want your organism to look for:

important minerals, gases, fluids, etc. to feed

One possibility is the reduction of sulfate. Sulfate is oxidised sulfur: SO₄^2-. There are bacteria that reduce it in order to feed. So you can have your soil rich in sulfate (for example gypsum, epsom) and then your organism feeds on that.

You can have sulfate seams in the ground that your organism follows, and then once a seam stops, the organism dies. Explaining the sulfate seams is easy: all you have to do is have a dry environment in the geological past to form evaporite deposits.

Now the fun part. What happens when the organism dies? You now have a decomposing mess of carbon and reduced sulfide (S^2-). Some of this reduced sulfur might form pyrite, also known as fool's gold. Spectacular shiny crystals of iron sulfide. Some of it might form hydrogen sulfide (H₂S), a very stinky and flammable gas. The carbon will serve as a sink for oxygen forming CO₂, lowering the availability of oxygen for burning of the sulfides. This allows the sulfides to accumulate.

Now comes the cave in! At an instant you expose everything to the atmosphere including oxygen. The hydrogen sulfide ignites, burning the pyrite as well. Some of the burnt gas might deposit as yellow crystal of native sulfur. The oxidation of the sulfides then leads to something that may look like modern acid mine drainage.