The smell of hydrothermal chemistry
This is a short, informal, and completely uncontrolled summary of what I smelled round Yellowstone Park in September 2011. Despite a huge number of papers on the chemical analysis of the gases from fumaroles and hydrothermal vents (my collection now has over 40), none of them seem to say what it is actually like to be there.
Yellowstone National Park has hundreds of hydrothermal features, including dozens of active geysers, powered by a mantle plume under the Yellowstone caldera. There is debate about why the plume is there, its history and so on, but the end result is that the rocks under Yellowstone are highly volcanically active, and volcanic gasses routinely escape into the atmosphere in
All volcanic gas contains steam, so ‘dry’ fumaroles can be dry for one of two reasons:
As a guide, my guess is that if you can hear the gas hissing or roaring out of a dry fumarole like a broken gas pipe, then the gas is hot and the water is there as steam. If it is a gentle fizz like a soda bottle being opened, then water might be being trapped underground. Do not put your fingers in the gas to check.
To smell a geothermal feature, stay on the board-walks! Wait until the plume of steam/gas drifts over you. This should be safe-ish. If you feel nauseous or dizzy, get out the plume. If you fall unconscious or die, ask someone else to take you out the plume. Just kidding.
These are some of the features that I happened to note, their characteristic smells, and some comments on the chemistry. Note that most of the characteristic smells are due to sulfur gases (discussed afterwards).