The antarctic fumaroles do have wide bases. I've got a nice photo collection courtesy of Phil Kyle who's actually been to Erebus and done work on them. I can probably send you a few if you're interested, or even one of my powerpoint presentations about them.

On Mars, any reasonable rate of warm water supply will lead to surface effusion of steam and/or warm water. This will condense/freeze to an icy ring or blob and start to build a tower or a cone.

One way to look at it is that ice is a rock-forming mineral on Mars, most of the time. You can build ice mounds and ice volcanoes, then cloak them in dust and preserve them for quite some time.

Imagine that alaskan man-made tower if spring never came - each tower would collapse and a new one grow on top of the debris, just as jms describes for deep sea sulphides and "black smoker" chimneys. The mound has to get quite large before the rate of sublimation over its whole surface area is equal to the rate of water supply.

(without someone to add pipe, though, it would tend to be less tall and more spread out, even allowing for Mars' lower gravity)

So a martian hot spring would be clearly visible from orbit as a bright patch, but they might not be obviously warmer than the surrounding rock - the icy carapace would mask the small amount of warm water, IMHO.