Námafjall near Myvatn in Iceland is a high-temperature geothermal area. Groundwater seeps down to a depth of 1000 m, where its temperature rises to above 200°C, and it finds its way upward as hot steam. Along with the steam comes hydrogen sulphide, which gives a nasty smell. Indeed it is one of the most offensively smelling places on earth.
This field is known as the Hverir, and we may discuss whether the “springs” are mudpots, fumaroles or sulfatares. A fumarole (from Latin fumus, smoke) is an geothermal spring, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from the ground. so these are fumaroles. A fumarole rich in sulphur gases is called a solfatara. The name solfatara comes from the Italian solfo, sulphur (via the Sicilian dialect). So they are sulfatares. A mudpot, mud pool or paint pot is a sort of hot spring or fumarole consisting of a pool of usually bubbling mud. Here is such a bubble just before bursting:
So they are mud pots. You just choose whatever you like. They are however not mofettes. Mofetta (Italian from Latin mephitis, a pestilential exhalation), is a name applied to a volcanic discharge consisting chiefly of carbon dioxide.
Sulphuric acid in the water dissolves the rocks and clay or mud is formed. Mudpots form in high-temperature geothermal areas where water is in short supply. The little water that is available rises to the surface at a spot where the soil is rich in volcanic ash, clay and other fine particulates. The thickness of the mud usually changes along with seasonal changes in the water table.
The hot springs at Hverir produce considerable sulphur deposits. In previous centuries sulphur was mined in Iceland to produce gun powder. This nearly lead to a war between Sweden and Denmark. In 1567 the Swedish king Eric XIV considered conquering Iceland (under Danish rule at the time), so that Denmark would be without sulphur to produce gun powder - an important ingredient for warfare at that time. The plans were skipped, however, for financial reasons.
For mud pots see also my post on Kamchatka - Geyser Valley