English: Active volcanoes not only spew lava, but also hot gases, some of them toxic, some of them just warm air with various amounts of steam and CO2. The vents for such gases are called fumaroles. They commonly form as magma brings groundwater to a boil, forming steam that mixes with volcanic gas and air from the cracks and pores in the volcano, and finally rising to the surface.

So what is special about fumaroles forming on an ice-covered volcano in an extremely cold environment like Erebus? Two types of fumaroles can be found depending on if they are located under the ice that cover the volcano or open cracks in exposed volcanic rock. Fumarole ice chimneys and caves are formed when steam vents on rocks builds up at the surface. These are easily amongst the most spectacular features on Mt. Erebus. There are hundreds of steaming ice towers around the summit region, and just as many caves, of all sizes, shapes and forms.

(text from https://web.archive.org/web/20210216183856/https://earthref.org/content/ice-caves-mt-erebus)