Where a volcanc eruption occurs underneath a glacier or icesheet. The heat of the eruption can cause large scale melting and result in floods , lahars and pillow lavas. Flat or table topped volcanoes ,called Tuyas, can form.Iceland is the best place for subglacial volcanic eruptions.
Pillow lavas can form as lava enters the subglacial meltwater, then phreatomagmatic activity deposits a breccia then lava flows over the top. Iceland has numerous subglacial volcanoes.
In 1996 the Grimsvotn volcano beneath tthe Vatnajokull Icesheet erupted melting the icesheet above melt. The water was stored beneath the ice sheet until it lifted the icesheet and burst through the base of the glacier and out across the coastal plane at a rate of 50 000 m3/s. This is called a jokulhlaup or glacial burst.