Subglacial mounds (SUGM) of the Mount Edziza volcanic complex are an unusual type of subglacial volcano formed when subglacial eruptions began melting overlying glacial ice at a time when this region was covered by glacial ice during the Pleistocene and early Holocene periods. These subglacial eruptions were not hot enough to melt a vertical pipe right through the overlying glacial ice, instead forming mounds of hydrated volcanic rock made up of volcanic fragments called hyaloclastite and lava that solidified into pillow-shaped masses called pillow lava deep beneath the glacial ice field. Once the glaciers had retreated, the subglacial volcanoes would be revealed, with a unique shape as a result of their confinement within glacial ice.