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The Basics
A variety of distinctive volcanic rocks stem from the unique interaction of lava, water and ice during a subglacial eruption. Some of the formations closely resemble those of underwater, or subaqueous, eruptions, as in the case of so-called ̶0;pillow lava.̶1; Lava directly exposed to ice or cold water quenches quickly, a rapid cooling that discourages crystal formation and creates vitreous or glass-like deposits such as hyaloclastite and palagonite. Often these volcanic glasses form a matrix with tuff -- welded volcanic ash -- and pillow lava.
Locations
The most well-known subglacial eruptions in modern times have occurred in Iceland, a geologically restless island along the great tectonic schism called the Mid-Atlantic Ridge. Antarctica has numerous volcanoes associated with its continental ice sheet, although the full extent of subglacial vents is not clear. Subglacial eruptions may occur anywhere volcanoes coincide with icecaps and glaciers, such as the Andes and the Cascade Volcanic Arc; they are also speculated to have occurred on Mars, possibly at a much vaster scale than on Earth.
Risks
Terrific floods can stem from subglacial eruptions, although they don̵7;t always exactly coincide in time. Volcanic activity under a glacier melts some quantity of overlying and surrounding ice, and commonly meltwater lakes occupy calderas beneath or upon the icecap. Periodically these lakes will rise high enough to float their ice dams and surge off the volcano in violent, fast-moving floods called ̶0;jökulhlaups,̶1; after their Icelandic name. Jökulhlaups stemming from eruptions of the Icelandic volcano Grimsvötn under the huge Vatnajökull glacier, such as those occurring in 1934, 1938 and 1996, have been truly enormous, some reaching 50,000 cubic meters per second at peak flow. Other hazards include the ejection of ash-laden steam from lava-ice-water contact high into the atmosphere, which can obstruct air travel.
Landforms
Subglacial volcanoes often take on distinctive shapes that betray their origin even in areas no longer swaddled in ice. Ice-capped eruptions commonly form steep-sided ridges of hyaloclastite, palagonite and pillow lava often called hyaloclastite ridges, moberg ridges or tindars. Eruptions attaining the surface of a glacier, filling meltwater lakes with lava deltas and then spewing in aboveground layers, may create a flat-topped caprock atop steep shoulders of moberg ridges. Such a mesa-like ̶0;table mountain̶1; is called a ̶0;tuya̶1; -- after a butte in British Columbia̵7;s Cassiar Mountains -- or, in Iceland, a ̶0;stapar.̶1; The existence of tuya- and tindar-like formations on Mars has suggested glaciovolcanic activity on that planet.