A
subglacial volcano, also known as a
glaciovolcano, is a volcanic form produced by
subglacial eruptions or eruptions beneath the surface of a
glacier or
ice sheet which is then melted into a
lake by the rising
lava. Today they are most common in
Iceland and
Antarctica; older formations of this type are found also in
British Columbia and
Yukon Territory,
Canada.
During the eruption, the heat of the lava from the subglacial volcano melts the overlying ice. The water quickly cools the lava, resulting in
pillow lava shapes similar to those of
underwater volcanoes. When the pillow lavas break off and roll down the volcano slopes, pillow
breccia,
tuff breccia, and
hyaloclastite form. The meltwater may be released from below the ice as happened in
Iceland in 1996 when the
Grímsvötn caldera erupted, melted 3 cubic km ice and gave rise to a large
glacial lake outburst flood.
The shape of subglacial volcanoes tends to be quite characteristic and unusual, with a flattened top and steep sides supported against collapse by the pressure of the surrounding ice and meltwater. If the volcano eventually melts completely through the ice layer, then horizontal lava flows are deposited, and the top of the volcano assumes a nearly-level form. However, if significant amounts of lava are later erupted
subaerially, then the volcano may assume a more conventional shape. In Canada the volcanos have been known to form both conical and nearly-level shapes. The more distinctly flat-topped, steep-sided subglacial volcanoes are called
tuyas, named after
Tuya Butte in northern British Columbia by Canadian geologist
Bill Mathews in 1947. In Iceland, such volcanoes are also known as
table mountains.
