A subglacial volcano, also called a glaciovolcano, is a type of volcano formed when eruptions happen under a glacier or ice sheet. The heat from the lava melts the ice above, creating a lake. These volcanoes are most commonly found today in Iceland and Antarctica. Older examples of this type are also found in British Columbia and the Yukon Territory in Canada.

During an eruption, the hot lava from a subglacial volcano melts the ice above it. The water from the melted ice quickly cools the lava, forming shapes called pillow lava, which look similar to those found underwater. When these pillow lava pieces break off and roll down the volcano’s slopes, they form rock types such as pillow breccia, tuff breccia, and hyaloclastite. In 1996, an eruption at the Grímsvötn caldera in Iceland melted 3 kilometers of ice, creating a large flood from the sudden release of meltwater.

Subglacial volcanoes often have a unique shape, with a flat top and steep sides. The ice and meltwater around them help prevent the volcano from collapsing. If the volcano melts completely through the ice, lava flows horizontally, creating a nearly flat top. However, if lava later erupts above the ice, the volcano may take on a more typical shape. In Canada, these volcanoes can form both cone-shaped and flat-topped forms. The flat-topped, steep-sided type is called a tuya, named after Tuya Butte in northern British Columbia by Canadian geologist Bill Mathews in 1947. In Iceland, these same volcanoes are also called table mountains.

Jökulhlaups

Subglacial eruptions can lead to jökulhlaups, which are large floods of water. In November 1996, the Grímsvötn Volcano, located under the Vatnajökull ice sheet, erupted and caused a jökulhlaup that covered more than 750 km (290 sq mi). This flood damaged or destroyed several bridges. Sonia Esperanca, a program director at the National Science Foundation, explained the risks of subglacial volcanoes: "When a volcano covered by ice erupts, the way lava, ice, and meltwater interact can lead to serious problems."

Antarctica eruption

In 2008, scientists from the British Antarctic Survey, led by Hugh Corr and David Vaughan, published a report in the journal Nature Geoscience. The report stated that 2,200 years ago, a volcano erupted beneath the Antarctic ice sheet. This discovery was made using radar images from airborne surveys. The eruption was the largest volcanic event in Antarctica in the past 10,000 years. Volcanic ash from the eruption was found on the ice surface in the Hudson Mountains, near Pine Island Glacier.

On Mars

Many scientists think that liquid water may be found far below the surface of Mars, but current rovers cannot dig that deep. Researchers Meredith Payne and Jack Farmer from Arizona State University have examined pictures taken by the Viking and Mars Orbiter cameras to look for possible volcanoes hidden under ice that might help move tiny living things to the surface.

Ice cores

Scientists can determine when large subglacial volcano eruptions happened by studying ice cores, such as the Vostok core. These eruptions are found by looking for layers that have high amounts of NO3 and SO4.