Intermediate subglacial volcanism

Intermediate composition subglacial volcanoes differ from basaltic ones in both morphology and internal stratigraphy. Typical landforms include flow-dominated tuyas and subglacial domes. Flow-dominated tuyas are flat-topped and steep-sided, and consist of stacked, flat-lying lava flows, with upper surfaces which are subaerial in origin. They commonly possess horizontal columns and irregular, fine-scale, columnar jointing at lateral margins; their shapes and joint patterns cannot be explained by known palaeotopography, suggesting that they formed through subglacial eruptions that ultimately breached the ice surface. Subglacial domes are dome-shaped, lava masses with fine-scale ( less than 25 cm) columnar or other jointing, horizontal columns, irregular or radiating joint patterns on the top surfaces or sides of flows, and little or no pyroclastic material (e.g. hyaloclastite); their shapes and joint patterns cannot be explained by the apparent palaeotopography. Subglacial domes are interpreted as lava masses that did not breach the ice surface during eruption.