Formation
Tuyas are a type of subglacial volcano that consists of nearly horizontal beds of basaltic lava capping outward-dipping beds of fragmental volcanic rocks, and they often rise in isolation above a surrounding plateau. Tuyas are found throughout Iceland, British Columbia, the Santiam Pass region in Oregon, the Antarctic Peninsula and beneath the West Antarctic Ice Sheet in Antarctica. Tuyas in Iceland are sometimes called table mountains because of their flat tops.
S. Holland, a geographer for the British Columbia government, described tuyas in the following way:
- "They have a most interesting origin ... formed by volcanic eruptions which had been thawed through the Pleistocene ice-sheet by underlying volcanic heat. The lavas capping the mountains were extruded after the volcanoes were built above lake-level, and the outward-dipping beds were formed by the chilling of the lava when it reached the water's edge."
Because they erupt under ice and water, tuyas have phreatomagmatic eruptions create layers of breccia and hyaloclastite above of pillow lavas. If the volcano breaches the surface of the glacier it will be topped by a subaerially erupted lava plateau.
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