Subglacial volcanoes are volcanoes that are located beneath an ice sheet. Due to the unique interaction of the overlying ice and the erupting magma, distinctive "tuya" formations are created during a subglacial eruption. Tuyas are steep-sided, flat-topped plateaus composed of loose volcanic material often capped by lava flows. Their steep-sides and flat tops are formed because the surrounding ice confines the erupting magma to a small area. The flat top corresponds to the height of the surrounding glacier at the time of formation.

Many of these tuya formations have been identified on the Northern Plains of Mars from images collected by the Viking orbiter, the Mars Global Surveyor, Mars Laser Altimeter and the Mars Orbiter camera. On Mars, to be sure that such features were formed volcanically, tuyas must show evidence of volcanic growth on top of their flat surfaces. This growth generally looks like small lumps on the tuyas surface.

This image shows an aerial photograph of a tuya structure in Iceland (a)
and a hypothesized tuya structure on Mars (b).

From such images, the thickness of prior ice sheets on Mars has been estimated to be between 100-1200 m. Unfortunately, other Martian data does not support the theory of such a large ice sheet. It has been suggested that a volcanic eruption beneath a layer of rock and ice can also create tuyas. Scientists now believed that the Northern Plains of Mars may once have been covered with a layer of rock and ice that has since melted exposing the tuya formations.

Don't take my word for it

Allen, C. C., 1979. Volcano-ice interactions on Mars. Journal of Geophysical Research, 84 (B14): 8048-8059.

Chapman, M. G., 2003. Sub-ice volcanoes and ancient oceans/lakes: a Martian challenge. Global Planet Change, 35 (3-4): 185-198.

Komatsu, G. and Y. Litasov, 2002. Subice volcanism on the Azas Plateau: A comparison with possible Martian tuyas. Conference on Lunar and Planetary Sciences, LPI Contribution, 1262.