Mount Garibaldi in southwestern British Columbia is the only major volcano in North America known to have formed upon a regional ice sheet during the last glacial period, which began 110,000 years ago and ended between 10,000 and 15,000 years ago.[17] Hoodoo Mountain in northern British Columbia was contained within basins thawed in the ice and assumed the flat-topped, steep-sided form of a tuya.[18] Pyramid Mountain, in the Shuswap Highland of east-central British Columbia, was formed under more than 1,000 metres (3,300 ft) of glacial ice to assume the form of a subglacial mound.[19] The Fort Selkirk Volcanic Field in central Yukon contains volcanic features that were erupted subglacially when the large Cordilleran Ice Sheet existed in this area between 0.8 and one million years ago.[20] Submarine eruptions Submarine eruption: 1: water vapor cloud, 2: water, 3: stratum, 4: lava flow, 5: magma conduit, 6: magma chamber, 7: dike, 8: pillow lava Submarine eruptions are eruptions that occur underwater.[21] The appearance of these eruptions is different from those that occur on land.[21] When lava erupts it will be quickly cooled by the unlimited supply of water surrounding a submarine volcano, creating pillow lava.[21] Explosive fragmentation of lavas forms hyaloclastites.[21] Deep-sea submarine eruptions usually occur where the ocean floor is being pulled apart by plate tectonic movements called mid-ocean ridges, where about 75% of the Earth's magmatic eruptions occur.[21] Shallow submarine eruptions can cause explosions of steam and volcanic ash called Surtseyan eruptions, named for the island of Surtsey off the southern coast of Iceland.[21] Explosive submarine eruptions usually eject large quantities of very light volcanic rock called pumice.[21] This very light volcanic rock can initially float on water, forming long-lived rafts of floating pumice carried long distances from the volcano by ocean currents.[21] Lava flows entering water can cause explosions that form piles of ash and rubble similar to cinder cones, although they were formed from rootless vents not located above a magma conduit.[21] The deformed volcanic sequences that form greenstone belts in the Canadian Shield contain hyaloclastite and pillow lavas, indicating these areas were once below sea level and the lava was rapidly cooled underwater.