Numerous subglacial eruptions beneath the Powder Mountain Icefield have formed many distinctive subglacial volcanoes in the Mount Cayley volcanic field and contain abundant glass and fine-scale jointing from rapid cooling of lava, such as Ember Ridge and Slag Hill. Mineralogically, the volcanics range from andesite to rhyodacite, and chemically, the rocks span a range from andesite to dacite. Glassy volcanic rocks are abundant, with glass contents as high as 70%. Volcanoes such as the Slag Hill tuya were formed when magma intruded into and melted a vertical pipe in the overlying Powder Mountain Icefield. The partially molten mass cooled as a large block, with gravity flattening its upper surface, forming its flat-topped, steep-sided subglacial volcanic edifice. The latest volcanic activity in the Powder Mountain Icefield has occurred in the past 10,000 years.