Its prominent and enigmatic edifice is composed of basaltic andesite to dacite of unknown age and may represent a group of dikes and lava domes that formed subglacially.[1] At least four dikes protrude its summit.[1][2] These form the ribs of 60 m (200 ft) to 180 m (590 ft) high lava spines, the tallest being The Castle, which contains horizontal and radiating columnar joints.[1] The spines are covered by welded breccia close to their bases and columnar jointing extends into the welded sequence.[1] The most elevated lava flows and spines are composed of dacite.[1]