Technically, Crevasse Crag is the remains of an extinct volcano that formed during the onset of Pemberton Belt volcanism 16 million years ago. This activity produced breccias, tuffs and plagioclase-phyric flows, all of which form the present day edifice of Crevasse Crag. Analyses of major, trace and rare-earth elements indicate that dacite, andesite and basaltic andesite lava flows form its lower flanks. These overlie Late Cretaceous and younger intrusive rocks that form the glaciated mountain ridge on which Crevasse Crag lies.