Dredging has yielded basaltic rocks which define alkali basalt and basanite suites. The flows contain phenocrysts of clinopyroxene, iddingsite, olivine, plagioclase and sometimes spinel. Lava flows are covered by manganese crusts less than thick. They often display traces of alteration and formation of glasses such as palagonite. Some dredge samples might come from neighbouring Cretaceous seamounts rather than the South Arch volcanic field, however.