Englacial vs lacustrine origin of volcanic table mountains: evidence from Iceland

Detailed facies analysis of hyaloclastites and associated lavas from eight table mountains and similar "hyaloclastite volcanoes" in the Icelandic rift zone contradict a rapid and continuous, "monogenetic", entirely subglacial evolution of most volcanoes studied. The majority of the exposed hyaloclas tite deposits formed in large, stable lakes as indicated by widespread, up to 300-m-thick, continuous sections of deep water, shallow water and emerge nt facies. Salient features include extensively layered or bedded successio ns comprising mainly debris flow deposits, turbidites, base surge and fallo ut deposits consisting of texturally and compositionally variable, slightly altered hyaloclastites, as well as sheet and pillow lavas. In contrast, ch aotic assemblages of coarser-grained, more poorly sorted and more strongly palagonitized hyaloclastite tuffs and breccias, as well as scoria and lava are interpreted to have formed under sub- or englacial conditions in small, chimney-like ice cavities or ice-bound lakes. Irregularly shaped and errat ically arranged hyaloclastite bodies produced at variable water levels appe ar to have resulted mainly from rapid changes of the eruptive environment d ue to repeated build-up and drainage of ice-bound lakes as well as the rest ricted space between the ice walls. We distinguish a "deep water" facies fo rmed during high water levels of the lake, a hydroclastic shallow water and emergent facies (leakage of the lake or growth of the volcano above the wa ter surface). Our model implies the temporary existence of large, stable la kes in Iceland probably formed by climatically induced ice melting. The hig hly complex edifices of many table mountains and similar volcanoes were con structed during several eruptive periods in changing environments character ized by contrasting volcanic and sedimentary processes.