A NEW MODEL FOR THE EVOLUTION OF TABLE MOUNTAINS - VOLCANOLOGICAL ANDPETROLOGICAL EVIDENCE FROM HERDUBREID AND HERDUBREIDARTOGL VOLCANOS (ICELAND)

We present a new model for the evolution of volcanic table mountains, based on volcanological and petrological studies of Herdubreid/Herdubr eidartogl, an upper Pleistocene volcanic complex within the active Ice landic rift zone. The evolution of these table mountains is highly com plex and influenced substantially by different eruptive environments a s well as changing magma sources and volcanic and magmatic processes. Whereas current models imply entirely subglacial conditions and contin uous compositional (''monogenetic'') evolution for these volcanoes, we subdivide their evolution into four eruptive periods characterized by different environments: (a) Subaerial lava flows erupted during an in terglacial period. (b) Lavas and voluminous hydroclastic deposits form ed in a lake during a waning period of the last glaciation. (c) Subgla cial eruptions during thickening of the ice sheet as a result of a cli matic deterioration built pillow lava piles overlain by steep-sided co mplexes of hydroclastic deposits. These deposits differ from those of the second eruptive period in structure, texture, and degree of altera tion. Subaerial lava flows and agglutinates covered these deposits aft er buildup above the ice sheet. (d) Subaerial lava flows and fallout d eposits during a postglacial period. The detailed analysis of volcanic table mountains appears to be a very sensitive indicator of climatic fluctuations. Although most deposits of the studied volcanoes were for med during the waning period of the last glaciation, the subglacial de posits in the upper part of the volcanic complex reflect a temporary, but major, thickening of the ice sheet.