SUBSURFACE SEDIMENTS TELL ABOUT THE HYDROLOGY OF A VOLCANO

Differences in permeability, parallel versus perpendicular to bedding, in e.g. a stratovolcano, direct the major part of the ground water fl ow to the near-surface deposits, where it may be collected in local, p erched aquifers. Percolation of meteoric water down to the basal groun d water aquifer is slow, but is enhanced by steep fault surfaces and d ykes. In much of the present literature in hydrogeology it is assumed that the ground water flow takes place by seepage, if tunnels such as lava tubes are not available. The presented data, from the slopes of T enerife and a table mountain, Blafjall, in northern Iceland, demonstra te that ground water flow is able to create its own tunnel network in the near-surface deposits of a volcano slope in order to enhance the d rainage of the meteoric recharge. As the volcano grows, new tephra bed s are deposited on top of the older units and a new gallery of tunnels will probably be formed in the new near-surface deposits. The older g alleries receive less water and the tunnels are filled by stratified s ubsurface sediments; eventually the tunnels will become abandoned. The groundwater tunnels are a part of the regulating system enhancing the drainage of a volcano; they also contribute to the erosion of the vol cano. Volcanoes are important aquifers in different parts of the world and are used as reservoirs far drinking water and irrigation. A groun d-water tunnel system preferentially drains the perched aquifers and a s such their role is in conflict with the interests of the users of gr ound water. On the other hand, they prevent overflow in the perched aq uifers, which otherwise might release devastating debris-flows down th e volcano slopes.