UNSTABLE TEPHRA DAMMED LAKES IN THE VEIDIVOTN AREA, ICELAND

The effects of external water on erupting basaltic magma have been dealt with from many points of view, covering all environments from deep ocean to shallow lakes and high ground water to glaciated areas. Relatively little attention has been focussed on how volcanism affects the ground water and surface water reservoirs within volcanic areas. The most obvious effects are perhaps damming or diversion of waterways, e.g. by accumulation of volcanic products or by volcano-tectonic movements. Such changes are normally small but in countries like Iceland, where major glacial rivers flow across or within the volcanic areas, the effects can be extensive.

Tephra dammed lakes are a category of ephemeral lakes which may be the most dangerous water bodies that form in volcanic areas outside ice caps. Large unstable tephra dammed lakes formed in a river and lake basin within a volcanic area following two subsequent hydroexplosive eruptions on the Veidivotn fissure swarm, Iceland.

A 140 km2 temporary lake, Langalon, formed behind >15 m high dam of poorly consolidated tephra in the late 9th century AD, as a consequence of an eruption on the Vatnaoldur volcanic fissure. Being the largest lake in Iceland at its maximum extent, it had dischargeable volume of about 1.5 km3 and a catastrophic flood resulted when the tephra dam failed. In the late 15th century AD, four temporary lakes formed in the same area as a consequence of an eruption on the Veidivotn volcanic fissure. Together the lakes had dischargeable volume of 0.4 km3 if drained in a single episode. Shorelines left by the largest lake imply draining in several steps. Although of less volume than the Langalon flood, uncontrolled discharge from such lakes could be hazardous to the present utilization of neighboring areas.