VOLCANISM AND GLACIERS - PROCESSES AND ENVIRONMENTAL IMPACT

Volcanic eruptions within glaciers and ice sheets have occurred in several parts of the world, notably in Antarctica, North-Canada and Iceland. Evidence from these regions shows that large subglacial eruptions can have a major environmental impact but their effects are very different from that of subaerial eruptions of similar size. Subglacial eruptions may rapidly melt large volumes of ice, temporarily change the shape of glaciers and ice sheets, form hyaloclastite mountains, cause widespread fallout of tephra and lead to extensive flooding in high discharge jokulhlaups. In subglacial eruptions the removal of ice by melting and draining of meltwater forms deep depressions in the glacier surface. This changes the flow field of a glacier as ice flow is diverted towards these depressions. Such effects may persist for decades or centuries. In Iceland, historical eruptions in the Katla volcano have caused catastrophic jokulhlaups carrying sediments that temporarily extended the shoreline outwards by a few kilometres. Volcanism under glaciers creates a topography of steep sided mountains, ridges and tuyas. Such topography has been forming over the centuries in frequent eruptions under Vatnajökull, Iceland. Some subglacial Pleistocene eruptions created mountains with volumes of order 10 km³. Such eruptions may have melted a few tens of km³ of ice and caused catastrophic jokulhlaups with the potential for high sediment transport and/or erosion. On a different scale, there is evidence supported by theoretical models that the loading and de-loading of the lithosphere by the growth and decay of ice sheets strongly affects volcanism. Support for this is found in the voluminous outpouring of lava at the start of the Holocene in Iceland when extensive lava fields and large shield volcanoes were formed. To a considerable extent these early Holocene lavas reshaped the landscape in the volcanic zones in Iceland.