The eruption environment of multi-tiered columnar basalt lava flows

Field studies on the eruption environment and prevailing cooling conditions of multi-tiered lava flows of the type exemplified by the Giant's Causeway were carried out at localities on the Columbia River Plateau, southern Iceland, on the islands of Mull and Staffa and around the Giant's Causeway, NE Ireland. Common features can be recognized in the volcanology of such hows in all the localities examined. These include palaeotopographic evidence of river valleys giving conditions conducive to the pending of flows and the damming of existing drainage; evidence of high rainfall regimes or abundant surface water; association of interlava lacustrine and fluviatile sediments; association of hyaloclastite flow foot breccias within multi-tiered basalt lava successions and evidence of large volume lava hows and high eruption rates. Examination of the petrography of the upper curvi-columnar parts of the lava (the entablature) shows textural signs of quenching, compared with the lower more regular columnar zone (the colonnade). These common held and petropraphic features confirm the model of multi-tiered flow formation by modification of the cooling regime in the upper part of the how following inundation of the hot lava surface by displaced river drainage.