Using Lava Inflation Structures to Estimate Eruption Duration in Fossil Lava Fields: the Helgafell Eruption 5900 BP

Lava inflation structures, such as tumuli and pressure ridges, are common features in subarial pahoehoe flow fields but has also been reported from submarine lava flows. Tumuli form by clogging of individual lava tubes inside a flow field or when the lava supply rate exceeds the flow front displacement, which causes inflation of previously formed crust and formation of the characteristic whale-back shape of tumuli. Axial and radial clefts cut the tumuli ("inflation-clefts"). Measurements on active lava flows has shown that the time (during which inflation occur) correlates posetively with the square of the measured inflation-cleft depth, and can therefore be used to calculating active time of inflation by measuring cleft depths in fossil flows. Over threehundred measurements of inflation cleft depths were collected from tumuli and pressure ridges located in the Helgafell lava field, Vestmannaeyjar, South Iceland. The Helgafell eruption occurred approximately 5900 BP, and emplaced the largest lava flow on the island covering 6.5 km² (~ 0.6 km³ DRE). The erupted lava are plagioclase-phyric alkali basalt, exhibiting considerable variation (7.0 wt% MgO to 4.4 wt% MgO) due to flow fractionation and incorporation of large (< 7 cm) plagioclase xenocrysts. Measurements of inflation cleft depths show that a minimum crustal thickness of 0.3 m is required to initiate tumulus growth. The deepest clefts are located furthest away from the vent, which coinsides with the largest elevation difference between tumuli and source (e.g. uppermost point of lava tube). The cleft measurements where combined with careful stratigraphic mapping in order to estimate the total duration of the Helgafell eruption. It is important to keep in mind that tumuli are surface features and only reflect inflation of the uppermost flows. The maximum time calculated for active inflation must therefore correspond to a minimum eruption duration. By doing these calculations, and adding measurements of tumuli from various stratigraphic levels in the exposed coastal sections, the total duration of the Helgafell eruption can be estimated to a minimum value of nine months. The ratio of erupted volume/duration (km³/month) for Helgafell (0.06) is well in range of both the 1973 Eldfell eruption (0.04) and the 1963-1967 Surtsey eruption (0.02), considering that this a estimate of minimum duration and any increase in time will lower the volume-time ratio.