Effusive activity in the 1963-1967 Surtsey eruption, Iceland: Flow emplacement and growth of small lava shields

Surtsey volcano of the south coast of Iceland (7 November 1963 to 5 June 1967), best known for its 'Surtseyan' eruptions, also featured prolonged episodes of effusive activity producing two partly overlapping pahoehoe lava shields. Here this effusive activity is examined to assess the overall volcanic architecture of the Surtsey lava shields and the mechanisms involved in their construction. The lava cone and apron are the principal structures of the shields. The cone is constructed during periods of relatively high magma discharge and vigorous fountain activity, producing surface flows of shelly pahoehoe and fast moving (up to 20 m s^-1) slabby pahoehoe to aa sheet flows. The apron is formed during periods of passive lava effusion when the level of the lava stands well below the crater rims and lava is discharged through preferred internal pathways (i.e. lava tubes) to active flow fronts where it breaks out as inflating sheet lobes. The volcanic structure and architecture of monogenetic pahoehoe lava shields elsewhere in Iceland is essentially identical to the shields at Surtsey and it is proposed that they were constructed in a similar manner. A key conclusion is that the proportional size of the lava cone is a function of the average effusion rate. This relationship implies that shields with large cones were produced by relatively short eruptions (years to decades?) characterized by elevated, but fluctuating, magma discharge. Conversely, shields with small cones were produced by prolonged eruptions (decades to centuries?) typified by low and steady magma discharge.