| Summary: | Research into pahoehoe lava flows are of great importance because they can indicate mode of emplacement and duration of eruption that produce such lava flows. Þingvellir lava is pahoehoe lava composed of many lava lobes. Each lava lobe consists of three parts, upper crust, lava core and basal crust. Thin crust forms on the surface of pahoehoe lava when it flows on the surface. When the upper crust has reached sufficient strength to slow down the advance of the flow front, hydrostatic pressure within the lava lobe increases and it inflates. Meanwhile the lava beneath the upper crust is constantly adding to the thickness of the crust. The Þingvellir lava is a 10.8 km3 flow field that covers the northern half of Þingvellir basin. The main objective of this project is to document the characteristic volcanic architecture of the flow field as well as gain better understanding of mode and duration of its emplacement. The fissures at Þingvellir, e.g. Almannagjá, provide excellent exposures into the compound nature of the Þingvellir lava and thus are ideal for examining the lateral and vertical arrangements of lava lobes along with the internal structures that typify individual lobes. For this study I examined a 300 m long section of the footwall to Almannagjá. The lobe geometry is documented, lobe thickness and length measured to determine the aspect ratio for each. Thickness of upper crust on each lobe is measured to determine the duration of emplacement for each. The vesicularity of the upper crust is determined via image analyses and the results are examined in relation to mode of emplacement to evaluate possible mechanisms for the growth of the upper crust. This data is used to establish the collective emplacement time for the lobe in the examined section of the footwall. The area covered by the lava represented in the section is estimated using the lobe geometry and aspect ratios, which in turn is used to estimate the duration of the eruption that formed Þingvellir lava. The results of these calculations indicate ...
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