Title: H4 and other acid Hekla tephra layers (H4 og önnur súr gjóskulög úr Heklu)
Abstract: The postglacial activity of Hekla and its vicinity is divided into three stages according to tlre type of volcanic activity and the chemical composition of the eruption products.The second stage is characterized hy large Plinian eruptions.Isopach maps of the five acid tephra layers produced during this stage are presented with calculations or estimates of their areal coverage, volume and age.The second oldest of the acid tephra layers, H4, is described in some detail.Ils age is about 4000 C14 years.It covers ca.78.000 km-on land and its total volume as freshly fallen on land and sea is ca.9 km3.The tephra consists of pumice ancl ash with SiO2 ranging from 74% at the bottom of the layer to 57% in the topmost part, minor amount of crystals and various lithics.H± is divided into four units according to the colour changes occurring as the pumice becomes more basic: white, greyish-yelloiu, greyish-broiun and brownish-black.The lowermost tuhite unit makes up about 50% of the volume of the layer.Grain size reaches maximum in the greyish-yellow unit.Apparent changes in wind direction during the fall of the tephra indicate that the bulk of the layer was probably produced in less than 11/2 day.The chemical data available on H± and the larger, but almost identical, H3 are briefly discussed.Detailed sampling of the H3 tephra and microprobe analyses of the glass reveal a fairly gradual change in Si03 from the bottom to the top of the layer.The occurrence of white veins in darker pimiice clasts from some units of H± and H3, most prominent at the boundaries be-tween the units, indicates a mixing process between two end-members.So far, this is also supported by the chemical data.Hekla is a central volcano in a youth stage.A deep position and limited widtli of its magma chamber may explain why none of its big explosive and acid eruptions have resulted in the formation of a caldera.