changes with time during
the eruption. The opening phase has the highest SiO2 content, and while the
eruption is ongoing the SiO2 lowers. This explains why tephra layers from Hekla
often have a light coloured base and a dark top, making ideal markers of the
layers. For more
information on the Hekla volcanic system,
visit Catalogue of Icelandic Volcanoes
/volcanoes/about-volcanoes/hekla/