• 34 million year old microscopic planktonic foraminifera fossils from Java, Indonesia. Photo by Helen Coxall
  • Coring operation on the aft deck of icebreaker Oden. Photo by Martin Jakobsson
  • Deformed limestone, Taimyr expedition. Photo by Vicky Pease
  • Folding on Crete. Photo by Alasdair Skelton
  • Iceland. Photo by Elisabeth Däcker
  • Iceland.Photo by Elisabeth Däcker
  • Oden's bridge, SWERUS expedition. Photo by Björn Eriksson
  • Oman. Photo by Alexandre Peillod
  • Punta di Maiata, Italy. Photo by Jan Backman
  • Siccar Point in Scotland. Photo by Alasdair Skelton
  • Swedish Icebreaker Oden, Lomrog expedition. Photo by Martin Jakobsson
  • Swedish Icebreaker Oden outside Svalbard. Photo by Martin Jakobsson

Geology of Lagoa das Furnas, a crater lake on São Miguel , Azores arquipelago
by Tommy Andersson

Date and time: Thursday, June 15 at 10.00
Place: DeGeersalen, Geohuset (link to the house plan)

Chair: PhD Eve Arnold
Examiners: Professor Erik Sturkell (Göteborg University) and PhD Richard Gyllencreutz

Cover illustration: Painting of the eruption in Lagoa das Furnas. Painting made by Manuel Carreiro
(with direction from Victor-Hugo Forjaz)

In this thesis, the results from a geophysical mapping and coring campaign of Lagoa das Furnas are presented. Specific focus is placed on the origin of a subaqueous volcanic cone mapped in the southern part of the lake. Lagoa das Furnas is a crater lake with in the Furnas volcanic centre which is located on the island of São Miguel in the Azores archipelago. The Furnas volcanic centre has a long history of earthquakes and volcanic activity. The area is relatively well-studied, except for the lake floor. Therefore, a high resolution geophysical and geological mapping survey was conducted at Lagoa das Furnas. Sidescan sonar was used to map the surface of the lake floor and single beam sonar was used to acquire sub-bottom profiles. In addition to the geophysical mapping, sediment surface sampling and core drilling were carried out followed by geochemical analyses of the retrieved material. The mapped data permitted a characterisation of the floor of Lagoa das Furnas and revealed several volcanic features including fumarolic activity and a volcanic cone in the southern part of the lake. In order to unravel the origin of this cone several methods were applied, including analyses of tephra and minerals collected from the cone itself and from nearby deposits of two known eruptions, Furnas I and Furnas 1630. Sedimentological, petrological, geochemical and geochronological studies of pyroclastic deposits from the cone suggest a subaqueous eruption linked to the Furnas 1630 eruption. The chemistry of glass and crystal fragments sampled from the cone suggests that it is composed of more evolved magma than that of the main Furnas 1630, implying that the lake cone is likely a product of the last eruptional phase.
According to historical records, two of three lakes were lost due the 1630 eruption. The results of this study show that the remaining lake is most likely Lagoa das Furnas and consequently did exist before the 1630 eruption.

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