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Master's lecture in geology - Daníel Freyr Jónsson 

When 
Wed, 30/05/2018 - 11:00 to 12:30
Where 

Askja

Stofa 131

Further information 
Everybody welcome

Master's student: Daníel Freyr Jónsson 
Title: Mid-Holocene eruptive activity in the Hekla volcanic system
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Faculty: Faculty of Earth Sciences 
Advisor:  Esther Ruth Guðmundsdóttir, Associate Professor at the Faculty of Earth Sciences 
Other members of the master's committee: Bergrún Arna Óladóttir, Project Manager at the Institute of Earth Sciences and Olgeir Sigmarsson, Research Scientist at the Institute of Earth Sciences

Examiner: Magnús Sigurgeirsson, Geologist at ISOR

Abstract

Hekla volcano is one of the most active volcano in Iceland and is known for its explosive eruptions. Studies on prehistoric tephra have focused on the large plinian eruptions, namely Hekla 5, Hekla 4 and Hekla 3. Here, the aim is to improve the knowledge of other prehistoric eruptions and the volcanic history of Hekla by investigating the mid-Holocene tephra layers Hekla DH (6650 cal yr BP), Hekla Mó (~6060 cal yr BP) and Hekla Ö (6060 cal yr BP).
The Hekla DH, Hekla MÓ and Hekla Ö tephra layers have been described, mapped and their composition analyzed in several soil sections proximal to Hekla proper. The magma composition of these three layers range from rhyolite through andesite and basaltic-andesite to basalt. Hekla DH represents the earliest confirmed eruption of basaltic andesite from the Hekla system, which subsequently has become the dominant product of the system. The basalts (SiO2 ~46-47%) observed in these three Hekla tephra layers consists of two types; one with TiO2 <3 wt% and Al2O3 >15 wt% and the other with TiO2 >3 wt% and Al2O3 <14 wt% for a MgO concentration of >6 wt% and <6 wt% respectively. Chemical composition of basaltic andesite in the Hekla DH tephra indicates that lower oxygen fugacity conditions may have existed in the magmatic system beneath Hekla prior to the Hekla DH eruption, compared to later times. Increased oxygen fugacity during mid-Holocene could have resulted from melting different regions of heterogeneous mantle source due to deglaciation of the area.
New isopach maps are compiled for all three tephra layers. Mapping of the dispersal of the Hekla DH tephra has confirmed a source in the Valagjá area, in the northern part of the Hekla system. With the chemical dataset obtained in this study, a possible relationship between Hekla Ö and a distal Hekla tephra in NW, W and SW-Iceland is suggested. Consequently, the Hekla Ö tephra possibly covers as much as 80% of terrestrial Iceland.
In addition, thirteen tephra layers of mid-Holocene age (4200-7100 cal yr BP) were investigated in a soil section in Tagl and analyzed for source provenance. Eleven of the layers originate from Katla, while the chemical composition of two other layers points to a source in Vatnafjöll. One layer is a silicic Katla tephra layer, with the composition and location in the stratigraphy resembling that of the SILK N-1 tephra, which was deposited around 5800 cal BP. 

Daníel Freyr Jónsson

Master's lecture in geology - Daníel Freyr Jónsson