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Field Guide for Trjaviðarlækur – Hekla tephra layers
Chapman Conference 2012
by
Thor Thordarson
Fig. 1 Distribution of active volcanic systems among volcanic zones and belts in Iceland: 1. Reykjanes, 2. Krýsuvík, 3.
Brennisteinsfjöll, 4. Hengill, 5. Hróðmundartindur, 6. Grímsnes, 7. Geysir, 8. Prestahnjúkur, 9. Hveravellir, 10. Hofsjökull, 11.
Tungnafellsjökull, 12, Vestmannaeyjar, 13. Eyjafjallajökull, 14. Katla, 15. Tindfjöll, 16. Hekla-Vatnafjöll, 17. Torfajökull, 18.
Bárðarbunga-Veiðivötn, 19. Grímsvötn, 20. Kverkfjöll, 21. Askja, 22. Fremrinámur, 23. Krafla, 24. Þeistareykir, 25.
Öræfajökull, 26. Esjufjöll, 27. Snæfell, 28. Ljósufjöll, 29. Helgrindur, 30. Snæfellsjökull. The bold fonted volcanic systems are
those of interest for the Þingvellir trip. The large open circle indicates the approximate centre of the Iceland mantle plume.
Dotted line shows the northern limits of the East Volcanic Zone, whereas the hachured line indicates the boundary between the
active and propagating rift segments of the zone.
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USEFUL MAPS OF SOUTH ICELAND
Fig. 2. The main geological features of South Iceland.
Fig. 3. Distribution of lowland areas invaded by the sea towards the very end of the Weichselian glaciation.
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Hekla-Trjáviðarlækur Excursion
Þjórsárdalur
Þjórsárdalur valley is cut into an extinct central volcano, which bears the same name. This is revealed by unusual
abundance of andesite and rhyolite dyke, lava and tephra formations in the Plio-Pleistocene strata of the surrounding
mountains. The valley is in fact an old caldera that has been extensively modified by glacial erosion. The volcano centre
is at the head of the valley near Mt. Fossalda, where many inclined cone dykes dissect strongly altered and colourful
rhyolite and andesite formations. Sections of the original caldera faults are seen at Mt Skeljafell on the east side of the
valley and at Grjótárgljúfur on the west side, showing that from rim to rim the caldera was at least 8 km wide. At these
localities, rhyolite dykes have intruded the caldera faults and connected to small rhyolite lava domes situated a bit
higher up in the sequence. At Mt Skeljafell the dyke is exposed in a small ravine just north of the main road, where it
ascends the western slope of the mountain and can be followed up to the lava dome (the pale coloured rock formation
on the upper slopes) where it sits on the caldera fault. Among the rocks that form the caldera fill are andesite tuffs and
pillow lavas that are closely associated with tillite beds. A good example of this can be seen at Mt. Reykholt in the
centre of the valley, where a thick pillow lava and móberg tuff sequence is sandwiched between two tillite layers. This
sequence clearly shows that, about 2 million years ago, while the volcano was still active, a glacier filled in the caldera.
Later the volcano was almost completely buried by a series of basaltic lava flows formed by eruptions outside the
Þjórsárdalur central volcano and then its caldera was carved out by erosion during subsequent glaciations forming the
valley we see today.
In the last 7000 years Þjórsárdalur has been repeatedly devastated by tephra fall from the nearby Hekla volcano, but
never more so than by the largest plinian eruption at Hekla 3100 (H3; Figure 3). The whitish specs on the surrounding
mountain slopes are not snow but the remnants of the pumice-fall deposit from this Hekla eruption. At the foothills of
Mt Skeljafell this fall deposit is more than 2.5m thick. When the first settlers arrived in Iceland in the ninth and tenth
centuries, the valley had completely recovered from its previous encounters with Hekla and it soon became quite
densely populated, hosting at least 20 farms. This settlement in Þjórsárdalur Valley was decimated by the tephra fall
from the 1104 eruption at Hekla. At the time, the valley was completely blanketed by 10-30 cm thick pumice-fall
deposit, which was enough to make the area inhabitable. Some of these ancient farm ruins were excavated by the
Nordic archaeological expedition in 1939 and one of them is the old Norse homestead at Stöng. The valley floor is
covered by an 3500-year-old lava flow – one of the youngest Tungnaár lavas – which originated from volcanic fissures
in the Veiðivötn area. It is a branch of a larger lava flow field, the Búrfell lava, which covers the high plateau to the east
Mt Skeljafell and Mt Búrfell. The lava found its way into the Þjórsárdalur Valley through a narrow ravine named Gjáin,
which is about 500 m to the east of the farm ruins of Stöng. In the centre of the valley of the Búrfell is a cluster of small
scoria cones, which were formed by rootless eruptions when the lava covered the wetlands that once existed in
Þjórsárdalur Valley, blankets lava. At Hjálparfoss the River Fossá has eroded a beautiful section through the lava,
exposing the roots and conduits of the rootless cones, along with spectacular fanning columns formed by waterenhanced cooling of stagnant lava.
Figure 3. Hekla 3 (H3) pumice fall deposits at Hafið, near Búrfell Mt.
The “Queen” of Icelandic volcanoes - Hekla (1491 m) is without doubt Iceland's most famous volcano and is the third
most active volcano in the country, behind Grímsvötn and Katla, with a tally of 18 eruptions in historical times (Figs 4,
5). Hekla is the only one of its kind because it is the only ridge-shaped stratovolcano known in Iceland where eruptions
occur repeatedly on the same fissure. Hekla magmas are erupted from an 8 km-deep chamber, and the magma system is
characterised by two end-member compositions, an andesite magma containing ~52 wt per cent SiO2 and a rhyolite
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magma with SiO2 in excess of 70 wt per cent. Intermediate magma compositions, which are common, are produced by
mixing of the end-member magma types in the chamber prior to eruption.
Fig. 4. Hekla volcano, view from the southeast.
Fig. 5. Tephra-dispersal direction for historical Hekla eruptions (numbers indicate year of eruption); the shaded area shows the extent
of the H3 tephra layer formed 2900 years ago by a plinian eruption at Hekla.
Hekla has produced a vast number of eruptions in postglacial time, although the three rhyolitic plinian eruptions H3
(3100 years BP), H4 (4200 years BP) and H5 (6500 years BP) stand out as they blanketed up to two-thirds of the
country with light-coloured pumice fall deposit (Fig. 6). The first eruption in historical times was a tremendous
explosive plinian eruption of 1104, emitting 2.5 km3 of rhyolite tephra that covered more than half of Iceland with
pumice fall. In doing so, it desolated the settlement in Þjórsárdalur Valley and destroyed more than 20 farms. More
commonly, Hekla events are mixed eruptions that begin with a short-lived (< 1 hour) explosive phase at the summit
crater, producing Plinian fall deposit of dacite or andesite composition. Then there is a sharp transition into a fissure
eruption, with a curtain of fire extending across the crest of the volcano producing andesite tephra and lava. This phase
typically lasts between several hours and a couple days, when the activity becomes confined to a short segment of the
fissure or a single vent that primarily produces lava.
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Fig. 6. Isopach maps of H3 and Hekal 1104 tephra layers showing their distribution and thicknesses.
Hekla erupted four times in the twentieth century in 1947, 1970, 1980 and 1991, and celebrated the turn of the
millennium by an eruption on 26 February 2000. Records of other historical Hekla eruptions show that the volcano has
erupted at least once every hundred years since 1104, except in the fifteenth century. All of these eruptions are
mentioned in the historical chronicles and thus facilitate more exact dating of the eruptions than would otherwise be
possible. Some eruptions are described in great detail, especially the younger ones (i.e. post-1693), and demonstrate a
surprisingly pragmatic attitude towards volcanic eruptions. This is clearly illustrated by the description of the eruption
at Hekla in 1300AD, written by Einar Hafliðason in the fourteenth century:
MCCC [i.e. 1300]. Coming up of fire in Mount Hekla with such violence that the mountain split, so that it will be
visible as long as Iceland is inhabited. In that fire, great rocks played like coals and when they hit together there
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were great crashes that were heard in the north of the country and in many places elsewhere. So much pumice flew
onto the homestead at Næfurholt that the roofs of the buildings were burnt off. The wind was from the southeast,
and it carried northwards over the country such dense sand between Vatnsskarð and Axarfjarðarheiði, along with
such great darkness, that no-one inside or outside could tell whether it was night or day, while it rained the sand
down on the earth and so covered all the ground with it. On the following day the sand was so blown about that in
some places men could hardly find their way. On these two days people in the north did not dare put to sea on
account of the darkness. This happened on 13 July.
The total volume of magma erupted by Hekla in historical times is about 10 km3, which is enough to build a 1 m wide
and 1.6 m-high wall around the 6000 km-long coastline of Iceland. The tephra fall from Hekla eruptions has repeatedly
caused great damage to pasture areas and cultivated farmlands, with more than 50 farms in the vicinity (< 70 km) of the
volcano damaged or destroyed in a single eruption. In more distant regions, the damage felt was different, where a large
part of the livestock was killed by fluorine poisoning via the intake of ash particles while grazing in fields affected by
ash fall. The greatest damage was caused by the eruptions in 1300, 1341, 1510, 1693 and 1766.
Trjáviðarlækur
The Trjáviðarlækur basin is situated NE of the Búrfell hydroelectric power plant which started operating in 1970.
Spillwater from the intake lake has cut a channel into the basin floor and exposed a Holocene (postglacial) sedimentary
sequence along channel banks. The deposits show that drastic environmental changes took place during the last 10,000
years. The radiocarbon (14C) age dating and tephrochronology were used for dating the sedimentary sequence. The main
points of the Holocene basin history are:
>10,000 years BP. Outwash plain was built up infront of the Weichselian ice sheet.
Around 10000 years BP. Lacustrine sediments accumulated in shallow lakes or ponds. Colonization by plants took
place.
10,000- ca. 4,200 years BP. The basin was covered with vegetation as evidenced by a 2.5 m thick peat layer. Birch
(Betula) was the most common plant. Within the peat is the fine grained acid tephra layer H5 from at ~7,000 years BP
(10-15 cm thick).
~4,200 ~ 1,200 years BP. The basin was covered with more than 2m thick H4 tephra (7 km3) around 4.2000 years BP,
which suffocated all vegetation. Winddrifted tephra and sand accumulated (ca. 4 m), until the H3 tephra (10 km3) fell
3,100 years BP. About 1,200 years BP loessial soil began to form and the basin was revegetated.
1104 AD - present. In 1104 Hekla produced another silicic plinian eruption – H1104 tephra (Hekla 2 km3). The tephra
fall caused desolation of farms in the area. The vegetation recovered relatively quickly and ancient records give
evidence of wood utilization within the basin. This activity culminated in 17th century. In the 18th and 19th centuries
the vegetation began declining due to overgrazing and deterioration of the climate. Soil erosion set in and the
Trjáviðarlækur basin became denuded once again in the Holocene. At present the basin is barren with the exception of
crooked birch bushes on adjacent slopes and some cultivated grass pasture.
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Fig. 7. Geological and topographical features in the Burfell area. Legend: 1) Tungnarhraun lavas. 2) Hekla lavas. 3) Outwash plain.
4) Pleistocene bedrock.
Fig. 8. Topographical map of the Trjáviðarlækur basin. Roman numbers indicate the location of the profiles shown on Fig. 10.
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Fig. 9. Schematic section of the sedimentary sequence in the Trjáviðarlækur basin. Legend : 1) Primary tephra. 2) Reworked tephra.
3) Loessial soil with tep hra layers. 4) Peat. 5) Lacustrine sediments. 6) Sand with gravel. 7) Tillite.
Fig. 10. Stratigraphic profiles. Locations are shown on Fig. 8.