Jökull - 01.01.2014, Side 54

J. Helgason and R. Duncan
Table 3. Main stages in the landscape evolution of Hafrafell. – Helstu stig í landslagsþróun Hafrafells.
Land- Erosion Period / Age Group For- Thickness of Dominating Relief1 (m)
scape surface Chron (Ma) mation volcanic activity
stage strata (m)
1st HR1 Neogene Gilbert 4.187–3.596 H1 HF1–HF5 161 SAV2 + 50?
2nd HR2 Neogene Gauss 2.581–3.596 H2 + H3 HF6–HF9 234 SGV3 + 100?
3rd HR3–HR6 Quaternary Lower Matuyama 2.581–1.945 H4 + H5 HF10–HF19 829 SAV >-829
4th HR7 Quaternary Lower Matuyama – – – Erosion – Hafrafell + 260
2.581–1.945 – – – valley formation
5th HR8– Quaternary Upper Matuyama 1.945– H6 HF20– 448 Hafrafell valley -448?
HR10 –0.781 HF31 filled up, SAV + SGV
6th HR11– Quaternary Brunhes < 0.781 H7 +H8 HF32– 1100 SGV +1000
HR12 HF39
Explanations: 1Relief: positive values indicate relief generation while negative values indicate landscape evening out due to
accumulation. 2SAV: Subaerial volcanism – lavas. 3SGV: Subglacial volcanism – subglacially formed lithologies.
Stage 2. Subglacially erupted volcanics in Hafrafell
date at least from the Gauss chron (ca. 2.6–3.6 Ma).
Relief in Hafrafell during stage 2 is controlled by the
sub-ice environment.
Stage 3. Volcanism during lower Matuyama (2.581–
1.945 Ma) produced a ≥800-m-thick sequence of
lavas, intercalated with a few erosion surfaces. This
extensive volcanic activity in the Hafrafell area filled
negative relief and smoothed landscape in relatively
short time, ≤0.64 Myr.
Stage 4. During Upper Matuyama time, or from about
the Olduvai subchron to almost the onset of Brunhes
(ca. 1.945–0.781 Ma), older lava flow bedrock was
carved by glacial erosion forming the >260-m-deep
Hafrafell valley.
Stage 5. The Hafrafell valley was filled during Upper
Matuyama time (ca. 1.945–0.781 Ma) with lava flows
that contain the Olduvai subchron. The filling during
this stage was a minimum of 260 m and may have
been as much as 448 m.
Stage 6. During Brunhes time, volcanism was exten-
sive from the Hrútfjallstindar and Öræfajökull vol-
canic centers (Helgason and Duncan, 2013; Steven-
son et al., 2006). Simultaneously, erosion continued
and valleys deepened by at least 1000 m.
Topographic evolution
When considering the topographic evolution of east-
ern Iceland Walker (1982) stated: "the contrast be-
tween the Austfirðir and inland plateau is attributed
to a departure about 5 m.y. ago from steady state con-
ditions, caused by a significant southward migration
of the locus of volcanism". This view does not con-
sider the influence of glaciers on the landscape. We
demonstrate that glaciers have generated over 2-km-
deep valleys that formed roughly during the last 3.5
Myr at Hafrafell. We conclude that until about 3 Ma,
glaciers had not set their mark on the region, either in
the form of a valley network or formed the deep de-
pression now present below the Vatnajökull ice sheet.
Keeping in mind that SE Iceland is regarded a rift
flank volcanic regime (Einarsson, 2008) it follows that
subsidence of the volcanic products there should be
much less than in the active accreting rift zones to the
west and north. We speculate that the major intru-
sive process and lack of crustal subsidence is likely
to generate local volcanoes that may reach high ele-
vation above the surrounding area and actually higher
elevation than comparable volcanic centers of the ac-
tive rift zones. Also, the formation of inclined sheet
swarms and major magma intrusion in SE Iceland has
added to the crustal build-up (Walker, 1975). There,
sheet intrusions commonly amount to over 50% of the
stratigraphic sequence. Referring to the net move-
ment caused by inclined sheets, Torfason (1979, p.
324) states: "the sheets and the major intrusions are
the major cause of the extensive uplift of south-eastern
Iceland, which is more than anywhere in Iceland."
These factors likely maintained the landscape in
SE Iceland at relatively high elevation above sea level.
Glacial erosion, on the other hand, may first have
counteracted the positive build-up caused by rift flank
volcanism and intrusions. Later, a deeply eroded val-
ley system with high peaks in between would have
54 JÖKULL No. 64, 2014