Jökulhlaups from Snæfellsjökull: Evidence from the Geomorphological and
Sedimentological Record
[*K. T. Smith*] (Institute of Earth Sciences, University of Iceland,
Sturlugata 7, Reykjavík, IS – 108, Iceland, ph: +354-894-9068; email:
kate@raunvis.hi.is)
Snæfellsjökull is an ice-capped stratovolcano (1446 m high) with a 200 m
deep summit crater infilled with ice. Three major phreatic (plinian)
eruptions are known of in the Holocene which produced lava and airfall
tephra, most recently c.1750 BP. Geological maps indicate that over 25
eruptions have occurred in the volcanic system in the last 10.000 years.
Snæfellsjökull is central to a national park. A future eruption there
could place villages, tourist centres, farms and transport infrastructure
at risk.
In terms of Holocene history and hazard perception, Snæfellsjökull has
often been overshadowed by the likes of Hekla and Katla, more active
volcanoes where eruptions have been witnessed, but as we know from the
tragic examples of Öræfajökull and Nevado del Ruiz, the eruptions of
stratovolcanoes can be dramatic and are worthy of assessment. The hazards
they pose are no less significant because when they erupt these volcanoes
produce jökulhlaups, lahars, pyroclastic flows and lava flows along short,
steep paths towards nearby communities. This poster presents new research
into Holocene jökulhlaups / lahars.
Geomorphological mapping, analysis of 36 sediment profiles around
Snæfellsjökull and geochemical analysis of tephra and putative flood
deposits using an electron microprobe show that there is evidence for
major postglacial flooding events on the northern, western and southern
slopes of the volcano, along at least five different flood routes. Fluvial
landforms radiating from the volcano include rock-cut and sedimentary
channels, polished micro-channels, potholes, cataracts and aligned and
imbricated boulders. Many of these landforms cut into or are deposited on
top of porous postglacial lava flows across which normal streamflow is
very limited.
Sedimentary investigations outside of present-day active (spring melt)
channels show extensive overbank deposits of rounded,
fluvially-transported volcanogenic material with pumice clasts up to 20 cm
(c-axis) within narrow stratigraphic windows, not repeated throughout the
profiles. This indicates that the depositional events were rare and larger
in discharge than normal precipitation or snowmelt floods. These deposits
are matrix-supported with coarse, well-rounded pale pumice clasts in a
sand to granule-sized tephra-dominated matrix. Structures are minimal but
include crude bedding and both fining upwards and coarsening upwards
sequences. These characteristics are found in hyperconcentrated flows and
types of fluidal flows. Geochemical analysis of pumice within these
putative flood deposits to the south of the volcano shows marked
similarity with published analyses of tephra from the youngest
Snæfellsjökull central volcano eruption. This data all placed together
points towards Holocene jökulhlaup activity at Snæfellsjökull in channels
radiating from the volcano. No sign has been found to date of extremely
extensive flooding over whole flanks of the volcano.
These overbank but spatially limited deposits are more similar to lahar
and jökulhlaup deposits found in confined valley systems rather than the
well studied sandur deposits associated with the largest historical
jökulhlaup events in Iceland. This reflects the limited source of water
from the smaller ice cap of Snæfellsjökull compared to Mýrdalsjökull and
Vatnajökull and also topography which constrains flows to narrow zones
rather than allowing spreading over large expanses immediately they leave
the glacier margin.
This poster presents new evidence of postglacial floods of volcanogenic
material, most likely associated with eruptions within the Snæfellsjökull
volcanic system, particularly the youngest eruption of the Snæfellsjökull
central volcano c.1750 BP. This is important for assessing future hazards
here and lessons learned about the nature of palaeojökulhlaup evidence at
Snæfellsjökull can be applied elsewhere in the world. Importantly these
results highlight the variability of evidence left behind by jökulhlaups
in the geological record resulting from differences in volcano morphology
and nature, extent of ice cover and topographic conditions.
POSTER
CORRESPONDING AUTHOR: K.T. SMITH