The eruption of a rhyolitic dyke into shallow ice: Hrafntinnuhryggur, Krafla,
Iceland
[*Hugh Tuffen*] (Department of Environmental Science, Lancaster University,
Lancaster, UK, LA1 4YQ; phone: +44-1524-593-571; fax: +44-1524-593-985; email:
h.tuffen@lancaster.ac.uk ); Jonathan Castro (Division of Petrology and Volcanology,
Smithsonian Institution, Washington, DC 20560-0119, USA, phone: +1-202-6331810; fax: +1-202-357-2476; email: castroj@si.edu).
Hrafntinnuhryggur (Obsidian Ridge) at Krafla volcano, northern Iceland was
generated when a rhyolitic dyke erupted into shallow ice towards the end of the last
glacial period. Several small-volume lava bodies budded from the feeder dyke,
forming a 2.5 km long ridge that is 200-400 m wide and 30-80 m in height (total
volume ~0.01 km3). Although the bulk of the ridge is made up of variably spherulitic
obsidian lava, tuffaceous hyaloclastites are also present and were probably formed at
the onset of the eruption. The dyke is locally exposed at one end of the ridge, where it
cut older basaltic hyaloclastite and fed a lava flow.
Lava bodies on the ridge crest have columnar-jointed sides, characteristic of
chilling against steeply-inclined ice walls, but display remarkably well-preserved
pumiceous flow tops. Lava exposed at lower elevations close to the base of the ridge
is mostly strongly perlitised and may have intruded waterlogged hyaloclastite. It thus
appears that the initially subglacial eruption melted through ~80 m of ice and lava
effusion then occurred in ice-confined but subaerial conditions.
Lava bodies on the ridge display striking textural zonations, with an outer
perlitised, hackly-fractured zone surrounding concentric zones of collapsed foam,
welded obsidian breccia and lithophysae-rich obsidian, which envelop a platyfractured core of pervasively spherulitised lava. These textural zones record the
penetration of meltwater into the margins, together with variations in the rate of strain
and cooling of the lava with distance from the margin. In some locations deep etching
of obsidian surfaces at lava margins indicates the action of particularly corrosive
hydrothermal fluids.
In addition to reconstructing the eruption mechanisms and patterns of magmaice interaction at Hrafntinnuhryggur, the following studies have been carried out:
1. Measurement of water contents in the obsidian. These have shown that the lavas are
predominantly degassed (0.10-0.15 wt %), consistent with field evidence for an iceconfined subaerial eruption. The water content of lava in the feeder dyke is
significantly higher, consistent with incomplete degassing at higher confining
pressures. Small-scale heterogeneities in water content are being measured with
synchrotron FTIR and used to reconstruct the timescale of diffusion around bubbles
and spherulites. This is helping to place constraints on the timescale of emplacement
and cooling.
2. Characterisation of the major element chemistry of the lava. Despite the broad
spectrum of textures in the obsidian, its compositions is very homogenous, being a
tholeiitic rhyolite with ~75.2 wt % SiO2 and minor proportions of clinoferrosilite and
Fe/Ti oxide microlites.
3. Experimental determination of the high-temperature fracture mechanics of the lava.
The compressive shear strength of the lava has been measured at a range of
temperatures and strain rates. The exceptionally high strength of flawless aphyric
obsidian shows the importance of crystals, bubbles and cooling cracks in
concentrating stresses and weakening the magma.
4. Documentation of brittle-ductile deformation structures and foam collapse textures
within the obsidian lava bodies. These textures record the brittle-ductile response of
the lava to stresses over a variety of timescales and mechanisms of degassing from the
lava, which are key controls on the behaviour of rhyolitic eruptions.
These studies are being combined in order to reconstruct the timescale of lava
emplacement and cooling, patterns of degassing and interaction with meteoric water
and ice. The eruption at Hrafntinnuhryggur is therefore of interest both from the
perspective of volcano-ice interaction and, more broadly, as an example of an effusive
rhyolitic eruption.
ORAL
CORRESPONDING AUTHOR: HUGH TUFFEN