Passage Zones, Tuyas and the Stability of Englacial Lakes
[*Magnús T. Gudmundsson*]
(Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland; ph.
+354-525 5867; fax: +354-562 9767; email: mtg@raunvis.hi.is)
It is generally accepted that basaltic tuyas form in eruptions within large glaciers or ice sheets.
Essentially identical landforms are created in sustained eruptions in lakes or the ocean, with
the island of Surtsey, formed off the south coast of Iceland in 1963-1967, being a good
example. Observed eruptions within glaciers have not built tuyas. These eruption were
relatively short-lived and did not make the necessary transformation from the
phreatomagmatic phase to the effusive phase. Although some variations exist, a striking
feature of most tuyas is the largely horizontal passage zone, indicating a semi-stable level of
the englacial lake in which a tuya grew through the progression of lava-fed deltas. The
apparent stability of these lakes is enigmatic, since ice-dammed lakes in present-day glaciers
are for the most part characterized by gradual rise in lake level, followed by rapid drainage in
jokulhlaups, resulting in a fast drop in lake level sometimes exceed 100 m. It has recently
been suggested that supraglacial drainage may provide an explanation for semi-stable
meltwater lakes during tuya-forming eruptions (Smellie, 2006). However, the mechanism by
which supraglacial drainage may be established and remain stable has not been resolved in a
satisfactory manner. It is likely that tuyas are mainly formed in long-lived low-discharge
eruptions similar to the presently on-going eruption at Kilauea. Most basaltic eruptions start
off with an initial high-discharge phase. A subglacial eruption of this kind will in its early
phases melt large volumes of ice with the meltwater draining away subglacially. If the
eruption continues for an extended period of time, magma discharge is likely to drop.
Considering the thermal regime within the growing tuya, and that the subglacial meltwater
tunnel may close, even if only temporarily, a supraglacial drainage path may form along the
depression in the ice surface overlying the initial subglacial drainage path. The rate of
incision of the surpraglacial channel will depend on the thermal regime of the glacier,
meltwater temperature and flow rate. At the moderate meltwater flow rates expected for low
magma discharge, the development of a supraglacial channel and the stability of the water
level at the eruption site will depend on the temperature of the upper layers of the glacier. In
a temperate glacier incision will lead to gradual lowering of water level and is unlikely to
result in a semi-stable water level. However, in a thick polythermal glacier, surface ice
temperatures well below zero will slow down or stop incision. This suggests that conditions
for stable water level with supraglacial drainage can develop in polythermal glaciers, possibly
explaining the horizontal passage zones in tuyas. This conforms to the fact that known tuyas
in Iceland were formed during glacial periods, within glaciers that must have had equilibrium
lines at much lower elevations than seen in present-day glaciers.
Smellie, J.L. 2006. The relative importance of supraglacial versus subglacial meltwater escape in basaltic tuya
eruptions: An important unresolved conundrum. Earth Science Reviews, 74, 241-268.