Page 36 - Fundamentals of Physical Volcanology

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VOLCANIC SYSTEMS 13

aspects of the associated deposits (e.g., the pres- September and October 1996 a spectacular sub-
ence of accretionary lapilli, small clumps of glacial eruption occurred along a fissure beneath
particles stuck together because they were damp) the extensive Vatnajökull ice cap. The fissure was
suggest that the eruption was hydromagmatic. located between two well-known subglacial volcanic
However, in other respects the deposits are like centers – Grímsvötn and Bárðarbunga. Initially the
those of “normal” Plinian eruptions. For example, evidence for the eruption was a depression 2 km
the 1875 eruption of Askja in Iceland lasted about wide and 200–300 m deep which formed in the ice
6.5 hours during which time it exhibited subPlinian, above the fissure. Eventually the eruption melted
Plinian and phreato-Plinian phases. The phreato- through the 400–600 m thick layer of ice above the
Plinian phase appears to have lasted about an hour vent and a steam column rose into the atmosphere.
and deposited the Askja-C ash which has a volume This changed into a dark eruption column which
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of about 0.2 km . Phreato-Plinian eruptions differ rose about 500 m and was produced by rhythmic
from the other eruptions involving interaction of explosions at the vent. The eruption column height
lake water with magma in that they appear to gen- increased to a maximum of 9 km late in the day on
erate steady, continuous eruption phases in which October 2 and a new depression formed in the ice
the external water simply enhances the fragmenta- 3 km away from the first one as a new subglacial
tion of the magma without dramatically altering the fissure started erupting. The eruptions continued
eruption style. During the AD 181 eruption at Taupo for a week with alternating periods of quiet and
(the eruption for which the term ultra-Plinian was explosive activity. The length of the active fissure
coined), the eruption style also varied in this way gradually increased to ∼9 km with eruptions occur-
between Plinian and phreato-Plinian depending on ring through pockets of melt water as much as 50 m
whether lake water could gain access to the magma. deep. The large amount of meltwater generated
in subglacial eruptions gives rise to a phenome-
non called a jökulhlaup (glacier-burst, an Icelandic
SUBGLACIAL ERUPTIONS
word), a giant flood of glacial meltwater. The 1996
These are eruptions in which the vent is situated eruption produced a jökulhlaup which reached
3 −1
beneath a glacier or ice sheet. The eruptions can peak flow rates of ∼45,000 m s and was able to
be effusive or explosive depending mainly on the transport ice blocks the size of houses. This did
thickness of the overlying ice, and usually involve considerable damage to bridges along the southern
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the production of large amounts of water as ice is coast of the island and affected an area of ∼750 km .

melted. Often the melting produces so much water It is estimated that the eruption produced about
3
that the eruption is, in effect, submarine, and pil- 0.7–0.75 km of volcanic products, including a new
3
low lavas form, creating a pillow mound around a subglacial ridge ∼7 km long, and melted ∼4km of
localized vent or a pillow ridge if the vent is an elon- ice.
gate fissure. In some fissure eruptions the dike
(the fracture through which magma is rising to feed
INTERACTIONS WITH GROUNDWATER
the vent) essentially overshoots the interface between
the rock surface and the base of the glacier so that So far we have talked about interactions of magma
magma actually penetrates some distance into a with surface water or ice. Magma can also interact,
crack in the ice. This leads to rapid chilling and frag- however, with water beneath the Earth’s surface,
mentation of the magma as it melts the surrounding i.e., with groundwater. Some Vulcanian explosions
ice, and the water produced undergoes chemical seem to be caused in this way. Other eruptions
reactions with the magma fragments to form a involving interactions with groundwater form a
rock called hyaloclastite. The fragments then fall type of wide, shallow, low-rimmed volcanic crater
through the water to accumulate alongside the vent called a maar (Fig. 1.20). Two such craters formed
forming a hyaloclastite ridge. during the 1977 eruption at Ukinrek in Alaska. The
Iceland is one place where it is quite common for first (West) maar formed over a period of 3 days
fissure eruptions to occur beneath glaciers, and in and was 170 m wide and 35 m deep, whereas the