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Australia: The Land Where Time Began |
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New Volcanic Province – Inventory of Subglacial Volcanoes in
West Antarctica
The West Antarctic Rift System is overlain by the West Antarctic Ice
Sheet, and as a result of the comprehensive ice cover, there is only
limited and sporadic knowledge of volcanic activity and its extent. It
is important to improve understanding of subglacial volcanic activity
across the province for both helping to constrain the way in which
volcanism and rifting may have influenced the growth and decay of ice
sheets over glacial cycles, and in light of concerns whether enhanced
geothermal heat fluxes and subglacial melting may contribute to
instability of the West Antarctic Ice Sheet. In this study ice sheet bed
elevation data were used to locate individual conical edifices that
represent subglacial volcanoes. In order to support this interpretation
aeromagnetic, aerogravity, satellite imagery and databases of confirmed
volcanoes were used to support this interpretation. The overall result
that is presented in this paper constitutes a first inventory of
subglacial volcanoes in west Antarctica. There are 138 volcanoes that
were identified in this study, 91 of which have not previously been
identified, and which are distributed widely throughout the deep basins
of west Antarctica, though they are especially concentrated, and are
oriented along, the more than 3,000 km central axis of the West
Antarctic Rift System.
One of the most extensive regions of stretched continental crust on
Earth is present in West Antarctica, which in dimensions and settings is
comparable to the East African Rift System and the Basin and Range
Province in the US (e.g. Behrendt et
al., 1991; Dalziel, 2006;
Kalberg et al., 2015).
According to van Wyk de Vries et
al. it is important to obtain improved knowledge of the geological
structure of the region as it provides the template over which the West
Antarctic Ice Sheet has waxed and waned over multiple glaciations (Naish
et al., 2009; Pollard &
DeConto, 2009; Jamieson et al.,
2010), and a first order control on the spatial configuration of the ice
dynamics of the WAIS (Studinger et
al., 2000; Jordan et
al., 2010; Bingham et
al., 2012). At the present an
extensive, complex network of rifts characterise the subglacial region,
and this is likely to have initiated at various times since the
Cainozoic (Fitzgerald, 2002; Dalziel, 2006; Siddoway, 2008; Spiegel et
al., 2016), and which may
still be active in some locations (Behrendt et
al., 1998; LeMasurier, 2008;
Lough et al., 2013; Schroeder
et al., 2014). This series of
rifts beneath the WAIS has collectively been termed the West Antarctic
Rift System (WARS), and the Transantarctic Mountains bounds it to the
south.
Rift interiors with thin, stretched crust are associated, in other major
rift systems of the world, with considerable volcanism (e.g. Siebert &
Simkin, 2002). In West Antarctica, however, there have been only a few
studies that have identified subglacial volcanoes and/or volcanic
activity (e.g. Blankenship et al.,
1993; Behrendt et al., 1998,
2002; Corr & Vaughan, 2008; Lough et
al., 2013), with
comprehensive identification of the full spread of the volcanoes
throughout the WARS being deterred by the ice cover. It is important to
improve on this limited impression of this distribution of volcanoes in
the WARS for several reasons:
1)
The characterisation of the geographical spread of volcanic activity
across the WARS can complement wider efforts to understand the main
controls of rift volcanism throughout the world (Ellis & King, 1991;
Ebinger et al., 2010).
2)
Volcanic edifices contribute to the macroscale roughness of the ice
sheet beds, by their formation of “protuberances” at the subglacial
interface, and this roughness in turn forms a first-order influence on
the flow of ice (cf. Bingham & Siegert, 2009).
3)
Geothermal heat flow and, therefore, basal melting, also potentially
impacts on the dynamics of the ice (Blankenship et
al., 1993; Vogel, 2006).
4)
It has been argued that the recovery of palaeoenvironmental information
from the glaciations of the Quaternary, such as the thickness of the
palaeoice and thermal regime, can be achieved by using the subglacial
volcanic sequences.
This study presents a new assessment on a regional scale of the
locations of volcanoes that are likely to be in West Antarctica, based
on morphometric (or shape) analysis of the ice bed topography of West
Antarctica. The shape of a volcano depends on 3 principal factors:
1)
The composition of the magma that is erupted;
2)
The environment into which the magma has been erupted; and
3)
The erosional regime to which the volcano has been subjected since
eruption (Hickson, 2000; Grosse et
al., 2014; Pedersen & Grosse,
2104).
The composition of magma in large continental rifts generally has a low
to medium silica content with some more alkaline eruptions (Ebinger et
al., 2013). Most knowledge of
volcanoes is derived from subaerial outcrops in Marie Byrd Land, West
Antarctica, a region where volcanoes are composed of intermediate
alkaline lavas that were erupted onto a basaltic shield, and there are
smaller instances in which volcanoes are composed entirely of basalt and
a few more evolved compositions (trachyte, rhyolite; LeMasurier et
al., 2013).
Therefore, vans Wyk de Vries et al. consider that many structures
in the WARS are also basaltic. With regards to the environment of the
eruption, it is typical for subaerial basaltic eruptions to produce
cones that are broad shield types that protrude upwards from the
surrounding landscape (Grosse et
al., 2014). Monogenetic volcanoes often form steep structures that
have a flat top that are comprised of phreatomagmatic deposits draped on
pillow lava cores which are overlain by lava-fed deltas, termed tuyas,
under subglacial conditions (Hickson, 2000; Pedersen & Grosse, 2014;
Smellie & Edwards, 2016). A range of morphometries that reflect the
multiple events responsible for the formation of large, polygenetic
volcanic structures, though many have overall ‘conical’ structures that
are similar to conical structures or shield volcanoes (Grosse et
al, 2014; Smellie & Edwards,
2016).
The macrogeomorphology in the WARS is often dominated by elongated
landforms that result from geologic rifting and subglacial erosion. It
was proposed by van Wyk de Vries et
al.in this paper that the
most reasonable explanation for the presence of any ‘cones’ in this
setting is that they must be of volcanic origin. They defined ‘cones’ as
any features with a low length/width ratio when viewed from above;
therefore, for this study, they included cones that have very low slope
angles. They therefore view cones in this subglacial landscape as being
diagnostic of the presence of volcanoes. They also note that
identification of cones alone will not identify all volcanism in the
WARS. E.g., a likely feature of rift volcanism, such as eruptions of
volcanic fissures, will produce ridge forms, or ‘tindars’ (Smellie &
Edwards, 2016), rather than cones. Also, in the wet basal environment of
the WAIS, it is more likely that the older the cones the more likely it
will have lost its conical form as a result of subglacial erosion.
Cones, Therefore, that are present today are likely to be relatively
young – though van Wyk de Vries et
al. cannot use their method
to distinguish whether or not the features are volcanically active.
Morphometry as a tool for identifying subglacial volcanoes
van Wyk de Vries et al
consider in this paper implications that have arisen from their
findings.
1)
The approach of van Wyk de Vries et
al. demonstrates that it is
possible to use morphometry on the Digital Elevation Model (DEM),
crucially, together with relevant auxiliary information, in order to
identify potential subglacial volcanic edifices beneath West Antarctica.
2)
It highlights that subglacial West Antarctica, in essence, WARS – is
comprised on one of the largest volcanic provinces in the world (cf.
LeMasurier et al., 1990;
Smellie & Edwards, 2016), and it provides the basic metrics concerning
the locations and dimensions of the main volcanic zones.
3)
It highlights the wide spread of subglacial volcanism beneath the WAIS,
which may have an impact on how the WAIS responds to external forcing by
way of the coupling of the ice to its bed, and it may have implications
for volcanic activity in the future as the ice cover thins.
According to van Wyk de Vries et
al. as far as they know this study is the first to use morphometry
for the identification of volcanic edifices on the continental scale
beneath the Antarctic Ice Sheet. The extent of this volcanism has only
been inferred by geophysical studies that were carried out previously
(Behrendt et al., 2002).
Morphometry has been widely used elsewhere in volcanology: E.g., to
catalogue volcanic parameters, such as:
i)
height, base width and crater width (e.g. McKnight & Williams, 1977;
Pedersen & Grosse, 2014), or
ii)
to reconstruct volcanic edifices that had been eroded (Favalli et
al., 2014);
iii)
to resolve volcanic characteristics in subaerial, submarine settings
(e.g. Stretch et al., 2006)
and
iv)
Extraterrestrial (e.g. Broz et al., 2015) settings.
In all such cases, however, volcanic morphometry has been applied to
DEMs that had been assembled from elevation measurements, which were
distributed evenly, derived from sensors that were viewing surfaces that
were unobscured. In the case of subglacial Antarctica, having confidence
that the subglacial DEM, which has been constructed from elevation
measurements that are non-random, is of sufficient resolution for the
interpretation is key. Increasing glacial recovery of subglaciological
information from morphometry has occurred over recent years. Seeding
centres for glaciation of the WAIS (Ross et
al., 2014) and the East
Antarctic Ice Sheet (Bo et al.,
2009; Rose et al., 2013) have
been identified by the preponderance of sharp peaks, features that are
cirque-like and valleys that are closely spaced relative to other parts
of the subglacial landscape. In other places, landscapes of ‘selective
linear erosion’, which are diagnostic of former dynamism in regions of
ice that are now stable, have been detected from the presence of
significant linear incisions (troughs) into higher surfaces that are
otherwise flat (plateaux) (Young et
al., 2011; Jamieson et
al., 2014; Rose et
al., 2014). A feature that
all of these studies have in common is that they have auxiliary evidence
to the morphometry that is closely considered and, therefore, have not
relied only on the shape of the surface in coming to interpretations
concerning formation of the landscape. It was shown by this study that
such a combined approach is also valid for the locating and mapping of
many volcanic edifices that were previously unknown across the
ice-shrouded WARs.
Subglacial volcanism – extent and activity
At least 138 likely volcanic edifices that were distributed throughout
the WARS were identified by this study. A significant advance is
represented by this on a total of 47 volcanoes that had been identified
across the whole of West Antarctica, most of which are visible at the
surface and are situated in Marie Byrd Land and the Transantarctic
Mountains (LeMasurier et al.,
1990). It was noted by van Wyk de Vries et
al. that the wide
distribution of volcanic structures throughout the WARS, as well as the
presence of clusters of volcanism that is concentrated within the Marie
Byrd Land dome, shows a remarkable similarity to that of the East
African Rift System, which is also more than 2,000 km long and is
flanked by the Ethiopian and Kenyan domes (Siebert & Simkin, 2002;
Ebinger, 2005). Morphologically, the volcanoes have characteristics of
height-volume and basal diameters that match closely those of the rift
volcanoes around the world. According to van Wyk de Vries et
al., the total region that
has undergone volcanism is likely to be considerably larger than that
which has been identified by this study, keeping in mind that beneath
the Ross Ice shelf the paucity of data precludes meaningful analysis of
a significant terrain that is also considered to be part of the WARS.
There has been a longstanding debate concerning the activity of the
WARS, with some advocating a rift that is largely inactive (LeMasurier,
2008) while others suggest volcanism on a large scale (Behrendt et
al., 2002). The arguments
favouring an inactive rift are based on the elevation that is
anomalously low, of the WARS compared to other continental rifts
(Winberry & Anandakrishnan, 2004; LeMasurier, 2008) and the relative
absence of pebbles of basalt that were recovered from boreholes
(LeMasurier pers. Comm., 2015). High regional heat fluxes, conversely,
(Shapiro & Ritzwoller, 2004; Schroeder et
al., 2014), geomagnetic
anomalies (Behrendt et al.,
2002) and evidence of recent subglacial volcanism (Blankenship et
al., 1993; Corr & Vaughn,
2008) suggest that the rift is currently active. Evidence of a large
number of subglacial volcanoes has been provided by this study, which
has their quasi-conical shield type geometries still intact. The nature
of the cones that are largely uneroded suggest that many may date to the
Pleistocene or younger, which supports that argument that the rift is
still active at present.
The results of this study do not allow a determination of whether the
different volcanoes are active or not; however, the identification of
multiple new volcanic edifices, as well as the improved sense of their
spread and concentration across the WARS, may guide future investigation
of their activity. The Marie Byrd Land massif has been studied several
times previously, the findings of which suggested that massif is
supported by mantle that is of particularly low density, possibly
comprising a volcanic ‘hotspot’ (Hole & LeMasurier, 1994; Winberry &
Anandakrishnan, 2004). Tephra layers that have been recovered from Byrd
Ice Core near the WAIS divide suggest that multiple volcanoes from Marie
Byrd Land were active in the late Quaternary (Wilch et
al., 1999), and there is
seismic activity in Marie Byrd
Land that has been interpreted as volcanism that is currently active
(Lough et al., 2013). Strong
radar-sounded englacial reflectors in the catchment of the Pine Island
Glacier have been interpreted as evidence of a volcanic eruption that
occurred about 2,000-2,400 years ago (see Corr & Vaughn, 2008) while Mt
Erebus on the opposite rift flank in the transarctic Mountains comprises
a volcano that is known to be active that is located above another
potential volcanic hotspot (Gupta et
al., 2009). Across the region
volcanism is also likely to contribute the elevated heat fluxes that
have been inferred to underlie much of the WAIS (Shapiro & Ritzwoller,
2004; Fox Maule et al., 2005;
Schroder et al., 2014). In
order to recover the structure of the mantle beneath the WAIS the
deployment of broadband seismics is now showing great promise (e.g.
Heezel et al., 2016), and the
map of van Wyk de Vries et al.
of potential locations of volcanoes could help target further
installations directed towards improving monitoring the subglacial
volcanic activity of the
continent.
Stability and future volcanism – implications
Potential influences on the stability of the WAIS is provided by the
wide spread of volcanic edifices and the potential of extensive
volcanism throughout the WARS. Basins that descend from sea level with
distance inland, that underlie many parts of the WAIS, lend the ice
sheet a geometry that is prone to runaway retreat (Bamber et
al., 209; Alley et
al., 2015). The likelihood
that extensive retreat occurred in the WAIS during the glacial minima of
the Quaternary (Naish et al.,
2009) and concurrently contributed to several metres of global sea level
rise (O’Leary et al., 2013).
The WAIS may be currently undergoing another such wholesale retreat, as
ice in the sector facing the Pacific Ocean has been retreating
consistently from the time of the earliest aerial and satellite
observations (Rignot, 2002; McMillan et
al., 2014; Mouginot et
al., 2014). van Wyk de Vries
et al. do not consider it
likely that volcanism has had a significant role in the triggering of
the current retreat, for which there is compelling evidence that the
forcing has initiated from the margins (Turner et
al., 2017), but they do not
propose that subglacial volcanism has the potential to influence future
rates of retreat by;
1)
Producing enhanced basal melting that could impact of the basal motion
of the ice and
2)
Providing edifices that may act to pin retreat.
Some authors have suggested, with regard to the first of these
possibilities, that the active subglacial volcanism could play a role in
instability of the WAIS (Blankenship et
al., 1993; Vogel et
al., 2006; Corr & Vaughn,
2008). Subglacial volcanism in Iceland has provided an analogy, where
subglacial eruptions have been known to melt basal ice, which floods the
basal interface and induces periods of enhanced ice flow (e.g. Magnússon
et al., 2017; Einarsson et
al., 2016); however, in the
ice caps of Iceland the ice is considerably thinner than it is in the
WAIS and, therefore, more prone to subglacial-induced uplift. There is,
nevertheless, evidence suggesting changes to subglacial distribution of
water can occur beneath the WAIS, and that they can sometimes have
profound impacts on the dynamics of the ice; examples are ice-dynamic
variability over subglacial lakes (e.g. Siegfried et
al., 2016) or the suggestion
that subglacial pulses of water may have been responsible for historical
occurrences of piracy of ice streams (e.g. Anandakrishnan & Alley, 1997;
Vaughn et al., 2008). There
has been much recent attention on the drainage of subglacial lakes
comprising plausible triggers for such dynamic changes, though
subglacial eruption may represent another source of pulsed water where
the occurrence is seen only rarely, if ever, has been factored into
models of ice sheets. There is a potential for inactive or dormant
volcanism to influence ice flow by increasing heat flux to the
subglacial interface; a basal melt cavity may be produced by this and
the flow of ice enhanced (Bourgeois et
al., 2000; Schroder et
al., 2014).
Volcanic edifices, on the other hand, stand as significant
protuberances, whether active of not, which may act geometrically as
stabilising influences on the retreat of the ice. It is shown by
numerical models that are used to project potential rates of WAIS
retreat, once initiated, as long as the ice bed is smooth and downslopes
inland, retreat will continue unabated, but retreat will be delayed or
stopped by any increase in roughness or obstacle in the bed (Ritz et
al., 2015; Nias et
al., 2016). van Wyk de Vries
et al. identified in this
study a number of volcanic edifices situated in the deep basins within
the WAIS; these edifices which are likely to
result from volcanism, could
represent some of the most influential pinning points for retreating
ice, past and future.
In the future, the thinning and potential removal of ice cover from the
WARS volcanic province could have profound impacts for volcanic activity
across the region in the future. It has been shown by research in
Iceland that when the ice cover thins the production of magma increased
at depth in response to decompression of the underlying mantle (Jull &
McKenzie, 1996; Schmidt et al.,
2013). Also, evidence has been found that, worldwide, volcanism is most
frequent in deglaciating regions as the overburden pressure of the ice
is first reduced and then removed (Huybers & Langmuir, 2009; Praetorius
et al., 2016). Therefore,
significant potential to increase partial melting and eruption rates
throughout the rifted terrain is offered by unloading of the WAIS from
the WARS. The concentration of volcanic edifices along the WARS could be
construed as evidence that such enhanced volcanic activity was a feature
of Quaternary minima. The possibility is raised by this that a thinning
of the ice cover and glacial unloading over the WARS in the future may
increase and this, in turn, may lead to an enhanced production of water
and contribute to further potential ice-dynamical instability.
Conclusions
If morphometric analysis is applied to a digital elevation model of the
West Antarctic Rift System, and assessment of the results with respect
to auxiliary information from expressions of ice surface to
aerogeophysical data, van Wyk de Vries et
al. have identified 138
subglacial volcanic edifices that are spread throughout the rift. In the
broad rift zone the volcanoes are distributed widely, with particular
concentrations in Marie Byrd Land and along the axis of the central
WARS. It is demonstrated by the results that one of the world’s largest
volcanic provinces, which is on a similar scale to the East African Rift
System, is shrouded by the West Antarctic Ice Sheet. Beneath West
Antarctica the overall density of volcanoes was found to be 1 edifice
per 18,500 ± 500 km2, with a central belt along the central
sinuous ridge that contains 1 edifice per 7,800 ± 400 km2.
The presence of such a volcanic belt that traverses the deep marine
basins beneath the centre of the WAIS could prove to be a major
influence of behaviour in the past and the stability of the ice sheet in
the future.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |