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Australia: The Land Where Time Began |
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Large Igneous Provinces
–
explosive Basaltic Volcanism One of the key concepts that have emerged from
scientific studies of Large Igneous Provinces over the last 25 years is
the importance of volcaniclastic deposits and the implications for
palaeoenvironmental reconstructions, eruption dynamics and impact on
climate. It was recognised and highlighted (Ross et
al., 2005) that the
occurrence of mafic volcaniclastic deposits is near-ubiquitous, forming
an integral component of LIPs. Contained in these deposits, is
information, some unique, on the mechanisms of primary fragmentation,
eruptive processes, and depositional environments. A record of what is
now recognised as complex temporal and spatial volcanic heterogeneity in
LIPs, allowing the reconstruction of their tectonic and physical
evolution as a complementary story that is equally significant, to that
of the geochemical evolution of magmatism. Mafic
volcanic-derived clastic deposits A wide variety of mechanisms spanning the full
range of volcanic and sedimentary processes can generate clastic
deposits composed of mafic volcanic particles in any proportion ranging
from partly to entirely, resulting in textures and morphologies that are
also variable. There are 3 genetic categories of clastic deposits that
are mafic volcanic-derived, that is based on the mechanism of formation,
Primary and
reworked volcaniclastic deposits,
and epiclastic deposits
(White & Houghton, 2006). Primary
volcaniclastic deposits According to Ukstins Peate & Elkins-Tanton1
primary volcaniclastic deposits are formed from fragmented material that
is deposited as a direct result of explosive or effusive eruptions.
There are 4 types of deposits: (White & Houghton, 2006; White et
al., 2009). The main factors
that control the formation of magma eruption rates are concentration of
magmatic volatiles, presence and relative abundance of external water,
either freestanding waterbodies or in sediments that are saturated.
Compared to other processes of sedimentation depending exclusively to
gravity, mobilisation of particles that are magma-generated is unique,
because transport energy can be acquired by primary volcanic particles
from their source, e.g. explosive expansion, flow velocity of lava, and
may be initially independent of slope or depositional base level (Fisher
& Smith, 1991). Autoclastic
Deposits – produced by auto brecciation, and generated as effusive
lavas that exceed viscosity-strain rate threshold and fragment (Peterson
& Tilling, 1980); groundmass crystallisation and crust disruption are
promoted by rapid cooling (Cashman et
al., 1999). Brecciated upper
and lower surfaces characterise the morphology of a’a lava flow; slabby
or rubbly pahoehoe have upper crusts that are broken or brecciated and
are transitional between pahoehoe and a’a (e.g. Guilbaud et
al., 2005). Pyroclastic
Deposits – are formed from explosive volcanic eruption plumes and
jets or pyroclastic density currents, and they can be generated by
magmatic or
phreatomagmatic mechanisms, or a complicated interaction between
both (e.g. Graettinger et al.,
2013). A minor mechanism for generating mafic volcaniclastic deposits in
Large Igneous Provinces (LIPS), documented examples of which are rare,
is represented by magmatic fragmentation. The vent sites of the Columbia
River Basalts in the Roza Member have scoria fall deposits of densely
agglutinated and welded spatter, which are highly vesicular (Brown et
al., 2014). Large-scale phreatomagmatic volcanism results from
the involvement of water, either from aquifers or surface water, in
mafic eruptions which are capable of generating deposits of up to 102
to 105 km3 (Ross et
al., 2005). The full spectrum
of products from the interaction of magma and water is represented by
Phreatomagmatic pyroclastic and hyaloclastic deposits, as well as
peperite (Wohletz, 2002). For formation, external water is integral,
though magmatic fluids are not precluded, and may also play a role in
fragmentation in the case of phreatomagmatic eruptions. During
hydromagmatism clast-forming processes include 4 primary mechanisms:
magmatic explosivity, steam explosivity, cooling-contraction
granulation, and dynamic stressing. All of which are dependent on the
ratio of magma to water (Wohletz, 1983; Kokelaar, 1986). According to
Ukstins Peate & Elkins-Tanton1 the optimal fuel (magma) to
coolant (water or sediment-laden water) mixture to generate explosivity
(magma to pure water mass ratio of ~ 0.33: White. 1996) produce
phreatomagmatic pyroclastic deposits (White, 1996), whereas water
volumetrically dominates hyaloclastic deposits and wet sediment (wet
sediment to magma mass ratios > 1: dominates peperite (Wohletz, 2002)
Hyaloclastic Deposits; these are generated solely by quench
fragmentation during the reaction between water and magma, and are
produced by the contact between effusive magma and abundant water in
either marine or continental settings. The most typical products of
mafic magma and spalling in a subaqueous environment are pillow lavas,
pillow-palagonite breccias, and hyaloclastites. Peperite
Deposits These deposits are the result of the interaction with
unconsolidated, water-bearing clastic deposits in shallow water
intrusions, subaqueous or surface environments (White et
al., 2000; Skilling et
al., 2002). Ukstins Peate &
Elkins-Tanton1 say that it is suggested by experimental and
theoretical studies that mechanisms of interactions between water and
magma and magma-sediment-water may be similar (Kokelaar, 1986), and
properties rely on fluidisation and vigorous injection
and mixing of water-saturated sediments and lava (Kokelaar,
1982). The contemporaneity of sediments and volcanism is indicated by
the recognition of peperite (Busby-Spera and White, 1987). There are
relatively few documented examples of LIPs, given the ubiquity of
environments that could generate peperite, though this may be a function
of identification and not their actual absence. The Paraná-Etendeka LIP
was emplaced into a predominantly arid desert, and even in such an
environment peperites were formed where pahoehoe lava flows interacted
with wet lacustrine silts and clays, which had collected in low-lying
topography of surfaces of lava flows (Waichel et
al., 2007). Reworked
volcaniclastic and epiclastic deposits Particles sourced from primary volcaniclastic
deposits that have been redeposited by surface processes (wind, water,
ice, gravity) concurrent with eruption or after being immobile for a
period of time, comprise the reworked volcaniclastic deposits. In
reworked volcaniclastic deposits the volcanic processes that generated
the particles are different from those that transport the particles to
their final site of deposition. Epiclastic (or volcanogenic) sediments
are formed from the weathering and erosion of volcanic rocks, including
rocks that were previously lithified volcanoclastic. Lithification can
be part of volcanic emplacement (e.g. welding) or as a secondary process
of cementation or compaction. Mafic
volcaniclastic deposits – spatial and temporal occurrence The record by volcaniclastic deposits of
tectono-volcanic facies and the evolution over time of the architecture
of provinces is one of their strengths. Ukstins Peate & Elkins-Tanton1
say the recognition of pre-volcanic doming on a kilometre scale is not
an unequivocal feature of LIPs, coupled with recent numerical modelling
that indicates that substantial and complex patterns of pre- and syn-volcanic
subsidence and/or uplift (+/- hundreds to thousands of metres) can be
generated by LIP emplacement. It has been suggested that tectonic
evolution may be a significant factor that controls the broad scale
distribution of these deposits (e.g. Czamanske et al., 1998; Ukstins
Peate & Elkins-Tanton, 2009; Ukstins Peate & Elkins-Tanton, 2010;
Sobolev et al., 2011).
Provinces containing significant volcaniclastics include the Kirkpatrick
section, from the Middle Jurassic, of the Ferrar flood basalts in
Antarctica, with tuff-breccias up to 400 m thick (Elliot & Flemming,
2008; the Kachchh region in the northwest of the Deccan Flood Basalts,
with lapilli and lithic blocks (Kshirsagar et
al., 2011); and the Karoo
(McClintock et al., 2008).
East Greenland
– North Atlantic Igneous Province
Emeishan Large
Igneous Province Evidence of
volatile loads, temperatures and height of plumes If the chemicals released by volcanism reach the
stratosphere they will have the greatest effect on the global climate,
both in terms of destructive chemical reactions and longevity, though
they are rapidly removed from the troposphere by rain. There is
generally a lower gas content basaltic magmas, possibly with the
exception of sulphur, and they are less viscous than eruptions that are
more silicic, and they are generally less explosive without interactions
with external volatiles, though basaltic Hawaiian style fire fountains
are capable of injecting material into the stratosphere (Stothers et
al., 1986; Woods, 1993). The
Laki eruption (Iceland 1783-1784, Thordarson et
al., 1996) confirmed this,
Laki being largely effusive, but it had significant episodes of
fire-fountaining. An eruption column of more than 9 km high was
described in witness accounts (Thordarson & Self, 2003), and the
detection of chemicals from Laki in Greenland ice cores confirms that
material reached the stratosphere (Fiacco et
al., 1994; Wei et
al., 2008). However, basaltic eruptions that produce
volcaniclastic deposits may have more capacity to inject material into
the stratosphere, particularly at northern latitudes, where the
tropopause is at a lower altitude (~9 km near the poles compared to 17
km at the Equator, though sensitive to a wide variety of external
parameters (Thuburn & Craig, 1997; see also Ross et
al., 2008)). It is common for
basaltic volcaniclastic deposits to be Phreatomagmatic, and a range of
effects can result from the incorporation of water into an eruptive
plume. If the water vaporises the addition of steam to the plume
decreases the density of the plume through the consumption of latent
heat. A higher plume with a lower eruptive velocity may develop as a
result of reduced density. If the added water cannot vaporise the eruptive
fountain will collapse and flow laterally (Koyaguchi & Woods, 1996).
Phreatomagmatic basaltic eruptions may transition between these states
as they proceed. It has been suggested (Koyaguchi & Woods, 1996) that
both the fountains and wet lateral flows can form lapilli, and they
found that the initial eruptive velocity of 100 m/s, a temperature of
1,000 K, and a volatile content of 3 wt% water, a plume may reach up to
35 km, by which time its temperature would be lower than 300 K. (Walker
et al., 1984) in agreement,
has estimated a vent exit velocity of 259-300 m/s for the basaltic
plinian Tarawera eruption of 1886, which generated a column of ash that
rose to ~30 km. The broad thermal perturbation of a flood basalt
province may produce its own weather pattern of atmospheric thermals,
though individual eruptions may not always reach these conditions. This
concept was first investigated (Emanuel et
al. 1995) in which they
proposed that very large bolide strikes and volcanic eruptions on a
large scale could produce storms that were exceptionally violent termed
hypercanes which were capable
of injecting large amounts of mass into the stratosphere. It has been
described in work that is more recent (Kaminski et
al., 2011) which described
penetrative convection above large lava flows, where large upwellings
past the tropopause are driven by broad temperature perturbations. Summary:
potential for a changing climate A significant fraction of LIP eruptive volume is
comprised of mafic volcaniclastics, including the Siberian, Emeishan,
North Atlantic, Karoo, Ferrar, and Columbia River flood basalts. The
relative impacts of volcanism and tectonism on a region can be
determined by the type and distribution of volcaniclastics. It is now recognised that globally volcaniclastic
eruptions have the potential for climate- changing atmospheric effects.
These eruptions can inject material into the stratosphere, either from
the eruptive plume alone or with the help of regional weather effects
that are produced by the LIP itself. Some effects of these eruptions in Siberia have
been studied which have demonstrated that significant sulphur, chlorine
and fluorine from the fluids in their eruptive-driving aquifer were
carried by Siberian volcaniclastics (Black et
al., 2012). It was also found
that halocarbons sufficient to destroy the ozone are naturally produced
by these eruptions Black et al.,
2013) through reactions taking place in the bedrock and the eruptive
plume (Svensen et al., 2009;
Black et al., 2014). The major missing link between flood basalts and
extinctions may be the volcaniclastics in flood basalts. Ukstins Peate &
Elkins-Tanton1 say the underlying cause may be volatiles,
that are climate changing, are sourced from continental crustal rocks
that chamber the flood basalt magmas, which are missing in ocean basins.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |