Australia: The Land Where Time Began
Large Igneous Provinces – explosive Basaltic VolcanismLarge Igneous Provinces (LIPs) are known from the Precambrian at 3.79 Ga (Ernst, 2013), extending trough examples that are well-preserved from the Mesozoic and Cainozoic (Ross et al., 2005; Bryan & Ferrari, 2013; and references therein). LIPs were originally inferred to consist of massive and effusive basaltic lava flows that were laterally continuous in a layer cake sequence. It has now been shown by detailed volcanostratigraphy, which has generated a more nuanced view of the architecture, which highlighted the fact that in many provinces there is included a significant component of clastic material derived from mechanisms of volcanic and sedimentary formation. In contrast, some of the volumetrically largest basaltic volcanoclastic deposits have been found to apparently be associated with Large Igneous Provinces (Ross et al., 2005).
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).
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.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|