September 2010 LIP of the Month

Large Igneous Provinces: Sites Of The Largest Volcanic Eruptions In Earth’s History*

*This is based on the paper: “The Largest Volcanic Eruptions on Earth” to be published in Earth Science Reviews 102 (2010) 207–229; doi:10.1016/j.earscirev.2010.07.001

Scott Bryan¹, Ingrid Ukstins Peate², Stephen Self³, David Peate², Dougal Jerram⁴, Mike Mawby⁴, Goonie Marsh⁵, Jodie Miller⁶

1. Biogeoscience, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001 Australia. Email: scott.bryan@qut.edu.au
2. Department of Geoscience, 121 Trowbridge Hall, University of Iowa, Iowa City, IA 52242, USA
3. Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA United Kingdom
4. Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE United Kingdom
5. Department of Geology, Rhodes University, PO Box 94, Grahamstown 6140 South Africa
6. Department of Geology, Stellenbosch University, Private Bag X1, Matieland, Western Cape 7602 South Africa

Summary

Large igneous provinces (LIPs) are sites of the most frequently recurring, largest volume basaltic and silicic eruptions in Earth history. The magma volumes, eruptive mechanisms, frequency and associated aerosol emissions of these eruptions are critical for understanding any interpreted climate forcing and environmental change by LIPs. The largest volume (>1000 km³ dense rock equivalent) and magnitude (>M8) eruptions produce areally extensive (10⁴-10⁵ km²) basaltic lava flow fields and silicic ignimbrites and are the main building blocks of LIPs. Available information on the largest eruptive units are primarily from the Columbia River and Deccan provinces for the dimensions of flood basalt eruptions, and the Paraná-Etendeka and Afro-Arabian provinces for the silicic ignimbrite eruptions. In addition, three large-volume (675-2,000 km³) silicic lava flows have also been mapped out in the Mesoproterozoic Gawler Range province (Australia), an interpreted LIP remnant. Magma volumes of >1000 km³ have also been emplaced as high-level basaltic and rhyolitic sills in LIPs, and may contribute substantial aerosol emissions through shallow degassing and crystallisation. The data sets indicate comparable eruption magnitudes between the basaltic and silicic eruptions, but due to considerable volumes residing as co-ignimbrite ash deposits, the current volume constraints for the silicic ignimbrite eruptions may be considerably underestimated. Magma composition thus appears to be no barrier to the volume of magma emitted during an individual eruption. Despite this general similarity in magnitude, flood basaltic and silicic eruptions are very different in terms of eruption style, duration, intensity, vent configuration, and emplacement style. Flood basaltic eruptions are dominantly effusive and Hawaiian-Strombolian in style, with magma discharge rates of ~10⁷-10⁸ kg s^-1 producing dominantly compound pahoehoe lava flow fields. The major flood basalt eruption durations are most likely >10 yrs. Effusive and fissural eruptions have also emplaced some large-volume silicic lavas, but discharge rates are unknown, and may be up to an order of magnitude greater than those of flood basalt lava eruptions for emplacement to be on realistic time scales (<10 years). Most silicic eruptions, however, are moderately to highly explosive, producing co-current pyroclastic fountains (rarely Plinian) with discharge rates of 10⁹-10¹¹ kg s^-1 that emplace welded to rheomorphic ignimbrites and co-ignimbrite ash deposits. Ash and aerosol injections into the stratosphere may be greater from co-ignimbrite ash clouds than eruption plumes. At present, durations for large-magnitude silicic eruptions are unconstrained; at discharge rates of 10⁹ kg s^-1, equivalent to the peak of the 1991 Mt Pinatubo eruption, the largest silicic eruptions would take many months to evacuate ~5000 km³ of magma. The generally simple deposit structure is more suggestive of short-duration (hours to days) and high intensity (~10¹¹ kg s^-1) eruptions. These extreme discharge rates would be facilitated by multiple point, fissure and/or ring fracture venting of magma. Eruption frequencies are much elevated for large-magnitude eruptions of both magma types during LIP-forming episodes. However, in basalt-dominated provinces (continental and ocean basin flood basalt provinces, oceanic plateaus, volcanic rifted margins), large magnitude (>M8) basaltic eruptions have much shorter recurrence intervals of 10³-10⁴ years, whereas similar magnitude silicic eruptions have recurrence intervals of up to 10⁵ years. The huge volumes of basaltic and silicic magma erupted in quick succession during LIP events raises several unresolved issues in terms of locus of magma generation and storage (if any) in the crust prior to eruption, the paths and rates of ascent from magma reservoirs to the surface, and relative aerosol contributions from the flood basaltic and rhyolitic eruptions.

Introduction

It is the volume of magma emitted during individual LIP eruptions, the frequency of such large-volume eruptions, and the total volume of magma intruded and released during the main igneous pulses that make LIP events so exceptional in Earth history, and called upon to explain environmental and climatic changes and mass extinctions (e.g., Rampino & Stothers, 1998; Courtillot, 1999; Courtillot & Renne, 2003; Wignall, 2001; 2005; Self et al., 2005; Kelley, 2007). Despite the total cumulative erupted volumes and timing of LIP events being reasonably well-constrained (Coffin & Eldholm, 1994; Bryan & Ernst, 2008), our current understanding of the size, duration and frequency of individual LIP eruptions is very limited. Considerable focus has been on flood basalt eruptions in the continental flood basalt provinces, which are the best exposed and studied examples of LIPs. Almost all information on the size of individual flood basaltic eruptions comes from the many studies undertaken on the Columbia River Flood Basalt Province, which is the smallest (~0.234 Mkm³) and youngest example of a continental flood basalt province (e.g., Swanson et al., 1975; Reidel et al., 1989; Tolan et al., 1989; Self et al., 1997; Camp et al., 2003; Hooper et al., 2007). It is only recently that some understanding has been made on the magnitude of flood basalt eruptions from other flood basalt provinces (Deccan: Jay & Widdowson, 2006; Self et al., 2008; Chenet et al., 2009; NAIP: Single & Jerram, 2004).

By contrast, similarly large-volume silicic volcanic eruptions are known from a number of tectonic regimes, but which are exclusively continental in crustal setting. Extension of active continental margins, whether in narrow, rifted arc or back-arc settings (e.g., Taupo Volcanic Zone) or broader extensional belts (e.g., Basin & Range Province, western USA) and intraplate to rifted continental environments (e.g., Afro-Arabian province) have been the most productive settings for large-volume (>1,000 km³) silicic eruptions since the middle Tertiary (Mason et al., 2004). Consequently, unlike flood basalt eruptions, large-volume silicic eruptions are not exclusive to LIPs and not restricted to discrete eruptive episodes such as LIP events throughout Earth history (Thordarson et al., 2009). The presently determined average recurrence rate of one silicic eruption of Magnitude 8 or greater (Pyle, 1995; 2000) every 100,000-200,000 years (Self, 2006) reflects the contribution from sources in a variety of tectonic settings. This relatively higher frequency for large magnitude silicic eruptions means that they pose a greater hazard to human civilization than flood basaltic eruptions (Thordarson et al., 2009). What is distinctive regarding large-volume silicic eruptions from LIPs is their association with large-magnitude basaltic eruptions, their enhanced frequency and the cumulative volume of silicic magma emplaced (up to 10 Mkm³) when compared to other tectonic settings (Bryan et al., 2002; Mason et al., 2004; Bryan & Ernst, 2008).

Super-eruptions have recently been defined as those yielding more than 1x10¹⁵ kg of magma (Sparks et al., 2005; Self, 2006). For rhyolitic eruptions, this is equivalent to ~410 km³ (at a magma density of 2,450 kg m^-3). However, super-eruption has not yet been strictly applied to basaltic eruptions, where only 360 km³ of erupted magma is required, given their higher magma density of ~2,750 kg m^-3. Here we present a compilation of known eruption volumes for the very largest (>Magnitude 8.5, equivalent to ~1,160 km³ of basalt lava, or ~1,280 km³ of dense silicic lava or ignimbrite) basaltic and rhyolitic eruptions from LIPs. This is to complement recent compilations for example, on the largest Tertiary-Quaternary silicic explosive eruptions (Mason et al., 2004), and provide a basis for improved long-term eruption rate estimates (e.g., White et al., 2006), which are currently based on sparse data from LIPs. Consequently, our understanding of what are the largest eruptions is limited and biased to late Tertiary and Recent volcanic activity. In these recent compilations, only one silicic eruption of magnitude 9 (1x10¹⁶ kg, or ~5,000 km³ of dense magma) has been recognized, which occurred 28 Ma (Mason et al., 2004). Issues investigated with the data set presented here are: What are the largest eruptions? What are the physical limits that may exist for eruption magnitude and whether magma composition imposes any limitation? How do basaltic and silicic “super-eruptions” differ in terms of eruption mechanisms, rates, durations and frequencies during LIP-forming events? And what implications do these issues have for the generation and storage of such prodigious magma volumes?

Determining the Products of Single Eruptions

Determining what deposits constitute the products of an individual eruption and assessing erupted volumes are not straightforward in LIPs given exposure problems (e.g., concealment, burial or uplift and erosion), the potential great extent of (often thin) eruptive units (10⁴-10⁶ km²), tectonic deformation and fragmentation, and subtle lithologic or geochemical distinction. As a consequence of these factors, each LIP will have a different prospectivity potential for delineating the products of large-magnitude eruptions (Table 1). Important tools for discriminating individual eruptive units are superposition, the presence or absence of internal palaeosoils, sedimentary or other lithologically distinct deposits (Figs 1, 2), petrography, mineral chemistry, bulk rock or juvenile component compositional characteristics, and paleomagnetic character (see Milner et al., 1995; Self et al., 1997; Jay et al., 2009; Chenet et al., 2009). Geochemistry has been useful at regional scales in defining stratigraphic units of basaltic lavas with distinctive geochemical features (e.g., Mangan et al., 1986; Devey & Lightfoot, 1986; Peate et al., 1992; Hooper, 1997; Marsh et al., 2001) but greatest success is achieved where a combination of approaches are used (e.g., Chenet et al., 2008; 2009; Jay et al., 2009). Examples of significant regional correlations include the dissected and tectonically fragmented silicic eruptive units of the Paraná-Etendeka flood basalt province (Milner & Duncan, 1987; Milner et al., 1992; 1995) and on-land ignimbrites of the Afro-Arabian province with distal co-ignimbrite ash deposited in marine basins (Ukstins Peate et al., 2003; 2008). Additionally, the recognition of key volcanic facies and facies distributions can be used to link proximal to distal units where emplacement style might change over long distances, and importantly in offshore seismic sequences where direct access is not possible (Jerram et al., 2009; Nelson et al., 2009).

A complication in unravelling the products of an individual eruption in LIPs is caused by factors such as the great areal extent of volcanism, multiple vent activity, potentially long crustal transport distances, and in particular, the duration of flood basalt eruptions. Given these factors, synchroneity of contrasting magmas (Fig. 3) and of widely separated eruptions are a distinct possibility. Because of the high frequency of large eruptions during a LIP-forming event, the products of the majority of eruptions cannot be discriminated on the basis of current radiometric dating techniques. Heterogeneous granitoids produced by mingled gabbroic and granitic magmas in associated intrusive complexes (e.g., Messum; Fig. 3A) record the simultaneous existence of mafic and silicic magmas in LIP plumbing systems (Vogel, 1982; Ewart et al., 2002). Consequently, it remains unclear where interbedded units do exist whether this interstratification implies either a substantial timebreak or simultaneous (basalt-rhyolite or basalt-basalt) eruptions from different vent regions within the province.

Two pertinent examples of the above occur in the Paraná-Etendeka flood basalt province (Fig. 4): 1) two basaltic magma types (Urubici and Gramado) are interbedded in the southern Paraná (Peate et al., 1999); and 2) two discontinuous and chemically different basalt lavas are interbedded within the Goboboseb Quartz Latite of the Etendeka Province (Milner & Ewart, 1989; Milner et al., 1992; 1995). In the Paraná example, compositionally identical Urubici-type basaltic lava flow units 9 and 10, exposed along the coastal escarpment, were divided into two units as they are separated by a Gramado-type basaltic lava flow near Urubici. We suggest that units 9 & 10 were part of a single eruptive event (sourced from a vent region to the north where Urubici-type magmas predominate) that coincided with the simultaneous eruption of a Gramado flow field from the south.

For flood basalt eruptions, the term lava flow field refers to the aggregate product of a single eruption and can be formed of one or more lava flows, each the product of a vent or a group of vents along a fissure segment (Self et al., 1997). Consequently, all flood basalt lava fields are compound pahoehoe lavas composed of 1000's of lava sheet lobes, and innumerable smaller lobes, stacked/superposed in some places and laterally arranged in others. By contrast, the stratigraphy of silicic eruptive units appears much simpler, as exemplified by the Paraná-Etendeka quartz latites (Fig. 5), which crop out as relatively featureless extensive sheets with a simple internal structure and jointing, leading to the interpretation as single flow or cooling units (Milner et al., 1992). A simple deposit structure, an extremely chemically uniform or distinctive character and chemical distinction from other silicic units are used here to support the interpretation of silicic units in flood basalt provinces such as the Paraná-Etendeka, Karoo and Afro-Arabian provinces as being the products of individual eruptions.

Magnitude of LIP eruptions

LIPs are dominantly basaltic igneous events and the primary volcanic building blocks are extensive (10³-10⁵ km²), sheet-like lava flow fields (Fig. 6; e.g., Self et al., 1997; Jerram, 2002; White et al., 2009). Detailed work in the NAIP has identified lava flows at a number of scales, many of the units being only several 10s of km³ in examples from the Skye main lava series (Single & Jerram, 2004), up to much larger sequences as imaged in offshore seismic. Within these offshore sequences, lavas can be seen to have flowed long distances from subaerial to subaqueous environments producing large hyaloclastite sequences (Jerram et al., 2009). Additionally, in continental LIPs (flood basalt provinces and volcanic rifted margins), mafic volcaniclastic deposits (Ross et al., 2005; Ukstins Peate & Bryan, 2008), sill and dyke intrusions (Jerram, 2002; Elliot et al., 2008) and silicic ignimbrites (Bryan et al., 2002; Bryan, 2007) are also, areally and volumetrically significant and important architectural components (White et al., 2009).

Deposit dense-rock-equivalent volumes from known large-volume LIP eruptions are listed in Tables 2 & 3. These should be considered as minima. Constraints on flood basalt eruption magnitudes come mostly from the well-studied Columbia River Basalt (CRB) province, which is the product of many, dominantly pahoehoe flow fields varying in size from 1 to >2,000 km³ in volume (Tolan et al., 1989). The recent study of Reidel (2005) of proposed chemically correlated flow types such as the McCoy Canyon or Cohassett (Table 2) of the Grande Ronde Basalt Formation, indicates much larger volume flows may have been emplaced during the interval when >60% of the volume of the CRB province was erupted (Tolan et al., 1989; Camp et al., 2003). Also, studies on the Ambenali and Mahabaleshwar Formations of the Deccan indicate single formations have volumes similar to the entire CRB Group (~230,000 km³, Camp et al., 2003), with volumes of individual flow fields ranging from ~2,000 to >8,000 km³ (Self et al., 2006; 2008).This general upper magnitude for flood basalt lavas is similar to the dimensions of associated sills in flood basalt provinces, such as the enormous Peneplain Sill in the Dry Valleys, Antarctica, (19,000 km²; Gunn & Warren, 1962) with an estimated volume of 4,750 km³, the Dufek-Forrestal intrusions (10,200-11,880 km³; Ferris et al., 1998), and the 1,500 to 5,000 km³ Palisades Sill of the Central Atlantic Magmatic Province (Husch, 1990; Gorring & Naslund, 1995).

The magnitude of silicic eruptions in LIP events, by contrast, has received little attention with a presumption that the flood basalt eruptions were larger in volume and more likely to perturb climate (cf. Cather et al., 2009). The potential scale of silicic eruptive units emplaced during LIP events was first realised by the work of Milner et al. (1992, 1995) in the southern Etendeka continental flood basalt province and through correlations with its rifted counterpart, the Paraná province in South America. This database of extremely large silicic eruptions during LIP events (Table 3) has recently been expanded following detailed studies in the northern Etendeka (Marsh et al., 2001; Ewart et al., 2004b) and Afro-Arabian province (Ukstins Peate et al., 2003, 2005; 2008).

Areal extents of correlated quartz latite units from the Paraná-Etendeka LIP are shown in Figure 7. The areal extents of the quartz latite units indicated by cross-South Atlantic correlations of Milner et al. (1995) and Marsh et al. (2001) are huge (>0.1 Mkm²), ranging up to 0.17 Mkm² and greatly exceeding areas of the largest mapped flood basalt lavas from the CRB Province (0.01-0.1 Mkm²; Tolan et al., 1989; Self et al., 1997). Lateral extents of the Paraná-Etendeka quartz latites are also correspondingly large (up to 650 km; Milner et al., 1995; Marsh et al., 2001) and equivalent to the longest run-out lengths of flood basalt lavas (Tolan et al., 1989). As emphasised in previous correlations (Milner et al., 1995; Marsh et al., 2001), a distinct asymmetry exists in the quartz latite distributions, occupying more area on the South American continent, as well as there being a strong linear distribution to most of the quartz latite units with a preferred NW-SE orientation (Fig. 7). Additionally, a spatial variation exists with the high-Ti quartz latites restricted to the northern area of the Paraná-Etendeka province (Bellieni et al., 1986; Peate et al., 1992; Marsh et al., 2001; Ewart et al., 2004b).

Table 3 demonstrates that dense rock equivalent eruptive volumes for many silicic eruptive units in LIPs range between 1,000 and 4,000 km³ (>M8.5), and are at least equivalent to the dominant erupted volumes for the largest flood basalt lavas from the CRB Province (Table 2). Importantly, the data indicate the Paraná-Etendeka LIP has been the site of up to five silicic eruptions (Table 3) larger in magnitude than the Fish Canyon Tuff (>4,500 km³ dense rock equivalent), the most commonly cited example of the largest known ignimbrite eruption (Mason et al., 2004).

In addition to large explosive silicic eruptions, recent studies of the Mesoproterozoic Gawler Range Volcanic Province have delineated three >M8 silicic lava eruptions from this LIP remnant. The large areal extent of these units has been aided by eruptions from multiple point or fissure vents and contemporaneous eruption of chemically distinct magma batches (Allen et al., 2008; McPhie et al., 2008). However, despite the similar eruption magnitudes (Table 3), these lavas have run-out lengths that are over an order of magnitude less than the ignimbrite eruptive units. As with basaltic sills intruded into the flood basalt piles, the 2.47 Ga Woongarra Rhyolite sill of the Hammersley Basin in Western Australia (Trendall, 1995) reinforces the point that large-volume batches of silicic magma are emplaced at high stratigraphic levels and available for effusive or explosive eruption during LIP events. The Woongarra Rhyolite is a composite sill of 15,400 km³ emplaced at depths of a few hundred metres from the paleosurface. Sill emplacement occurred in two pulses thought to be separated by a few hundred years, with the pulses recording injection of 6,200 and 9,200 km³ of magma (Trendall, 1995).

In terms of cumulative erupted volumes, the silicic LIPs (Bryan et al., 2002; Bryan, 2007; Bryan & Ernst, 2008) contain the largest volumes (~0.25-3 Mkm³) of silicic volcanic rock and are equivalent to many continental flood basalt provinces. Because of this large and rapidly emplaced cumulative volume, the silicic LIPs must also be host to high frequency, large volume (>1,000 km³ and >M8) eruptions (Mason et al., 2004; Bryan & Ernst, 2008). However, at present, there are virtually no constraints on the dimensions of individual eruptions from the silicic LIPs. Mid-Tertiary examples from the western USA (e.g., the Fish Canyon Tuff and other monotonous intermediates) give some insight into the potential magnitude of silicic LIP eruptions, as these were erupted at the same time and along strike to the north of the ~0.4 Mkm³ Sierra Madre Occidental silicic LIP, the largest ignimbrite-dominated volcanic province in North America (Swanson et al., 2006; Ferrari et al., 2007). Several examples of similarly crystal-rich and potentially large-volume, caldera-related rhyolitic ignimbrites occur within the Sierra Madre Occidental (e.g., Copper Canyon and Vista Tuffs, Swanson et. al., 2006).

Discharge Rates of LIP Eruptions

The two very different vent configurations and eruptive styles (Table 4) play a significant role in the orders of magnitude difference in discharge rates for the basaltic and silicic eruptions in LIPs. For the flood basalt eruptions, early workers inferred fast eruption rates to emplace the huge volumes of magma in a matter of days to a few weeks (Shaw & Swanson, 1970; Swanson et al., 1975; Mangan et al., 1986; Tolan et al., 1989). Determining the discharge rate has obvious implications in terms of the requirement for vast plumbing systems and magma reservoirs for the magma to be delivered quickly to the surface. The identification of lengthy fissure systems of 70-200 km long in the CRB Province (e.g., Swanson et al., 1975) provided the mechanism for the rapid evacuation of a huge magma reservoir with the constraint that the mass discharge rate was not so high as to generate a large eruption column. However, the vent system for only one moderate-sized flood basalt eruption (Roza) has been studied in detail, and the nature and extent of activity along other fissure vent systems and dykes for other flood basalt flow fields remain poorly known.

The studies of Self and coworkers on the Roza flood basalt eruption have demonstrated that the eruptive volume can be accounted for by an ~10 year duration at an averaged effusion rate of ~4,000 m³ s^-1 (1.12 x 10⁷ kg s^-1), equivalent to the peak rate of the 1783-1784 Laki basaltic eruption in Iceland (Self et al., 1997). However, given the ~150 km length of the total fissure system for the Rosa flow, at these effusion rates, only part of the fissure system can have been active at any one time. The eruption rate, if averaged over the fissure length, would be exceedingly low (~0.0267 m³/s/m length of fissure) and result in magma freezing in the dykes/fissures in transit to the surface. Such magma freezing may be one mechanism by which effusion becomes concentrated or localised along the fissure system. Voluminous sheet lava flows, therefore, do not require the rapid extrusion of mafic magma at rates much higher than in historic eruptions except for the few cases of young flood volcanism, nor do long fissures imply high eruption rates as only segments of the fissure may be active at any one time (cf. Hooper et al., 2007).

Few constraints are available for discharge rates of silicic eruptions during LIP events. This is in part due to the lack of information on the nature of source vents and the virtual absence of Plinian fall deposits in LIPs from which most constraints on magma discharge rates for silicic explosive eruptions are made (e.g., Wilson et al., 1978; Sparks, 1986; Wilson & Walker, 1987; Carey & Sigurdsson, 1989). The large-volume silicic eruptive units in LIPs are dominantly ignimbrite or rheoignimbrite (e.g., Milner et al., 1992; Bryan et al. 2002; Ukstins Peate et al. 2005; Bryan, 2007) and the general lack of widespread Plinian fall deposits suggest that in general, mass discharge rates are sufficiently high (>10⁸-10⁹ kg/s) to prevent stable and buoyant Plinian eruption columns forming at the onset of, and during eruptions. Other factors that would contribute to co-current eruption dynamics include large, wide or multiple vents, such as fissures or along ring faults, and lower gas content (particularly for the rheomorphic ignimbrite examples) that in turn help lower eruption velocity (Wilson et al., 1980; Woods, 1995; Freundt, 1999). In general, the formation of single cooling units and in some cases, absence of internal erosion surfaces or sedimentary deposits produced by epiclastic processes within ignimbrite sheets have been interpreted to indicate ignimbrite emplacement within a period of no more than a few hours or days (e.g., Christiansen, 2001).

The inferred intensities for a number of prehistoric and historic Plinian eruptions vary between 1.6 x 10⁶ kg s^-1 to 1.1 x 10⁹ kg s^-1 (Carey & Sigurdsson, 1989), whereas the magma discharge rate for the June 15, 1991 eruption of Mt Pinatubo was between 4 x 10⁸ and 2 x 10⁹ kg s^-1 (Koyaguchi, 1996). Assuming a discharge rate of 1 x 10⁹ kg s^-1 and given the deposit volumes of the large silicic eruptive units in Table 3, the estimated duration of these eruptions varies from ~44 days for the smallest volume ignimbrite listed (Sana'a Ignimbrite, Afro-Arabian LIP) to ~248 days for the Guarapuava-Tamarana/Sarusas quartz latite (Paraná-Etendeka LIP). If mass eruption rates were an order of magnitude lower, then eruption durations would be in the order of 1-10 years, and approach the inferred durations of the flood basalt lava eruptions. The duration of the first pulse of the Woongarra Rhyolite sill emplacement has been estimated at ~240 years, equating to a magma discharge rate of 5 x 10⁶ kg s^-1 (Trendall, 1995), which approaches discharge rates of Quaternary sub-Plinian eruptions (Carey & Sigurdsson, 1989). In such long-lived eruptions, we would expect to see unsteadiness in the eruption that would be reflected in bedding or multiple eruptive units in the deposits, which are rarely observed (cf. Sarusas quartz latite, Ewart et al., 2004b). Alternatively, if eruption durations approach those from well-documented Quaternary eruptions (ie. hours to days), then eruption intensities of 10¹⁰ to 10¹¹ kg s^-1 are required. Such high eruption intensities without the development of a tall Plinian eruption column can be achieved by multiple vents or ring fracture fissure eruptions. The simple deposit structures of the silicic eruptive units thus supports the notion for high eruptive fluxes (10⁹-10¹¹ kg s^-1) and short duration (<1 month) eruptions. These higher rates are supported by recent work on giant ash clouds (Baines & Sparks, 2005) that suggest eruption intensities approach 10¹⁰ kg s^-1 resulting in durations between 2-10 days for the largest eruptions (M8-9). Additional supporting examples for short durations include the 450 km³ Bishop Tuff eruption from Long Valley, California, about 770,000 years ago that has been estimated to have lasted about 4 days (Wilson & Hildreth 1997), and the compositionally zoned 2,200 km³ Huckleberry Ridge Tuff erupted from the Yellowstone volcanic field, which occurs as a single cooling unit and likewise been suggested to result from an eruption that took days at most (Christiansen, 2001).

Frequency of Large-Magnitude (>M8) Eruptions From LIPs

It is both the volume of magma emitted during individual eruptions in LIP events and the total volume of magma released (>0.1-80 Mkm³) that make LIP events so exceptional in Earth history (Self et al., 2005; Self, 2006). It is this combination of large erupted volumes and high frequency that lead to the rapid construction of thick (1->3 km) areally extensive plateaus (0.1-2 Mkm²), which internally, show few signs of major time breaks, erosion surfaces and regional unconformities (Fig. 6A). The high-frequency of large-magnitude eruptions also distinguishes LIP events from other tectonic settings and processes where igneous rocks are formed. Importantly, without LIP-forming igneous events, basalt super-eruptions would not have occurred through Earth history, but in contrast, silicic super-eruptions have occurred independently of LIP events (Sparks et al., 2005).

From a volcanological viewpoint, geochronological studies of LIPs, summarized in studies such as Rampino & Stothers (1988), Courtillot & Renne (2003), Kelley (2007), Bryan & Ernst (2008) and Chenet et al. (2008), have revealed two main features relevant to the timing of eruptions and LIP formation. These are that: 1) much (70-90%) of the eruptions are produced during one or two main pulses of eruptive activity; and 2) that the pulse or pulses, or even the whole duration of activity in the LIP, can be very brief geologically, <5 Ma, and possibly even < 1 Ma in some cases. For LIPs of any age, the errors on the age estimates cannot resolve individual formations within these pulses of activity, and certainly cannot resolve individual eruptions. In many cases, the errors encompass the age range of almost all eruptive units from top to bottom of the LIP pile (e.g., Barry et al., 2010).

For the flood basalt eruptions, evidence from the CRB LIP suggests that during the main pulse and emplacement of the Grande Ronde and Wanapum Basalt Formations (~16-5-15.3 Ma), a high frequency of the largest magnitude eruptions existed producing the most voluminous flow fields (Tolan et al., 1989). For example, the Grande Ronde Basalt forming >60% of the total volume of the CRB LIP (Camp et al., 2003), comprises at least 110 individual eruption packages or flow fields (Barry et al., 2010) with an averaged volume of 1,238 km³ (Tolan et al., 1989). New ⁴⁰Ar/³⁹Ar dates for Grande Ronde lavas reveal they were emplaced within a maximum time range of 0.42 ± 0.18 Myr (Barry et al., 2010), corresponding to an averaged frequency of ≥M8 eruptions of 220/Myr or one ≥M8 eruption every ~4,200 yrs. For larger continental flood basalt provinces such as the Deccan, recent studies (Self, 2006; Self et al., 2008; Chenet et al., 2009) indicate that individual formations emplaced over similar time scales of hundreds of thousands of years, have volumes either equivalent to the main phase lavas (Grande Ronde Formation, ~0.15 Mkm³) or to the entire CRB LIP (~0.23 Mkm³, Camp et al., 2003). Therefore, LIP formations must also be characterised by even higher frequencies of M8 and larger eruptions.

For most LIPs other than the CRB, however, the number of eruptions is unknown. Even within the CRB, the number is not known precisely but is probably around 200. Fitting the number of eruptions within a 1-2 Ma timeframe still gives average eruption intervals of 1,000s - 10,000 yrs (Self et al., 2006; Barry et al., 2010). While the accumulated lava pile, and the thickness added by each eruption, is impressive, considered from a modern or historic perspective, LIP eruptions probably were not necessarily ‘hyperactive’, even during the main pulse. Much more needs to be known about the rates of lava production and the lengths of hiatuses between eruptions in LIPs, and various approaches are being undertaken to estimate this (Chenet et al., 2008), but such studies are in their infancy.

The recent compilation on large-volume silicic explosive eruptions by Mason et al. (2004) revealed 42 known eruptions of >M8 over the past 36 Ma. This yielded a minimum time-averaged estimate of eruption frequency of 1.1 events/Myr since the beginning of the Oligocene, but over this time, such large magnitude eruptions have clustered in two pulses at 36-25 Ma and 13.5 Ma to present. The older pulse corresponds to two LIP events: the Afro-Arabian and Sierra Madre Occidental provinces but with most dimensional data for eruptions drawn from related large-volume silicic ignimbrite volcanism to the north of the Sierra Madre Occidental in the Great Basin region of western U.S.A. (e.g., Gans et al., 1989; Best & Christiansen, 1991). During these pulses, large eruption frequencies were slightly higher at ~2 events/Myr. However, virtually no data are currently available on the magnitude of individual silicic eruptions within the Sierra Madre Occidental, and eruption frequencies will have been much higher for this period. In contrast to the eruptive record for the last 35 Myrs, large eruption frequencies were at least 9 events/Myr during the Paraná-Etendeka LIP event, and for the Afro-Arabian province, the equivalent of 12 events/1 Myr. Consequently, the frequency of silicic super-eruptions is greater during LIP events than when compared to global, long-term averaged frequencies of silicic super-eruptions. Importantly though, based on available age data for the continental flood basalt provinces, these high frequencies appear sustained only for very brief periods of ≤1 Myr.

Generation & Storage of Large Magnitude LIP eruptions

The eruption of such exceptionally voluminous magmas and the often remarkable chemical homogeneity in on-land deposits have led to the general interpretation that LIP eruptions require very large magma reservoirs, which are rapidly evacuated. The very large volumes for individual eruptions from LIPs raise a number of space-volume issues in terms of the storage and dimensions of, and connectivity and interactions between holding chambers within the crust. Not only do issues of space arise from the storage of such huge volume magma bodies in the crust, but additional complications arise in a temporal sense because many >M8 basaltic and silicic eruptions may occur within 1 Myrs, some or all of which may be genetically unrelated and represent new episodes of large-volume magma generation. For example, a number of large volume (>1,000 km³), closely-spaced (≤10⁵ yrs) and genetically related silicic eruptions (Goboboseb and Springbok quartz latites) occurred from the Messum igneous complex, and which overlapped in space and time with flood basaltic eruptions (Ewart et al., 1998b; 2002). How do these magma reservoirs spatially overlap and how does this impact on the thermal and rheological character (e.g., de Silva & Gosnold, 2007) of the crust? The architecture and spatial-temporal relationships of flood basaltic and rhyolitic magma reservoirs in the crust during LIP events remain poorly understood. In our paper we have considered that the variety of large-volume basaltic and silicic eruptions in LIPs can be envisaged in terms of four end-member magma petrogenetic pathways (Fig. 8). These pathways reflect the varying importance of crustal storage for low-pressure crystallisation, crustal assimilation and large-scale partial melting.

References