The largest volcanic eruptions on Earth
Introduction
The generation and emplacement of large igneous provinces (LIPs) are anomalous transient igneous events in Earth's history resulting in rapid and large volume accumulations of volcanic and intrusive igneous rock (Coffin & Eldholm, 1994, Bryan & Ernst, 2008). LIP events have been estimated to have had a frequency of one every 20 Myrs since the Archean (Ernst and Buchan, 2001), but when the current oceanic LIP record dating back to 250 Ma is also included, this frequency is reduced to one per 10 Myr (Coffin and Eldholm, 2001; see also Prokoph et al., 2004). The volcanic and intrusive products of individual LIPs collectively cover areas well in excess of 0.1 Mkm2, and typically, extruded volcanic deposit volumes are ≥ 1 Mkm3. Oceanic plateaus define the upper limits of the areal and volumetric dimensions of terrestrial LIPs, with reconstruction of the Ontong-Java, Hikurangi and Manihiki plateaus (Taylor, 2006) having a pre-rift areal extent of ~ 3.5 Mkm2, larger than the Indian sub-continent, a maximum crustal thickness of 30 km and a maximum igneous volume of 59–77 Mkm3 (Kerr and Mahoney, 2007).
A distinguishing feature of LIPs, as exemplified by continental flood basalt provinces, is the high magma emplacement rates (e.g., Storey et al., 2007) where aggregate magma volumes of ≥ 1 Mkm3 are emplaced from a focussed source during 1 to 5 million year-long periods or pulses (Bryan and Ernst, 2008). In detail, most LIP eruptions had magnitudes significantly greater than those of historic eruptions, tending towards extraordinarily large-volume eruptions (> 103 km3; Tolan et al., 1989, Jerram, 2002, White et al., 2009, Chenet et al., 2009), making LIP volcanism exceptional. However, to produce such tremendous cumulative volumes of erupted magma, the short-lived, main eruptive pulses of LIP events must consist of many and frequently recurring, large-volume eruptions, each evacuating 102–103 km3 of magma. Consequently, it is the volume of magma emitted during these individual 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, 1988; Courtillot, 1999, Courtillot & Renne, 2003, Wignall, 2001, Wignall, 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).
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 (> 1000 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, Pyle, 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 1 × 1015 kg of magma (Sparks et al., 2005, Self, 2006). For rhyolitic eruptions, this is equivalent to ~ 410 km3 (at a magma density of 2450 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 ~ 2750 kg m−3. Here we present a compilation of known eruption volumes for the very largest (> Magnitude 8.5, equivalent to > 1160 km3 of basalt lava, or > 1280 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 (1 × 1016 kg, or ~ 5000 km3 of dense magma) has been recognized, which occurred 28 Ma (Lipman et al., 1970, 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?
Section snippets
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. Important tools for discriminating individual eruptive units are superposition, the presence or absence of internal
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). However, 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 & Fleming, 2008) and silicic ignimbrites (Bryan et al., 2002, Bryan, 2007) are
Magnitude of LIP eruptions
Constraints on flood basalt eruption magnitudes come mostly from the well-studied CRB province, which is the product of many, dominantly pahoehoe flow fields varying in size from 1 to > 2000 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
Discussion
Constraining the dimensions of LIP eruptive units are vital to determining potential rates of magma eruption from vent systems, the duration of eruption and emplacement, and potentially, the rates of magma production and storage (Tolan et al., 1989). This discussion focuses on these aspects and examines what fundamental differences or similarities exist between the largest basaltic and silicic eruptions.
Conclusions
Large igneous provinces have been the loci for both basaltic and silicic super-eruptions (> M8) throughout Earth history, and are therefore important for understanding their potential for driving environmental and climate change and causing mass extinctions, melt production rates from the sublithospheric mantle and crust, the thermal, mechanical and compositional evolution of the lithosphere, and what upper limits there may be to the volume and rate of magma eruption. LIPs are unique for their
Acknowledgements
Simon Milner, Andy Duncan, Richard Ernst, Luca Ferrari, Alexei Ivanov and particularly Tony Ewart are thanked for extensive discussions on aspects of this manuscript. Scott Bryan acknowledges support from Kingston University, and Ingrid Ukstins Peate and David Peate acknowledge support from NSF grant EAR0439888 for this work. Additional support has been provided by National Research Foundation (South Africa) grant FA2006032400017 to Jodie Miller, Chris Harris & Scott Bryan. Namibia fieldwork
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