Large Igneous Provinces

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Large Igneous Provinces

(The following is adapted from an article in 'Elements' by Andy Saunders. A full version is available here)
It has been nearly 15 years since the term ‘large igneous province’ was introduced by Coffin and Eldholm in 1991. An umbrella term to include continental flood basalt provinces, oceanic plateaus, volcanic rifted margins and the aseismic ridges, it rapidly entered common parlance even though it was, and remains, loosely defined. The key aspect of LIPs is that they represent anomalously high magmatic fluxes. The magma is usually basaltic, but may be rhyolitic. They are usually large in areal extent, covering many thousands if not millions of square kilometres, and they testify to unusual geological processes, involving large amounts of thermal energy. Where did this energy come from – deep within the Earth as a mantle plume, a meteorite impact, or from delamination of continental lithosphere?

There is considerable debate about whether the mantle source of basaltic LIPs was anomalously hotter than 'normal' mantle (e.g., that feeding active spreading ridges). A key issue is the presence of ultrabasic (high-magnesium) melts. Many LIPs show evidence of these (e.g., North Atlantic, Caribbean, Madagascar). It is difficult to make these melts without a mantle that is substantially hotter than the norm. However, some LIPs do not contain evidence of such melts. This may be a real feature, and suggest that their source is not particularly hot, but it should be remembered that high-magnesium melts are dense and may pond deep in the crust or in basaltic magma chambers.

Figure Caption. Map of the main Mesozoic and Cenozoic large igneous provinces, and the main ages of their emplacement, modified after Coffin and Eldholm (1992). Both oceanic and continental LIPs are shown, but volcanic rifted margins are omitted, as are the major rhyolitic LIPs. If your favourite LIP is missing (Afar, Ethiopia, Rajamhal etc.) - apologies! Shown are the locations of present-day hotspots, mostly underlain by active mantle plumes, that may be the descendants of the associated LIP. The extension of the Siberian Traps, shown in feint colour, is based on evidence of basalt subcropping beneath the West Siberian Basin (see Geological Setting). The International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) has a commission devoted to promoting the study and understanding of LIPs.

The sheer volume, and eruptive rates, of magma in many LIPs is evidence for many of the high source temperatures, but equally the source may be more 'fertile' (ie contain more potential melt) than normal mantle. Similarly, special convection systems may process the mantle more quickly through the melt zone. And, of course, a catastrophic event like a meteorite may create huge amounts of melt virtually instantaneously. The bias of this author (ADS) is that most LIPs are produced by mantle plumes. The evidence of a hot source, the tell-tale trail leading from many LIPs to active volcanic centres, the uplift of the crust associated with LIP emplacement, all point to a plume process. This is not to say that all LIPs were produced by plumes! LIPs also have the potential to create environmental havoc by release of gases and blocking oceanic circulation systems. See here for more.