Thematic Articles

Oceanic plateaus represent large areas (~1 ×× 106 km2) of thickened oceanic crust formed from rapidly erupted lava (<3 Myr). These plateaus have formed throughout most of geological time. They generally correlate with periods of environmental catastrophe characterised by oceanic anoxia, leading to black shale formation and mass extinction events. Such correlations are particularly evident in the Cretaceous and can be partly attributed to the release of CO2 during oceanic plateau formation, which ultimately resulted in a runaway greenhouse effect. Additionally, sea level rise and disruption of oceanic circulation patterns by displacement of seawater during plateau formation contributed to increased environmental stress and biotic extinction.

Subaerial continental flood basalt volcanism is distinguished from all other volcanic activity by the repeated effusion of huge batches of basaltic magma (~102–103 km3 per eruption) over short periods of geologic time (<1 Myr). Flood basalt provinces are constructed of thick stacks of extensive pahoehoe-dominated lava flow fields and are the products of hundreds of eruptions. Each huge eruption comes from a dyke-fed fissure tens to hundreds of kilometres long and lasts about a decade or more. Such spatial and temporal patterns of lava production do not occur at any other time in Earth history, and, during eruptions, gas fluxes of ~1 Gt per year of SO2 and CO2 over periods of a decade or more are possible. Importantly, the atmospheric cooling associated with aerosols generated from the SO2 emis- sions of just one flood basalt eruption is likely to have been severe and would have persisted for a decade or longer. By contrast, warming due to volca- nogenic CO2 released during an eruption is estimated to have been insignifi- cant because the mass of CO2 would have been small compared to that already present in the atmosphere.

Ameteorite impacting on the surface of the Earth produces not only a crater but also, if the impactor is sufficiently large, high melt volumes. Computer simulations suggest that, in addition to shock- induced melting produced by impact, additional decompression melting of the hot target mantle beneath the crater can produce melt volumes comparable to those found in large igneous provinces (LIPs). The coincidence between the expected frequency of such impact events combined with the similarity in magma volumes of LIPs suggests that large meteorite impacts may be capable of triggering LIPs and mantle hotspots from a point source which is subsequently buried. Can the impact model explain any LIP? What are the distinctive macroscopic criteria predicted from an impact model, and how may they be recognised or rejected in the geological record of the Earth?

When continental crust gets too thick, the dense eclogitic bottom detaches, causing uplift, asthenospheric upwelling, and pressure- release melting. Delamination introduces warm blocks of lower crust with a low melting point into the mantle; these eventually heat up, ascend, decompress, and melt. The mantle below 100 km depth is mainly below the melting point of dry peridotite, but its temperature will be above the melting point of recycled fertile (basaltic or eclogitic) components, obviating the need for excess temperature to form “hotspots” or “melting anomalies”. When plates pull apart or delaminate, the mantle upwells; entrained crustal fragments of various ages are fertile and create melting anomalies along developing mid-ocean ridges, fracture zones, and old suture zones. Eclogites associated with delamination are warmer and less dense than subducted oceanic crust and more susceptible to melting and entrainment.

Mantle plumes are columns of hot, solid material that originate deep in the mantle, probably at the core–mantle boundary. Laboratory and numerical models replicating conditions appropriate to the mantle show that mantle plumes have a regular and predictable shape that allows a number of testable predictions to be made. New mantle plumes are predicted to consist of a large head, 1000 km in diameter, followed by a narrower tail. Initial eruption of basalt from a plume head should be preceded by ~1000 m of domal uplift. High-temperature magmas are expected to dominate the first eruptive products of a new plume and should be concen- trated near the centre of the volcanic province. All of these predictions are confirmed by observations.

Episodically, the Earth erupts large quantities of basaltic magma in geologically short periods of time. This results in the formation of large igneous provinces, which include continental flood basalt provinces, volcanic rifted margins, and giant oceanic plateaus. These fluctuations in the Earth’s system are still poorly understood. Do they owe their origin to mantle plumes, meteorite impacts, or lithosphere-controlled processes? Whatever their origin they correlate closely with major changes in oceanic and atmospheric chemistry and may trigger global mass extinctions.