The four largest extinction events of the Phanerozoic (late Devonian, 370 Ma; end-Permian, 251 Ma; end-Triassic 201 Ma; end-Cretaceous , 65 Ma) each coincide with periods of rapid and voluminous continental flood-basalt volcanism. There is also evidence from the extinction horizons that each coincided with a major impact event as well, most widely accepted for the end-Cretaceous event. Geological time is so long that pure chance cannot be ruled out entirely to explain coeval impacts and CFB events, but is unlikely (a 1 in 8 chance for one coincidence, but 1 in 3500 for four). So there has been a long-running controversy over a volcanic or an extraterrestrial cause for extinctions, together with speculation that large impacts can somehow trigger CFB events. The last does not work for the end-Cretaceous extinction, because the Deccan volcanism began somewhat before the formation of the “smoking-gun” Chicxulub crater, and a linking mechanism is not clear. Taking into account lesser extinctions and CFB events, there is a rough periodicity of 30 Ma and similar ages for both. Geoscientists at the Geomar Institute of the University of Kiel in Germany have stoked up the controversy by taking a very different view of events (Phipps Morgan, J. et al. 2004. Contemporaneous mass extinctions, continental flood basalts, and ‘impact signals’: are mantle plume-induced lithospheric gas explosions the causal link? Earth and Planetary Science Letters, v. 217, p. 263-284) albeit not a completely new one. They consider the processes at depth that presage CFB events, where rising mantle material impacts at the base of thick continental lithosphere. Each of the CFB provinces linked in time to the four large extinctions lies on an ancient craton, devoid of tectonic activity for over a billion years, and greatly depleted in heat-producing elements. Lithosphere beneath them is over 300 km thick and might have acted in the manner of the lid on a pressure cooker, building up gas pressure during the delay in breaking through overlying rock. Eventually pressure would be sufficient to breach the lithosphere, and gases (CO2 and SO2) would be explosively vented, perhaps creating globally toxic conditions. Release of the pressure would lead to collapse above the plume head that would propagate upwards, at hypersonic speeds according to the authors. Maybe that would fling enormous amounts of rock into the stratosphere. Some chunks might be large enough to cause big impact structures at the surface when they fell back, so explaining the coincidence. They account for the pre-extinction start of CFB outpourings, as in the case of the Deccan traps, by lateral and upwards migration of part of the plume to locally thinned lithosphere. The power involved in such an event extending through the entire lithosphere could account for the shocked grains, microspherules and fullerenes in known extinction horizons. Being sourced in mantle rock that may once have resided near the core-mantle boundary, such a process could also eject high iridium concentrations that were the signs that first led to the Alvarez’ hypothesis of impact-induced extinctions, but without an extraterrestrial culprit. Despite the attractions of the impact theory, no sign of meteoritic debris has been found in any of the ejecta horizons or the craters themselves. On Phipps Morgan and colleagues’ account that is not surprising, because the impacting objects would have been common Earth rock. The authors decided to dub these hypothetical events “Verneshots” after Jules Verne’s book From the Earth to the Moon, which involved a giant gun firing the space craft moonwards. If there is anything in the idea, then surely there would be spectacular evidence of the source of the blasts, but perhaps they are conveniently buried by later CFBs. Geophysical studies do show signs of circular features beneath both the Deccan and Siberian Traps. However, the associated seismic shock waves would pervade large volumes of crust outside the blast vent, and signs of that, such as shatter cones, are perhaps an easier target. As with all departures from “accepted wisdom”, the Geomar group’s ideas will come in for a lot of stick, quite possibly from the fans of giant impacts, who not so long ago were themselves dismissed as “whizz-bang kids” by many geoscientists.
That gas build-up might lead to catastrophic crustal collapse gets some support from a modelling study on the processes involved in volcanic collapse (Reid, M.E. 2004. Massive collapse of volcano edifices triggered by hydrothermal pressurization. Geology, v. 32, p. 373-376), albeit in miniature. Mark Reid of the USGS focuses on those volcano collapses that occur without any warning signs from eruptions and seismicity. His study examines the effects of deep intrusion of magma on the groundwater systems within stratovolcanoes. This could promote increases in gas pressures deep within the edifice. Their upward propagation would destabilise the entire volcanic structure, leading to its collapse in extreme situations. The modelling indicates increased likelihood of over-pressuring where permeability is low; a crude analogy to Phipps Morgan and colleagues’ pressure lid of inert cratonic lithosphere. Gas-rich magmas can emerge explosively in continental flood basalt provinces, normally regarded as forming by episodic, quiet outpourings from fissure systems. That is well demonstrated by the Ethiopian-Yemeni CFB province. The main basaltic trap sequence is followed by very widespread felsic ignimbrites on both sides of the Red Sea that formed by lateral blasts of incandescent debris and felsic lava shards. Only one example of an ignimbrite centre is known from the province. Lying about 60 km south of Sa’ana, near the small town of Mabar, it is a circular structure about 18 km across with clear concentric zoning. Interestingly the zones dip steeply towards the centre of the structure, in an inverted cone, that is possibly due to collapse even more dramatic than in the calderas that sourced the more familiar ignimbrites of the Andes.
See also: Ravilious, K. 2004. Four days that shook the world. New Scientist * may 2004, p. 32-35.