Background

Ever since the acceptance of the Huttonian principle of uniformitarianism, it has been difficult for Earth scientists to accept any explanation of a geological event that relies on a catastrophic mechanism. This problem is epitomized by the time that was taken to recognize the importance of meteoritic impacts in planetary evolution. Thus, even the origin of lunar craters was strongly debated until the middle part of the 20th century, and until the 1950s, geologists were reluctant to accept an impact origin for Meteor Crater (Arizona), which is arguably one of the best preserved of all terrestrial craters. Since then, the number of accepted terrestrial craters has risen at a rate of about 3 per year, and currently stands at ~150, with no indication of a reduction in the rate of detection.

The recognition of a geological feature as having an impact origin generally rests on observing a number of characteristic shock-related effects in the country rock. Some researchers, however, have gone beyond these accepted limits to suggest that several larger geological features had an impact origin, but have auto-obliterated the traditional evidence of impact by subsequent large-scale igneous activity. Examples of such suggestions include the Bushveld Complex (Hamilton, 1970, Rhodes, 1975), the Deccan Traps (Rampino, 1987; Negi et al., 1993), the breakup of tectonic plates (Seyfert & Sirkin, 1979; Price, 2001), the formation of oceanic plateaus and large igneous provinces (LIPs) (Rogers, 1982; Jones et al., 2002; Coffin & Ingle, 2003, Ingle & Coffin, 2003a,b), and the coronae of Venus (Stewart et al., 1993; Vita Finzi & Howarth, 2003; Hamilton, 2005, 2007). These suggestions have usually been rejected on the grounds that an impact model is less plausible than the widely accepted plume model (Mahoney & Coffin, 1997; Richards et al., 1989).

Melosh (1989) contended that there is no firm evidence that impacts can induce volcanic activity in the impact crater region, although studies of the Addams Crater on Venus and the Sudbury complex in Canada seem to be at odds with this claim. Figure 1: Addams crater is remarkable for the extensive outflow that extends 600 km from the crater rim.

Revisiting the hypothesis

Hydrodynamic modelling could investigate whether the degree of melting generated by a large impact into hot lithosphere produces more melting than predicted from classic scaling laws (Jones et al., 2002). Central to our analysis is the contention that the phenomenon of pressure-release melting, or decompression melting, is the key to understanding the volumes of melt generated during large impacts and that in part this process has been overlooked or wrongly de-emphasized. We conclude that decompression melting of the sub-crater mantle may initiate almost instantaneously, but the effects of such a massive melting event may trigger long-lived mantle upwelling. The energy released is largely derived from gravitational energy. This is outside (but additive to) the conventional calculations of impact modelling, where energy is derived solely from the kinetic energy of the impacting projectile, be it comet or asteroid. Therefore the empirical correlation between total melt volume and crater size no longer applies, but instead is non-linear above some threshold size, depending strongly on the thermal structure of the lithosphere.

We used  indicative hydrocode simulations to identify regions of decompression beneath a dynamic, large impact crater. The volume of melting due to decompression was then estimated, from comparison with experimental phase relations for the upper mantle, and depends on the geotherm.

Figure 2: Indicative hydrocode model of a simulated impact designed to show regions where decompression melting should occur.  Model conditions: 300 x 300 km cell, impactor = 10 km radius iron, velocity 10 kms^-1, orthogonal impact, target = basalt (homogeneous), pressure gradient = PREM ;  See Jones et al. (2002).

We suggest that the volume of melt produced by a 20-km-diameter iron projectile travelling at 10 km/s into hot oceanic lithosphere may be comparable to a LIP (i.e. of the order of ~10⁶ km³). The mantle melts will have "plume-like" geochemical signatures, and rapid mixing of melts from sub-horizontal sub-crater reservoirs to depths where garnet and/or diamond is stable is possible. Direct coupling between impacts and volcanism is therefore a possibility that should be considered in the context of global stratigraphic events in the geological record.

Maximum melting would be produced in young oceanic lithosphere and could produce oceanic plateaus, such as the Ontong Java plateau at ~120 Ma. The end-Permian Siberian Traps could also have resulted from volcanism triggered by a major impact at ~250 Ma, onto continental or oceanic crust. Auto-obliteration by volcanism of all craters larger than ~200 km would explain their anomalous absence on Earth compared with other terrestrial planets in the solar system. This model provides a potential explanation for the formation of komatiites and other high-degree partial melts. Impact reprocessing of parts of the upper mantle via impact plumes is consistent with models of planetary accretion after the late heavy bombardment and provides an alternative explanation for most primitive geochemical signatures currently attributed to plumes as originating from the deep mantle or the outer core.

Discussion

Recently Ivanov & Melosh (2003) concede: "Consider an impact that creates a transient cavity approximately twice as deep as in our numerical simulation (depth ~100 km). Such an impact is, indeed, big enough to raise hot mantle rocks close to surface. This impact corresponds to a final crater diameter of 400 to 500 km ­ a very rare event in the current post-heavy bombardment period. Such a huge event IS possible.." (and would trigger volcanism).

The question that needs still to be refined is how large an impactor is needed to generate large scale melting, and what is the effect on the thermal structure of the lithosphere on the volume of melt generated. It is to be expected that considerably more melt will be generated when an impact penetrates thin oceanic lithosphere (e.g., under the Ontong Java plateau) than if the same impact landed on thick continental crust. We also note that Glikson (1999) pointed to the planetary-scale role of mega-impacts in the history of development of the Earth’s crust, and drew attention to the likely preferential melting efficiency of mega-impacts in oceanic lithosphere due to their higher geothermal gradients and thinner crust. Many of Glikson’s ideas and fundamental implications are substantiated by our results for decompression melting. 

We finish by quoting Boslough et al. (1986), who stated "the impact-produced flood basalt hypothesis is attractive because it is potentially testable on the basis of predictions of features that have not yet been discovered...unlike current plume models for flood basalts and hotspots".  In conclusion, we assert that the concept of impact-induced volcanism has not been adequately examined and may offer a new framework for the interpretation of large-scale igneous and geological processes.

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