The words on our LIPs: Impact Theories?

Asteroids pounding the Earth. Torrents of lava billowing from the Earth’s interior. Some would say these are mutually exclusive phenomena. Or can large impacts trigger major volcanic episodes? And where does a 1979 film starring Michael Douglas fit into this?

Our efforts to try and better understand our little ball of rock brings answers wrapped in questions. The layered character of our discoveries mimics the wonderful stratified complexity of this ellipsoid on which we reside. The outer layer of crustal rocks is in a constant state of regeneration and destruction according to plate tectonic theory. Much of the volcanic activity and rocks on this Earth can be attributed to this mechanism, but some rather significant portions demand an alternative explanation.

Hotspots

Hotspots are interesting little patches of volcanic activity that cannot be explained by plate tectonics. They generally pop up within a plate rather along plate boundaries. Hotspots are believed to stem from melting anomalies in the upper mantle, the layer directly below the Earth’s crust. Hotspots in some cases can produce Large Igneous Provinces (LIPs) and associated aseismic ridges. LIPs are massive volumes of lava that form characteristically massive (2000 – 2500 km across) basaltic edifices on land and in the sea (White & Mackenzie, 1989). Aseismic ridges are tectonically inactive sea floor mountains.

Many believe these occurrences can be explained by the Mantle Plume Theory formulated by Wilson and Morgan in the 60s and early 70s. The plume theory is believed to be a consequence of the super-hot iron-nickel rich core. The core reacts with cooler lower mantle above and creates a thermal gradient between the adjacent sections. The boundary layer then rises buoyantly through the overlying mantle producing conduits with a shape closely resembling a hoover attachment which eventually reaches the surface (Campbell, 2007).

Plume

Alternatives to this theory invoke processes more closely associated with tectonics and shallower processes. Anderson (2005) suggests delamination; where crust becomes too thick then detaches, leading to mantle upwelling and melting. Foulger (2005) suggests subduction (sinking of one tectonic plate under another) related contamination is the culprit for the Icelandic hotspot…..while others have looked to the skies.

Impact-induced volcanism

How does an impact lead to a volcano?

First ideally you would like to have evidence that an actual impact took place. Here you might look for evidence of shock metamorphism (impact related alteration of a rock/mineral). Normally this can be observed in the hardy quartz grains. Unfortunately, projectiles from space can be quite inconsiderate and sometimes choose to strike rocks like basalt (oceanic crust). Here you need to search for alternative evidence such as geochemical anomalies. Some are sceptical that large asteroids (250 – 300 km) could generate even small volcanic episodes (Ivanov and Melosh, 2003).

Adrian Jones has a different idea. He believes that the initial shock of an impact would indeed produce melt and that additional processes could then lead to the the big LIPs described earlier (Jones, 2005).

Basically the Earth’s mantle is made of what is called peridotite (a rough coarse igneous rock). It responds interestingly to changes in pressure. Jones et al. (2002) simulated that the total melt volume would soar in response to decreasing pressure, a process known as decompression melting. He shares the opinion of many that several areas attributed to hot spot volcanism and mantle plumes may be explained by impact events.

The largest Oceanic LIP in the world is called the Ontong Java Plateau. It is a spectacular underwater expanse that’s pretty close to the size of Alaska. It is believed to be about 120 million years old.

Stephanie Ingle and Millard Coffin (2004) presented the possibility that the OJP may have been formed after a large impact. This emplacement of the OJP coincides rather interestingly with an oceanic anoxic event and a mass die-off of some animal species. The authors believe their impact may be the root cause. They also note how scarce impact craters are on the Ocean floor. They believe volcanic events may be hiding evidence of these  (Ingle & Coffin, 2004).

The China Syndrome

A 1979 hit movie. The plot: A nuclear meltdown where the worst case scenario is that the Nuclear components could melt their way through the Earth ‘all the way to China’. Seems plausible right? Well try this one.

The observation that volcanic hotspots appear to crop up in antipodal pairs (on opposite sides of the Earth) was first given serious consideration in the early 90s (Rampino & Caldeira, 1992).

John Hagstrum, inspired by this suggested the antipodal impact model. The idea is basically that seismic waves generated from an impact would propagate at the other side of the world, perhaps acting as the precursor to antipodal hotspots (Hagstrum, 2005). He suggests that impacts might also generate catastrophic tsunamis. The antipodal nature of the Chicxulub crater in Mexico and the Deccan traps in India is interesting (Fig. 1). Many believe that this impact and this mega volcanic episode was the one-two punch that ended the dinosaurs.

Geophysicists at Princeton have recently put some computers on the case. While previous studies made the assumption that the Earth is a perfectly symmetrical sphere (Boslough et al. 1996; Ni & Ahrens 2006), the Princeton simulations took a whole range of other factors into account; mainly the elliptical nature of the Earth. Their simulations indicated that it may be possible that antipodal impacts could be the precursor to volcanism on the other side of the world. The mega-tsunami is a definite possibility too (Meschede et al., 2011).

Hotspots
Hotspots on Earth. Primary hotspots are represented by circles, secondary by triangles, and grey patches are LIPs. Deccan and Chicxulub are highlighted – could they be an antipodal pair?

Imagination fuels discovery

‘I never once came upon any of my discoveries through the process of rational thinking.’ – Albert Einstein

It has recently come to light that the Sudbury impact was generated by a comet and not an asteroid (Petrus et al., 2014). The main difference between comets and asteroids is that comets are mostly balls of ice with while asteroids have more rocky or metallic make-ups. This means that it probably disintegrated on impact.

It raises some interesting thoughts. Our planet is a pretty closed system so far as we know, but it was born by accretion of cosmic particles. We are those bits of dust. Could comets be responsible for providing much of our water? Could projectiles smashing into our earth be responsible for some of our most enigmatic planetary phenomena? Watch this space… all around us.

By Donal O’Farrell, postgraduate student, Trinity College Dublin.

References

  1. White R. & Mckenzie D. (1989) Magmatism at rift zones: The generation of volcanic continental margins and flood basalts. Journal of Geophysical Research: Solid Earth.
  2. Campbell I.H. (2007) Testing the plume theory. Chemical Geology.
  3. Anderson D.L. (2005) Large Igneous Provinces, Delamination, and Fertile Mantle. Elements Magazine.
  4. Foulger G.R. (2005) A cool model for the Iceland hotspot. Journal of Volcanology and Geothermal Research.
  5. Jones A.P. (2005) Meteorite Impacts as Triggers to Large Igneous Provinces. Elements Magazine.
  6. Ingle S.& M.F. Coffin (2004) Impact origin for the Greater Ontong Java Plateau. Earth & Planetary Science Letters. 
  7. Hagstrum J.T. (2005) Antipodal hotspots and bipolar catastrophes: Were oceanic large-body impacts the cause? Earth and Planetary Science Letters.
  8. Meschede M.A. & Myhrvold L. & Tromp J. (2011) Antipodal focusing of seismic waves due to large meteorite impacts on Earth. Geophysical Journal International.
  9. Petrus J.A. & Ames D.E. & Kamber B.S. (2014) On the track of the elusive Sudbury impact: Geochemical evidence for a chondrite or comet bolide. Terra Nova.

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