In last week’s blog I outlined the evidence put forward for a worldwide glaciation in the Neoproterozoic, supported by geophysical and geochemical evidence. However, as with all ground breaking developments, other geologists look at the exposures and geophysical data and come to rather different conclusions.
The basis of the idea is generated by the Neoproterozoic diamictites which have been described as being glacial tillites, but others believe them to be the results of mass flow, turbidites and even volcanic lahars. There is no denying that there was an extensive glaciation at the time, and the evidence of striated pavements and boulders cannot be explained in any other way, but this does not mean that the glaciation was worldwide. The Port Askaig formation in Scotland has been interpreted in two different ways by different geologists; glacial and sub-aqueous mass flow.
The palaeomagnetic data which fixes the position of these rocks in tropical areas is also questioned. Igneous rocks may give a palaeomagnetic direction as they cool through the Curie point, which is over 500°C. If in any subsequent tectonic activity the rock is heated above the Curie point, this initial direction may be lost and replaced by a more recent direction. We are also dealing with the Earth over 570mya; can we be sure that the magnetic field was the same as today, a dipole close to the axis of rotation? The core would have been hotter, which may have led to multiple poles. The further we go back in geological time the more difficult it becomes to interpret the behaviour of the Earth. The Earth’s tilt could have been considerably more than now, and as has been pointed out, if the tilt was more than 45º, the tropics would be less exposed to insolation than the poles and so more prone to glaciation.
The geophysics of the dating has also been questioned; minerals containing uranium that decays into lead are used, mainly in the mineral zircon. But diamictites do not contain these minerals and so geologists use suitable igneous rocks found in the same sequence, often volcanic ash. Snowball Earth demands synchronised ice cover, but the dating could better match multiple glaciations spread over time. There is no denying that this was a time of severe glaciation, but it is argued that there is a great deal of evidence for moving and floating ice, striated boulders and surfaces, and dropstones for example. The phrase ‘slushball Earth’ has been used.
Rodinia, the ancient super-continent, is the key to the alternative model. The glaciation(s) occurred at a time when this landmass was breaking up by rifting. This would create big fault scarps, with unstable areas and down-slope mass movement giving the diamictites, turbidites and other sedimentary rocks deposited under water. The rifting would also give large uplifted blocks which would be high enough, and so cold enough to give more local glaciations, and so the glacial sediments and structures are neatly explained. This happens even now, to a lesser extent, in high tropical regions. The rifting would not all happen simultaneously, and so the problem of the exact timing of the events is overcome.
This debate will be resolved as more research is carried out, hypotheses tested, and as geophysical and geochemical techniques develop. But the Snowball Earth controversy illustrates one of the attractions of studying geology. We do not know all the answers; our knowledge expands and ideas adapt, and every so often there is a big shift in our interpretation and knowledge that causes tremors in the established understanding of the subject.
Sources and references
There are some good sites on the internet covering the topic:
www.snowballearth.org has a comprehensive explanation of the main theory. The BBC put out a very good Horizon programme in 2001 and this can be followed up on www.bbc.co.uk/science following links to Horizon. They also have an interesting news item from 2001, about the Port Askaig tillite on http://news.bbc.co.uk/1/scotland and follow links to snowball Earth related items.
The academic papers concerning the topic are:
Proposing snowball Earth:
Paul F. Hoffman and Daniel P. Schrag: The snowball Earth hypothesis: testing the limits of global change. (In Terra Nova, 14, 129-155, 2002)
Questioning snowball Earth:
Nick Eyles: Glacio-epochs and the supercontinent cycle after ~3.0 Ga: Tectonic boundary conditions for glaciation. (In Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89-129)
For a very good overview:
Ian J. Fairchild and Martin J Kennedy: Neoproterozoic glaciation in the Earth System. (In Journal of the Geological Society, London, Vol. 164, 2007, pp. 895-921.)
Ian Fairchild’s research on this topic can be seen on the University of Birmingham web site; Schools and Departments → Geography, Earth and Environmental Sciences → Staff.