|The Mesoproterozoic Era|
|Proterozoic||The Stenian Period|
Most of the important events we mentioned in the Mesoproterozoic actually occurred in the Stenian, the last Period of the Mesoproterozoic Era. These include the final assembly of the supercontinent of Rodinia, the evolution of sex probably earlier), and the acme of stromatolite development.
After much searching we have found a site bold enough to make a stab at a map of the Stenian globe. This reconstruction is adapted from the site of Prof. Joseph Meert at the University of Florida, and presumably derives from Pesonen et al 2003). Prof. Meert, if anyone, appreciates the uncertainties of such a reconstruction. Meert & Torsvik (2003). Nevertheless, this is the state of the art.
As Meert & Torsvik explain, the current reconstructions are based largely on paleomagnetic data which are relatively scarce for rocks this old and which are subject to very significant errors. In fact, the central assumption that the Earth's magnetic field can be modeled as a simple dipole running through the center of the planet has been seriously questioned for dates on the order of 500+ Mya. Meert et al. (2003).
Indeed, unique, almost bizarre, tectonic mechanisms are possible when dealing with supercontinents. Meert & Tamrat (2004). These authors offer (how seriously it is hard to tell) a "Hand O'God" or "HOG" hypothesis. They point out that a supercontinent acts as a large thermal cap on the Earth's surface which tends to trap heat. A supercontinent also represents a physical distortion of the planetary surface with sufficient mass to alter, very slightly, the gravitational field of the Earth. In effect, the continent sits on top of a hill of its own making -- almost literally pulling itself up by its own bootstraps.
The build-up of heat under the mantle initiates, at some point, the formation of convection. The point at which this occurs depends critically on a dimensionless constant, the Rayleigh Number of the mantle material. Unfortunately, this number is not known, even within an order of magnitude, since it is a bit difficult to perform quantitative physical analysis on molten rock at several thousand °C. and millions of atmospheres of pressure. However, reasonable figures are believed to lie in the range of 106 to 107. In this region convection will occur and will descend to induce a plume within a reasonable geological time, i.e. on the order of 100 My.
This plume rises back up towards the supercontinent (on a similar time scale) and fractures the supercontinent along old lines of continental subduction. The hot plume also becomes trapped under the thickened crust. Thus, within 200 My, a supercontinent creates its own massive hot-spot, setting off catastrophic tectonic activity. Further, the ultra-hot plume effectively grabs the offending continent and throws it off the raised hot spot like the proverbial divine anterior autopodium, wrenching the doomed land mass apart in the process. Meert & Tamrat (2004).
We should pause here to emphasize that the timing is quite uncertain. It seems evident from Honda et al. 2000), on which Meert & Tamrat rely, that the relevant timescale could be anywhere from 107 up to 1010 years. Without a closer estimate of the Rayleigh number, it is impossible to say. Further, Honda et al. assume constant viscosity, which is an important element of the Rayleigh number. A brief survey of abstracts and lecture notes on the web suggests that mantle viscosities may vary by about two orders of magnitude (by a factor of about 100) depending on depth, heat and chemical composition. In fact, the high temperature, and thus low viscosity, of the plume material may account for the sudden increase in the speed of plate movements as the plume material builds up under the continental mass. So, this may be less the hand o'god than a divine banana peel.
We would put this hypothesis in the filing cabinet marked "interesting but unprovable" were it not for the interesting similarity with the events which are now believed to have occurred at the end of the Permian. At that time, inexplicable and almost unprecedented vulcanism turned much of Siberia into a magmatic bubble-bath. Pangea was ultimately destroyed, and the atmosphere was poisoned, causing Earth's greatest mass extinction. Benton (2003). Prof. Meert has suggested (pers. comm.) that Pangea wasn't around long enough for the HOG effect to apply. However, given the quantitative uncertainties of variable viscosity and Rayleigh Number, the HOG hypothesis may have more generality than its authors give it credit for.
ATW041007. Text public domain. No rights reserved.
Joe's Paleomag Home Page: Prof. Meert's very useful home page. This stop is Rodinia Central on the Time Line.