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All at sea?

Dr Joe McCall

JoeMcCall has problems with Panthalassa

Geoscientist 18.10 October 2008

Some 20 years ago I expressed doubts about Panthalassa, a term coined long before plate tectonics appeared. Today, my doubts have increased.

Pangaea is a reality and provides the role model for a tribe of other supercontinents, which have been breeding like rabbits (Ur, Arctica, Nena, Nuna or Columbia, Vaalbara, Kenorland, Rodinia, Pannotia and the 250 million-year-hence Novopangea and Pangea Ultima/Proxima). Geotectonists would seem to have concentrated on the assembly in the past and the break up, still going on, of Pangaea, effected by Wilson Cycles; which require a continental split and an ocean formed between the halves. Little attention seems to have been given to the outside of Pangaea - a vast tract of ocean crust covering more than half the globe. Panthalassa just seems to have gone along with Pangaea, because – well, unless you go for an expanding Earth (see last month's Soapbox) - you have to fill the entire globe!

However, a simple equation tells us that, if Earth/globe dimensions remain constant, and there has been the same amount of continental crust since way back in the Precambrian, as you shrink to form Pangaea by destroying ocean crust, more has to be created elsewhere, outside Pangaea; hence Panthalassa. Also, one cannot invoke a change in global size to one entirely covered by continental crust - the oceans must go somewhere. Panthalassa is a necessity, even if we know little

What we do not seem to know is how this vast spread of ocean crust was generated. In a great tract with no continents, you cannot invoke Wilson cycles starting with continental rifting. Even more difficult is the problem of how this vast tract of ocean crust was destroyed. It must have been present in entirety throughout the hundred million years of Pangaea's life from the Permian into the mid-Jurassic; then it seems to have vanished in a moment of geological time. I have a map of the Earth which accompanied Condie’s Plate Tectonics and Crustal Evolution 1976,and all the present oceans depicted on it appear to be post-middle Jurassic. It has been suggested to me that there is a trace of Panthalassa ocean crust in the Pacific, but I am dubious about this. Furthermore, to destroy by subduction the vast tract of ocean crust that was Panthalassa would take millions and millions of years at present rates of subduction. There is also the problem of the nature and geometry of the process acting externally to Pangaea with no continents there; and one cannot even see how the process of destruction could have started up, externally to Pangaea, in the mid-Jurassic.

There is support for the eventual destruction in time of all ocean crust in Impact Structure statistics. Against a prediction of 300 in the Earth's seas, there are 22 known in shelf and only one, Eltanin - 25km diameter of Pliocene age - in the deep ocean (Bellinghausen Sea), with meteorite particles.

Much of my geological career has been devoted to assembling "ground truth". I think we need a conference, entirely devoted to Panthalassa, bringing in the real brains of modern geotectonics. I suspect that the proceedings will have to rely very much on modelling, rather than direct evidence, but I may be wrong. How much evidence is there in the geological record of Panthalassa’s existence - surely it should at least have produced ophiolites and deep sea sediments with radiolarians?

* Review, Special Publication 37 Gondwana & Tethys: Audley-Charles & Hallam, eds.