Mantle Waves Form Mountains

This ARTICLE, based on THIS PAPER in Nature, suggests that the splitting apart of a continental plate affects not only the area near the rift zone, but also the interior of the continent. 30 or 40 km of the roots of the continent are stripped away and this wave of root erosion moves towards the interior of the craton. And this results in an increase in altitude of the land surface.

I always thought that high land required deep roots - think of the deep crust below the Tibetan plateau. Here the suggestion is that the removal of roots allows the land to rise, like a baloon rising when the sandbags are thrown overboard.

The example cited is the Drakensberg of South Africa. This, and similar, if less impressive formations, form escarpments around the cratons formed at the breakup of Gondwana. The paper suggests that the coastal escarpments of Brazil and the Western Ghats of India have a similar origin.

a, Rifting causes edge-driven convection in the mantle, rift-flank uplift and escarpment formation. b, Rayleigh–Taylor instability migrates along the lithospheric root, resulting in convective removal of the TBL of the lithospheric keel, driving kimberlite volcanism11, isostatic uplift and denudation. c, Escarpment becomes a pinned drainage divide that is locally breached by the main rivers draining the plateau. Meanwhile, the convective instability continues to migrate towards the continental interior, leading to isostatic uplift, a shift in the locus of erosion and plateau formation (Fig. 4).

The authors believe that they explain why the interior of the cratons is so high and also the occurrence of kimberlites.

This is a very complex subject and I do not pretend to understand much of the arguments but read the papers and come to your own conclusions.

 Down to Earth Extra September 2024

The September 2024 edition of Down to Earth Extra has been published. You can download it HERE or you can red it below.

 Orcadian Basin Rock at Stonehenge!

I expect you all have heard that the Altar Stone at Stonehenge has been proved to be from the Old Red Sandstone (ORS) of North East Scotland. The original paper is in Nature and can be read HERE. 

The Altar Stone, seen here underneath two bigger Sarsen stones, which new research suggests is actually Scottish not Welsh © Professor Nick Pearce/PA Wire

Nature has also provided a video which is well worth viewing.

The origen has been established using extremely sophisticated analysis techniques on individual crystals in a couple of thin sections. These proved that the zircon, rutile and apatite crystals present (and there were not many of them!) could only come from the Orcadian Basin. The Nature paper goes into excruciating detail on this, but it is summed up in the following figure.

How the stone got to Stonehenge will, no doubt, lead to furious debate. Tha Nature article posits that it was "anthropogenically transported"! What I find more interesting is why the builders of Stonehenge thought that a stone from so far away was necessary for the site.

 Mantle Drilled

Several sources have alerted me to this item of news. The longest core sample of the mantle (1,286m) has been drilled and will soon be available for examination. It was drilled by the well known drill ship "Joides Resolution" in the North Atlantic near the Mid-Atlantic Ridge, and not far from the Lost City hydrothermal field, which is also well known.

You can read all about it HERE (Nature), HERE (Science) and HERE (The Conversation). What you can read is that it is serpentinized abyssal mantle peridotite interleaved with thin gabbroic intrusions. Other than that they have little to say because, as yet, little has been done with the core. No doubt that will change!

A sample of mantle rock viewed under a microscope.Credit: Johan Lissenberg

 Phosphorus Presence leads to Macrofossils?

It is thought that the lack of phosphorus (P) was what stopped the evolution of unicellular life into complex life in the Pre-Cambrian. The increase in P allowed the Ediacarian and Cambrian fossils to evolve.

In Gabon, the 2100 Ma Paleoproterozoic Francevillian formation, due to peculiar circunstances had high phosphorus (and other element) concentrations. And there are structures which have been interpreted as macrofossils.

Did complex life start 1500 Ma before the Ediacarian and Cambrian? If it did, it was a false start as the peculiar conditions ended and life resumed it unicellular course until the Ediacarian.

As you can imagine, there is much controversy about this. You can see examples of the suspected biota HERE. Some say they are colonial organisms, others that they are not.

You can read all about it HERE. This based on THIS PAPER.

The Fraceville "biota". Are they fossils or are they something else?