The Answer Is Blowing in the (Mantle) Wind


Geology went through a Kuhnian revolution in the 1960s and 1970s, with the emergence of plate tectonics as an explanation, as a mechanism, driving what geologists were observing in the field. Plate tectonics has settled in as a credible, definitive and confirmable explanation of everything from vulcanism to oceanic trenches.

But it’s also an incomplete explanation.

It doesn’t explain, for example, why the plates are moving. Yes, they are demonstrably moving, but why? It explains why there are ranges of volcanoes 60 to 100 miles inland from places where oceanic plate is being stuffed under a continental plate, but what about vulcanism much further from the subduction zone? Some of the most dramatic volcanic episodes in the Earth’s history were 200-300 miles or more from a subduction zone: the Deccan Traps in India and the Washington and Oregon flood basalts, to name two. Plate tectonics nicely explains the apparent movement of volcanic “hotspots” like the Yellowstone hotspot, but isn’t much help in explaining why there are hotspots in the first place.1 Why did the Oregon Cascades arise almost immediately after subduction, while the Andes and Tibet waited tens of millions of years to rise? Plate tectonics has a long list of things it fails to explain.

Plate tectonics is a wonderful and amazing, but also incomplete, explanation for what geologists observe. So far, geology is struggling to fill in the blanks.

A new geologic theory attempts to fill in some of those gaps.

WC has written earlier about “seismic tomography,” using earthquakes and a wide network of seismometers, combined with some serious computing power, to construct an image of Earth’s interior. That technique is being increasingly refined and used across wider scales, giving a larger and more detailed idea of the Upper Mantle and Lower Mantle. And, because this is geology, it’s a lot messier than anyone expected.

Geologists had thought that when subducted plate got into the middle portions of the Upper Mantle – the Mantle Transition Zone – it would melt and mix with the other mantle, losing its differentiation from the surrounding mantle. The seismic tomography suggests otherwise. Here’s a sample from under the Andes.

A slice through Earth’s mantle under the Andes. © Jonny Wu, University of Houston

A slice through Earth’s mantle under the Andes. © Jonny Wu, University of Houston

Geologists assumed that sinking oceanic plates, in the Mantle Transition Zone, would melt completely. The seismic cat scans suggest otherwise, that plate fragments – what the scientists called “slabs” – remain differentiated from the surrounding mantle and continue to sink, descending all the way to the Core-Lower Mantle boundary.

Recent work on how rock acts at high temperatures and under immense pressure – the conditions that exist deep in the Earth’s mantle –  suggest that rock there is neither a solid nor a liquid. Rather, it’s a kind of extremely stiff fluid. The theory is that as those ancient, subducted slabs sink very, very slowly downwards as a consequence of their higher density, they create equally slow currents in the mantle. Call it “slab tectonics,” to differentiate it from the business in the crust of the earth, “plate tectonics.”

Geologists hypothesize that this “mantle current” or “mantle wind” is what drives long-subducted slabs so far under the crust, and what brings the anomalous rock chemistry believed to exist in the Lower Mantle up to the surface. That’s still mostly speculation, but it is fuzzily apparent that slabs remain differentiated from the mantle and sink far lower than anyone had known.

And that geology has a new tool that can peer where science has never seen before.

(For more on slab tectonics and mantle CAT scans see Howard Lee’s article, “Tectonics runs deeper than we thought.”)

 


  1. Sure, geologists talk about “mantle plumes” to explain the hotspots, and talk about “convection in the deep mantle.” But it’s all been handwavium, speculation and theories not supported or ambiguously supported by available evidence. See, for example, Gillian Foulger’s Plates v. Plumes, which shows the flaws in the “deep convection” idea but doesn’t really have a better answer.