A Revolution in Plate Tectonics?

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Chiloe

Trad climber
Lee, NH
Topic Author's Original Post - Sep 10, 2014 - 02:41pm PT
OK, by popular request here's a whole new thread to consolidate things I started on two others. First post is a recap.

A study just published in the Proceedings of the National Academy of Sciences aims to overthrow scientific orthodoxy in a field that I grew up around, and still sometimes follow -- plate tectonics. My sympathies are with the rebels.

This new paper by Anderson and Natland argues that mantle plumes, those cute little upwellings that cause so many cartoon volcanoes, do not exist and never have. If the authors are right (and there is a "small but vocal" group of other geologists and geophysicists who have been making this argument for years) then much of mainstream geology has gone badly astray over the past several decades.

The PNAS abstract and complete paper are here. From the more colorful Huffington Post writeup,

Have scientists had volcanoes all wrong?

A popular theory has it that, at least in certain types of volcanos, eruptions occur when molten rock known as magma gushes up from deep inside the earth via narrow jets known as mantle plumes. But a new study of seismic data has identified one very big hole in the theory:

Mantle plumes don't exist.

"Mantle plumes have never had a sound physical or logical basis," study co-author Dr. Don L. Anderson, professor emeritus of geophysics at Caltech in Pasadena, California, said in a written statement released by the university. "They are akin to Rudyard Kipling's 'Just So Stories,' a reference to the British author's tales offering silly explanations for how giraffes and other animals got their peculiar anatomies."

Mantle plumes were first hypothesized in 1971 and widely adopted among geologists around 1990, Anderson told The Huffington Post in an email. But despite significant research activity over the past couple of decades, the seismic data available to researchers were too spotty either to prove or disprove the existence of the plumes.

According to the new study--co-authored by Dr. James Natland, a professor emeritus of marine geology and geophysics at the University of Miami--robust new data and improved theory show once and for all that those plumes are nowhere to be seen.
Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 02:43pm PT
The Anderson & Natland paper does not come out of the blue; these ideas have been around for years among geologists, but rejected and even actively suppressed by the mainstream. Here is an earlier earlier and extra feisty version of the argument, from someone I know well. Published in Lithos 2011:

Plate tectonics began in Neoproterozoic time, and plumes from deep mantle have never operated

The widely accepted concepts that plate tectonics has operated throughout all or most of geologic time and that plumes from deep mantle have delivered heat and material to the crust are contradicted by powerful multidisciplinary evidence, some of which is summarized here. Pre-Neoproterozoic rocks, individually and as associations, and their geologic and crustal structures, are very different from modern ones, and include none of the definitive indicators of plate interactions that are abundant in the Phanerozoic record. The assumption that plumes rise from deep mantle is derived from false 1950s assumptions that Earth has evolved slowly from a cold start, and survives as dogma despite disproof of its early basis and of its subsequent generalizations and predictions. The history of science contains many gaps between consensus and truth, but most mainline literature presumes that consensus favoring ancient plates and plumes obviates the need for evaluation of assumptions, evidence, and alternatives.
Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 02:47pm PT
A key publication from anti-plume geologists (who have often had to find their own publication venues) is this book by Gillian Foulger. Not light reading (emphasis added).


Since the advent of the mantle plume hypothesis in 1971, scientists have been faced with the problem that its predictions are not confirmed by observation. For thirty years, the usual reaction has been to adapt the hypothesis in numerous ways. As a result, the multitude of current plume variants now amounts to an unfalsifiable hypothesis.

In the early 21st century demand became relentless for a theory that can explain melting anomalies in a way that fits the observations naturally and is forward-predictive. From this the Plate hypothesis emerged–the exact inverse of the Plume hypothesis. The Plate hypothesis attributes melting anomalies to shallow effects directly related to plate tectonics. It rejects the hypothesis that surface volcanism is driven by convection in the deep mantle.

Earth Science is currently in the midst of the kind of paradigm-challenging debate that occurs only rarely in any field. This volume comprises its first handbook. It reviews the Plate and Plume hypotheses, including a clear statement of the former. Thereafter it follows an observational approach, drawing widely from many volcanic regions in chapters on vertical motions of Earth's crust, magma volumes, time-progressions of volcanism, seismic imaging, mantle temperature and geochemistry.
Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 02:54pm PT
Actually the argument is that there *was* no plate tectonics before (depending on sources) something like 600 to 1000ma.

If you check out the Lithos paper you'll see the evidence is very broad based. Seismic data inform the new Anderson paper, but much before that draws on other information, like coastal geology constraints showing the Pacific Plate just did not change direction by 60 degrees, as required for the hot-spot explanation of the Emperor/Hawaii chain.
Jaybro

Social climber
Wolf City, Wyoming
Sep 10, 2014 - 02:55pm PT
Makes sense. So what then, made Hawaii?
Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 02:56pm PT
So what then, made Hawaii?

A propagating crack or weakness, which unlike the whole Pacific plate really could make sudden change in direction.
Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 02:58pm PT
So how to they reconcile the seemingly stationary nature of the Hawaiian hotspot relative to shallow mantle convection?

It's not actually stationary, that turns out to have been an assumption. If you try to define a stationary georeference system using Hawaii and other alleged hotspots, you find out they don't work. All are moving relative to each other, hence they can't be fixed from deep sources. All kinds of fancy hypotheses such as mantle "winds" were invented to explain things away as hotspot predictions failed every empirical test.
Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 03:01pm PT
What drives this?

Good question, good picture. See that 60 degree bend? It's conventionally explained as a change in direction of movement by the whole Pacific plate, gliding over a fixed hotspot. Except we know from coastal geology, which constrains Pacific plate movements pretty well, that such a change did not happen.

So the alternative is that you're looking at something like a propagating crack or weakness. Volcanism does not require a new heat source, rocks below the crust are already hot. What it requires is a release of pressure above.
squishy

Mountain climber
Sep 10, 2014 - 03:01pm PT
http://www.liveleak.com/view?i=4b9_1410369832
Clint Cummins

Trad climber
SF Bay area, CA
Sep 10, 2014 - 03:08pm PT
Cool. Thanks for sharing, Larry.

Is there a theory for what started the plates moving?
[Edited:] I see it is something about density inversion.
Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 03:16pm PT
Like I said, not light reading, but here is an excerpt from the Lithos paper cited above, explaining why everything you learned about Hawaii is wrong. Note the reference in this 2011 paper to "Anderson, in press." That's the same Anderson who wrote today's new PNAS paper; not sure whether it is the same paper.

All published tomographic models of purported deep plumes are severely flawed, but I discuss here only Hawaii, which provides the type example for rationalization of a “plume track” while disregarding both observed tectonic controls of magmatism and failure of geophysical predictions in plume speculations (Anderson, in press). Pro-plume tomographers Wolfe et al. (2009) depicted a low-velocity plume rising through much of the upper mantle beneath the Hawaiian region, and a disconnected narrow plume rising obliquely northwestward toward it from a depth of 1500 km in the lower mantle. Wolfe et al. modeled only steeply rising teleseismic S waves to calculate uppermantle structure, and only steeply rising SKS waves to calculate midmantle structure with rays that came through the liquid core via phase conversions. The narrow seismometer spread precluded sampling deep mantle beneath the islands with moderately and gently inclined crossfire, and Wolfe et al. did not utilize any other steeply rising S and P rays that would have increased coverage, nor did they incorporate any surface waves, receiver functions, or Vp/Vs derivatives to constrain depths, amplitudes, and characters of possible anomalies.Wolfe et al. truncated their published model downward at 2000 km, but the narrow bundle of SKS rays that alone defined their purported lower-mantle plume rose northwestward through a poorly known lowermost-mantle region of low velocity (likely recording high iron content and high density, not high temperature), which they acknowledged could be modeled as their plume — but they claimed a plume to provide the “simplest” explanation. Wolfe et al. forced their S-wave time delay deep into the upper mantle by assuming that only moderate retardation occurred within either the crust or a shallow magma-generating system. Leahy et al. (2010; Wolfe was second author) showed, with receiver-function analysis of the same seismometer records, that the upper-mantle “plume” of Wolfe et al. (2009) was the product of downward smearing of the time delay within thickened Hawaiian Swell crust. Leahy et al. acknowledged previous observational proof that the Hawaiian region lacked plume-predicted high heat flow.
Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 03:20pm PT
But that would be upper mantle, wouldn't it? Well above the 660km discontinuity?
klk

Trad climber
cali
Sep 10, 2014 - 03:25pm PT
I hadn't thought of the plume model as the dominant model. But then I'm not in the field. Most of what I knew about this debate came from Anderson and Natland, "A brief history of the plume hypothesis and its competitors" (2005):

http://books.google.com/books?id=0z74GC0rA5kC&pg=PA140&lpg=PA140&dq=william+dickinson+plate+plume&source=bl&ots=f6gTddZ3Vo&sig=VGYUwUXGc_BlFZWvSwI0zh3E7ak&hl=en&sa=X&ei=eM4QVMLAPKa5iwK06IHQCw&ved=0CBwQ6AEwAQ#v=onepage&q=william%20dickinson%20plate%20plume

skcreidc

Social climber
SD, CA
Sep 10, 2014 - 03:33pm PT
Just to throw my 2 cents out there, this falls in the realm of what was called geo-fantasy or arm waving by the guys and gals who look at rocks and fossils. It IS about time we moved past concepts like Mantle winds if you ask me. I will also suggest that this is more of a beginning than an end of the discussion of how our planet works.
NutAgain!

Trad climber
South Pasadena, CA
Sep 10, 2014 - 03:43pm PT
If we have any real scientists who publish in peer reviewed journals here, please speak up to refute or deny this. I've heard that PNAS is not really a peer reviewed journal; rather, it is a forum for people who have already established themselves in their field through other peer-reviewed publications (like Science, Nature, Cell, etc), people who have been admitted to the National Academy of Sciences.

So it is better than cheezeball or corporate publications, in that it reflects the integrity of a real successful scientist, but the methods and data from which conclusions are drawn are not subject to acceptance by their peers before publication.

Is this true?
Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 03:50pm PT
I've heard that PNAS is not really a peer reviewed journal; rather, it is a forum for people who have already established themselves in their field through other peer-reviewed publications (like Science, Nature, Cell, etc), people who have been admitted to the National Academy of Sciences.

I know PNAS is peer reviewed because I've done peer reviews for them, not on this topic of course. But here's Wikipedia's better response.

All research papers published in PNAS are peer-reviewed.[1] The standard mode is for papers to be submitted directly to PNAS rather than going through an Academy member. Members may handle the peer review process for up to 4 of their own papers per year—this is an open review process because the member selects and communicates directly with the referees. These submissions and reviews, like all for PNAS, are evaluated for publication by the PNAS Editorial Board. Until July 1, 2010, members were allowed to communicate up to 2 papers from non-members to PNAS every year. The review process for these papers was anonymous in that the identities of the referees were not revealed to the authors. Referees were selected by the NAS member.[1][2][3] PNAS eliminated communicated submissions through NAS members as of July 1, 2010, while continuing to make the final decision on all PNAS papers.[4]

The Lithos paper was certainly peer reviewed, and others include a newer one in Tectonophysics that I'll get 'round to citing later. But part of the problem is that major publication outlets in the field have been controlled by people with career investment in plumology.
Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 04:03pm PT
I will also suggest that this is more of a beginning than an end of the discussion of how our planet works.

Maybe so, if the revolution gains traction now that it's out in the open. From "An Alternative Earth" (dig the title!) published in GSA Today (2003). Emphasis added.

The standard Earth of geodynamics and geochemistry is rationalized from assumptions that the mantle is compositionally inverted—still-unfractionated lower mantle beneath volatile-depleted upper mantle—and that material circulates easily from bottom to top. Multidisciplinary data better fit a less-volatile and less-radioactive planet wherein depleted lower mantle, fractionated early and irreversibly, is decoupled from upper mantle plus crust that evolve and circulate separately. Early Archean fractionation produced global(?) felsic crust and refractory upper mantle. Later Archean granite-and-greenstone upper crust formed atop this ancient crust, which remained hot and weak; distinct continents and oceans did not exist, and upper mantle was much hotter than now. Plate tectonics began ca. 2.0 Ga when continents could stand above oceans and oceanic lithosphere could cool to subduction-enabling density and thickness. Upper mantle has since become more fertile and new increments of continental crust more mafic as continental crust has been progressively diminished by recycling into cooling mantle. Plate circulation is driven by subduction, which is enabled by density inversion produced by sea- water cooling from the top of oceanic lithosphere, is self-organized, and is confined to upper mantle. The matching rates of hinge rollback and of advance of fronts of overriding plates are keys to dynamics. Slabs sinking broadside from retreating hinges drive both subducting and overriding plates and force seafloor spreading in both shrinking and expanding oceans. An Antarctica-fixed framework depicts prediction-confirming “absolute” plate motions that make kinematic sense, whereas hotspot and no-net-rotation frames do not. Plumes from deep mantle, subduction into deep mantle, and bottom-up convective drive do not exist.

More recent work brings the start of plate tectonics a billion years or more closer to the present.
skcreidc

Social climber
SD, CA
Sep 10, 2014 - 04:05pm PT
Hmmmm. This could wreak havoc on the interpretation of igneous source areas.
klk

Trad climber
cali
Sep 10, 2014 - 04:20pm PT
PNAS is a peer-reviewed journal and one of the prestigious ones, at that, in part because it publishes work from a variety of disciplines.

Stuff that appears there is often the kind of work that will or at least could be of interest to folks working in fields outside the author's home discipline.


is this behind the pay wall?

http://www.pnas.org/site/aboutpnas/index.xhtml#PNAS_Online


Chiloe

Trad climber
Lee, NH
Topic Author's Reply - Sep 10, 2014 - 04:31pm PT
If you're asking about the Anderson/Natland PNAS paper I don't think that's paywalled:

http://authors.library.caltech.edu/49341/1/PNAS-2014-Anderson-1410229111.pdf

And the Hamilton Lithos paper is here:

http://www.mantleplumes.org/WebDocuments/Hamillton2011.pdf

There is a lot more of this stuff, none of it light reading.
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