Guest “Did you know?” by David Middleton
An upwelling of rock beneath the Atlantic may drive continents apart
The Mid-Atlantic Ridge may play a more active role in plate tectonics than thoughtBy Maria Temming
FEBRUARY 4, 2021
An upsurge of hot rock from deep beneath the Atlantic Ocean may be driving the continents on either side apart.
The Americas are moving away from Europe and Africa by a few centimeters each year, as the tectonic plates underlying those continents drift apart. Researchers typically think tectonic plates separate as the distant edges of those plates sink down into Earth’s mantle, creating a gap (SN: 1/13/21). Material from the upper mantle then seeps up through the rift between the plates to fill in the seafloor.
But new seismic data from the Atlantic Ocean floor show that hot rock is welling up beneath a seafloor rift called the Mid-Atlantic Ridge from hundreds of kilometers deep in Earth’s mantle. This suggests that material rising up under the ridge is not just a passive response to tectonic plates sliding apart. Rather, deep rock pushing toward Earth’s surface may be driving a wedge between the plates that helps separate them, researchers report online January 27 in Nature.
A better understanding of plate tectonics — which causes earthquakes and volcanic eruptions — could help people better prepare for these natural disasters (SN: 9/3/17).
[…]
Science News
Spreading centers drive continents apart? Who could have guessed? Uhm… Just about everyone since at least the late 1960’s.
What drives the plates?
From seismic and other geophysical evidence and laboratory experiments, scientists generally agree with Harry Hess’ theory that the plate-driving force is the slow movement of hot, softened mantle that lies below the rigid plates. This idea was first considered in the 1930s by Arthur Holmes, the English geologist who later influenced Harry Hess’ thinking about seafloor spreading. Holmes speculated that the circular motion of the mantle carried the continents along in much the same way as a conveyor belt. However, at the time that Wegener proposed his theory of continental drift, most scientists still believed the Earth was a solid, motionless body. We now know better. As J. Tuzo Wilson eloquently stated in 1968, “The earth, instead of appearing as an inert statue, is a living, mobile thing.” Both the Earth’s surface and its interior are in motion. Below the lithospheric plates, at some depth the mantle is partially molten and can flow, albeit slowly, in response to steady forces applied for long periods of time. Just as a solid metal like steel, when exposed to heat and pressure, can be softened and take different shapes, so too can solid rock in the mantle when subjected to heat and pressure in the Earth’s interior over millions of years.The mobile rock beneath the rigid plates is believed to be moving in a circular manner somewhat like a pot of thick soup when heated to boiling. The heated soup rises to the surface, spreads and begins to cool, and then sinks back to the bottom of the pot where it is reheated and rises again. This cycle is repeated over and over to generate what scientists call a convection cell or convective flow. While convective flow can be observed easily in a pot of boiling soup, the idea of such a process stirring up the Earth’s interior is much more difficult to grasp. While we know that convective motion in the Earth is much, much slower than that of boiling soup, many unanswered questions remain: How many convection cells exist? Where and how do they originate? What is their structure?
Convection cannot take place without a source of heat. Heat within the Earth comes from two main sources: radioactive decay and residual heat. Radioactive decay, a spontaneous process that is the basis of “isotopic clocks” used to date rocks, involves the loss of particles from the nucleus of an isotope (the parent) to form an isotope of a new element (the daughter). The radioactive decay of naturally occurring chemical elements — most notably uranium, thorium, and potassium — releases energy in the form of heat, which slowly migrates toward the Earth’s surface. Residual heat is gravitational energy left over from the formation of the Earth — 4.6 billion years ago — by the “falling together” and compression of cosmic debris. How and why the escape of interior heat becomes concentrated in certain regions to form convection cells remains a mystery.
[…]
USGS
From the time I graduated from college in 1980, up until a few minutes ago (when I read the Science News article), I assumed that every geoscientist knew that mantle convection drove seafloor spreading, which in turn drove continents apart.
However, it appears that this was turned bass-ackwards while I was busy looking for oil & gas, rather than reversing cause and effect. In much the same manner that atmospheric CO2 suddenly became the geological driver of climate change in 1988, subduction apparently became the driver of seafloor spreading…
Until the 1990s, prevailing explanations about what drives plate tectonics have emphasized mantle convection, and most earth scientists believed that seafloor spreading was the primary mechanism.
Cold, denser material convects downward and hotter, lighter material rises because of gravity; this movement of material is an essential part of convection. In addition to the convective forces, some geologists argue that the intrusion of magma into the spreading ridge provides an additional force (called “ridge push”) to propel and maintain plate movement. Thus, subduction processes are considered to be secondary, a logical but largely passive consequence of seafloor spreading. In recent years however, the tide has turned. Most scientists now favor the notion that forces associated with subduction are more important than seafloor spreading. Professor Seiya Uyeda (Tokai University, Japan), a world-renowned expert in plate tectonics, concluded in his keynote address at a major scientific conference on subduction processes in June 1994 that “subduction . . . plays a more fundamental role than seafloor spreading in shaping the earth’s surface features” and “running the plate tectonic machinery.” The gravity-controlled sinking of a cold, denser oceanic slab into the subduction zone (called “slab pull”) — dragging the rest of the plate along with it — is now considered to be the driving force of plate tectonics.
We know that forces at work deep within the Earth’s interior drive plate motion, but we may never fully understand the details. At present, none of the proposed mechanisms can explain all the facets of plate movement; because these forces are buried so deeply, no mechanism can be tested directly and proven beyond reasonable doubt. The fact that the tectonic plates have moved in the past and are still moving today is beyond dispute, but the details of why and how they move will continue to challenge scientists far into the future.
USGS
Having graduated in 79 and 82 with geology and geophysics rectangular sheets of paper, I thought I had learned everything about the evolution of continental drift to plate tectonics at that time. However, the name Arthur Holmes is new to me. Thanks. You’ve contributed to my goal of learning something new every day.
Some geologists predict the formation of another single super continent in the far future. The Pacific northwest has a Paciic plate diving under a North American plate and the N. American plate is dragged downward until the pressure builds and results in a snap back ….earthquake…followed by a tidal wave….it is almost clock work for long term geology. Drill cores can be taken near the coast and sand layers indicate the tidal waves.
As long as we are telling tales about plate tectonics (Sea Floor Spreading) I may as well add one about a disappointed scientist. Morley was a Canadian geologist who had gotten stuck in the administration of scientific grants and programs as I recall. About a year before the famous Vine and Matthews paper, he had noticed the odd pattern of remanent magnetism in sea floor data, and sent a short note to Nature or perhaps it was science, outlining the idea of sea floor spreading which was rejected as too speculative. I see now that the Vine and Matthews hypothesis is occasionally called the Matthews Morley hypothesis. There is some justice after all.
Civil Engineering 101: No matter what your structural analysis might come up with, you can not push on a rope or cable. If you’ve got a solution that has a cable or rope in compression, you did something very wrong.
I’ll also add that putting concrete in tension is also usually very bad idea unless it has lots of rebar and possibly pre/post-tensioning.
I suspect the same is true for Earth’s plates.
Rocks are not very similar to rope… But if they were, mantle convection cells would be pulleys.
See the Pattern raises serious questions about the mechanics of plate tectonics https://youtu.be/8qoTs7w22r4
This brings to mind a study I read as an undergrad way way back in the early ’80s. It was titled something like ‘What kind of tool do we need to pick up Texas?’ It was an analysis from a rock mechanics point of view. Small scale rocks can be brittle and strong, but scaled up hugely they are very much weaker. Various hypothetical tools were examined for lifting Texas, such as drilling holes, putting in pins and lifting it with a gigantic crane. Not surprisingly, it would crumble. Turns out the only way to lift it is with a giant shovel.
Now applying this lesson to plate tectonics we should see that a tectonic plate is too weak to be pulled or pushed, it needs support (or force) applied all over, like Texas, i.e. a convection cell.
Bravo.
Is the circular movement of earth’s molten interior a 3-dimensional Coriolis effect due to the rotation of the earth, analogous to the 2-dimensional Coriolis effect as seen on storms on earth’s surface?
There are about 40,000 miles of mid-ocean ridges all over the Earth. For example, there is a mid-ocean ridge that surrounds Antarctica and so on.
The physical evidence for mid-ocean ridges is robust and overwhelming.
The physical evidence for subduction is much less robust.
There are two locations, one in the North Pacific and one in the South Pacific Ocean off the Americas where observations and measurements have shown that the plates go back and forth (East and West).as measured after moderate earthquakes in the areas.
The pressure needed to “push” the plates under the continents would be huge.
So much that a back and forth motion would only happen with tremendous counter forces and resulting huge earthquakes, not moderate ones.
Geology has a limited knowledge of what goes on below the Earth’s surface.
Consider: the Earth grows.
There is a jump up and down ‘breakthrough’ in geology. I kid you not. The earth is changing in real time. See observations below that the mid ocean ridges all over the planet suddenly started to increase their spreading rate in 1997. 300% increase in mid-ocean earthquakes at the ocean ridges all over the planet.
All over the planet. How is that even possible?
The explanation of tectonic plate motion is the same explanation for the origin of the water that covers 70% of the surface of the earth.
The tectonic plates are moved by CH4 that is extruded from the liquid core of the earth when it crystallizes. The CH4 at high temperatures and pressure binds with metals. That property created a sheath around the liquid CH4 that is extruded from the liquid core.
The sheath keeps the CH4 in a tube like structure that cares the CH4 and the force from the core to the surface of the planet.
The earth’s core started to crystallize about 1 billion years ago. That started the massive increase in CH4 that was pushed up to the surface of the planet.
The start of core extruded CH4 created the earth’s deep oceans. And explains why advanced life did not appear on the earth until after core crystallizations and at the time of the first appearance of deep oceans on the earth.
Why deep oceans gave life to the first big, complex organisms
What happened on 570 million years ago for the earth ….
To produce the ‘first’ deep oceans on the earth?
There is an old unsolved paradox to explain Cambrian Explosion of advance life, 570 million years ago.
https://www.sciencedaily.com/releases/2018/12/181212134354.htm
Why deep oceans gave life to the first big, complex organisms
Why did the first big, complex organisms spring to life in deep, dark oceans where food was scarce? A new study finds great depths provided a stable, life-sustaining refuge from wild temperature swings in the shallows.
In the beginning, life was small. For billions of years, all life on Earth was microscopic, consisting mostly of single cells.
Then suddenly, about 570 million years ago, complex organisms including animals with soft, sponge-like bodies up to a meter long sprang to life. And for 15 million years, life at this size and complexity existed only in deep water
All over the planet starting in 1994, there was suddenly a 300% increase in the frequency of earthquakes of magnitude 4 to 6 all along the ridges where the sea floor is being pushed apart.
The mid-ocean ridges are thousands of miles long. At these very long regions, the ocean ridge is pushed apart in two directions. At ridges where there is sufficient force (Pacific ocean) to move the ocean plates apart fast, there is constant magma coming up at the ridge.
This increase in the frequency of mid ocean earthquakes, physically absolutely must have a cause and the only physically possible cause is there must be a physical system in the earth that suddenly can cause unbelievably massive ocean ridges to be pushed apart.
So, what we have ‘discovered’ (this is an observational discovery not a theory): There is mysterious unknown ‘force’ that Geology has not found that is moving the ocean plates and …
This is the logical implications of the observations, kicker. This mysterious force that moves the ocean plates can increase by a 300%. The mysterious force is changing in real time.
Curiously and unbelievably, It is a fact that Geology does not have an explanation as to what ‘moves’ the massive ocean plates and what moves the massive continental plates.
“As detailed in those studies, increasing seismic activity in these submarine volcanic complexes is a proxy indicator of heightened underwater geothermal flux, a forcing mechanism that destabilizes the overlying water column. This forcing accelerates the thermohaline circulation while enhancing thermobaric convection [3-6]. This, in turn, results in increased heat transport into the Arctic (i.e., the “Arctic Amplification”), a prominent feature of earth’s recent warming [7-9].”
https://www.omicsonline.org/open-access/have-global-temperatures-reached-a-tipping-point-2573-458X-1000149.pdf
https://www.newgeology.us/presentation21.html
“In recent years, the kinematics of continental drift and sea-floor spreading have been successfully described by the theory of plate tectonics.
However, rather little is known about the driving mechanisms of plate tectonics, although various types of forces have been suggested”14. Seven years later, in 1982, the assessment was:
“At the present time the geometry of plate movements is largely understood, but the driving mechanism of plate tectonics remains elusive”3. By 1995 we find that:
“In spite of all the mysteries this picture of moving tectonic plates has solved, it has a central, unsolved mystery of its own:
What drives the plates in the first place? ‘[That] has got to be one of the more fundamental problems in plate tectonics,’ notes geodynamicist Richard O’Connell of Harvard University.
‘It’s interesting it has stayed around so long’ “25. In 2002 it could be said that: “Although the concept of plates moving on Earth’s surface is universally accepted, it is less clear which forces cause that motion.
“The driving force of plate movements was initially claimed to be mantle deep convection currents welling up beneath midocean ridges, with downwelling occurring beneath ocean trenches.
Since the existence of layering in the mantle was considered to render whole-mantle convection unlikely, two layer convection models were also proposed.
Jeffreys (1974) argued that convection cannot take place because it is a self-damping process, as described by the Lomnitz law.
Plate tectonicists expected seismic tomography to provide clear evidence of a well-organized convection-cell pattern, but it has actually provided strong evidence against the existence of large, plate-propelling convection cells in the upper mantle (Anderson, Tanimoto, and Zhang, 1992).
Many geologists now think that mantle convection is a result of plate motion rather than its cause and that it is shallow rather than mantle deep (McGeary and Plummer, 1998).”
Your 570 Ma is Pre-Cambrian, ie Ediacaran Period (635-541 Ma), last of the Neoproterozoic Era of the Protetozoic Eon. The Cambrian (541-485 Ma) is the first period of the Paleozoic Era of our present Phanerozoic Eon.
The fact of subduction has been observed and can be measured.
Ah, the old slab-pull diagram … allegedly transmitted through fault and joint desiccated crust, above an occasionally partially-melted crustal root … that does not seem to move with it … oh well … the show must go on …
I don’t know whether to laugh or cry.
An upwelling of rock beneath the Atlantic may drive continents apart
The Mid-Atlantic Ridge may play a more active role in plate tectonics than thought – quote
No, really? I learned that in high school back in the Dark Ages when kids were taught Real Science. What happened to that strange idea – teach the REAL stuff instead of twaddle?
Mantle convection does not exist (there is zero evidence for it occurring and there is a computer simulations that show it does not happen in the earth. The mantel motion is from falling slabs and moving plates.) and mantle convection does not move the earth’s plates.
A simple calculation indicates the force moves the ocean plate is orders of magnitude more that the greatest possible force a convection drive system could generate.
“Computer models showed that internally heated (and/or surface cooled) systems have no upwelling sheets or plumes and that all concentrated flow originates in the upper cold boundary layer, which stirs the interior as it sinks.
Thus it became natural to regard plates of lithosphere as driving themselves and, incidentally, stirring the rest of the mantle”5.
“Another difficulty is that if this is currently the main mechanism, the major convection cells would have to have about half the width of the large oceans, with a pattern of motion that would have to be more or less constant over very large areas under the lithosphere.
This would fail to explain the relative motion of plates with irregularly shaped margins at the Mid-Atlantic ridge and Carlsberg ridge, and the motion of small plates, such as the Caribbean and the Philippine plates”19.
Some researchers make the point emphatically: “convection does not drive plates.” Upper mantle convection is a product, not a cause, of plate motions20. Thus the location and orientation of a sinking slab is the best indicator of which way upper mantle flows.”
This is an excellent summary of this issue.
https://www.newgeology.us/presentation21.html
What drives the plates?
Study of the motions of plates is called kinematics, while study of the driving forces is called dynamics. “A key to the simplicity of plate tectonics is that the strength of lithospheric plates enables the analysis of their kinematics to be isolated and treated separately from the dynamic processes controlling plate motions; relative velocities of plates can be analysed without reference to the forces that give rise to them”34.
Around the end of the first decade of dominance by plate tectonics, in 1975, the situation was described this way: “In recent years, the kinematics of continental drift and sea-floor spreading have been successfully described by the theory of plate tectonics. However, rather little is known about the driving mechanisms of plate tectonics, although various types of forces have been suggested”14. Seven years later, in 1982, the assessment was:
“At the present time the geometry of plate movements is largely understood, but the driving mechanism of plate tectonics remains elusive”3.
By 1995 we find that: “In spite of all the mysteries this picture of moving tectonic plates has solved, it has a central, unsolved mystery of its own: What drives the plates in the first place?
‘[That] has got to be one of the more fundamental problems in plate tectonics,’ notes geodynamicist Richard O’Connell of Harvard University. ‘It’s interesting it has stayed around so long’ “25.
In 2002 it could be said that: “Although the concept of plates moving on Earth’s surface is universally accepted, it is less clear which forces cause that motion. Understanding the mechanism of plate tectonics is one of the most important problems in the geosciences”8.
A 2004 paper noted that “considerable debate remains about the driving forces of the tectonic plates and their relative contribution”40.
“Alfred Wegener’s theory of continental drift died in 1926, primarily because no one could suggest an acceptable driving mechanism. In an ironical twist, continental drift (now generalized to plate tectonics) is almost universally accepted, but we still do not understand the driving mechanism in anything other than the most general terms”2.
The problem has always been that it is hard to discern what is going on deep in the Earth, motion is almost imperceptably slow, and different combinations of forces, perhaps varying over time, could apply to particular areas.
“When the concepts of convection and plate tectonics were first developing, many thought of mantle convection as a process heated from below, which in turn exerts driving tractions on the base of a relatively stagnant ‘crust’ (later, ‘lithosphere’) to cause continental drift.
In the early 1970s, more sophisticated understanding of convection led to the opposite view. It was realized that only a fraction of the Earth’s heat flow originates in the core, while most results from radioactivity and/or secular cooling of the mantle.
Computer models showed that internally heated (and/or surface cooled) systems have no upwelling sheets or plumes and that all concentrated flow originates in the upper cold boundary layer, which stirs the interior as it sinks.
Thus it became natural to regard plates of lithosphere as driving themselves and, incidentally, stirring the rest of the mantle”5.
Some researchers make the point emphatically: “convection does not drive plates.” Upper mantle convection is a product, not a cause, of plate motions20. Thus the location and orientation of a sinking slab is the best indicator of which way upper mantle flows.
“The advent of plate tectonics made the classical mantle convection hypothesis even more untenable. For instance, the supposition that mid-oceanic ridges are the site of upwelling and trenches are that of sinking of the large scale convective flow cannot be valid, because it is now established that actively spreading, oceanic ridges migrate and often collide with trenches”14.
I beg to differ with you on your statement that mantle plumes do not exist. Especially using a computer model as one of your primary pieces of evidence. Models, as we all know, will give you any answer you want based on the inputs. Real geology relies hard facts and earth tomography has shown very strong evidence of low density (hot) material rising from the upper mantle into the lower lithosphere. Call it a mantle “plume” of something else, I don’t care but you can call it evidence that a mechanism exists to get hot (possibly melted) mantle rock into high crustal positions (see Yellowstone, etc.)
Ok, so…”Mantle convection does not exist.”
But a few sentences later…
“Upper mantle convection is a product, not a cause, of plate motions.”
Which is it?
The correct answer is, neither.
Both of those assertions are incorrect.
Do not bother thanking me.
I suggest that the writer, a 1980 grad, reread Forsyth and Uyeda (1975), which demonstrated that plate forces (and gravity) are the primary drivers of plate motion (i.e. “geodynamics”), and that convection is a secondary consequence, and is not itself the primary driver.
https://academic.oup.com/gji/article/43/1/163/586101
Also, since we’re talking about plate tectonics here, it’s important to keep in mind that the original theory of plate tectonics by McKenzie, Le Pichon, et al. was entirely kinematic, i.e. it merely described plate motion and was entirely agnostic with respect to the cause.
Demonstrated, or asserted speculatively?
As a geologist I’m not particularly upset with “slab pull” as a mechanism as I am with the wildly uninformed “news report”. All intro geology classes taught include plate tectonics (at least the simplified version). Even someone that has not had intro geology can easily find basic plate tectonics online. Where was the editor?
30 years ago (and as far as I know until recently had remained) the leading candidate for spreading was gravity pull. This was accomplished by the increasing density of oceanic crust as it moved from the thermal inflated spreading center into the deeper ocean basin. Gravity is fairly weak so the process was enhanced by convection in the mantle The idea that magma was “pushing” the plates apart never worked given the divergent/tensional process that it is. All rock evidence from spreading ridges and ophiolite complex investigations showed that the spreading process is overwhelmingly passive (axial riff valleys anyone?).
I am now “woke” to the “slab pull” mechanism idea which has been boosted by sophisticated mantle tomography analysis that shows the intactness (is that a word?) of subducted slabs to great mantle depths. By running-back subduction there has been great insight into former oceanic plate positions and configurations among others. The short answer are slab walls where the subducted piles up like soft-serve ice cream or frozen yoghurt anchoring the subducted slab which provides a new insight as to why continental plates converge on oceanic plate/oceanic plate subduction zones (they appear to remain stationary).
As to the brittleness of oceanic crust, concrete and shallow upper crust rock are very poor analogues for modeling oceanic plate lithosphere behavior. There are many factors involved in the cohesiveness of a 5 KM thick piece of complicated oceanic crust (not all basalt but including gabbro, diabase, etc.). Obviously given time and temperature that crust can bend/flex or we would not have subduction zones mappable in the lithosphere and the mantle.
Part of the reason why people have so many misconceptions is that the diagrams in textbooks are interpreted literally, even though they have hugely exaggerated proportions and are many orders of magnitude out of scale.
If the Earth was shrunk to the size of a cue ball, it would feel as smooth as polished glass if held in in one’s hands.
The crust would be a skin so thin that…
Well, let’s consider that there question.
Oceanic crust is 3 to 6 miles thick, and ocean plates areas are hundreds to thousands of miles in horizontal extent.
Continents are thicker but still mostly many tens to as much as hundreds of times wider than they are thick.
Earth is about 8000 miles wide.
The crust is a skin than is 24,000 miles in circumference, but on average roughly ten miles thick.
So the planet is somewhere around a thousand times wider than the skin encasing it which we call the crust.
So the crust is proportionally as thick compared to the whole Earth, as the curb outside a hundred story high building is to the building itself.
How about if the Earth was an apple?
How thick is the crust compared to the skin of an apple?
Let’s go metric to make this more better to calculate.
An apple is about 3 or 4 inches in diameter.
8 to 10 centimeters of shall we just say 100 millimeters for an apple that is four inches high, roughly.
Skin of an apple is variable, but many have skin about a millimeter thick, and sometimes less than that.
For the Earth’s crust to be the same proportion as an apple skin, the apple would have to have a skin a tenth of a millimeter thick or so.
That is about the same as a human hair…if the person has very thick hair…IOW not a blonde or a redhead.
So the crust of the Earth is proportionally about as thick as hair is to an apple.
And it is far from uniform.
The part where all of the pushing and pulling is going on is far thinner.
The thinnest is near the spreading centers.
We are talking fine haired blonde for the crust in these areas. Silky.
Even the thickest and densest oceanic crust near the subduction zones is thinner than the average used in the above rough calculation. So yeah…the crust is several kilometers thick…but it is thousands of KM wide.
It is weak enough so that a inch a year of motion…as fast and fingernails grow, tears open the whole skin of the planet and let’s the juicy tootsiepop center come pouring out.
And when two pieces run into each other at that velocity (fast as fingernails grow)…it crumples up into miles high jagged mountains of solid rock. Some of them crumples are as high as commercial jets fly.
Ocean floor plate is very weak rock. Just look how easily and cleanly it shears at all of the many transverse faults.
It is weak for one thing b/c of how it forms.
It is like Ritz cracker crumbs held together with spit.
I’m glad you raised this. I get “Science” too – and I thought I was seeing things! (I had to check the cover in case I might have mistakenly picked-up “Woo Scientist”)
I’ve acquired the paper now. (It wasn’t in “Science”, it was in “Nature”.)
It is titled “A thin mantle transition zone beneath the equatorial Mid-Atlantic Ridge”.
The authors are:
Matthew R. Agius (Ocean and Earth Science, University of Southampton, Dipartimento di Scienze, Università degli studi Roma Tre)
Catherine A. Rychert (Ocean and Earth Science, University of Southampton)
Nicholas Harmon (Ocean and Earth Science, University of Southampton)
Saikiran Tharimena1,(Institute for Meteorology and Geophysics, University of Vienna)
J.-Michael Kendall (Department of Earth Sciences, University of Oxford)
FWIW the abstract reads: (emphasis mine)
The location and degree of material transfer between the upper and lower mantle are
key to the Earth’s thermal and chemical evolution. Sinking slabs and rising plumes are
generally accepted as locations of transfer1,2, whereas mid-ocean ridges are not
typically assumed to have a role3. However, tight constraints from in situ
measurements at ridges have proved to be challenging. Here we use receiver
functions that reveal the conversion of primary to secondary seismic waves to image
the discontinuities that bound the mantle transition zone, using ocean bottom
seismic data from the equatorial Mid-Atlantic Ridge. Our images show that the seismic
discontinuity at depths of about 660 kilometres is broadly uplifted by 10 ± 4
kilometres over a swath about 600 kilometres wide and that the 410-kilometre
discontinuity is depressed by 5 ± 4 kilometres. This thinning of the mantle transition
zone is coincident with slow shear-wave velocities in the mantle, from global seismic
tomography4–7. In addition, seismic velocities in the mantle transition zone beneath
the Mid-Atlantic Ridge are on average slower than those beneath older Atlantic Ocean
seafloor. The observations imply material transfer from the lower to the upper
mantle—either continuous or punctuated—that is linked to the Mid-Atlantic Ridge.
Given the length and longevity of the mid-ocean ridge system, this implies that
whole-mantle convection may be more prevalent than previously thought, with ridge
upwellings having a role in counterbalancing slab downwellings.
References 1, 2 & (especially!) 3 are:
1. van der Hilst, R. D. Complex morphology of subducted lithosphere in the mantle beneath
the Tonga trench. Nature 374, 154–157 (1995).
2. Montelli, R. et al. Finite-frequency tomography reveals a variety of plumes in the mantle.
Science 303, 338–343 (2004).
3. Hofmann, A. W. Mantle geochemistry: the message from oceanic volcanism. Nature 385,
219–229 (1997).
This is tantamount to arguing that the streams of rope lava flowing down the slopes of Kilauea are “pulling” the lava up out of the volcano. Nonsense!
Yuppers!
Almost as bad as suggesting glaciers pull snow out of the sky.
A beautiful visualization, uploaded 13 years ago, showing a growing earth.