This is geology B with all of us trampled.
Ever since Alfred Wegener proposed the theory of
continental drift in 1912, we've been aware that blocks of the
Earth's lithosphere. Are moving with respect to each
other with the Advent of plate tectonics in the 1960s, these
moving blocks became identified with the tectonic plates that
tile the Earth's surface. We now have accurate
measurements of plate, motion speeds, which range from about
half a centimeter per year to about 10 centimeters per year.
But there is still no General consensus as to what makes
plates move, broadly speaking, there are two competing
explanations in the first, the plates ride on top of convection
cells of a vigorously convecting mantle in the second it is the
forces acting on plate boundaries principally the pull
of dense lithospheric slabs subducting into a less dense,
mantle that drive the plates. That white van hints.
Bergen is a professor of global tectonics and paleogeography at
the University of Utrecht. He has reconstructed the history
of plate motions in various locations around the world.
With the primary goal of using this history to understand the
Dynamics of the mantle. And his latest research is
directed to shedding light on the long-standing question.
As to what drives tectonic plates that were even hints.
Bergen, welcome to geology B. It's a pleasure to be here.
Before we talk about what drives the motion of tectonic plates.
I'd like to ask you about your plate motion reconstructions?
Are they Global in nature like those of deepmind Willows Group,
which we discussed in an earlier podcast or rather a regional the
rigor structures that I make are mostly based on Mountain belts
and they are there for regional. So we try to reconstruct the
plates that we have lost because they have subducted.
What regions have you reconstruct?
Did almost all regions that disappeared in the last 200
million years. So that is, especially the
Mediterranean region, the area between Arabia Eurasia, the
Himalaya and Tibet southeast Asia and the sucker Pacific
regions. How did you go about performing
these reconstructions? And What observations did you
use what? We try to figure out is the
paleogeography of plates that existed and the plate boundaries
that existed in areas that have fully subducted.
And the rappers that we can use for that are now making larger,
Creationary Mountain belts, and with a Grecian room and melts.
I'm referring to table scrapes relics that form the upper few
kilometers of plates that have now subducted and you find them
in Mountain belts like we have in Japan or in New Zealand but
also the Himalaya or the Alps and many of the mountain belts
are the Mediterranean region are made of those off scrape relics
and in those Mountain belts in those off scrape relics, we see
all sorts of rock types. And the nature of the rock types
of fossils in the rock types, the ages of the Rocks.
Give us a lot of information about what the geographic
features were that these Rock times formed on.
For instance, we can find Barrier Reef type platforms
carbonate platforms and that will probably tell you that
you've been on large Continental fragments.
Whereas if it is a local at all, like we may find around Hawaii,
you will get a very different geological record.
So the relics that we find in these Mountain belts, Tell us
the type of rock the age of The Rock and the moment that that
rock arrived at a trench and that is the type of data that we
use to let's say to put them back on the now, selected
lithosphere and make reconstructions with them.
That's fascinating. So just by identifying the rock
types in the mountain belts which are as you put it off
scraped relics literally scraped off the top few kilometers of
plates when they subducted you can infer the extent of those
subducted plate. And when their various parts
reached, the trench and subducted.
So what we can see in a mountain belt is what is still there.
And that probably is the biggest problem that we have in geology,
geologists can only study, what is there?
Particularly in these Mountain belts?
These are minimum records if there was a notion 5000 km wide
of which no rock ever accreted. We will never be able to tell
that. That ocean was there.
So, as a first starting point, For these reconstructions we use
the reconstructions like deeper Muller has made did mer and his
group are the world's leading experts in making
reconstructions of the oceans that we do have today.
Magnetic stripe things in the ocean floor and large faults on
the ocean floor of the fracture zones.
In the ocean floor, they tell us what the formation history and
the growth history of these ocean basins have been if we go
back to 150 million years ago, only about half of the surface
of the Earth is covered by rocks.
That we I have today, and the other half is lost.
So, the first thing that we do is estimate, using Rico
structures, like deep, Mars how much lithosphere of loss is not
only the ocean. I was lithosphere was lost on
Witchblade was lost and it tells us how fast blades were moving
together and in what direction, they were moving together.
And then on those plates that we, let's say pulled back up
from the mantle, we stick the bits and pieces that we find the
mountain belts, systematically one-by-one, across the planet.
And then we get the Sure of let's say 90% white void, no
information and 10% little fragments in bits and pieces and
those fragments are little bits and pieces, still make for a
quite an accurate geography, and because you can have an
educated. Guess what is missing in
between? Is there a way to test these
reconstructions with observations of a different
nature to make sure that consistent there are most
directly? You can see where those plates
might currently. We reside in the Earth's mantle,
we have seismological techniques and the cat-scans of seismic
tomography that allows us to see where we have anomalously, high
wave, speed, anomalies in the mantle, that could be subducting
lithosphere. So we can predict whether we
find subducting lithosphere and places.
Where are we go? Structured tells us there is
subducted lithosphere and clearly, if we do that at modern
subduction zones, then it works because, you know, that's how we
know that that's abducted literature in the first place
because we can calibrate Tomorrow motions, but in
Mountain belts, like in Iran or in the Mediterranean region, we
see the relics of subduction zones that formed and died in
the geological past in locations.
Far away from the motor mount belt.
And for those reconstructions, we can use tomography as a test.
Another independent test to see whether we place a deformed Rock
unit that we now have a mountain belt in the right position on
the Earth's lithosphere is through paleomagnetic data, the
played, That we have. We can place relative to the
Earth's spin axis by using Bailey magnetic data from stable
continents. And then we can see whether we
put the rock units that we have in Mountain belts in the right
position at the right latitude and the geological past, that's
really interesting. So you actually don't use the
seismic tomography or the paleomagnetic data as inputs to
your reconstruction, you just use them as tests once you've
done it using your examination of Belts in the other logic that
you described earlier exactly what I try to do is predict
based on the structure of mountain belts and based on the
history of the seafloor, I try to predict what unit must have
been where and with the information that we have in
Mountain belts on fault motions, plus simple geometry.
You know, it has to fit if you take something as complex as the
Mediterranean region, there are so many constraints from so many
different Corners that each Eventually may have multiple
options to interpret, but if you put them all together, there are
not that many options where you can put everything, you know, if
two regions both receive sediments, they cannot have been
on top of each other. So, by using geometry, we
normally end up with a reconstruction that has limited
degrees of freedom and then we predict Bailey magnetism and in
some cases like for instance, in Corsica Sardinia in the
Cretaceous and the Mediterranean region Bailey, magnetism told us
they had really a Current position for what you
reconstructed. It was permitted by what I knew
but there was no way to tell from the get-go so that is an
example where we changed it. But if it is possible to avoid
to use Paley magnetism as input than I do also because
paleomagnetism has a much larger uncertainty than we have
appreciated in the past. The mantle I specifically do not
use as input because it is precisely the evolution of the
mantle and the Dynamics of the mantle that I try to
reconstruct. So, I force my reconstructions
to fit a pattern in the mantle that I think should belong to
it. Then I lose my independent
control on what happened in the mantle.
And if I want to understand how the mantle is moving collecting
or not, and I used more refuse input from our reconstructions,
I inevitably end up in a circular reasoning.
Let's talk about how reconstructions can shed light
on what drives plate motions. So can you recap for us?
What the The two competing theories are about what drives
tectonic plates. The two theories are both more
than 50 years old. The first one essentially
originates from the famous Arthur Holmes.
Who was the first to realize that radioactive decay in the
Earth's mantle must be a heat engine and he envisaged run over
boiling pan of milk willaura feet with collecting mental,
trying to lose its heat and he supposed that the continental
drift hypothesis of a gunner at I'm, which was lacking a
mechanism that he supposed that the horizontal Flowage as he
called it would be a driver of that horizontal motion of blocks
or whatever they were because they didn't really know.
Then, when plate tectonics started to become the Paradigm
Forsyth in a way that I think, is 75, had the perfectly logical
notion that if you have ocean floor and that ocean floor cools
and becomes denser than the mantle, that it will sink.
And if it sinks at subduction zones is - buoyancy will act as
a pull force on the Dow going plate.
Apart from that the Aging of oceanic lithosphere, also comes
with the topographic effect that will give a horizontal Force
known as Ridge push, which is thought to be subordinate to
slap. Ooh, but that suggests that.
It's the density structure of the moving plates rather than
the boiling of the mantle underneath convection in the
mantle of myth. But there's one thing that both
ideas have in common, or two things.
First of all, they kind of make sense.
And the second is, they were both Made when we knew nothing
of the mantle, we had no direct observations of the mantle below
her feet. And that is, I think the
interesting point where we are now with all the Decades of
seismological work, geochemical work, the interpretation of the
geological history of the mantle that came with the connection of
seismic tomography to records. That we see a mountain belts and
we're now coming into the point that we cannot only see how
please move over the surface, but also how they move through
the Earth's mantle and we start to see a history below our feet.
We can start to evaluate to what extent.
These ideas of pulling slabs and whirling mantels are actually
consistent with what we see what are the observations of the
mantle the bear on the question of plate drivers.
The first thing that comes to mind is seismological images
that map out where seismic waves in the mantle are fast and
whether slow one way of explaining fast and slow.
And that is the way that is most common is that Is that are
colder than average will have a seismic velocity that is higher
than average and reaches that are halter have slower.
Seismic waves but seismology is also explained that
compositional, differences or water differences may also have
such effect. What we see in the mantle is
that there are regions that are elongated, large bodies of
anomalously, fast stuff, whatever it might be that when
you compare it to play triggers, Structures have volumes that go
inside with a history of lithosphere consumption.
So with seismic tomography, we see a current snapshot of where
the bodies in the mantle are that might be slams and the next
step that we took his new interpretation step whereby, you
connect the interpreted geological history of Seduction
with those bodies in the mantle, if you correlate the shallowest
bodies in the mantle The youngest records of Seduction
and you assume that older seduction records tend to
correlate to deeper slabs deeper anomalies, then you get a very
consistent pattern whereby. All subduction zones of the last
200 to 250 million years are accounted for in the mantle and
there are no major anomalies left unaccounted for.
On the other hand, the geochemists have been working on
the chemical composition of volcanic rocks that apparently
come from different reservoirs. In the mantle, there's volcanism
correlated to reaches of Seduction for organism
correlated to regions of extension, a mid-ocean ridges
and their volcanism that is intraplate.
And that is not just intraplate. But is intraplate a very
specific regions relative to a normally structures in the
deepest mantle and those different volcanic provinces.
Have different signature suggesting that there are
different reservoirs in the mount but also that the mantle
is much more heterogeneous than we would expect from a mantle
that is very quickly mixing and convicting.
So that is a constraint. Then a third constraint that I
think is very spectacular is that there are places in the
mantle where people in the last 20, 25 years, I found zircons
minerals that normally forming magmatic rocks and we find
zircons that are we You're older than the volcanoes.
Are the crust the oceanic crust on, which they form, in places,
where they should not be like, on the Mid-Atlantic Ridge.
These are features that are now so often observed that we start
to be able to piece together a picture of the mantle directly
from observations that was not available at the time that we
came up with the ideas of the drivers of plate tectonics.
So there's the seismic tomography which is one
constraint, there's the Geo. So chemical signature which we
get by analyzing the trace element and the isotopic
composition of lavas Source from the mantle below at mid-ocean
ridges above subduction zones. And at intraplate hot spots,
which all point to a hydrogen are T in the mantle and there is
the presence of crystals of zircon crystals, which we know
really could only a formed in the magmatic rock that Than
crystallised that's coming up from the mantle so those three
things together, our new data that tell us about the mantle
that we didn't have at the time that these two competing
theories were formulated initially.
Yeah, I'm sure there are more, I think particularly that the
seismologist and a mineral physicists have a few other
tricks up their sleeve that I have yet to learn more about,
but there are now quite a lot of observations that will constrain
at least how the Earth is operating.
Now, it doesn't mean that it Is always operating that way.
But I think we can get a better idea about to what extent.
The mantle is vigorously, convecting or not.
So how does all this relate to the plate motion reconstructions
that we discussed earlier on the mental is a geological product
itself. It's the product of a geological
history. So if we find zircons in the
mantle in a location where they cannot have formed recently,
like in the Mid-Atlantic Ridge, Would you get them there?
These are kids that are 3 billion years old up all the way
until a couple hundred million idea.
Get all those are kids together in the first place but you do
that in the Sandstone. That's how you get crystals
together on a geological surface.
How do you get sand in a mental? Well, you need to have a
subduction zone for that. Sand doesn't sink through
mid-ocean ridge into the mantle. So the plate reconstruction
should predict, which anomaly that we see in tomography when
that anomaly. Could have formed and where but
we know where the tomography tells is not where that thing is
today, and it seems reasonable to assume so far that one slabs
break from lithospheric plates. They do not move a lot anymore.
Horizontally, so the just sing vertically.
So if we have a slab deep in the mantle somewhere and we want to
figure out when the last opportunity has been to form
that thing with subduction and we need a plate reconstruction
and we needed to know that bladebreakers Action relative to
the Earth's mantle which is not straight forward, but there are
ways to estimate that. So there was a paper last year
of an Australian group led by Mariana veto who found something
quite unexpected. On the North Pole they found on
the GAC, origin active Ridge. Today, they found a signature of
Seduction in the geochemistry and they noted that in the
mantle below their feet, but at a depth of 15 hundred
kilometers, or so, there is an anomaly that suggests that there
has been Seduction on and North Pole.
And if you then take a plate reconstruction, the youngest
subduction zone that was reconstructed.
At that location is something like 130 million years old which
suggests that that signature survived in the location where
it Formed still right above that slab and over 3000 years ago and
we made a similar case with rocks that had a magnetic
signature that we see on New Guinea but they've seemed to
have formed in absence of Seduction.
And we also notice that we start to find those rocks.
As soon as are reconstructions Dallas, the New Guinea started
to move over as tobacco-related, anomaly in the mantle over slab
on the mantle which formed because of interoceanic
Seduction, 30 million years. Prior what?
That might suggest is that those anomalies do not move much
horizontally in the mantle over a very long period of time.
And if that is true, then it means that the mantle is moving
that much, at least not in the last time in the 30 million
years. Okay, so let me see if I fully
understand the logic here. So when you have a subduction
that goes down into the mantle and because of a certain amount
of melting and interaction with the mantle that will change the
chemistry of the mantle to a certain extent, which we can
detect later using our modern geochemical techniques.
So by using your reconstructions, we can tell the
history. Turi, and the location of pasts
abductions. And what we seem to be finding
is that the geochemical imprint of those paths.
Abductions has survived through geological time in roughly, the
same locations, where that imprint was originally created
by the subduction itself. If I got it right.
It's better explain than I do. But yes.
So by putting together the reconstructions, with the
modern, Geochemistry and tomography.
We are now beginning to piece together.
A picture. That maybe the mantle is really
pretty still, maybe not stagnant but really not moving very much.
Is that the conclusion? Yeah, that is the conclusion
that I'm slowly starting to draw.
And so the implications are that the mantle is not like a boiling
pail of milk with vigorous convection cells.
Therefore plate motions would not really be driven because
there's no convecting sell to drive them.
Yeah. That a search may not be a
shocking conclusion because people have argued that blade
motions themselves can be explained by slabs going down,
but even models that have slabs driving.
The plates, have those same slabs.
Also driving convection. So that the mantle itself is
rigorously convecting from a geodynamic point of view is
still the Paradigm. The question is only been, is
that convection driving it or is that convection passive?
And what these observations only?
I think we're only at the start of this whole thing with these
observations seem to suggest is that not only are the place
driving themselves at least in the last few Animo years, but
they're not doing much to the mantle underneath the man alone
needs appears to be a more or less sluggish passive body of
Rock. That melts if you decompress it
or you had some water that will certainly move as swirl where
you move your pedal through the washer where your slabs are.
Moving through the mountain, they certainly are starting the
monthly fee. Slab is retreating, which we can
demonstrate happens. I'm not last to move around so
it's not a solid body of rock. It can move if somebody pushes
it but it's on the large scale. It seems not to be doing too
much. So in other words the convection
Driven by the radioactive heat is not really driving these
plate motions. As Arthur Holmes might have
suggested originally when you proposed the theory, but what
motions? There are would be responsive to
the Dynamics of the plates themselves and the forces acting
on the edges. Whether it's push or pull, yes,
and those motions are local and on the large scale, minor one or
two orders of magnitude slower. Then the plates are moving.
I don't say that. I understand the dynamic
consequences of that statement. I do get a Mattox.
I tried to piece out how stuff moved.
Not so much wide moved. Sure, I'm do that and with
colleagues and with my research I'm either blessed or
compromised by a very poor knowledge of mathematics and
physics. So my limitations tell me that I
can figure out when stuff moved and where it moved but not why.
So from that point Point of view.
I came to this conclusion and I think that on the other hand,
this can be a beautiful constraint on trying to figure
out what is driving things because whatever is driving.
It has to fulfill these observations or perhaps we
misinterpret these observations. If this is correct, it gives a
rather unique constraint on the workings of the planet that we
didn't realize before. Yeah, it's fascinating.
I mean, obviously, this is a question that's been around for
many, many decades, and now appears this.
Kind of a new line of evidence that might bear on on the
question of what drives plate tectonics.
So if the mantle is, let's say relatively stagnant and the
driver is the forces acting on plate, boundaries, what does
that tell us about individual plate motions that we see in.
Clit help us understand why some plates are moving fast and some
plates are moving slowly. It must be all our experiments
are numerical experiments or analog experiments of plate,
motion sale. I have a plate and there's
something buoyant on that plate like a continent and incontinent
hits the trench. Then I shoot you see a change in
my plate motion. But I'm not sure whether we do
there, are many places Australia has collided twice with
interoceanic subduction zones, and slabs attached to it, and
they're broken off and nothing happened in its plate, motion
history, just happily kept going and India.
We see the same thing, India still moving northward and the
last phases of slab, break of have barely any effect on plate,
motion Mediterranean region. There's been Continental
subduction for 100 million years on and off Off.
We don't really see it in the plate motions.
So even though slab pool must be the driving factor of plate
tectonics, as we think the loss of slab does not really have the
predicted result. So I'm not sure whether we
understand and to make life even more difficult, there is a paper
that just came out by Chuck the mats.
And so I got Mercury of who are well known for high resolution
reconstructions of plate, motions, we know that India went
super fast 50 million Oh 60 more years ago up to 150 km from the
year or even more and then it slowed down and everybody has
always been saying something to do with Collision.
Well, they showed is actually that in the time, that it went
super fast. India was speeding up and
slowing down by fifty to a hundred percent of their speed
in time. Scales of 23 million years that
we really cannot explain and these are crazy or high
resolution records. So, as usual, I think that the
more we learn, the more, More. We figure out that we do not
understand what's going on again, if you look at where
these Concepts came from, they make perfect sense.
But in the light of everything we know now there are a couple
of observations that show that it's either incorrect or
incomplete and we can speculate a little bit about what might be
driving it. I think it's an exciting time
and you learn Amex. That woman hits buggin.
Thank you very much. You're welcome for more about
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