This is Geology Bytes with Oliver Strimple.
Every half billion years or so, most of the Earth's continental
lithosphere appears to have converged to form a
supercontinent, A phenomenon known as a supercontinent cycle.
The most recent supercontinent was Pangaea, which lasted from
about 330 million years ago until about 170 million years
ago. Until recently, the amalgamation
and subsequent breakup of supercontinents was thought to
result from processes happening on or near the Earth's surface.
Specifically, the idea was the subduction of cold oceanic
plates consumes the oceans lying between the continents, a
process that only stops when continents bordering the
subducting oceanic plates collide and a supercontinent is
formed. Once assembled, the
supercontinent was thought to behave like a thermal blanket
that traps the heat generated in the mantle immediately beneath
it, causing it to uplift thin and eventually break apart.
But over the past decade or two we've learned a great deal about
the mantle, mostly from seismic tomography and computer
modelling. We now believe, for example,
that subduction of cold oceanic plates can reach the base of the
mantle and that subduction of the oceans that close up to form
a supercontinent may interact with a deep mantle in such a way
as to produce rising plumes of hot mantle material that impinge
on the base of the supercontinent and foster it's
break up. This has LED us to overhaul our
ideas about what drives the long term motions and configurations
of the continents and even whether we should be calling it
the supercontinent cycle. Damien Nantz studies tectonic
activity and the large scale dynamics of the lithosphere.
Recently, he has concentrated his research on reconstructing
past geographies of the continents, and particularly on
whether or not a supercontinent called Panosha formed towards
the end of the Proterozoic, before Pangaea, but after the
previous supercontinent called Rodinia.
He is Distinguished Professor Emeritus of Geological Sciences
at Ohio University. Damien Nance.
Welcome to Geology Bytes. Thank you, Oliver.
It's a great pleasure to be here.
Could you remind us as to how we reconstruct the past positions
of the continents and thus infer the existence of
supercontinents? Let's start with the most direct
evidence. Well, the two most important
sources of data come from paleomagnetism, which allows us
to reconstruct the former position of continents at a
particular time, at least with respect to latitude, and from
absolute age constraints which allow us to determine when that
time was. So those are the two key areas.
But in reconstructing supercontinents, it's a bit like
putting together a jigsaw puzzle.
The geologic picture, if you like, on each continent, must
also make sense in their reconstructed positions.
There are also several indirect sources of evidence or proxies
for the existence of supercontinents.
Can you describe the most important of those?
Yes, in fact there are a lot, but the two most important are
the most obvious. Since the assembly of a
supercontinent requires continents to collide, then
their assembly should coincide with global orogeny and the
things that go with it, like granitoid magnetism and zircon
age peaks. Conversely, supercontinent
breakup requires continents to rift, and so it should be marked
by the things that indicate rifting, such as mafic dike
swarms and the emplacement of usually hotspot related large
igneous provinces, which are referred to as lips.
But assembly and breakup influence a whole host of
things. They should, for example, have a
profound effect on sea level and climate, and therefore are
likely to influence things like extinctions and major
evolutionary radiations. They also have major impacts on
things like the sedimentary record, the chemistry of
seawater, the composition of the atmosphere, the list goes on and
on. Do all these proxies then
approximately line up and agree as to when a particular
supercontinent formed and then broke up?
They certainly do for Pangaea. They appear to for Panosha and
those that we can use for earlier super continents appear
to coincide. Some we lose.
It's very difficult to judge sea level.
As you get further back in time, things like major evolutionary
radiations and extinctions become very difficult when all
you're dealing with is little tiny creatures.
But yes, they do. In my introduction, I briefly
mentioned that until recently we explained the formation and
breakup of supercontinents in terms of surface processes or
near surface processes. Can you fill out that picture a
bit more? Yes.
Well, you have to remember that when the idea of the
supercontinent cycle was first introduced, we knew precious
little about the mantle. And we were of the view that
supercontinents, as you mentioned, acted like thermal
blankets on the underlying mantle, that this trapped mantle
heat beneath them and that that caused the supercontinent to
thermally uplift and eventually break up.
That process may be part of the story, but I think today we
would say it is only part of the story.
Now, on the other hand, the oceans that opened when a
supercontinent breaks up can be expected to eventually close.
They're going to subduct and eventually close, and hence they
may reunite another supercontinent.
So we were sort of the view that supercontinents sowed the seeds
of their own destruction, if you like, and would eventually break
up. But in doing so, they would
create new oceans that themselves had limited life
expectancies and would ultimately close.
But the cycle was very much viewed as a lithospheric upper
mantle process. Everything we're talking about
is relatively shallow, and we thought of it as a governing
mechanism for plate tectonics. Things like what happened to
subducting slabs after they subducted and what was going on
in the deep mantle were completely unknown quantities at
that time. OK, so we've learned a lot more
about the mantle in the last few decades, and this has indeed
caused us to rethink the picture.
First of all, how did we actually gain this new knowledge
of the Mantle? And then what did it reveal
about the mantle? As you mentioned, the
breakthroughs really came from 2 areas, from seismic tomography
and from computer modelling. For example, seismic tomography,
which is really just a means of X raying the Earth's deep
interior, revealed that subducting oceanic slabs didn't
simply sort of dissipate in the upper mantle as was generally
presumed, but actually sink all the way down to the core mantle
boundary. And the core mantle boundary
seismic tomography revealed the existence of two Thermo chemical
features that were antipodal to each other and which affected
seismic velocities and were known as large low shear
velocity provinces, or LLSVPS. I'll interject here that while
seismic tomography reveals the enormous extent of these large,
low shear velocity provinces, each of which extends several
1000 kilometres across and rises about 1000 kilometres up from
the core mantle boundary, we don't really know why the
seismic shear wave slowed down there.
It could be a higher temperature, a different denser
composition, or some of both. And importantly, one of these
LLSVPS was centred directly under the former position of
Pangaea, So that in itself strongly suggested a link
between supercontinents and the deep mantle, which was something
we certainly never considered. And that connection has in fact
been strengthened with the recognition that some of the
hotspots, most of the hotspots that formed in the wake of
Pangea breakup actually represented the surface
manifestation of mantle plumes that emanated from the edges of
these LLSVPS. Again, another connection
between the Deep Mantle and Pangaea breakup, suggesting a
Deep Mantle involvement, which we hadn't considered.
How, then, do we use this knowledge to look at the
mechanism of supercontinent formation, and particularly of
the supercontinent cycle? It's hugely important because it
demonstrates a link between supercontinence or the
supercontinent cycle and the deep mantle And in doing that it
actually elevates the importance of the cycle significantly from
simply being a near surface phenomena that influences or
controls plate tectonics to being a whole mantle process.
And what we take from this is that the assembly of
supercontinents somehow influences a mantle in a way
that causes a major region of downwelling to become a major
region of upwelling. So that the mantle downwelling,
which we would expect to be associated with the closing
oceans of the assembly of a supercontinent, somehow evolves
into mantle upwelling, which is evident in the rise of these
mantle plumes responsible for the breakup of the
supercontinent. So somehow the cycle has taken
an area of major mantle downwelling and caused it to
evolve into one of upwelling. So the original idea of
supercontinents acting as thermal blankets on the
underlying mantle may still be part of the story, and maybe an
important part of the story. But that would not involve the
deep mantle. So it does not explain this
relationship, and instead it is more likely that this change is
linked to the seducting slabs of the oceans that close to form a
supercontinent. And if you think about it, as
you are bringing a supercontinent together, you've
got seduction in the closing oceans and subducting slabs
sinking down towards the core mantle boundary beneath what
will be the future position of the supercontinent once it
assembles. So that process clearly
contributes to the downwelling beneath the supercontinent.
But once these subducting slabs reach the core mantle boundary,
they somehow managed to cause plumes to rise beneath the
supercontinent that foster its breakup.
Now, exactly how they do this is not yet known.
There are a variety of possibilities, all or none of
which may be correct. They may interact, for example,
with these LLSVPS and existing LLSVP in such a way as to
trigger the rise of mantle plumes at its edges.
Or they may actually form an LLSVP at the core mantle
boundary, and in doing so generate edge effect mantle
plumes. Or they may simply act as
thermal blankets themselves, but this time on the core ponding
heat that way that then rises in the form of mantle plumes.
Now, in addition to all that, once a supercontinent assembles,
all of the seduction that was going on in the closing oceans
beforehand, stops and must move. It has to relocate, and it will
relocate to the edges of the supercontinent, to the ocean
surrounding the supercontinent. And if you imagine that you now
have a supercontinent with a sort of girdle of subduction all
the way around it, or most of the way around it, and a bunch
of new subduction slabs or subducting oceanic slabs now
sinking down towards the core mantle boundary as a curtain
around the Super Canton. And that downward motion alone
might be sufficient to cause a return flow in the form of
upwelling beneath the Super Canton.
So that's another way you might be able to convert this
downwelling to an upwelling. But whatever the path is, the
effect is to cause the mantle downwelling associated with
assembly to evolve into mantle upwelling responsible for break
up. Why should the subduction when
it shuts off underneath the downwelling that has brought the
supercontinent together? What's the mechanism?
Why should that flip into this curtain or girdled around the
supercontinent? Why doesn't the subduction just
shut off and happen in some random place somewhere else on
the planet? This is a tendency of subduction
systems to jump back towards the ocean continent margin.
So there is a likelihood then that and certainly modeling
shows this, that the subduction system would bounce out to the
margins of the supercontinent. This is certainly what happened
with Pangaea. It was almost entirely
surrounded by subduction systems, and some of those still
survive in the modern Pacific Ocean.
So you said that we find that there's one of these large below
shear velocity provinces sitting underneath the former location
of Pangaea. I wasn't clear if that province
was thought to have predated the formation of Pangaea and somehow
rather caused it to form above it or the other way round, or
what exactly is the connection thought to be?
We're not certain about that. What we can see, however, is
that there is an amazing coincidence between the position
of one and the reconstructed position of Pangaea that sits
slap dab underneath it and currently sits under Africa,
which is the one piece of Pangaea that hasn't moved very
much since it broke up. Everything is moved away from
Africa and Africa hasn't done very much and the current LLSVP
in that hemisphere sits right under Africa.
So it's a kind of correlation that cries out for explanation,
and we don't really have it yet. Exactly so.
And when you couple that with the fact that all of these
hotspots that are clearly linked to Pangaea breakup coincide with
the edges of this thing, that likewise begs an explanation.
We don't have one yet. We have ideas, but we certainly
don't know yet. And I think that is probably
where the future of this whole story will take us into trying
to resolve those kind of issues in today's world rather than
trying to do it in the distant past.
What is the role of computer modelling being in helping us
understand what's going on here? Computer modelling confirms the
idea that supercontinents form over areas of mantle dam
welding, but that they preferentially do so in what is
known as a degree. One structure that is a
supercontinent will form over an area of mantle dam welling that
is antipodal to an area of upwelling.
So you have a mantle with a relatively simple overall
convection system, a single area of dam welling, and an antipodal
area of upwelling. But as it sits there, the effect
of the amalgamation is to start generating these plumes at the
core mantle boundary underneath it, which of course rise and
foster its break up. But in doing so they change the
overall structure of the mantle from a degree one structure to
what is known as a degree two. That is a mantle with antipodal
areas of upwelling, because the original area of upwelling is
still there and you've now created another one underneath
the supercartinent, antipodal to it.
And between these, what you see is a girdle of downwelling,
which is where subduction is going to be focused, and that
forms a girdle all the way around.
It's referred to as the subduction girdle.
And interestingly, that then bears on how supercontinents
might assemble after they break up.
Because in a world like that, once your supercontinent breaks
up, the dispersing continents are going to move towards an
area of mantle damwelling. And that area of mantle
damwelling is now this girdle. And so the dispersing continents
might be expected to collect over this girdle of mantle
damwelling. And what they do then depends on
what that girdle does in terms of movement.
If it moves away from the former supercontinent by a process of
rollback, for example, like we see in the Western Pacific, it
may continue to move until it reaches the other pole antipodal
to the former supercontinent. In which case the continents are
going to coalesce there and you will create a new supercontinent
antipodal to the former one. And in doing so you will have
closed the ocean surrounding the supercontinent.
So in the modern world, if we think about the breakup Pangaea,
the effect of that would be to close the Pacific and reunite
the continents as a result of doing that.
And that's referred to as extroversion closure of the
ocean surrounding the supercontinent.
Conversely, it may collapse, this girdle may collapse on
itself, and the continents will then come back to their former
positions, more or less, and you will recreate a supercontinent
in much the same position as the one you've just broken up.
That process known as introversion.
And in today's world, that would be like saying the next
supercontinent forms as a result of the closure of the Atlantic
Ocean, which would reunite the continents to form a
supercontinent in much the same position that Pangaea was in.
Now, 1/3 of possibility is a girdle doesn't move, and that
the supercontinent effectively assembles on the girdle
somewhere. And if that's the case, then the
new supercontinent forms sort of about 90° to the former one, and
that's referred to as author version.
And in today's world, that would be akin to forming a
supercontinent by closing up the Arctic Ocean and closing up the
Caribbean, for example, and bringing the continents together
that way. So those are the three methods
of reuniting A supercontinent that are often banded around at
the moment. Do we know enough about the
actual positions of former supercontinents?
Going back, maybe Pangaea, possibly Panosha, Radinia, maybe
Canola? And before that to know if they
did indeed reassemble in the same place, Antipodele, or at
90°? In other words, which of these
three theories is borne out by the data?
There are certainly folks out there who have argued, yes, that
the relationship has been one of 90° and that we see author
version in the past. When we first advocated the
cycle, the one we thought most likely was introversion.
And the reason was not because we had any great idea of how
these things happened, but we simply looked at Pangaea and
Pangaea formed by the closure of oceans like Iapetus, the
Tonquist, the Raik, et cetera. And these were all oceans that
formed when what we thought was the previous supercontinent,
Panosha, broke up. And so from that perspective, it
looked as if Pangaea had formed by closing oceans produced by
the breakup of the previous supercontinent.
And that's introversion. But in truth, I don't think we
can be specific about it. The relative positions of
successive supercontinents seems to be quite a important
observation to resolve these large scale mantle processes
that we're talking about here, and I'm wondering whether either
the global distribution of these relict or slab graveyards might
give us some indication, or whether indeed it's the computer
modeling that can come to the rescue here.
I think it's more likely to be the computer modelling.
You could argue, for example, that the reason we have
antipodal slab graveyards or large low velocity provinces is
because one formed under Pangaea and the other one formed under
the previous supercontinent. It hasn't dissipated yet.
And if that was the case, of course we're dealing with
extroversion because they're on opposite poles.
And our problem with establishing the precise
existence of supercontinents is we don't have much longitudinal
constraints. Paleomagnetism will get us
latitudinal constraints, but it's often rather difficult to
know whether we're on this side of the world or that side of the
world. So exactly, positioning 1
supercontinent with respect to another is not an easy process.
And on top of that, we have this issue of true polar wonder,
which may very well impact all supercontinents.
And that is a feature of physics that requires the mass imbalance
that is produced when you put all the continents together in
one place to cause the Earth to rotate in such a fashion as to
bring that mass imbalance to the equator.
It's a whole Earth rotation of the mantle and the lithosphere.
So to detach somewhere at the core mantle boundary, which
brings the entire mass of a super continent, say one that
assembled at the pole. This process of true polar
wonder would slowly but surely bring that down to the equator,
and in fact I think brings it down quite fast.
I believe the rates of true polar wonder are quite a bit
faster than the rates of plate motion, for example.
So that means that all super continents, given enough time,
will end up on the equator, regardless of where they formed.
There is a lot of debate as to whether Panosia existed during a
period between about 630 million years ago and 530 million years
ago, pretty much midway between Rodinia and Pangaea, and you're
an advocate of its existence. What brought you to this
conclusion? Well, I would certainly like to
think Panosia existed, but I may be wrong.
And interestingly, whether or not it did, Penosha is central
to some of the issues we've been talking about.
It's also a bit ironic because poor old Penosha was the first
pre Pangaean supercontinent to be proposed, and when we were
putting the case of the supercontinent cycle together,
it seemed to us the one for which the case was strongest.
But the problem is that are two key tools for establishing
supercontinents, namely paleomagnetism and absolute age
dating, have failed to establish its existence.
Paleomagnetic data for the IDIAC run, that is, the period during
which this supercontinent would have existed if it did, is very
inconclusive. The data are notoriously
dispersed, and that may well mean that the Earth's magnetic
field was misbehaving at that time.
There may have been true polar wander going on, or it may not
have been a dipole, but something odd was going on with
the magnetic field, which means it's difficult to use
paleomagnetic data to say yay or nay.
If we look at the age dating, the problem there is age dating
suggests that the supercontinent was breaking up before it ever
got together. And if that's the case, then it
never really existed. And even if it did, I think even
I would concede that if it did exist, it was a very short lived
supercontent. But why did we think it was a
supercontent? The reason is because the proxy
signals are unmistakable and these include all the ones that
you'd expect. Things like global hierogeny,
zircon age peak, evidence of widespread rifting and mayfic
dyke, swarms and lips and changes in sea level and climate
and extinctions and radiations, and just a host of geochemical
and isotopic tracers, they're all there.
The problem is, while they are collectively compelling, they
are individually contestable and therefore they cannot be
considered definitive. For example, you could argue
that the orogenic activity and zircon age peak, which are very
real, record the assembly of Gondwana, not Pernosa, and
conversely the evidence of rifting and lips, et cetera.
This was just a continuation of the break up of Rodinia and not
the break up of a subsequent supercon.
So there is an argument against each of them that can be made so
that they're contestable. So although the proxy evidence,
which was the only evidence we had at the time, was strong, it
is not definitive. But interestingly, even if
Penosha did not exist, what this proxy evidence suggests is there
is actually a far more important question going on here, and that
is, is a full blown supercontinent needed to make
the supercontinent cycle work? Or might a close gathering of
continents be sufficient to change mantle circulation in the
way we've been talking about and cause the converging continents
to reverse their convergence? So the key here is to convert
that downwelling that's underneath where the newly
forming supercontinent is forming into an upwelling.
And that's why you're really saying this is perhaps more of a
deep mantle cycle rather than a supercontinental, which implies
a surface cycle. And the question then is, I
suppose, what does it take to get that reversal?
Do you actually have to close them up completely and produce
that thermal blanket with no cracks or gaps in it, or is just
proximity good enough? Exactly so.
And if you don't need a supercontinent, then it isn't a
supercontinent cycle. But although it involves the
deep mantle as you just mentioned, it is not a mantle
cycle either because the supercontinent or this near
supercontinent obviously plays a vital role.
So it is a mantle lithosphere cycle, but I think we're going
to have to come up with a sexier title than that if we want it to
catch on. Why are we so concerned as to
whether Panosha actually happened or not?
The existence of Panosha is critical for a number of
reasons. If we don't actually need a
supercontinent, and Panosha is an example of, well we didn't
have one and yet we change mantle circulation, then the
question is, are there other times when this has happened?
Do we have other proximate supercontinents that have
affected the cycle in the past that we haven't even thought
about? There are other sort of key
issues. The existence or non existence
of Panosha obviously affects the interval between supercontinents
and therefore it influences whether or not the cycle has
experienced secular change through time.
Without Panosha, the interval between supercontinents has
remained pretty much constant since the end of the Archaean at
around 606, fifty million years or so, suggesting A broadly
steady state cycle. If we put Panosha in the picture
and say it was a supercontinent, then the interval looks to be
shortening towards perhaps 300 million years or so.
And this would imply that the cycle is accelerating.
And if that's the case, it's telling us something very
important about mantle evolution.
And the other thing that it influences is a question that
geologists are tantalised by. And that is OK, we've got a
cycle, when's the next one? So when does the next
supercontinent assemble? Well that depends very much on
the interval between them and without Kenosha we can expect
another supercontinent in 300 million years or so, but with
Kenosha it might form in as little as 100 million years and
so a significant difference. So there are lots of quite
important questions hinging on the existence or non existence
of Kenosha, and I think perhaps more important than did it
actually all come together to form a supercontinent?
Is the answer to the question did Panosha, whether it formed
or not, impact mantle circulation in such a way as to
turn an area of downwelling into upwelling.
If it did, then it is a supercontinent as far as a
supercontinent cycle is concerned, regardless of whether
or not it is a supercontinent by definition.
I'm struck that these ideas are based on rather small number
statistics and some of the sources of evidence we discussed
have significant uncertainties associated with them.
I'm wondering what observations or computer modelling or other
line of research would you most like to see to help us get more
confidence that we know what the long term dynamics of the
continents are and what drives them.
Well, you're absolutely right, I'm afraid.
The topic is plagued by the fallacy of small numbers, but we
have to work with what we've got.
And as you mentioned, it's also plagued by uncertainties
associated with the proxy record which bedeviled its
interpretation. So there are a lot of issues
here, but as to where we go from here, as I mentioned before, I
think the answer lies not in scratching our heads over deep
time, but it lies in the modern world and the improvements of
the techniques we already have for studying the thermal
structure of the mantle and modelling its evolution through
time. And I'm fairly confident that
within the next five years, certainly within the next
decade, the increased resolution in seismic tomography and
increased sophistication in modelling will allow us to
understand the relationship between things like plate
tectonics, mantle circulation, subducting slabs, mantle plumes.
This will finally come together in a comprehensive picture.
And once we've got that, it will provide us with a powerful new
insight for interpreting the dynamics of the Earth in deep
time. So once we've sorted out the
present, then we can go back and have a look at all this
information and see if we can't make much better sense of it.
Damien Nance, thank you very much.
Well, thank you, Oliver. It's been a real pleasure.
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