This is geology B with Oliver's trampled.
After a decade of vigorous debate, the mantle plume
hypothesis first proposed in a paper by Jason Morgan in 1971.
Became the widely accepted explanation for the existence of
volcanoes located far away from plate, boundaries.
Plumes are posited to be upwellings of hotter than
average mantle arising from a great depth.
Most probably from the very bottom of the mantle where it
meets the Earth's core Hawaii, and other mid-ocean islands are
pointed to as the classic examples of plume induced or so
called, Hot Spot volcanoes. However, even as the plume
hypothesis has become more and more entrenched, the evidence
for them remains inconclusive. Of and seismic tomography has
yet to attain the resolution needed to detect them in the
lower mantle. Jillian.
Folger is Emeritus, professor of geophysics at Durham University,
she is a leading. Exponent of the plate theory for
volcanism in which the theory of plate.
Tectonics is able to account for all of the Earth's volcanism
dispensing with the need for additional hypotheses, including
mantle plumes Julian Folger. Welcome to geology B.
Thank you, Oliver and thank you for doing this podcast.
Can you remind me what it was exactly that led Jason Morgan
another's to propose the existence of mantle plumes.
Well during the 60s, when plate tectonics was developed, it
became immediately clear to scientists that most volcanism
on Earth was immediately explained as occurring at plate
boundaries. So much of this was occurring
along mid-ocean ridges, where the plates are moving apart, and
some was occurring behind subduction zones, Island arcs.
But then there was Hawaii, it was smack in the middle of the
biggest plate on Earth and it was as far away from any plate
boundary as you could possibly get anywhere on the surface of
the Earth, and it's a huge and very productive cluster of
volcanoes. So this was kind of a glaring
exception. Shun to the general rule and it
needed. Some kind of explanation.
Did you espouse the mantle plume hypothesis?
When you started your career as a research, geologist guilty,
your honor When I Was An undergraduate.
And after again, it was presented to me by all my
colleagues and teachers and mentors.
As though, this was just a fact, and this is well established as
plate. Tectonics of, which were very
confident. Of course.
But I never questioned them for plumes and in the first part of
my teaching career, as a lecturer at Durham University, I
taught the undergraduates the mantle plume hypothesis and I
taught them that the Hawaii. Emperor seamount chain, Bend was
due to a change in the direction of the Pacific Plate and we all
assumed that this was correct. It wasn't till I looked into it
that I started to find that. Not everything was quite as
clear and straightforward. So we'd all been led to believe,
was that something in particular that brought on your misgivings
about the existence of plumes? What triggered me?
Really. And this was a kind of cathartic
experience. I did a large seismic tomography
experiment covering the whole of Iceland's.
I worked with Jason Morgan at Princeton in the u.s.a., we
covered the island of Iceland with very modern Broadband
seismic stations and we gathered a lot of seismic data, Data.
And we performed an inversion, to study, the three-dimensional
structure, down to about 600 kilometers.
And what we saw there wasn't consistent with what we
expected, a mantle plume to look like.
So I took these results to an International Conference in San
Francisco in 1999, and I discussed it with colleagues
there. And one colleague that I said
earlier, this month of plume is From what we expected, as a
different kind of mantle plume. And he said, well, maybe there
isn't a mantle plume there and I was completely poleaxed by this,
incredible suggestion. It hadn't occurred to me ever
before that. Such a thing could be so and I
just started asking questions. What about this?
What about that, what about the other and that just led me into
a 10-year long roller coaster ride where I realized that we
had to just drop all these assumptions.
We just hadn't been. I'm thinking what we were doing
was the conclusion that I came out with?
But surely since that 1990s, Iceland study of yours, there
have been a ton of other studies that support the existence of
mantle plumes. There's a difference between
being consistent with a hypothesis and requiring it.
So many things could be claimed to be consistent with a mantle
plume hypothesis, but on the other hand, if there are half a
dozen other, Yes, and you've had to change the plume.
Hypothesis beyond all recognition to cram your foot
into the glass slipper that wasn't made for it.
Then we have to really think what we're doing.
So what exactly are the geological features that would
provide conclusive evidence of the existence of a mantle plume
in the early days of the development of mantle plumes
dot-org, the website that really hosts The debate regarding the
existence of mantle plumes, I thought it would be helpful to
decide exactly what a mantle plume was because there are so
many different variants. So I reached out to Professor,
Ian, Campbell of the Australian National University, Professor
Campbell as well. Known for his work simulating,
plumes in large tanks. Full of fluid in his laboratory
there. And I asked him what he can.
So that the plume hypothesis to predict what are the
predictions. If we go to study an area, what
should we look for? And what things should we expect
to observe, a scientific hypothesis should be forward?
Predictive he very obligingly clarified.
That in his view, there were five primary predictions that we
should look for if we're testing for the existence of a mantle
plume. So the first one is that when
the mantle plume, First Rises up, through the mantle and it
reaches the base of the lithosphere.
It domes the lithosphere up so we should see kilometer scale,
lithospheric, uplift on a scale of some hundreds of kilometers
broad. That's the first one following
this uplift. We should see the outpouring of
a flood Basalt. For example, the Deccan traps,
something like that. The third thing is that there
should be a tail, a plume tail stretching, I'm from the surface
down to the core-mantle boundary and hopefully we should be able
to image this with methods such as seismic tomography.
The fourth prediction is that this plume along with its tail
is fixed relative to the Earth's geomagnetic pole.
In other words, the tectonic plates which are moving relative
to the geomagnetic pole. These tectonic plates are moving
over the fixed plume and they should carry away with them.
A chain of extinct, volcanoes of volcano erupts, it's carried
away. The next one, erupts that's
carried away. So a plume region should have a
Time Progressive trail of volcanism and the fifth
expectation is that there should be evidence that the source of
the eruptive was hot. It was some 200 degrees hotter
than say, mid-ocean ridge Basalt sources.
So those are the five predictions.
Uplift flood Basalt tail down to the core mantle boundary Time
Progressive trial and high temperature.
And how did these stack up in Hawaii?
In Hawaii, the emperor chain, the oldest part of the a chain
is, right? At the far north of the Pacific
Ocean, and it's right by the Aleutian trench.
So anything older than about 80 million years, has gone down the
Aleutian trench. So if there was initially
uplift, And a flood Basalt, then there's no evidence for it.
People have looked at the chemical composition of lavas in
Kamchatka to try to see if there's some geochemical
signature of a subducted flood Basalt.
But they have not so far found any such evidence.
So, basically, there's no evidence for the first two
predictions. So then there's the plume tail
down to the core-mantle boundary, and there have been a
number of Of studies of this. And although almost every study
has reported some evidence that supports that, the problem is
every study, sees it in a different place or going down to
a different depth or tilting in a different direction.
So, there's no repeatability between the studies.
It's very unfortunate that Hawaii is right in the very
middle of the Pacific and there's a relatively small
amount of land. Mass there, because it makes it
very difficult to do seismic experiments, and it would be a
lot easier if Hawaii was in the middle of a continent.
And we could spread out seismometers over a very broad
area. So that's the status of the
third prediction, the fourth prediction of a Time Progressive
Trail. This is spectacularly, well,
reproduced the Hawaiian seamount chain, and north of that.
The emperor seamount chain space.
Secular Lee time Progressive but unfortunately, there's a sting
in the tail there and that is that the time progression,
doesn't match the motion of the Pacific Plate.
This is very curious. When the emperor chain was in
place, this is roughly between 80 million years and 45 million
years ago. The Pacific Plate was
approximately stationary and the locus of volcanism migrated
across Ross the stationary plate at approximately Bend time,
that's about 45 million years say the Pacific Plate, began to
move and then the locus of volcanism stopped.
So the whole chain is indeed time Progressive, but not
because the Pacific Plate was carrying the volcanoes away.
It's just for some reason, this volcanic, Locus, this active
volcanic, Locus wants to move relative to the Plate.
And if the Pacific Plate is moving, it will stop.
And if the Pacific Plate stops, it moves.
So this is just the biggest puzzle in.
Geology really to move on to the fifth prediction which is high
temperature. This prediction is fulfilled at
Hawaii. There is pretty unequivocal
evidence that the source of Magma's at Hawaii is hotter than
at Mid. Ocean ridges.
So there we have it. One out of five in earlier,
geology, B podcasts, Barbara Romano, it's talked about the
large low Shear velocity provinces seen clearly in the
tomography and Alan McNamara showed fluid.
Dynamical models that seem to predict the existence of such
regions. Surely those are real.
The lsv peas are of course real but the Question is, what are
they? They were originally noticed to
have low seismic wave speeds and they were nicknamed the super
plumes but it was immediately discovered that instead of being
hot and buoyant they're dense and probably not hot at all.
They were quickly renamed seismic piles because they're
basically piles of dense material sitting on the
core-mantle boundary And it was quickly, hypothesized that
instead of generating plumes themselves, the plumes were
generated outside the LLS VPS and that they trickled up the
edges of the ls VPS and Rose up to the surface around their
edges. As you say, we can really only
see seismic wave velocity from the seismic data.
So what are the factors that affect seismic wave speed many
workers? Tomb, that the speed at which
seismic waves travel can be considered to be a proxy of
temperature. Well, this is not the case.
There are three things that can affect seismic wave speeds and
in order of their strength, if you like the first one, is the
presence of partial melt. If we have a trace of partial
melt, this can drop seismic wave speeds enormously without any
change in. Temperature.
So that is the strongest effect. Partial melt the second most
strong effect, which can lower weight, the seismic waves boots
or indeed, raise them is composition.
So if you're in a region where you feel you can, absolutely
rule out, partial melt explaining your observations,
then the next thing to ask yourself is, could there be
geochemical variations, petrological variations in the
material down there. Only if that can be rolled out,
should we really turn to temperature?
Because temperature is by far the weakest effect on seismic
wave speeds. So if you go out and do an
experiment and then you measure the three dimensional variation,
for example in the S wave speed of seismic waves, you have one
variable there and you have three possible ways of
explaining it or indeed, a combination of those three.
So it really is not robust. Interpretation to assume that a
seismic tomography. Image is a proxy for
temperature. Okay, that's enough on plumes.
Tell us about the alternative hypothesis, which is called the
plate hypothesis. So the plate hypothesis suggests
that the stress field in the lithosphere is variable, both
horizontally, and vertically, and that volcanism occurs away
from plate boundaries where the lithosphere is in extension.
Where the lithosphere is in extension, it can allow
pre-existing Melt to leak up either from the lower crust or
the mantle. And if the lithosphere goes into
extension, to the extent, that it may even Rift all the way
through, for example, when we have supercontinent break up,
then you could have asthenosphere rising up, maybe
one or two hundred kilometers in height and then you could boost,
whatever. Pre-existing melt there is with
decompression, upwelling mantle melting.
So this is the plate hypothesis. It's basically an extension of
the existing plate tectonic hypothesis, except it's really
saying that the Earth is not either 100% rigid plates and
narrow plate boundaries, but the Interiors of the plates also
have variable stress and in some places they're extending and
there can be leakage of melt in those areas to so the neat.
Thing about the plate hypothesis really is that?
As you said in your introduction, it means that all
the volcanism on Earth and earthquakes can be explained by
one hypothesis. The the plate tectonic
hypothesis in its relaxed mode. If you like, and we don't need
to completely separate hypotheses with this volcanism
explained by plate tectonics and that volcanism needing a
completely separate, unrelated hypothesis for it.
And then, All these volcanic regions, which might be half and
half, causing us headaches in between, it's like a unifying
hypothesis if you like. So, according to the play type
offices, we would expect to see evidence of extension in the
lithosphere. Whenever volcanism has occurred,
or is currently occurring away from a plate, boundary.
Yes, that's correct. Okay.
Let's see whether we can test the predictions of the play type
officers. You're an expert on Iceland and
that's where you initially found the evidence for plumes lacking,
even though it's on a mid-ocean spreading Ridge at the edge of a
plate. The fact that there's enough
magma generated there to form land in contrast to the rest of
the Mid-Atlantic Ridge, requires some explanation, which of
course, the plume theorist provide by positing a plume
under Iceland. The observations can't be
explained by this the crust is 30, E km thick is even as much
as 40 km thick in some places. And if this was all Plum juice,
if this was a stack of basaltic material Iceland should stand
for kilometers high. But Iceland doesn't stand for
kilometers. High is stands approximately one
kilometer High and the only way of explaining this is that the
lower crust under Iceland the bottom 20 or 30, What is the
crust in Iceland is much denser than we would expect if it was
basaltic and really the only way that can be explained is if it's
largely Continental, this again is like, heresy.
Oh my God, you know, people will say that I haven't read anything
for the last 50 years because this was initially suggested 40
or 50 years ago, by Russian seismologists, everything
essentially can be explained by this, Adding the thermal field.
So although this sounds completely turned upside down.
I am virtually 100% certain that the majority of the crust
beneath Iceland is largely continental.
And that what we're looking at in Iceland is basically a
volcanic passive margin in formation and the reason there's
a volcanic passive margin in formation there is because
there's a micro-continent under Iceland.
I didn't quite understand how if Iceland was made entirely out of
Basalt that came out of a mantle plume, it would have to be 4
kilometers high, but if it's got Continental material underneath
it, which I thought was lighter than Basalt.
Anyway, it might only have to be one kilometer, which is what it
is. How does that work?
Look, if the lower crust contains a lot of Continental
mid and lower crustal material, then it will have a higher
density than if it was just unusually thick Oceanic layer 3
and it will have the correct seismic velocity, so the seismic
velocity of the lower crust under Iceland is approximately
7.3, which is consistent with crust.
But the density is close to the mantle This can be explained by
Deep crustal. Continental petrology is ultra
basic petrology she's but it can't be explained by Oceanic
type basaltic petrology. She's you mentioned are volcanic
passive margin in Iceland, what's that?
Well it's easiest to give an example of that.
And this is what lies along the coastline of Norway and the east
coast of Greenland. It's essentially stretched T',
continental crust which is capped with lava flows which
were produced in great volume when the Atlantic Ocean first
opened up. So I suppose this picture of a
rifting extending micro-continent forming a
passive. Volcanic margin is consistent
with the plate. Hypothesis for Iceland.
What about Hawaii then, which is exhibit a for the plume
hypothesis? Can we see extension there?
That would be predicted by the plates.
Hypothesis, Hawaii is not only a massive challenge to either
falsifying or verifying the plume hypothesis because of its
great remoteness. And the fact that a lot of
evidence would like to see is either very difficult to find or
it's gone, but the same goes As for the point hypothesis, as you
rightly point out, we would expect to see substantial
evidence of extension there. But we're not really in a
position to be able to find that as yet.
So Hawaii is a tremendous challenge for both hypotheses
and it's just something that's going to be sorted out in the
future and I hope that happens quickly enough that I'm still
alive to see what happens. Do we have any independent line
of reasoning to That the lithosphere around Hawaii might
be extending there have been some studies done where people
have calculated the stress field in the Pacific Plate.
Simply put the Pacific Plate, covers one third of the surface
of the planet. It's absolutely huge.
It is shaped roughly like the continent of Africa.
So it has a big gap, bulge to the West, which is the Western
Pacific and then, there's The part of the East, which is
bordered by the East Pacific Rise and the South Pacific
spreading Ridge and the part to the east.
As I say, it's got a spreading Ridge along most of its length.
And then there's the San Andreas fault Zone, and then there's
more spreading in the Northeast Pacific.
So, that part of the Pacific is, got a lot of spreading Ridge on
it. The West Pacific.
On the other hand is surrounded on three sides by subduction.
Bones. So there's a strong slab, pull
Force to the north down, the Aleutian trench to the west and
1/8 China, and to the South. So the Western Pacific is in
more extension. If you like due to being largely
surrounded by subduction zones where as the East Pacific has
got more spreading Ridge so it's subject to Less in the way of
extensional stresses. So these two parts of the
Pacifica really under the influence of very different
stress fields and Hawaii lies exactly on the border between
the East and West Pacific. So one kind of Smoking Gun if
you like, is that this volcanic region is active exactly where
Pacific Plate material is drifting from the Eastern
Pacific. Stress regime into the Western
Pacific. Stress regime, and people have
modeled this, some colleagues have done relatively simple
models of the stress field due to Thermal contraction to
Thermal effects cooling contraction.
And also due to these slab forces, there have been some
promising results, which suggests that there are regions
of extension smack in the middle of the plate there.
And that this comes about About because of the variation is
Tresses, I've described this work.
Really needs to be done to the utmost accuracy that we can has
probably our best shot while we wait for seafloor geodesy to
become more mature. What about the other mid-ocean
volcanoes are there? Reasons to Suppose there might
be extension going on there as well Tristan da cunha for
example that might be an interesting place to look at as
been a lot of work done there, true?
The colonel is a relatively small island in the middle of a
very broad, scattering of seamounts.
In the Atlantic Ocean, we have a lot of major active fracture
zones. We have a lot of scattered
continental crust. It's clear that there's a lot of
Continental fragments there and the volcanism their, the
scattered volcanism is probably greatly influenced by the
inhomogeneous structure of the Atlantic in that region.
Which is bringing about differential movements.
The plate is not stable basically, okay?
So it's a complicated picture which may be consistent with the
plate hypothesis. How does the plate hypothesis
address the volcanism in the middle of continental plates
such as Yellowstone in North America?
The plates are not fully rigid. As was initially proposed in the
plate, tectonic hypothesis. But we now know that the plates
are deformable to some extent. We have features like the East
African Rift Zone, we have the Rhine graben and we also have
the Yellowstone area in North America and the Basin and Range
Province, for example. So these are regions inside the
plate which are deforming and in To some extent and Volcanic
regions like Yellowstone, and the other volcanism more,
southerly and the Basin and Range Province.
This, I would attribute to defamations internally in the
plate. The one thing about Yellowstone
that is heavily emphasized, is that there is a Time Progressive
volcanic Trail. So this time Progressive
volcanism along the Eastern Snake River plain is in The
great solicit calderas under the Eastern State River plain there.
The basaltic volcanism in that region is not time progressed if
it's the solicit volcanism which obviously comes from melting of
the lower crust. So I thought to myself, well, if
this part of the volcanism is time Progressive than the plate,
hypothesis predicts that extension must be time
Progressive So I just pulled up Google and started to type in
time Progressive volcanism in the Basin and Range Province.
And within five minutes I had found Papers written by
structural geologists who proposed that the activity of
large normal faults in the northern Basin and Range
Province was time. Progressive along the Eastern
Snake River, plain all the information is out there.
It just has not been Protected in terms of the plate
hypothesis. How long do you think it'll be
before observations will be good enough to provide a rigorous
test of the plate? Hypothesis?
I think we have enough observations on the table
already, one. Excellent thing about the plume
hypothesis is that it certainly has inspired and in gendered and
Justified a tremendous lot of research into very interesting
places like the Galapagos like Easter, Ireland reunions Tristan
da cunha. The only thing that's really
lacking is for people to come along and say, okay, I'm going
to look at hotspots such and such and I'm going to gather
together all the data that we've got so far.
I'm going to see how this fits into the plate hypothesis.
And then this will show what gaps there are in.
Testing the plate hypothesis and maybe we can propose an
experiment to fill in that Gap. Julian Folger.
Thank you very much. It was a great pleasure. sir
Oliver, it's been tremendous, fun, talking to you for more
about geology, b, as well as pictures and illustrations, that
support this podcast, you can go to geology B.com