This is John G B with all of us trampled.
The lower reaches of the Earth's mantle extend all the way down
to the boundary with a metallic core which lies about 2900 km
below. The surface, we knew almost
nothing about this. Highly inaccessible region until
good seismic measurements became available in the 1970s.
That coincided with a rapid increase in computer power,
which enabled seismologists to generate images, or be it at
very low resolution of the mantle, the images surprised Us
by revealing some dramatic features in the lower mantle
Alan McNamara is a professor of geological Sciences at Michigan
State University. He uses computer based, fluid
mechanical models to investigate the behavior of the mantle
working in tandem with the seismologists to understand the
origin and dynamics of the features we see in the mantle,
Alan McNamara, welcome to Georgie B.
Do they view all over? Am happy to be here in my
introduction. I mentioned some dramatic
features that the seismologists are found in their images of the
mantle. But before we talk about these
features, how is it even possible to see into the depths
of the mantle and create an image out of seismic waves?
Well, there are a couple different ways we can image the
inside of the earth kind of an analogous way as you would take
a CAT scan. The physics is a little
Different but the concepts similar on the surface of the
Earth. We have earthquakes and we also
have seismic stations located all around the globe.
And whenever there's an earthquake, those seismic waves
travel through the interior of the Earth, to the seismometers,
all around the globe. And by looking at those travel
times how long it took to go from the earthquake to the
seismometer, we can invert the problem and determine what the
inside of the earth looks like, basically we Can find, which
areas are the seismic waves move faster than average?
In which area is the seismic waves move slower than average.
And usually for seismic wave, moves faster than average, that
usually means the area is colder and if a seismic wave moves
slower than average that usually means the area is hotter.
But through that Technique, we can put together a rough image
of the Earth's interior. Now I'm like a CAT scan where
the sensors and the detectors can be distributed equally
around an earth were limited earthquakes.
We occur at plate boundaries and the big earthquakes which are
the most useful ones mostly occur at subduction zones and
the Earth is only thirty percent covered by land.
So that's where most of our seismometers are.
So there are huge gaps and heterogeneity and receivers and
and earthquakes leads to a very blurry image of the Earth's
interior. Now, there's another technique
that we can use to image very small things at the bottom of
the Earth's mantle and the sharpest, contrasting material
properties in the earth is actually the mantle and the core
at that boundary seismic waves reflect off that boundary very
easily. Now if you have other materials
above that boundary, or even below that boundary, you'll
often see little bounces that come back before or after that,
main core mantle boundary wave bounce and we call those
precursors or post cursors, and those can be used to detect the
presence of very small. X along the core-mantle
boundary. What do we see in these seismic
tomography maps in the regions where we've had subduction or
plates sink back into the mantle.
We see that those seismic waves move faster than average in
those areas so that indicates that those areas are colder than
average. And that's consistent with what
we think is happening at the surface of the plates are cold,
they sink into the mantle and they retain that coolness
compared to the rest of the mantle for a very long time.
So we see that in the Seismic tomography between those areas,
we have two large regions in the lower mantle that exhibit lower
than average seismic wave speed and we call those the large low
Shear velocity provinces and those are located beneath Africa
and the Pacific Ocean when we're such as started working on
understanding what these features mean and what's
actually going on in the mantle, did they have any observational
data? Other than the seismic data to
go on. This is a very interesting
question. So before we had a good
understanding of what the interior of the Earth look like
in a 3D sense, we had geochemical observations and
there are two main environments on the surface of the Earth
where you get the formation of Basalt.
So if you melt the mantle, you form a rock called Basalt and
the ocean floor is all just covered in basaltic rock.
So one area where you get the creation of Basalt is at
mid-ocean ridges. Is where the plates are
separating and new plates are being formed.
In another area is in Ocean islands called hot spots.
Hawaii is the prime example of a hot spot Iceland is also another
example of a hot spot and they also are made of Basalt.
And so those results to first order, look the same, but if you
do detailed geochemical analysis of these Basalt, you'll find
that they're quite different. So, the first thing you notice
is and the mid-ocean ridges where the plates are Being a new
plate is being created. Those basalts tend to have very
similar chemistry all around the world, the chemistry were
talking about is Trace element, chemistry?
So just the elements that are just a minor fraction of the
composition of those basalts so they seem to be relatively
consistent around the world and my understanding that chemistry,
it seems that those basalts are coming from rock that has melted
before. So there, it's not the first
time that mantle melted to form those It's now at the hotspot,
the trace element geochemistry is quite different.
First of all between each hotspot.
The chemistry is quite variable. I'm like, the mid-ocean ridges
where they chemistry is very similar from Ridge to Ridge.
Another interesting thing is, if you look into the geochemistry
that indicates that those basalts were formed from mantle
that has not melted before or maybe melted less than the
mid-ocean ridge basalts. So it seems like that much.
Real is coming from a different place.
An early idea was that the mantle was layered with a lower
mantle, in an upper mantle and the upper mantle caused all of
the mid-ocean ridge basalts? Whereas the lower mantle was
isolated from the upper mantle, but there would be mantle plumes
rising up from the lower mantle to the surface bringing up that
lower mantle material to the hotspots.
Once the seismological observation started coming out,
we noticed that there probably isn't a layered mantle but it is
intriguing. Seeking to think that these
large low Shear velocity provinces beneath Africa.
And the Pacific could be this source of this different
geochemistry that we see at the hot spots.
So actually we have a kind of marriage between the geochemical
and the seismic observations, what are the main scientific
questions were trying to answer about these large low Shear
velocity provinces? I would say the big question is
what are they that we don't know.
So what we do know is they are seismic Emily's additional
seismic studies that I haven't talked about here indicate that
they probably have very sharp edges to that.
Meaning that likely that they have a compositionally sharp
edge to them. So we think they're a different
composition than the rest of the mantle but we don't know what
they are. What they're made out of how
they got there, whether they are growing or whether they're
shrinking. The main thing we don't
understand is how they interact with the mantle to affect mantle
convection. In.
So one big question is, are they just passive features that are
just kind of moving around in response to mantle convection?
Or are they active features that are participating in the mantle?
Convection itself and perhaps even guiding mantle convection.
So those are the big questions that we have.
Okay. So let's talk about the question
of what these things are, particularly whether they're
just temperature anomalies or also temperature anomalies with
Different composition from the surrounding mantle.
How do you go about shedding light on this question?
My research is in computational fluid dynamics, basically
modeling the fluid dynamics of mantle convection, there are a
couple things that we can help in this process and one is we
can first test, whether hypotheses we may come up with.
For example, let's say if there's a hypothesis that the
large low Shear velocity provinces are caused by denser
than average. Positional reservoirs in the
mantle, we can put those ideas into a computer model.
An observation will spark an idea and oftentimes those ideas
are incorrect. They're just not consistent with
physics and the modeling can help show that it's not.
The second thing we can do is once we put those hypotheses
into computer models, we can investigate what additional
predictions, would those hypotheses make.
So again using the example of hypothesizing, that the large
low Shear velocity provinces Are made of compositional reservoirs
of denser than average material. If that's the case, does that
work? And it does work and computer
models. And then the question is, how
can it affect the core-mantle boundary topography?
How can it affect other things going on?
In the mantle in ways that seismologists can later detect
to determine whether the hypothesis is valid one or not?
Can you give an example of a specific predictions?
Say of your models? That could Could at least in
theory distinguish low Shear velocity provinces that contain
denser material from those that do not sure I had a student
several years ago named Theresa lasek her main PhD research was
to explore a question like this. She looked at two different
conceptual models for what causes the large low Shear
velocity provinces and one model?
She modeled the hypothesis that these are areas of mantle plumes
small mantle plumes that are rising up beneath For a kind of
Pacific, but when viewed through the blurry lens of tomography,
they look like a large anomaly. As opposed to a lot of little
plumes, she perform calculations and a hypothesis and also on a
hypothesis that the large low Shear velocity provinces are
denser than average compositional reservoirs.
And for each of those models, she looked at the influence that
those different ideas would have on the Topography of the
core-mantle boundary. And she found very Stark
differences between the two. To.
So if the large low Shear velocity provinces are caused by
thermal anomalies you would expect to see kind of like a
spidery ridge pattern of core-mantle boundary topography
where beneath each plume you would have the core-mantle
boundary uplifted a little bit and it would lead to a spidery
pattern. Because if you have a lot of
plumes upwelling, a plume has roots, kind of like the roots of
a tree and so that spider web pattern on the other hand.
If the Sheer velocity provinces were just piles of compositional
reservoirs of more dense material, the core-mantle
boundary typography beneath them was a very flat Plateau so a
very different pattern of typography all together and
along with that Plateau, you had a little Ridge of elevated
topography along the margins of that Plateau.
So if the seismology can look at the core-mantle, boundary
topography at that level of detail to detect the difference
between those shapes. Oops, that would provide a lot
of insight into which of those hypotheses are more credible.
Unfortunately, the seismic results aren't there yet these
low share velocity provinces. In fact they're called large low
Shear velocity provinces because there are thousands of
kilometers across with one of them extending under the whole
of Southern and Eastern Africa and the other under about a 3000
km wide region of the Pacific. Can we actually see any
structures smaller than that, in the lower mantle?
There are other structures called ultra low, velocity
zones, and these are compared to the large low Shear.
Velocity provinces are very teeny so they may be around 100
km wide anywhere from 20 to 40, maybe up to 60 kilometers thick.
So really teeny structures compared to the size of the
mantle that are on the core-mantle boundary.
Now they're probably not very important in terms of affecting
mantle convection and Anyway, but what they can be very useful
for is to act as is little tracers to help us see what's
happening on the larger scale. So we detect these ultra low
velocity zones by these precursors and post.
Cursors, Reflections that I mentioned before, they've been
known for quite some time. Since probably mid 90s, one
idea, is that they're just regions of the lower mantle that
are undergoing partial melting. And another idea is that they're
ultra dense materials. And I think that's probably my
favorite hypothesis and where they are.
So just like if you had a swimming pool with water flowing
around in it, and you have a little sand or dirt in the
swimming pool, you'll find that if there's currents in the water
that that sand will form little patterns, and that's what dense
stuff does in a fluid dynamical. Environment, dense material will
collect in the convergent areas of flow.
And one thing that we hope for, is that We can use ultra low,
velocity zones to tell us what the mantle flow is doing in the
lower mantle, just like sand in the swimming pool.
So if ultra low velocity zones are made of a different denser
material, where would that have come from?
And is there anything in your models to support or refute
these ideas? So just like the surface of the
Earth is very heterogeneous in composition, we have low
density, sedimentary Rear exit the surface.
We have granitic crust, we have basaltic ocean floor and it
seems like the closer you look the more heterogeneity.
You see we expect the same thing to happen in the lower mantle as
well. So it's the lighter things float
to the top of the mantle and the denser things will sink to the
bottom. So one thing we don't know is
are all ultra low velocities owns.
The same thing maybe just like the surface of the Earth is
complicated. It's not unreasonable to think
that the lowermost. Mantle of the earth is
complicated in terms of its chemical, hydrogen 80s.
One idea, is that these are reaction products from
interaction of mantle rock with iron core.
Another idea is that these are very dense rocks that formed and
the surface of the Earth billions of years ago.
Fact two and a half billion years ago.
There was the formation of a lot of iron-rich rocks called banded
iron formations that are very dense rocks and contain a lot of
iron, some People have proposed that the ultra low velocity
zones that we see in the bottom of the mantle are actually those
rocks that formed at the surface that's abducted into the mantle
sinking plates and are now in the lower mantle.
Do you have a personal favor? My personal favorite?
Is that there there banded iron formations that formed on the
surface of the Earth two and a half billion years ago and it's
my favorite because that's the most tangible.
I've seen those kinds of rocks the ideas intriguing.
You know, all of the different other hypotheses regarding
chemical reactions and stuff. That's a little bit less
tangible. How well can you model the
mantle Dynamics? When we're trying to model the
core, we can't even come within orders of magnitude of the
correct viscosity without overtaxing the available
Computing resources. You know, there used to be a big
problem in the mantle Geo Dynamics Community.
For example, when I was a graduate student Our models were
very simple. We had to model fluids that were
many orders of viscosity higher than what we think the Earth's
mantle is just because we didn't have the compute power, but in
the last 20 years computers have increased their speed quite a
bit and with the Advent of parallel Computing.
We can now do what we consider to be realistic problems of
Earth's. Mantle.
Convection people, that model, the Earth's outer core are still
many many, many years away. From being able to do that.
So, our problem right now is not to compute power, our main
problem right now is the unknowns.
So there's a lot about the mantle that we don't know a lot
about its material properties. The biggest thing that we don't
know is the viscosity, many different Studies have been done
that. Try to tackle what the average
viscosity is in the lower mantle and so we have some rough
numbers, I would say our error. Bars are several orders of
magnitude And that's pretty big. And probably the biggest issue
is we don't understand the grain size of the mantle, particularly
the lower mantle and any formulation of viscosity,
depends on the grain size. So we don't know what the
average grain size is. And more importantly is we don't
know how heterogeneous that grain sizes throughout the lower
mantle, so this really limits the types of problems that We
can study and so what I like to do, instead of trying to
simulate the Earth's mantle, I try to just tackle scientific
questions and with a particular scientific question, just tried
to design the simplest experiment.
I can that can give us understanding of the process.
Normally the question is related to some dynamical process that's
occurring. So, by making the simplest
experiment as possible. To understand that process, we
can take that information and then Add that to all of the
other information that we know to put together our story of how
the mantle Works. Can we actually do any
experiments in the lab that maybe recreate the pressures and
temperatures of the mantle and help nail down?
One of these things such as the viscosity or the grain size?
Yes we do. So there's a branch of
geophysics called mineral physics.
Now, there are a couple issues with that, all of the
experiments have to be done at many.
Orders of magnitude higher strain rates than the earth is
convecting to do the experiment at Earth deformation rates.
Would just, you know, take way too long, take lifetimes to
complete. So we're already a little bit
out of the range of reality when we do those experiments but
still that's the best we could do.
And I think the information we get from that is very
enlightening, the biggest issue still is, we don't understand
the grain size. So even if mineral physicists
finally, came up with the best for Hello for the viscosity of
mantle materials in the earth is a function of temperature
pressure and a grain size. And all of that.
We're still limited by the fact that we don't know what the
grain size is. Well, now in the upper mantle,
we have some samples, there are some places where there have
been volcanic explosions that rip up part of the top of the
upper mantle and bring it to the surface.
And so we can see those. I have one of those rocks on my
desk at work and their grain size is around a millimeter or
so. But in the lower mantle, we have
I've no idea what the grain size is it all, when you say, we
can't reproduce The Strain rates.
Roughly, how fast all these motions?
How fast is the strain rate going on in the mantle.
So in the earth's upper mantle, at least the estimates for
strain rate are around 10 to the minus 15 per second.
Most people don't deal with strain rates so that's not a
very tangible thing to put in your mind.
But keep in mind that the plates are moving at a rate of a Own cm
per year. So for example, the North
America and Europe is separating at about the same rate as your
fingernail is growing and the minerals are being deformed over
millions of years. So the strain can be quite high
but to do a mineral physics experiment where you're
straining, things at the rate that your fingernails growing
can take a long time to come up with enough strength that you
would need to put together a formula for the viscosity, a lot
longer than the duration of a typical PhD.
I'm show. Yeah absolutely you mentioned
that the plate motions tells us something about the mantle
motions or the order of centimeters per year.
Does the fact that we really actually know the motion of the
plate pretty well from geodesy and so on does that provide a
constraint or like a top boundary condition on your
mantle models? That's helpful that's probably
one of the most powerful constraints we have to
understand convection in the Earth's mantle.
It actually provides an Before us that we especially now with
GPS, we know with high reliability.
So this number of centimeters per year, it's the only place
that we have in the Earth's mantle.
That provides any kind of number that will tell us how vigorous
the mantle is conducting. Now we don't know how that
extends down word. For example, we think we have
reasons to think that the lower mantle has a higher viscosity
and then the upper mantle. So maybe the lower mantels
flowing at much lower than centimeters per year.
Just the fact that plates are moving.
That was the main motivation to suspect?
We have mantle convection in the first place.
Okay. I want to ask you about
something. You mentioned earlier, which is
that one of the things we hope to shed light on is what the
origin of these large low Shear velocity provinces might be, are
they primordial or are they somehow created and sustained by
ongoing processes? What's the research on that?
And how might one distinguish? These two ideas.
There are two very different hypotheses as to the formation,
both may be occurring. So one idea is that the large
low Shear velocity, provinces are remnants of something more
ancient. So perhaps after the Earth
formed either during the differentiation of the Earth or
through processes that occurred in those first billion years,
when the Earth was undergoing, a lot of changes that perhaps a
layer, a more dense layer formed in the earth, and Through Time.
I'm that layer is slowly being entrained into the upper layer
and as a result, the density contrast between the two layers
get smaller and that layer ends up eroding, what we see now is
just the remnants or pieces of what was something much larger
and this is kind of motivate advised that Trace elements
geochemistry that we see at hot spots that indicates that the
source material for those basalts is something that's a
little bit more primitive. Thing that looks a little bit
more. Like what we think the original
Earth look, like the original mantle.
Look, like now, another idea, which is also supported by the
geochemistry, by the way is that these large low Shear velocity
provinces are created by the subduction of that basaltic,
oceanic crust through time. So we know that the surface is
always subducting into the mantle sinking into the mantle
and that layer of crust. On the surface of the ocean
floor, which bus six to seven kilometers thick.
We think that becomes more dense at mantle pressures.
And this is based on Mineral physics experiments that have
been done in the laboratory, you take a Basalt and put it under
high pressure. It will change its structure a
little bit and become more dense than the rest of the surrounding
mantle. And so they could be the
accumulations of billions of years of this oceanic crust
that's accumulating. So it's a big question for us as
to whether these large. Shear velocity provinces are
being created or they're being destroyed.
One idea that you mentioned when we were talking earlier is that
if you have as a consequence of say, one of your models that you
find that these large low Shear velocity regions would be
throwing up, big plumes to create hot spots, then isn't
there a challenge of how that, if it's a primordial feature how
that could be sustained. Absolutely.
First, let me back up and say the mantle Is a very viscous
place, it's very different from a lava lamp that.
You may have in a lava lamp, the two materials have a very
different viscosity and they don't stir together very well.
The Earth's mantle is very different because it's so
viscous. It stirs extremely well.
So it's very hard to have different compositions rising up
and not getting stirred with the rest of the mantle at the same
time. So if these large low Shear
velocity provinces, Are rising up through the mantle as some
think that they are, then it's very difficult for that process
to occur for four and a half billion years.
So that would argue for a hypothesis where these things
are accumulating with time. For my last question, if you had
access to totally Unlimited part of research funds, how would you
spend it? Definitely ocean bottom
seismometers, the Earth's Surface is only 30% land and
that's where most of the seismometers are, there are
islands around the oceans where there are seismometers as well.
So they're mean, there is some coverage in the ocean.
I think the best thing we can do is just get a better image of
what's going on down there, each new Advanced from seismology
radically changes, our ideas of what's going on.
You have money wasn't an issue, just more of those improve our
coverage. Let's get better pictures of
what's going on in the lower mantle.
Alan McNamara. Thank you very much.
Thank you Oliver. It's been fun for more about
geology, b, as well as pictures and diagrams that illustrate.
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