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This is John G B with all of us 
trampled. 

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The lower reaches of the Earth's
mantle extend all the way down 

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to the boundary with a metallic 
core which lies about 2900 km 

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below. 
The surface, we knew almost 

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nothing about this. 
Highly inaccessible region until

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good seismic measurements became
available in the 1970s. 

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That coincided with a rapid 
increase in computer power, 

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which enabled seismologists to 
generate images, or be it at 

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very low resolution of the 
mantle, the images surprised Us 

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by revealing some dramatic 
features in the lower mantle 

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Alan McNamara is a professor of 
geological Sciences at Michigan 

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State University. 
He uses computer based, fluid 

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mechanical models to investigate
the behavior of the mantle 

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working in tandem with the 
seismologists to understand the 

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origin and dynamics of the 
features we see in the mantle, 

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Alan McNamara, welcome to 
Georgie B. 

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Do they view all over? 
Am happy to be here in my 

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introduction. 
I mentioned some dramatic 

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features that the seismologists 
are found in their images of the

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mantle. 
But before we talk about these 

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features, how is it even 
possible to see into the depths 

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of the mantle and create an 
image out of seismic waves? 

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Well, there are a couple 
different ways we can image the 

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inside of the earth kind of an 
analogous way as you would take 

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a CAT scan. 
The physics is a little 

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Different but the concepts 
similar on the surface of the 

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Earth. 
We have earthquakes and we also 

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have seismic stations located 
all around the globe. 

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And whenever there's an 
earthquake, those seismic waves 

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travel through the interior of 
the Earth, to the seismometers, 

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all around the globe. 
And by looking at those travel 

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times how long it took to go 
from the earthquake to the 

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seismometer, we can invert the 
problem and determine what the 

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inside of the earth looks like, 
basically we Can find, which 

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areas are the seismic waves move
faster than average? 

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In which area is the seismic 
waves move slower than average. 

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And usually for seismic wave, 
moves faster than average, that 

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usually means the area is colder
and if a seismic wave moves 

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slower than average that usually
means the area is hotter. 

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But through that Technique, we 
can put together a rough image 

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of the Earth's interior. 
Now I'm like a CAT scan where 

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the sensors and the detectors 
can be distributed equally 

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around an earth were limited 
earthquakes. 

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We occur at plate boundaries and
the big earthquakes which are 

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the most useful ones mostly 
occur at subduction zones and 

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the Earth is only thirty percent
covered by land. 

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So that's where most of our 
seismometers are. 

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So there are huge gaps and 
heterogeneity and receivers and 

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and earthquakes leads to a very 
blurry image of the Earth's 

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interior. 
Now, there's another technique 

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that we can use to image very 
small things at the bottom of 

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the Earth's mantle and the 
sharpest, contrasting material 

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properties in the earth is 
actually the mantle and the core

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at that boundary seismic waves 
reflect off that boundary very 

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easily. 
Now if you have other materials 

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above that boundary, or even 
below that boundary, you'll 

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often see little bounces that 
come back before or after that, 

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main core mantle boundary wave 
bounce and we call those 

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precursors or post cursors, and 
those can be used to detect the 

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presence of very small. 
X along the core-mantle 

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boundary. 
What do we see in these seismic 

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tomography maps in the regions 
where we've had subduction or 

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plates sink back into the 
mantle. 

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We see that those seismic waves 
move faster than average in 

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those areas so that indicates 
that those areas are colder than

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average. 
And that's consistent with what 

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we think is happening at the 
surface of the plates are cold, 

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they sink into the mantle and 
they retain that coolness 

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compared to the rest of the 
mantle for a very long time. 

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So we see that in the Seismic 
tomography between those areas, 

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we have two large regions in the
lower mantle that exhibit lower 

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than average seismic wave speed 
and we call those the large low 

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Shear velocity provinces and 
those are located beneath Africa

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and the Pacific Ocean when we're
such as started working on 

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understanding what these 
features mean and what's 

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actually going on in the mantle,
did they have any observational 

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data? 
Other than the seismic data to 

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go on. 
This is a very interesting 

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question. 
So before we had a good 

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understanding of what the 
interior of the Earth look like 

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in a 3D sense, we had 
geochemical observations and 

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there are two main environments 
on the surface of the Earth 

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where you get the formation of 
Basalt. 

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So if you melt the mantle, you 
form a rock called Basalt and 

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the ocean floor is all just 
covered in basaltic rock. 

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So one area where you get the 
creation of Basalt is at 

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mid-ocean ridges. 
Is where the plates are 

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separating and new plates are 
being formed. 

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In another area is in Ocean 
islands called hot spots. 

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Hawaii is the prime example of a
hot spot Iceland is also another

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example of a hot spot and they 
also are made of Basalt. 

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And so those results to first 
order, look the same, but if you

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do detailed geochemical analysis
of these Basalt, you'll find 

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that they're quite different. 
So, the first thing you notice 

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is and the mid-ocean ridges 
where the plates are Being a new

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plate is being created. 
Those basalts tend to have very 

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similar chemistry all around the
world, the chemistry were 

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talking about is Trace element, 
chemistry? 

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So just the elements that are 
just a minor fraction of the 

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composition of those basalts so 
they seem to be relatively 

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consistent around the world and 
my understanding that chemistry,

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it seems that those basalts are 
coming from rock that has melted

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before. 
So there, it's not the first 

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time that mantle melted to form 
those It's now at the hotspot, 

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the trace element geochemistry 
is quite different. 

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First of all between each 
hotspot. 

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The chemistry is quite variable.
I'm like, the mid-ocean ridges 

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where they chemistry is very 
similar from Ridge to Ridge. 

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Another interesting thing is, if
you look into the geochemistry 

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that indicates that those 
basalts were formed from mantle 

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that has not melted before or 
maybe melted less than the 

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mid-ocean ridge basalts. 
So it seems like that much. 

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Real is coming from a different 
place. 

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An early idea was that the 
mantle was layered with a lower 

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mantle, in an upper mantle and 
the upper mantle caused all of 

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the mid-ocean ridge basalts? 
Whereas the lower mantle was 

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isolated from the upper mantle, 
but there would be mantle plumes

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rising up from the lower mantle 
to the surface bringing up that 

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lower mantle material to the 
hotspots. 

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Once the seismological 
observation started coming out, 

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we noticed that there probably 
isn't a layered mantle but it is

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intriguing. 
Seeking to think that these 

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large low Shear velocity 
provinces beneath Africa. 

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And the Pacific could be this 
source of this different 

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geochemistry that we see at the 
hot spots. 

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So actually we have a kind of 
marriage between the geochemical

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and the seismic observations, 
what are the main scientific 

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questions were trying to answer 
about these large low Shear 

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velocity provinces? 
I would say the big question is 

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what are they that we don't 
know. 

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So what we do know is they are 
seismic Emily's additional 

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seismic studies that I haven't 
talked about here indicate that 

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they probably have very sharp 
edges to that. 

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Meaning that likely that they 
have a compositionally sharp 

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edge to them. 
So we think they're a different 

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composition than the rest of the
mantle but we don't know what 

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they are. 
What they're made out of how 

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they got there, whether they are
growing or whether they're 

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shrinking. 
The main thing we don't 

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understand is how they interact 
with the mantle to affect mantle

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convection. 
In. 

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So one big question is, are they
just passive features that are 

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just kind of moving around in 
response to mantle convection? 

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Or are they active features that
are participating in the mantle?

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Convection itself and perhaps 
even guiding mantle convection. 

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So those are the big questions 
that we have. 

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Okay. 
So let's talk about the question

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of what these things are, 
particularly whether they're 

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just temperature anomalies or 
also temperature anomalies with 

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Different composition from the 
surrounding mantle. 

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How do you go about shedding 
light on this question? 

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My research is in computational 
fluid dynamics, basically 

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modeling the fluid dynamics of 
mantle convection, there are a 

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couple things that we can help 
in this process and one is we 

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can first test, whether 
hypotheses we may come up with. 

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For example, let's say if 
there's a hypothesis that the 

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large low Shear velocity 
provinces are caused by denser 

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than average. 
Positional reservoirs in the 

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mantle, we can put those ideas 
into a computer model. 

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An observation will spark an 
idea and oftentimes those ideas 

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are incorrect. 
They're just not consistent with

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physics and the modeling can 
help show that it's not. 

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The second thing we can do is 
once we put those hypotheses 

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into computer models, we can 
investigate what additional 

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predictions, would those 
hypotheses make. 

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So again using the example of 
hypothesizing, that the large 

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low Shear velocity provinces Are
made of compositional reservoirs

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of denser than average material.
If that's the case, does that 

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work? 
And it does work and computer 

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models. 
And then the question is, how 

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can it affect the core-mantle 
boundary topography? 

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How can it affect other things 
going on? 

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In the mantle in ways that 
seismologists can later detect 

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to determine whether the 
hypothesis is valid one or not? 

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Can you give an example of a 
specific predictions? 

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Say of your models? 
That could Could at least in 

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theory distinguish low Shear 
velocity provinces that contain 

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denser material from those that 
do not sure I had a student 

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several years ago named Theresa 
lasek her main PhD research was 

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to explore a question like this.
She looked at two different 

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conceptual models for what 
causes the large low Shear 

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velocity provinces and one 
model? 

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She modeled the hypothesis that 
these are areas of mantle plumes

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small mantle plumes that are 
rising up beneath For a kind of 

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Pacific, but when viewed through
the blurry lens of tomography, 

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they look like a large anomaly. 
As opposed to a lot of little 

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plumes, she perform calculations
and a hypothesis and also on a 

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hypothesis that the large low 
Shear velocity provinces are 

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denser than average 
compositional reservoirs. 

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And for each of those models, 
she looked at the influence that

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those different ideas would have
on the Topography of the 

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core-mantle boundary. 
And she found very Stark 

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differences between the two. 
To. 

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So if the large low Shear 
velocity provinces are caused by

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thermal anomalies you would 
expect to see kind of like a 

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spidery ridge pattern of 
core-mantle boundary topography 

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where beneath each plume you 
would have the core-mantle 

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boundary uplifted a little bit 
and it would lead to a spidery 

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pattern. 
Because if you have a lot of 

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plumes upwelling, a plume has 
roots, kind of like the roots of

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a tree and so that spider web 
pattern on the other hand. 

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If the Sheer velocity provinces 
were just piles of compositional

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reservoirs of more dense 
material, the core-mantle 

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boundary typography beneath them
was a very flat Plateau so a 

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very different pattern of 
typography all together and 

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along with that Plateau, you had
a little Ridge of elevated 

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topography along the margins of 
that Plateau. 

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So if the seismology can look at
the core-mantle, boundary 

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topography at that level of 
detail to detect the difference 

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between those shapes. 
Oops, that would provide a lot 

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of insight into which of those 
hypotheses are more credible. 

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Unfortunately, the seismic 
results aren't there yet these 

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low share velocity provinces. 
In fact they're called large low

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Shear velocity provinces because
there are thousands of 

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kilometers across with one of 
them extending under the whole 

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of Southern and Eastern Africa 
and the other under about a 3000

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km wide region of the Pacific. 
Can we actually see any 

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structures smaller than that, in
the lower mantle? 

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There are other structures 
called ultra low, velocity 

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zones, and these are compared to
the large low Shear. 

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Velocity provinces are very 
teeny so they may be around 100 

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km wide anywhere from 20 to 40, 
maybe up to 60 kilometers thick.

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So really teeny structures 
compared to the size of the 

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mantle that are on the 
core-mantle boundary. 

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Now they're probably not very 
important in terms of affecting 

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mantle convection and Anyway, 
but what they can be very useful

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for is to act as is little 
tracers to help us see what's 

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happening on the larger scale. 
So we detect these ultra low 

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velocity zones by these 
precursors and post. 

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Cursors, Reflections that I 
mentioned before, they've been 

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00:13:20,500 --> 00:13:24,800
known for quite some time. 
Since probably mid 90s, one 

232
00:13:24,800 --> 00:13:27,200
idea, is that they're just 
regions of the lower mantle that

233
00:13:27,200 --> 00:13:31,200
are undergoing partial melting. 
And another idea is that they're

234
00:13:31,200 --> 00:13:34,500
ultra dense materials. 
And I think that's probably my 

235
00:13:34,600 --> 00:13:36,900
favorite hypothesis and where 
they are. 

236
00:13:36,900 --> 00:13:41,300
So just like if you had a 
swimming pool with water flowing

237
00:13:41,300 --> 00:13:45,600
around in it, and you have a 
little sand or dirt in the 

238
00:13:45,600 --> 00:13:48,300
swimming pool, you'll find that 
if there's currents in the water

239
00:13:48,300 --> 00:13:54,300
that that sand will form little 
patterns, and that's what dense 

240
00:13:54,300 --> 00:13:58,300
stuff does in a fluid dynamical.
Environment, dense material will

241
00:13:58,300 --> 00:14:01,100
collect in the convergent areas 
of flow. 

242
00:14:01,100 --> 00:14:05,700
And one thing that we hope for, 
is that We can use ultra low, 

243
00:14:05,700 --> 00:14:10,200
velocity zones to tell us what 
the mantle flow is doing in the 

244
00:14:10,200 --> 00:14:12,600
lower mantle, just like sand in 
the swimming pool. 

245
00:14:13,100 --> 00:14:18,600
So if ultra low velocity zones 
are made of a different denser 

246
00:14:18,600 --> 00:14:21,200
material, where would that have 
come from? 

247
00:14:21,400 --> 00:14:26,200
And is there anything in your 
models to support or refute 

248
00:14:26,200 --> 00:14:29,300
these ideas? 
So just like the surface of the 

249
00:14:29,300 --> 00:14:33,600
Earth is very heterogeneous in 
composition, we have low 

250
00:14:33,600 --> 00:14:35,800
density, sedimentary Rear exit 
the surface. 

251
00:14:35,800 --> 00:14:40,400
We have granitic crust, we have 
basaltic ocean floor and it 

252
00:14:40,400 --> 00:14:42,900
seems like the closer you look 
the more heterogeneity. 

253
00:14:42,900 --> 00:14:47,300
You see we expect the same thing
to happen in the lower mantle as

254
00:14:47,300 --> 00:14:49,500
well. 
So it's the lighter things float

255
00:14:49,500 --> 00:14:52,000
to the top of the mantle and the
denser things will sink to the 

256
00:14:52,000 --> 00:14:55,000
bottom. 
So one thing we don't know is 

257
00:14:55,200 --> 00:14:57,200
are all ultra low velocities 
owns. 

258
00:14:57,200 --> 00:15:00,000
The same thing maybe just like 
the surface of the Earth is 

259
00:15:00,000 --> 00:15:03,400
complicated. 
It's not unreasonable to think 

260
00:15:03,400 --> 00:15:05,600
that the lowermost. 
Mantle of the earth is 

261
00:15:05,600 --> 00:15:08,200
complicated in terms of its 
chemical, hydrogen 80s. 

262
00:15:08,200 --> 00:15:12,400
One idea, is that these are 
reaction products from 

263
00:15:12,400 --> 00:15:15,600
interaction of mantle rock with 
iron core. 

264
00:15:16,400 --> 00:15:21,400
Another idea is that these are 
very dense rocks that formed and

265
00:15:21,400 --> 00:15:24,200
the surface of the Earth 
billions of years ago. 

266
00:15:24,400 --> 00:15:26,200
Fact two and a half billion 
years ago. 

267
00:15:26,200 --> 00:15:30,200
There was the formation of a lot
of iron-rich rocks called banded

268
00:15:30,200 --> 00:15:33,200
iron formations that are very 
dense rocks and contain a lot of

269
00:15:33,200 --> 00:15:37,000
iron, some People have proposed 
that the ultra low velocity 

270
00:15:37,000 --> 00:15:41,200
zones that we see in the bottom 
of the mantle are actually those

271
00:15:41,200 --> 00:15:44,800
rocks that formed at the surface
that's abducted into the mantle 

272
00:15:44,900 --> 00:15:47,300
sinking plates and are now in 
the lower mantle. 

273
00:15:47,800 --> 00:15:50,900
Do you have a personal favor? 
My personal favorite? 

274
00:15:50,900 --> 00:15:53,700
Is that there there banded iron 
formations that formed on the 

275
00:15:53,700 --> 00:15:56,500
surface of the Earth two and a 
half billion years ago and it's 

276
00:15:56,500 --> 00:15:58,500
my favorite because that's the 
most tangible. 

277
00:15:58,700 --> 00:16:02,300
I've seen those kinds of rocks 
the ideas intriguing. 

278
00:16:03,100 --> 00:16:05,900
You know, all of the different 
other hypotheses regarding 

279
00:16:06,000 --> 00:16:08,000
chemical reactions and stuff. 
That's a little bit less 

280
00:16:08,000 --> 00:16:12,000
tangible. 
How well can you model the 

281
00:16:12,000 --> 00:16:15,300
mantle Dynamics? 
When we're trying to model the 

282
00:16:15,300 --> 00:16:19,900
core, we can't even come within 
orders of magnitude of the 

283
00:16:19,900 --> 00:16:23,900
correct viscosity without 
overtaxing the available 

284
00:16:23,900 --> 00:16:27,000
Computing resources. 
You know, there used to be a big

285
00:16:27,000 --> 00:16:30,600
problem in the mantle Geo 
Dynamics Community. 

286
00:16:30,800 --> 00:16:33,700
For example, when I was a 
graduate student Our models were

287
00:16:33,700 --> 00:16:37,000
very simple. 
We had to model fluids that were

288
00:16:37,000 --> 00:16:40,600
many orders of viscosity higher 
than what we think the Earth's 

289
00:16:40,900 --> 00:16:44,700
mantle is just because we didn't
have the compute power, but in 

290
00:16:44,700 --> 00:16:48,700
the last 20 years computers have
increased their speed quite a 

291
00:16:48,708 --> 00:16:51,800
bit and with the Advent of 
parallel Computing. 

292
00:16:52,100 --> 00:16:56,900
We can now do what we consider 
to be realistic problems of 

293
00:16:57,000 --> 00:16:57,600
Earth's. 
Mantle. 

294
00:16:57,600 --> 00:17:01,100
Convection people, that model, 
the Earth's outer core are still

295
00:17:01,100 --> 00:17:04,400
many many, many years away. 
From being able to do that. 

296
00:17:04,800 --> 00:17:07,900
So, our problem right now is not
to compute power, our main 

297
00:17:07,900 --> 00:17:10,800
problem right now is the 
unknowns. 

298
00:17:11,500 --> 00:17:14,700
So there's a lot about the 
mantle that we don't know a lot 

299
00:17:14,700 --> 00:17:18,500
about its material properties. 
The biggest thing that we don't 

300
00:17:18,500 --> 00:17:21,700
know is the viscosity, many 
different Studies have been done

301
00:17:21,700 --> 00:17:24,900
that. 
Try to tackle what the average 

302
00:17:24,900 --> 00:17:28,500
viscosity is in the lower mantle
and so we have some rough 

303
00:17:28,500 --> 00:17:32,300
numbers, I would say our error. 
Bars are several orders of 

304
00:17:32,300 --> 00:17:37,000
magnitude And that's pretty big.
And probably the biggest issue 

305
00:17:37,000 --> 00:17:42,100
is we don't understand the grain
size of the mantle, particularly

306
00:17:42,100 --> 00:17:47,300
the lower mantle and any 
formulation of viscosity, 

307
00:17:47,500 --> 00:17:50,100
depends on the grain size. 
So we don't know what the 

308
00:17:50,100 --> 00:17:53,900
average grain size is. 
And more importantly is we don't

309
00:17:53,900 --> 00:17:58,200
know how heterogeneous that 
grain sizes throughout the lower

310
00:17:58,200 --> 00:18:02,900
mantle, so this really limits 
the types of problems that We 

311
00:18:02,900 --> 00:18:05,500
can study and so what I like to 
do, instead of trying to 

312
00:18:05,500 --> 00:18:10,800
simulate the Earth's mantle, I 
try to just tackle scientific 

313
00:18:10,800 --> 00:18:15,100
questions and with a particular 
scientific question, just tried 

314
00:18:15,100 --> 00:18:17,600
to design the simplest 
experiment. 

315
00:18:17,700 --> 00:18:21,600
I can that can give us 
understanding of the process. 

316
00:18:21,900 --> 00:18:25,200
Normally the question is related
to some dynamical process that's

317
00:18:25,200 --> 00:18:27,500
occurring. 
So, by making the simplest 

318
00:18:27,500 --> 00:18:30,800
experiment as possible. 
To understand that process, we 

319
00:18:30,808 --> 00:18:34,000
can take that information and 
then Add that to all of the 

320
00:18:34,000 --> 00:18:37,200
other information that we know 
to put together our story of how

321
00:18:37,200 --> 00:18:40,300
the mantle Works. 
Can we actually do any 

322
00:18:40,300 --> 00:18:45,200
experiments in the lab that 
maybe recreate the pressures and

323
00:18:45,200 --> 00:18:49,000
temperatures of the mantle and 
help nail down? 

324
00:18:49,200 --> 00:18:51,900
One of these things such as the 
viscosity or the grain size? 

325
00:18:52,400 --> 00:18:55,300
Yes we do. 
So there's a branch of 

326
00:18:55,300 --> 00:18:57,300
geophysics called mineral 
physics. 

327
00:18:57,700 --> 00:19:00,700
Now, there are a couple issues 
with that, all of the 

328
00:19:00,700 --> 00:19:02,600
experiments have to be done at 
many. 

329
00:19:02,700 --> 00:19:05,700
Orders of magnitude higher 
strain rates than the earth is 

330
00:19:05,700 --> 00:19:10,000
convecting to do the experiment 
at Earth deformation rates. 

331
00:19:10,100 --> 00:19:13,500
Would just, you know, take way 
too long, take lifetimes to 

332
00:19:13,500 --> 00:19:15,900
complete. 
So we're already a little bit 

333
00:19:15,900 --> 00:19:19,600
out of the range of reality when
we do those experiments but 

334
00:19:19,600 --> 00:19:20,800
still that's the best we could 
do. 

335
00:19:20,800 --> 00:19:23,900
And I think the information we 
get from that is very 

336
00:19:23,900 --> 00:19:27,000
enlightening, the biggest issue 
still is, we don't understand 

337
00:19:27,000 --> 00:19:29,300
the grain size. 
So even if mineral physicists 

338
00:19:29,800 --> 00:19:34,700
finally, came up with the best 
for Hello for the viscosity of 

339
00:19:34,700 --> 00:19:37,900
mantle materials in the earth is
a function of temperature 

340
00:19:37,900 --> 00:19:40,900
pressure and a grain size. 
And all of that. 

341
00:19:41,100 --> 00:19:43,400
We're still limited by the fact 
that we don't know what the 

342
00:19:43,400 --> 00:19:45,600
grain size is. 
Well, now in the upper mantle, 

343
00:19:45,600 --> 00:19:48,900
we have some samples, there are 
some places where there have 

344
00:19:48,900 --> 00:19:52,500
been volcanic explosions that 
rip up part of the top of the 

345
00:19:52,500 --> 00:19:54,400
upper mantle and bring it to the
surface. 

346
00:19:54,700 --> 00:19:57,100
And so we can see those. 
I have one of those rocks on my 

347
00:19:57,100 --> 00:20:00,000
desk at work and their grain 
size is around a millimeter or 

348
00:20:00,000 --> 00:20:02,600
so. 
But in the lower mantle, we have

349
00:20:02,700 --> 00:20:06,400
I've no idea what the grain size
is it all, when you say, we 

350
00:20:06,400 --> 00:20:08,500
can't reproduce The Strain 
rates. 

351
00:20:09,000 --> 00:20:12,300
Roughly, how fast all these 
motions? 

352
00:20:12,400 --> 00:20:15,500
How fast is the strain rate 
going on in the mantle. 

353
00:20:15,900 --> 00:20:19,900
So in the earth's upper mantle, 
at least the estimates for 

354
00:20:19,900 --> 00:20:23,900
strain rate are around 10 to the
minus 15 per second. 

355
00:20:24,300 --> 00:20:27,200
Most people don't deal with 
strain rates so that's not a 

356
00:20:27,200 --> 00:20:29,700
very tangible thing to put in 
your mind. 

357
00:20:29,900 --> 00:20:33,600
But keep in mind that the plates
are moving at a rate of a Own cm

358
00:20:33,600 --> 00:20:36,200
per year. 
So for example, the North 

359
00:20:36,200 --> 00:20:39,600
America and Europe is separating
at about the same rate as your 

360
00:20:39,600 --> 00:20:44,000
fingernail is growing and the 
minerals are being deformed over

361
00:20:44,000 --> 00:20:46,600
millions of years. 
So the strain can be quite high 

362
00:20:47,000 --> 00:20:49,900
but to do a mineral physics 
experiment where you're 

363
00:20:49,900 --> 00:20:53,000
straining, things at the rate 
that your fingernails growing 

364
00:20:53,800 --> 00:20:56,700
can take a long time to come up 
with enough strength that you 

365
00:20:56,708 --> 00:21:00,200
would need to put together a 
formula for the viscosity, a lot

366
00:21:00,200 --> 00:21:02,500
longer than the duration of a 
typical PhD. 

367
00:21:02,700 --> 00:21:06,900
I'm show. 
Yeah absolutely you mentioned 

368
00:21:06,900 --> 00:21:10,100
that the plate motions tells us 
something about the mantle 

369
00:21:10,100 --> 00:21:12,400
motions or the order of 
centimeters per year. 

370
00:21:12,900 --> 00:21:17,100
Does the fact that we really 
actually know the motion of the 

371
00:21:17,100 --> 00:21:21,700
plate pretty well from geodesy 
and so on does that provide a 

372
00:21:21,708 --> 00:21:24,200
constraint or like a top 
boundary condition on your 

373
00:21:24,200 --> 00:21:27,400
mantle models? 
That's helpful that's probably 

374
00:21:27,400 --> 00:21:29,400
one of the most powerful 
constraints we have to 

375
00:21:29,400 --> 00:21:31,200
understand convection in the 
Earth's mantle. 

376
00:21:31,300 --> 00:21:35,200
It actually provides an Before 
us that we especially now with 

377
00:21:35,200 --> 00:21:37,900
GPS, we know with high 
reliability. 

378
00:21:38,000 --> 00:21:41,200
So this number of centimeters 
per year, it's the only place 

379
00:21:41,200 --> 00:21:42,800
that we have in the Earth's 
mantle. 

380
00:21:42,800 --> 00:21:47,400
That provides any kind of number
that will tell us how vigorous 

381
00:21:47,400 --> 00:21:50,100
the mantle is conducting. 
Now we don't know how that 

382
00:21:50,100 --> 00:21:53,300
extends down word. 
For example, we think we have 

383
00:21:53,300 --> 00:21:56,800
reasons to think that the lower 
mantle has a higher viscosity 

384
00:21:56,800 --> 00:21:59,400
and then the upper mantle. 
So maybe the lower mantels 

385
00:21:59,500 --> 00:22:02,300
flowing at much lower than 
centimeters per year. 

386
00:22:02,700 --> 00:22:04,400
Just the fact that plates are 
moving. 

387
00:22:04,500 --> 00:22:06,200
That was the main motivation to 
suspect? 

388
00:22:06,200 --> 00:22:08,100
We have mantle convection in the
first place. 

389
00:22:08,600 --> 00:22:10,300
Okay. 
I want to ask you about 

390
00:22:10,300 --> 00:22:12,600
something. 
You mentioned earlier, which is 

391
00:22:12,600 --> 00:22:16,300
that one of the things we hope 
to shed light on is what the 

392
00:22:16,300 --> 00:22:21,000
origin of these large low Shear 
velocity provinces might be, are

393
00:22:21,000 --> 00:22:25,900
they primordial or are they 
somehow created and sustained by

394
00:22:25,900 --> 00:22:29,100
ongoing processes? 
What's the research on that? 

395
00:22:29,100 --> 00:22:32,100
And how might one distinguish? 
These two ideas. 

396
00:22:32,700 --> 00:22:36,600
There are two very different 
hypotheses as to the formation, 

397
00:22:37,100 --> 00:22:40,900
both may be occurring. 
So one idea is that the large 

398
00:22:40,900 --> 00:22:44,600
low Shear velocity, provinces 
are remnants of something more 

399
00:22:44,600 --> 00:22:47,300
ancient. 
So perhaps after the Earth 

400
00:22:47,300 --> 00:22:51,500
formed either during the 
differentiation of the Earth or 

401
00:22:51,500 --> 00:22:55,000
through processes that occurred 
in those first billion years, 

402
00:22:55,000 --> 00:22:58,200
when the Earth was undergoing, a
lot of changes that perhaps a 

403
00:22:58,200 --> 00:23:02,400
layer, a more dense layer formed
in the earth, and Through Time. 

404
00:23:02,500 --> 00:23:06,300
I'm that layer is slowly being 
entrained into the upper layer 

405
00:23:06,900 --> 00:23:10,900
and as a result, the density 
contrast between the two layers 

406
00:23:10,900 --> 00:23:16,200
get smaller and that layer ends 
up eroding, what we see now is 

407
00:23:16,200 --> 00:23:19,400
just the remnants or pieces of 
what was something much larger 

408
00:23:19,900 --> 00:23:23,700
and this is kind of motivate 
advised that Trace elements 

409
00:23:23,700 --> 00:23:28,900
geochemistry that we see at hot 
spots that indicates that the 

410
00:23:28,900 --> 00:23:31,000
source material for those 
basalts is something that's a 

411
00:23:31,008 --> 00:23:34,100
little bit more primitive. 
Thing that looks a little bit 

412
00:23:34,100 --> 00:23:35,900
more. 
Like what we think the original 

413
00:23:35,900 --> 00:23:37,400
Earth look, like the original 
mantle. 

414
00:23:37,400 --> 00:23:41,200
Look, like now, another idea, 
which is also supported by the 

415
00:23:41,200 --> 00:23:44,500
geochemistry, by the way is that
these large low Shear velocity 

416
00:23:44,500 --> 00:23:48,800
provinces are created by the 
subduction of that basaltic, 

417
00:23:48,800 --> 00:23:53,800
oceanic crust through time. 
So we know that the surface is 

418
00:23:53,800 --> 00:23:56,900
always subducting into the 
mantle sinking into the mantle 

419
00:23:57,600 --> 00:24:01,200
and that layer of crust. 
On the surface of the ocean 

420
00:24:01,200 --> 00:24:04,200
floor, which bus six to seven 
kilometers thick. 

421
00:24:04,500 --> 00:24:08,100
We think that becomes more dense
at mantle pressures. 

422
00:24:08,100 --> 00:24:10,700
And this is based on Mineral 
physics experiments that have 

423
00:24:10,700 --> 00:24:13,700
been done in the laboratory, you
take a Basalt and put it under 

424
00:24:13,700 --> 00:24:17,000
high pressure. 
It will change its structure a 

425
00:24:17,008 --> 00:24:20,800
little bit and become more dense
than the rest of the surrounding

426
00:24:20,800 --> 00:24:23,100
mantle. 
And so they could be the 

427
00:24:23,100 --> 00:24:26,500
accumulations of billions of 
years of this oceanic crust 

428
00:24:26,500 --> 00:24:30,900
that's accumulating. 
So it's a big question for us as

429
00:24:30,900 --> 00:24:33,700
to whether these large. 
Shear velocity provinces are 

430
00:24:33,700 --> 00:24:37,300
being created or they're being 
destroyed. 

431
00:24:37,900 --> 00:24:41,900
One idea that you mentioned when
we were talking earlier is that 

432
00:24:41,900 --> 00:24:45,500
if you have as a consequence of 
say, one of your models that you

433
00:24:45,500 --> 00:24:49,300
find that these large low Shear 
velocity regions would be 

434
00:24:49,700 --> 00:24:54,600
throwing up, big plumes to 
create hot spots, then isn't 

435
00:24:54,600 --> 00:24:57,700
there a challenge of how that, 
if it's a primordial feature how

436
00:24:57,700 --> 00:25:00,500
that could be sustained. 
Absolutely. 

437
00:25:00,600 --> 00:25:03,400
First, let me back up and say 
the mantle Is a very viscous 

438
00:25:03,400 --> 00:25:07,900
place, it's very different from 
a lava lamp that. 

439
00:25:07,900 --> 00:25:10,900
You may have in a lava lamp, the
two materials have a very 

440
00:25:10,900 --> 00:25:14,200
different viscosity and they 
don't stir together very well. 

441
00:25:14,900 --> 00:25:17,200
The Earth's mantle is very 
different because it's so 

442
00:25:17,200 --> 00:25:20,000
viscous. 
It stirs extremely well. 

443
00:25:20,000 --> 00:25:24,700
So it's very hard to have 
different compositions rising up

444
00:25:24,800 --> 00:25:28,700
and not getting stirred with the
rest of the mantle at the same 

445
00:25:28,700 --> 00:25:31,200
time. 
So if these large low Shear 

446
00:25:31,200 --> 00:25:36,300
velocity provinces, Are rising 
up through the mantle as some 

447
00:25:36,400 --> 00:25:39,800
think that they are, then it's 
very difficult for that process 

448
00:25:39,800 --> 00:25:41,900
to occur for four and a half 
billion years. 

449
00:25:42,200 --> 00:25:45,400
So that would argue for a 
hypothesis where these things 

450
00:25:45,400 --> 00:25:51,100
are accumulating with time. 
For my last question, if you had

451
00:25:51,100 --> 00:25:57,100
access to totally Unlimited part
of research funds, how would you

452
00:25:57,100 --> 00:26:00,400
spend it? 
Definitely ocean bottom 

453
00:26:00,400 --> 00:26:05,500
seismometers, the Earth's 
Surface is only 30% land and 

454
00:26:05,500 --> 00:26:07,900
that's where most of the 
seismometers are, there are 

455
00:26:07,900 --> 00:26:10,700
islands around the oceans where 
there are seismometers as well. 

456
00:26:10,700 --> 00:26:12,900
So they're mean, there is some 
coverage in the ocean. 

457
00:26:13,500 --> 00:26:15,900
I think the best thing we can do
is just get a better image of 

458
00:26:15,900 --> 00:26:20,100
what's going on down there, each
new Advanced from seismology 

459
00:26:20,100 --> 00:26:22,800
radically changes, our ideas of 
what's going on. 

460
00:26:22,900 --> 00:26:26,100
You have money wasn't an issue, 
just more of those improve our 

461
00:26:26,100 --> 00:26:27,800
coverage. 
Let's get better pictures of 

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00:26:27,800 --> 00:26:29,500
what's going on in the lower 
mantle. 

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00:26:30,600 --> 00:26:33,000
Alan McNamara. 
Thank you very much. 

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00:26:33,400 --> 00:26:36,300
Thank you Oliver. 
It's been fun for more about 

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00:26:36,300 --> 00:26:39,700
geology, b, as well as pictures 
and diagrams that illustrate. 

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00:26:39,700 --> 00:26:43,200
This podcast, you can go to 
geology B.com