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This is geology B, with all of 
us crumpled. 

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The subfield of geology called 
geodynamics. 

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Most commonly refers to the 
Motions associated with 

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convection, in the mantle, these
are slowed by human standards 

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and need to phenomena such as 
plate, motions, seafloor, 

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spreading mountain, building and
volcanoes. 

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But the Earth's interior 
actually undergoes motions on a 

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whole range of time scales. 
The shortest of these occurs 

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within seismic waves in, which 
the vibrations triggered by 

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earthquakes cause tiny movements
of the material as they pass 

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through. 
And the longest of these are the

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large-scale viscous motions of 
the convecting. 

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Mantle driven by the temperature
difference between the bottom 

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and the top of the mantle But 
there are a whole range of 

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intermediate timescale, motions,
lasting hours, days years and 

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millennia. 
Harriet Lau is an assistant 

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professor in the department of 
Earth and planetary Sciences at 

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the University of California 
Berkeley, she's especially 

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interested in the Motions of the
solidarity, that take place over

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these intermediate time scales. 
And in what they can reveal 

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about, the Earth's interior. 
Harriet Lao. 

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Welcome to geology B. 
Thank you Oliver. 

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Thank you for having me. 
It's an absolute pleasure. 

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I don't think most of us even 
knew that the solid earth is. 

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In fact, moving all the time. 
Scales of ours, two Millennia. 

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What are the drivers of these 
motions? 

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Let's move from the short time 
scales to the longer time 

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scales. 
You've already mentioned. 

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It's Heidel forces when we call 
these solid earth or body Tides 

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when snow caps melt in the 
summer they grow in the winter 

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these changes in Macon Also walk
the planet subsurface and longer

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still Ice Ages which currently 
cycling through. 

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Right now, these redistribute 
Mass on a global scale on time. 

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Scales of tens to thousands to 
hundreds of thousands of years. 

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Only 25,000 years ago, much of 
the northern hemisphere was 

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covered entirely by Ice during 
the last ice age. 

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This is since melted leaving, 
only the Greenland ice sheet, 

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but the solid earth is still 
responding to this all 

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Ultimately Earth's gravitational
field is being changed, either 

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by masses shifting around on the
surface often to do with growing

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and melting ice sheets or due to
external masses like the moon 

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and the sun. 
Okay, let's talk more about each

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of these. 
Starting with the shorter ones, 

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we all know about the tides in 
the ocean caused by the moon, 

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but the solid earth also has 
titled motions in some ways. 

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They're not too dissimilar from 
Ocean Tides. 

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So, we have ocean tides because 
Is of gravitational variations 

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between the Sun, the Moon, and 
the Earth, and their effect on 

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the ocean Mass. 
These same forces also warp the 

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solid Earth surface. 
These happen exactly the same 

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frequencies, just as we have 
ocean tides at lap on and off 

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the shore twice a day. 
We also have defamation of the 

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ground beneath us, and this is 
up and down about twice a day by

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as much as 40 cm these types of 
motions have been, Acted since 

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the earliest, theoretical 
geophysicists, like George 

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Darwin in the 1800's, but 
measuring them. 

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Since they're very slow motions.
As Darwin tried took a little 

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bit more time. 
So Contraptions like taught M 

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which measured deviations of the
ground to the vertical. 

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These were first used and still 
are nowadays. 

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We look to space. 
Satellite orbits will be 

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slightly perturbed due to these 
forces. 

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We even use a Elegy known as 
vlbi or very long Baseline 

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interferometry to measure this 
type of defamation, this 

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involves really large radio 
telescopes, we can measure very 

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slight differences between the 
distances of these radio 

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telescopes and these distances, 
tell us how the Earth is 

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deforming more commonly. 
Now is the use of GPS the very 

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same technology in your 
smartphone. 

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That tells you where you are 
now. 

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Imagine a really sensitive EPS 
station stuck to the ground 

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measuring how the ground moves 
of time. 

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Some GPS stations have been 
there measuring this motion for 

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decades now. 
And so, if the ground moves up 

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and down by around, 40 cm twice 
a day every day for decades, do 

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you physicists stack these 
measurements to get precise 

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recordings of this defamation? 
Okay, so now that we have all 

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these great measurements does 
the exact title defamation of 

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That they revealed tell us 
anything about what's going on 

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in the Earth's interior, I 
mentioned we go up and down by 

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around 40 cm but think of that 
more as an average you're 

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calling from Massachusetts. 
And perhaps you might go up and 

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down by 40 point, one 
centimeters and here, in Oakland

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where I am, I might go up and 
down. 

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By thirty nine point nine, five 
centimeters, these are just 

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numbers I made up, but it gives 
you an idea of what the GPS 

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measurements have revealed their
Are timely submillimeter level 

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variations that exist across the
globe. 

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What these deviations are saying
is that some parts of the 

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Earth's interior a stiffer and 
some are less. 

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So, some are denser similar. 
So we can use these deviations 

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to teach us something about the 
structural variation of Earth's 

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interior. 
One Target in particular are two

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regions in the Deep mantle. 
So around 3,000 kilometers, 

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beneath our feet and these Our 
to dome-like structures that 

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rise from the base of the mantle
to around 1,000 kilometers 

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above. 
So they're massive, there's one 

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beneath Africa and the one 
beneath the Central Pacific and 

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we see these in seismic images 
of the mantle. 

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These two regions are often 
known as the large low Shear 

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velocity provinces and the known
that way, because they slow down

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seismic waves. 
Now understanding the source of 

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why seismic waves are Slow down,
within these regions is a really

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big goal, too. 
Many Global Geo dynamicists 

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Tides. 
Weirdly enough provide a new 

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angle to get at the story of 
these LS V. 

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PS art. 
That's fascinating, I remembered

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that in an earlier. 
Podcast, Alan Magnum are 

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explained that the large low 
Shear velocity provinces that we

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see in the seismic imagery could
be caused either by higher 

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temperature in those regions or 
alternatively by different. 

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Such as a more iron rich one and
he modeled how these large 

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provinces would be expected to 
evolve and each scenario to see 

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which one might actually result 
in The observed shakes. 

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Unfortunately, the jury is still
out on that score because the 

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resolution of seismic tomography
is not up to the job of 

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resolving the differences yet, 
but I wonder does your title 

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analysis. 
Say anything about these large 

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low Shear, velocity provinces, 
and perhaps help us distinguish 

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between these Two 
interpretations very much like 

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seismic Imaging. 
We did a title version. 

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So in our case, the tides don't 
really care much about whether 

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seismic waves are sped up or 
whether they slow down through 

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these regions. 
But we focused on the density or

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more precisely the buoyancy of 
the deep mantle. 

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So whether there are regions of 
the mantle that would 

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preferentially rise or sink and 
so GPS measurements of the 

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variation in that title 
defamation seem to indicate that

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LS, Peas are negatively buoyant 
that is they are denser than the

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mantle surrounding them to get 
that negative buoyancy, 

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signature the elysée, peas have 
to be compositionally, distinct 

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from the mantle surrounding 
them. 

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And in order to increase the 
density, this would likely 

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involve some type of enrichment 
in a very heavy elements like 

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iron. 
In another podcast. 

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Chilean Folger said that seismic
speed is actually most 

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sensitive. 
Active to the presence of 

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partial melt more so even than 
it is the compositional or 

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temperature variations can your 
analysis of tidal motions infer,

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anything about the presence of 
partial melt in the lower 

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mantle. 
One caveat with this kind of 

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tidal analysis is that the 
spatial resolution is extremely 

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low. 
Imagine that the tides only see 

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a really blurred density map of 
Earth's interior. 

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So somewhere within the LSA 
piece is a compositional 

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difference. 
But the tides would find it very

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hard to pinpoint exactly where 
with partial melt. 

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It would be very sensitive to 
this but if that partial Mount 

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is in tiny little pockets, then 
they might be invisible to the 

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tides. 
You mentioned that the Marine 

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tithes and the solid earth Tides
go through about two cycles a 

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day. 
What other title periods are 

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there in the earth you might 
have heard of neap tides and 

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spring tides. 
So these also play a role The 

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solid earth Tides. 
So there are daily. 

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Tides and there are fortnightly 
tied and Eventide. 

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Every 18.6 years, these overtime
skills, will slightly change 

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factors on Earth's Tilt, as it 
rotates or other kinds of 

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orbital characteristics of our 
planet. 

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What these can teachers? 
Is that, how Earth's defamation 

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Behavior might change with 
frequency, if we think about 

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high frequency, seismic waves, 
which are periods of less than 

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ours, And compare those two 
Tides which act on hours to 

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weeks to years, we can really 
get a sense of how Earth's 

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Behavior changes on those 
timescales since GPS stations. 

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Now, been out there for a few 
decades, we can really start 

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detecting these signals and so 
there's quite an opportunity to 

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capitalize on this, okay? 
Moving up to a timescale of 

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months. 
We have motions caused by the 

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seasonal variation in the size 
of ice sheets, as snow caps, 

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melt and grow and That change in
mass. 

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Whoops, the Earth as well. 
And these are carefully watched 

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in many places like Iceland and 
across Scandinavia. 

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And we also have that the ice 
cover various over thousands of 

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years when we have the onset and
ending of ice ages and you said 

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the Earth is still reacting to 
the end of the last ice age over

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the last million years. 
Or so we've had ice ages. 

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Roughly every hundred thousand 
years and what these involve is 

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a slow No growth phase for most 
of that hundred thousand years. 

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And then you get a rapid melting
over 10 to 15 thousand years and

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we're currently in between an 
ice age right now. 

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Even though that excess mass, 
that was mainly stored in North 

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America, Scandinavia in Siberia.
Even though that's has long 

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since melted, Earth is still 
responding. 

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So these long time skill forces 
drive a much slower kind of 

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deformation. 
For example, Canada is still 

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rebounding after being way. 
Down by what we call the 

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laurentide ice sheet now extinct
and this motion is about 

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centimeters per year or so. 
So not too dissimilar from plate

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tectonics. 
But if you go to Hudson Bay what

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you'll see our beaches that are 
hundreds of meters, high way 

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above sea level, and they're up 
there because of this ongoing 

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uplift. 
So what do all these movements 

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of the daily seasonal and even 
Ice Age? 

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Timescales, teach us, you 
already mentioned that it's 

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Enough some new insight into 
what these large low Shear 

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velocity provinces are. 
But can it tell us anything 

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else? 
As you mentioned at the start of

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the podcasts, if can behave in 
many different ways at the 

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shortest time? 
Scales seismic waves are 

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triggered by earthquakes and 
vibrate almost entirely in an 

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elastic manner plates, that move
due to the flow of the 

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underlying mantle of treated in 
a completely viscous Manner. 

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And those happen at the longest 
time scales of millions to 

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billions of years. 
The time range, I focus on is in

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between these vastly different 
modes of defamation. 

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So of course, if isn't a 
perfectly elastic solid of a 

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wise seismic waves would keep 
going and never stop bouncing 

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around but as you move from 
these high-frequency processes 

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to lower frequency the Earth 
becomes increasingly viscous and

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you get hints at this on these 
hourly weekly to decayed or time

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scales. 
And finally, to those Ice Age 

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time skills the very After that 
North America is still 

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rebounding in response to an ice
sheet that completely melted 

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around. 7,000 years ago is an 
indication of this that motion 

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occurring today is delayed 
because of viscous forces within

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the Earth's interior. 
So how these transitions between

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these n members occur is 
precisely what these 

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intermediate processes could 
provide insight into. 

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So does that give us what the 
value of the viscosity is on 

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these different time scales? 
One thing that I've been kind to

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play with is the idea that 
viscosity is frequency dependent

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and we don't really treat 
viscosity that way. 

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But certainly, from our 
knowledge from Material Science,

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solid mechanics other fields, 
these ideas are prevalent. 

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And so the idea is to bring 
those kinds of ideas to earth 

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science when you're starting to 
see these different types of 

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data set. 
It must be real treasure Trove. 

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Now that we've finally got to 
the right kind of time scales to

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be able to actually mine all 
this data. 

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Something useful. 
There are some very clever Giada

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cyst so people who study the 
shape and the rotation of the 

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earth. 
We've spotted six-year, 

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variations in Earth's length of 
day so we roughly have 24 hours 

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a day but that slightly changes 
every six years and so these 

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rotational variations of for to 
reflect something again in the 

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Deep interior of the earth and 
so six years in order to really 

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have that signal, you've had to 
have Decades of GPS stations 

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measuring this, we Are an 
interesting time to start 

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thinking about these things that
actually, I've read papers in 

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the 70s and 80s of, folks that 
have theoretically predicted a 

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lot of these unique, vibrations 
of the earth. 

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And we've had to wait, this long
to get good measurements on 

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them. 
So, it's a good time to revisit 

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these problems. 
So, by measuring the amplitude 

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and the spatial variations of 
all these solid earth, motions 

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over all these many different 
time scales. 

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It's Is if we can learn a lot 
and we will learn even more 

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about the behavior and structure
of the Earth's interior, but can

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we integrate these observations 
with any other types of 

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observation or experiments that 
also have a bearing on the 

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interior structure of the earth?
So completely different way to 

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explore. 
This is in the lab and there are

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many people who work with rock 
samples. 

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So about the size of your hand 
and they conduct defamation 

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experiments on these rocks 
applying High pressures and 

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temperatures to mimic the 
conditions of Earth's interior. 

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So, in these studies are really 
focusing at the grain scale as 

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you apply these stresses. 
How do these grains that make 

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up? 
Rock interact with each other? 

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How does temperature affect 
this? 

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How does pressure? 
What if the rock is partially 

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molten? 
What about its porosity? 

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So, all these sorts of factors 
go into these lab measurements. 

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But even if Be enough. 
We can actually reproduce the 

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extreme temperatures and 
pressure conditions that Prevail

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it depth. 
We can't really even begin to 

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approach the very long time, 
scales involved. 

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Can we? 
Sadly, no, unless you're 

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extremely patient. 
So we've been talking about 

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processes that act on hours to 
weeks to years decades, right to

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millions of years and to get 
these long time skills within 

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the lab we have to extrapolate. 
We mentioned that these long 

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time skills. 
The mantle convects Flows like a

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very thick liquid and two 
important parameters that 

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dictate. 
This flow are the buoyancy which

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we've discussed, so that 
dictates whether fluids rise or 

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sink, but also the viscosity 
which we touched on earlier how 

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runny a fluid is now if we have 
less understanding on the 

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viscosity structure of the 
mantle, we don't have anything 

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like these kind of seismic 
images of Earth's interior 

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while. 
Traveling sideways Traverse 

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through the mantle, they're only
slowed and Edit by a few percent

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which practically speakings 
makes the math and the 

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computation much easier. 
There's also a lot more data 

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available viscosity on the other
hand, varies by orders of 

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magnitude, birth in depth and 
laterally. 

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So if you have a little bit of 
hot mantle like not willing 

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plume, that viscosity might be 
100 times less viscous than the 

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ambient mantle, and this makes 
simulating mantle flow quite 

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difficult. 
And so we really do want to 

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Combine, all these different 
types of information. 

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We have over us from the lab 
from modeling from observation 

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when you do actually go about 
integrating all these diverse 

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sources of measurements and I 
guess the applying the theory, 

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is there anything you can say 
about what it's told us yet. 

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So, that's a continuously 
developing goal of mine to 

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really bring together the work 
done at that kind of hand 

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sample, spatial scale of 
focusing on the greens time 

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skills that are necessary. 
Malden hours to the planetary 

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scale on time skills that are 
important for Earth history. 

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And so if we can integrate these
two perspectives coherently, we 

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might have a pathway to 
understand how the Earth deforms

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continuously with time scale. 
And really this kind of 

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understanding is quite timely. 
For example, I mentioned the 

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redistribution of Mass driving. 
A lot of these solid of 

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responses and if we go back to 
that ice example, Oh and look to

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a place. 
Like Western Antarctica. 

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Western Antarctica. 
The ice sheet that sits there is

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simultaneously undergoing a 
plethora of processes at various

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different time scales and so 
it's still deforming from the 

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last ice age. 25 thousand years 
ago, unlike the laurentide ice 

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sheet, it's still here, it 
didn't completely melt, but it 

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was significantly reduced. 
And so the solid earth 

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underneath the crust is still 
responding to that reduction in 

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ice the Antarctic. 
She is also undergoing study 

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changes from post-industrial 
climate change. 

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Its responding to the 
Centennial, scale melt and in 

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the news every so often we hear 
of Rapid ice collapses, we're 

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like, the Larsen B ice shelf, 
collapse in 2002. 

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Just chunks of the ice sheet 
will fall off near 

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instantaneously. 
And so, it turns out an 

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important part of how sea level 
changes when you melt ice and 

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dump it into the Ocean depends 
on just how the solid after 

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forms in response to this 
redistribution in order to 

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consider all these different 
types of processes that even 

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just Antarctica is undergoing. 
It's important to understand if 

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properties and how Earth will 
deform in response to both short

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and long time scale processes. 
If we really do want to 

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understand how to predict 
sea-level say properly. 

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That's right. 
Of course. 

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Sea level is a combination of 
any one. 

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Place of what. 
There's actually more water and 

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the Sea and whether the land is 
rising or sinking at the 

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coastline. 
Exactly. 

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What are you working on yourself
at the moment? 

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Well, tides are still going. 
Strong, as is my research on 

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solid earth tide processes. 
I have a graduate student who's 

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actually working to consider 
both tides and other types of 

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Earth. 
Vibrations simultaneously to 

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really hone in on that deep 
mantle structure and the kinds 

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00:20:14,800 --> 00:20:18,800
of vibrations, he's Working on 
called Stone, only modes, there 

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00:20:18,800 --> 00:20:22,100
are a special standing wave that
vibrates along the core-mantle 

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00:20:22,100 --> 00:20:27,000
boundary and we can now detect. 
Those also in seismometers, I'm 

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00:20:27,000 --> 00:20:30,500
working with experimentalists. 
Who do those hand sample 

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00:20:30,500 --> 00:20:32,400
experiments within the 
laboratory. 

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00:20:32,800 --> 00:20:35,300
We're working towards a 
theoretical framework. 

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That can consistently bring 
their micro scale studies to the

349
00:20:39,900 --> 00:20:43,300
macro scale applications that we
can use them for planetary 

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00:20:43,300 --> 00:20:45,700
problems and a lot of my work is
for you, right? 

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Performed on computer. 
So I've been very fortunate to 

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00:20:48,500 --> 00:20:51,300
continue much of this research 
through the pandemic. 

353
00:20:52,400 --> 00:20:56,600
What new observational or 
experimental results? 

354
00:20:56,700 --> 00:21:00,600
Would you most like to see any 
way we can get either 

355
00:21:00,600 --> 00:21:04,200
seismometers or GPS? 
Stations on the ocean floor 

356
00:21:04,200 --> 00:21:07,100
would be amazing. 
Of course, there's a small 

357
00:21:07,100 --> 00:21:10,600
obstacle known as the ocean 
itself but search Technologies, 

358
00:21:10,600 --> 00:21:13,200
do exist. 
And will only improve continents

359
00:21:13,200 --> 00:21:17,500
are really only three percent of
surface area. 

360
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And that's pretty much the limit
of where most It says, most GPS 

361
00:21:23,200 --> 00:21:26,400
stations are now and so we're 
missing a really big chunk of 

362
00:21:26,400 --> 00:21:30,400
the Earth's surface. 
But as I said, these ocean floor

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00:21:30,400 --> 00:21:33,900
technologies have been deployed 
here and there examples include 

364
00:21:33,900 --> 00:21:37,400
these Pacific Rise and off the 
coast of Hawaii. 

365
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So that would really serve to 
help us get an increased amount 

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of coverage for observations. 
Harriet Lau. 

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00:21:45,300 --> 00:21:47,300
Thank you very much, my 
pleasure. 

368
00:21:47,400 --> 00:21:52,100
Thank you, Oliver for more about
geology b, as well as pictures 

369
00:21:52,100 --> 00:21:55,800
and illustrations. 
That support this podcast, you 

370
00:21:55,800 --> 00:21:58,400
can go to geology B.com