This is geology B, with all of us crumpled.
The subfield of geology called geodynamics.
Most commonly refers to the Motions associated with
convection, in the mantle, these are slowed by human standards
and need to phenomena such as plate, motions, seafloor,
spreading mountain, building and volcanoes.
But the Earth's interior actually undergoes motions on a
whole range of time scales. The shortest of these occurs
within seismic waves in, which the vibrations triggered by
earthquakes cause tiny movements of the material as they pass
through. And the longest of these are the
large-scale viscous motions of the convecting.
Mantle driven by the temperature difference between the bottom
and the top of the mantle But there are a whole range of
intermediate timescale, motions, lasting hours, days years and
millennia. Harriet Lau is an assistant
professor in the department of Earth and planetary Sciences at
the University of California Berkeley, she's especially
interested in the Motions of the solidarity, that take place over
these intermediate time scales. And in what they can reveal
about, the Earth's interior. Harriet Lao.
Welcome to geology B. Thank you Oliver.
Thank you for having me. It's an absolute pleasure.
I don't think most of us even knew that the solid earth is.
In fact, moving all the time. Scales of ours, two Millennia.
What are the drivers of these motions?
Let's move from the short time scales to the longer time
scales. You've already mentioned.
It's Heidel forces when we call these solid earth or body Tides
when snow caps melt in the summer they grow in the winter
these changes in Macon Also walk the planet subsurface and longer
still Ice Ages which currently cycling through.
Right now, these redistribute Mass on a global scale on time.
Scales of tens to thousands to hundreds of thousands of years.
Only 25,000 years ago, much of the northern hemisphere was
covered entirely by Ice during the last ice age.
This is since melted leaving, only the Greenland ice sheet,
but the solid earth is still responding to this all
Ultimately Earth's gravitational field is being changed, either
by masses shifting around on the surface often to do with growing
and melting ice sheets or due to external masses like the moon
and the sun. Okay, let's talk more about each
of these. Starting with the shorter ones,
we all know about the tides in the ocean caused by the moon,
but the solid earth also has titled motions in some ways.
They're not too dissimilar from Ocean Tides.
So, we have ocean tides because Is of gravitational variations
between the Sun, the Moon, and the Earth, and their effect on
the ocean Mass. These same forces also warp the
solid Earth surface. These happen exactly the same
frequencies, just as we have ocean tides at lap on and off
the shore twice a day. We also have defamation of the
ground beneath us, and this is up and down about twice a day by
as much as 40 cm these types of motions have been, Acted since
the earliest, theoretical geophysicists, like George
Darwin in the 1800's, but measuring them.
Since they're very slow motions. As Darwin tried took a little
bit more time. So Contraptions like taught M
which measured deviations of the ground to the vertical.
These were first used and still are nowadays.
We look to space. Satellite orbits will be
slightly perturbed due to these forces.
We even use a Elegy known as vlbi or very long Baseline
interferometry to measure this type of defamation, this
involves really large radio telescopes, we can measure very
slight differences between the distances of these radio
telescopes and these distances, tell us how the Earth is
deforming more commonly. Now is the use of GPS the very
same technology in your smartphone.
That tells you where you are now.
Imagine a really sensitive EPS station stuck to the ground
measuring how the ground moves of time.
Some GPS stations have been there measuring this motion for
decades now. And so, if the ground moves up
and down by around, 40 cm twice a day every day for decades, do
you physicists stack these measurements to get precise
recordings of this defamation? Okay, so now that we have all
these great measurements does the exact title defamation of
That they revealed tell us anything about what's going on
in the Earth's interior, I mentioned we go up and down by
around 40 cm but think of that more as an average you're
calling from Massachusetts. And perhaps you might go up and
down by 40 point, one centimeters and here, in Oakland
where I am, I might go up and down.
By thirty nine point nine, five centimeters, these are just
numbers I made up, but it gives you an idea of what the GPS
measurements have revealed their Are timely submillimeter level
variations that exist across the globe.
What these deviations are saying is that some parts of the
Earth's interior a stiffer and some are less.
So, some are denser similar. So we can use these deviations
to teach us something about the structural variation of Earth's
interior. One Target in particular are two
regions in the Deep mantle. So around 3,000 kilometers,
beneath our feet and these Our to dome-like structures that
rise from the base of the mantle to around 1,000 kilometers
above. So they're massive, there's one
beneath Africa and the one beneath the Central Pacific and
we see these in seismic images of the mantle.
These two regions are often known as the large low Shear
velocity provinces and the known that way, because they slow down
seismic waves. Now understanding the source of
why seismic waves are Slow down, within these regions is a really
big goal, too. Many Global Geo dynamicists
Tides. Weirdly enough provide a new
angle to get at the story of these LS V.
PS art. That's fascinating, I remembered
that in an earlier. Podcast, Alan Magnum are
explained that the large low Shear velocity provinces that we
see in the seismic imagery could be caused either by higher
temperature in those regions or alternatively by different.
Such as a more iron rich one and he modeled how these large
provinces would be expected to evolve and each scenario to see
which one might actually result in The observed shakes.
Unfortunately, the jury is still out on that score because the
resolution of seismic tomography is not up to the job of
resolving the differences yet, but I wonder does your title
analysis. Say anything about these large
low Shear, velocity provinces, and perhaps help us distinguish
between these Two interpretations very much like
seismic Imaging. We did a title version.
So in our case, the tides don't really care much about whether
seismic waves are sped up or whether they slow down through
these regions. But we focused on the density or
more precisely the buoyancy of the deep mantle.
So whether there are regions of the mantle that would
preferentially rise or sink and so GPS measurements of the
variation in that title defamation seem to indicate that
LS, Peas are negatively buoyant that is they are denser than the
mantle surrounding them to get that negative buoyancy,
signature the elysée, peas have to be compositionally, distinct
from the mantle surrounding them.
And in order to increase the density, this would likely
involve some type of enrichment in a very heavy elements like
iron. In another podcast.
Chilean Folger said that seismic speed is actually most
sensitive. Active to the presence of
partial melt more so even than it is the compositional or
temperature variations can your analysis of tidal motions infer,
anything about the presence of partial melt in the lower
mantle. One caveat with this kind of
tidal analysis is that the spatial resolution is extremely
low. Imagine that the tides only see
a really blurred density map of Earth's interior.
So somewhere within the LSA piece is a compositional
difference. But the tides would find it very
hard to pinpoint exactly where with partial melt.
It would be very sensitive to this but if that partial Mount
is in tiny little pockets, then they might be invisible to the
tides. You mentioned that the Marine
tithes and the solid earth Tides go through about two cycles a
day. What other title periods are
there in the earth you might have heard of neap tides and
spring tides. So these also play a role The
solid earth Tides. So there are daily.
Tides and there are fortnightly tied and Eventide.
Every 18.6 years, these overtime skills, will slightly change
factors on Earth's Tilt, as it rotates or other kinds of
orbital characteristics of our planet.
What these can teachers? Is that, how Earth's defamation
Behavior might change with frequency, if we think about
high frequency, seismic waves, which are periods of less than
ours, And compare those two Tides which act on hours to
weeks to years, we can really get a sense of how Earth's
Behavior changes on those timescales since GPS stations.
Now, been out there for a few decades, we can really start
detecting these signals and so there's quite an opportunity to
capitalize on this, okay? Moving up to a timescale of
months. We have motions caused by the
seasonal variation in the size of ice sheets, as snow caps,
melt and grow and That change in mass.
Whoops, the Earth as well. And these are carefully watched
in many places like Iceland and across Scandinavia.
And we also have that the ice cover various over thousands of
years when we have the onset and ending of ice ages and you said
the Earth is still reacting to the end of the last ice age over
the last million years. Or so we've had ice ages.
Roughly every hundred thousand years and what these involve is
a slow No growth phase for most of that hundred thousand years.
And then you get a rapid melting over 10 to 15 thousand years and
we're currently in between an ice age right now.
Even though that excess mass, that was mainly stored in North
America, Scandinavia in Siberia. Even though that's has long
since melted, Earth is still responding.
So these long time skill forces drive a much slower kind of
deformation. For example, Canada is still
rebounding after being way. Down by what we call the
laurentide ice sheet now extinct and this motion is about
centimeters per year or so. So not too dissimilar from plate
tectonics. But if you go to Hudson Bay what
you'll see our beaches that are hundreds of meters, high way
above sea level, and they're up there because of this ongoing
uplift. So what do all these movements
of the daily seasonal and even Ice Age?
Timescales, teach us, you already mentioned that it's
Enough some new insight into what these large low Shear
velocity provinces are. But can it tell us anything
else? As you mentioned at the start of
the podcasts, if can behave in many different ways at the
shortest time? Scales seismic waves are
triggered by earthquakes and vibrate almost entirely in an
elastic manner plates, that move due to the flow of the
underlying mantle of treated in a completely viscous Manner.
And those happen at the longest time scales of millions to
billions of years. The time range, I focus on is in
between these vastly different modes of defamation.
So of course, if isn't a perfectly elastic solid of a
wise seismic waves would keep going and never stop bouncing
around but as you move from these high-frequency processes
to lower frequency the Earth becomes increasingly viscous and
you get hints at this on these hourly weekly to decayed or time
scales. And finally, to those Ice Age
time skills the very After that North America is still
rebounding in response to an ice sheet that completely melted
around. 7,000 years ago is an indication of this that motion
occurring today is delayed because of viscous forces within
the Earth's interior. So how these transitions between
these n members occur is precisely what these
intermediate processes could provide insight into.
So does that give us what the value of the viscosity is on
these different time scales? One thing that I've been kind to
play with is the idea that viscosity is frequency dependent
and we don't really treat viscosity that way.
But certainly, from our knowledge from Material Science,
solid mechanics other fields, these ideas are prevalent.
And so the idea is to bring those kinds of ideas to earth
science when you're starting to see these different types of
data set. It must be real treasure Trove.
Now that we've finally got to the right kind of time scales to
be able to actually mine all this data.
Something useful. There are some very clever Giada
cyst so people who study the shape and the rotation of the
earth. We've spotted six-year,
variations in Earth's length of day so we roughly have 24 hours
a day but that slightly changes every six years and so these
rotational variations of for to reflect something again in the
Deep interior of the earth and so six years in order to really
have that signal, you've had to have Decades of GPS stations
measuring this, we Are an interesting time to start
thinking about these things that actually, I've read papers in
the 70s and 80s of, folks that have theoretically predicted a
lot of these unique, vibrations of the earth.
And we've had to wait, this long to get good measurements on
them. So, it's a good time to revisit
these problems. So, by measuring the amplitude
and the spatial variations of all these solid earth, motions
over all these many different time scales.
It's Is if we can learn a lot and we will learn even more
about the behavior and structure of the Earth's interior, but can
we integrate these observations with any other types of
observation or experiments that also have a bearing on the
interior structure of the earth? So completely different way to
explore. This is in the lab and there are
many people who work with rock samples.
So about the size of your hand and they conduct defamation
experiments on these rocks applying High pressures and
temperatures to mimic the conditions of Earth's interior.
So, in these studies are really focusing at the grain scale as
you apply these stresses. How do these grains that make
up? Rock interact with each other?
How does temperature affect this?
How does pressure? What if the rock is partially
molten? What about its porosity?
So, all these sorts of factors go into these lab measurements.
But even if Be enough. We can actually reproduce the
extreme temperatures and pressure conditions that Prevail
it depth. We can't really even begin to
approach the very long time, scales involved.
Can we? Sadly, no, unless you're
extremely patient. So we've been talking about
processes that act on hours to weeks to years decades, right to
millions of years and to get these long time skills within
the lab we have to extrapolate. We mentioned that these long
time skills. The mantle convects Flows like a
very thick liquid and two important parameters that
dictate. This flow are the buoyancy which
we've discussed, so that dictates whether fluids rise or
sink, but also the viscosity which we touched on earlier how
runny a fluid is now if we have less understanding on the
viscosity structure of the mantle, we don't have anything
like these kind of seismic images of Earth's interior
while. Traveling sideways Traverse
through the mantle, they're only slowed and Edit by a few percent
which practically speakings makes the math and the
computation much easier. There's also a lot more data
available viscosity on the other hand, varies by orders of
magnitude, birth in depth and laterally.
So if you have a little bit of hot mantle like not willing
plume, that viscosity might be 100 times less viscous than the
ambient mantle, and this makes simulating mantle flow quite
difficult. And so we really do want to
Combine, all these different types of information.
We have over us from the lab from modeling from observation
when you do actually go about integrating all these diverse
sources of measurements and I guess the applying the theory,
is there anything you can say about what it's told us yet.
So, that's a continuously developing goal of mine to
really bring together the work done at that kind of hand
sample, spatial scale of focusing on the greens time
skills that are necessary. Malden hours to the planetary
scale on time skills that are important for Earth history.
And so if we can integrate these two perspectives coherently, we
might have a pathway to understand how the Earth deforms
continuously with time scale. And really this kind of
understanding is quite timely. For example, I mentioned the
redistribution of Mass driving. A lot of these solid of
responses and if we go back to that ice example, Oh and look to
a place. Like Western Antarctica.
Western Antarctica. The ice sheet that sits there is
simultaneously undergoing a plethora of processes at various
different time scales and so it's still deforming from the
last ice age. 25 thousand years ago, unlike the laurentide ice
sheet, it's still here, it didn't completely melt, but it
was significantly reduced. And so the solid earth
underneath the crust is still responding to that reduction in
ice the Antarctic. She is also undergoing study
changes from post-industrial climate change.
Its responding to the Centennial, scale melt and in
the news every so often we hear of Rapid ice collapses, we're
like, the Larsen B ice shelf, collapse in 2002.
Just chunks of the ice sheet will fall off near
instantaneously. And so, it turns out an
important part of how sea level changes when you melt ice and
dump it into the Ocean depends on just how the solid after
forms in response to this redistribution in order to
consider all these different types of processes that even
just Antarctica is undergoing. It's important to understand if
properties and how Earth will deform in response to both short
and long time scale processes. If we really do want to
understand how to predict sea-level say properly.
That's right. Of course.
Sea level is a combination of any one.
Place of what. There's actually more water and
the Sea and whether the land is rising or sinking at the
coastline. Exactly.
What are you working on yourself at the moment?
Well, tides are still going. Strong, as is my research on
solid earth tide processes. I have a graduate student who's
actually working to consider both tides and other types of
Earth. Vibrations simultaneously to
really hone in on that deep mantle structure and the kinds
of vibrations, he's Working on called Stone, only modes, there
are a special standing wave that vibrates along the core-mantle
boundary and we can now detect. Those also in seismometers, I'm
working with experimentalists. Who do those hand sample
experiments within the laboratory.
We're working towards a theoretical framework.
That can consistently bring their micro scale studies to the
macro scale applications that we can use them for planetary
problems and a lot of my work is for you, right?
Performed on computer. So I've been very fortunate to
continue much of this research through the pandemic.
What new observational or experimental results?
Would you most like to see any way we can get either
seismometers or GPS? Stations on the ocean floor
would be amazing. Of course, there's a small
obstacle known as the ocean itself but search Technologies,
do exist. And will only improve continents
are really only three percent of surface area.
And that's pretty much the limit of where most It says, most GPS
stations are now and so we're missing a really big chunk of
the Earth's surface. But as I said, these ocean floor
technologies have been deployed here and there examples include
these Pacific Rise and off the coast of Hawaii.
So that would really serve to help us get an increased amount
of coverage for observations. Harriet Lau.
Thank you very much, my pleasure.
Thank you, Oliver for more about geology b, as well as pictures
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