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The Earth's magnetic field changes over time.
In fact, there have been frequent reversals in the
direction of the field with the North and South magnetic poles
switching places. We tend to think of these
changes as applying to the whole globe.
But in addition to this temporal element to the magnetic fields
variability, there is also a spatial element with the field
changing very differently in different places across the
planet. Cathy Constable is a professor
at the Scripps institution of oceanography.
She measures the remnant magnetism in rocks and human
artifacts around the globe to reconstruct.
Not just the variations of the Earth's magnetic field as a
whole but also its spatial variations across the globe over
geological time Kathy Constable. Welcome to geology bites.
Hello. It's nice to be here.
It seems hard enough to reconstruct the history of the
Earth's magnetic field in one dimension.
The temporal one but I'm covering the spatial
distribution of the field over time seems really challenging.
Are you really able to create a spatial map of the field at
different times over the past? Yes, but the map is a little bit
crude the temporal Dimension as you describe.
It really only reflects the fuel that we might expect from a
magnetic dipole. If we Imagine The Familiar
permanent bar magnet at the center of the Earth.
But we really know that the field is more complicated
because it's driven by convection in the liquid outer
core. So if we have better mapping in
both space and time that's actually essential to getting
better acquainted with what's going on in the core and we've
been successful in building more detailed time-varying spatial
maps for the past 10,000 and the past hundred thousand years.
The temporal resolution is better for the more recent times
and in both cases as I said the spatial It's a bit crude.
It's still only on rather large scales.
So what are the materials you analyze to do this work?
There's a broad range of materials actually human
artifacts that have been heated to a high enough temperature to
reset the magnetization as they cool in place.
That would be things like fired Pottery bricks Hearth.
Swear. They've been high temperature
fires Kilns and there's a decreasing number of these human
artifacts as one goes back in time and Beyond about 3,000
years ago. There aren't that many of them
and the spatial distribution is limited by human activity rocks.
In fact cover a much broader temporal range and if we think
about lava flows, they solidify and cool and then they preserve
an instantaneous geologically speaking spot record of the
field like and Marine sediments. Give a more continuous record.
One that tends to be smoothed In Time by an amount that's
dependent on the sedimentation rate.
So all of these things contribute to our spatial
reconstructions so are such suitable materials conveniently
distributed across the globe for you to collect now that lost all
convenient as you know, there's about 70% of the earth's surface
is ocean. There's a lot more ocean in the
southern hemisphere than in the northern hemisphere.
So there's a Each challenge in the distribution between this in
the Southern Hemisphere and it's also true that there's less
human activity at high latitudes and fewer human artifacts there
and if we look at Marine sediments, which can be
recovered from the ocean. They tend to have lower
accumulation rates than the things that you get on land when
Lake sediments. So the temporal resolution isn't
as good it seems as if we can get pretty good record of
reversals from Many many magnetic stripes that are frozen
into the ocean crust as it's formed and spreads on either
side of the mid-ocean ridge. Are these some of your best
sources there are very good on some time scales.
What happens is that the magnetic stripes are limited by
the processes forming the ocean crust.
And also by the fact that measurements are usually made at
the ocean surface several kilometers from the source.
So what happens is that at the temporal resolution is actually
a lot better in lava flows and sediments with high accumulation
rate and they do a better job. And so there are some fortunate
geological circumstances where for example and steen's mountain
in Western North America 15 million years ago.
There were a series of lava flow eruptions that provided a
wonderful record of magnetic reversal and you can actually
see the evolution of this magnetic reversal through A
record preserved in these lava flows and they're also records
and sediments as an example from Italian segments.
And these two records are very interesting because even though
they are somewhat controversial they preserve very high rates of
change or appear to preserve very high rates of change in the
magnetic field by matter of degrees per year in terms of the
direction and that's something that is not necessarily
completely expected. And we're exploring a lot
further as part of the work that we're doing where we build a
global view of what's going on in four more recent time.
These things can really give us very good temporal resolution.
Tell us a bit more about why we are so interested in
reconstructing the spatial distribution of the field.
So my interest is purely in the geomagnetic record and getting a
better understanding of how the magnetic field is produced in
the Earth's core and what Rolls, its variability if we think
about the limitations of the seafloor Stripes, they tell us
about normal or reverse polarity protection rather than the
details of field strength variations.
And the resolution is typically no better than about 20 to 40
thousand years. They also only give us the
polarity of the field. They don't give us the details
of directional variability or even feel strength variability
around the globe and the spatial.
Structure of the field does vary with time and the rates of
change in the field also depend on location and strength.
So we have to go to a broader understanding of the spatial
scales to get this now numerical simulations of the core field
provide another way to look at how the field might vary in time
and space but they also have limitations we know the
equations to use but it's not currently possible to do the
computations with the right values for core properties.
And why is that The reason is that the core properties are
just very challenging for numerical simulations the
viscosity in the core turns out to be very low what that means
is that the numerical simulations would ideally have
very high spatial resolution. They take very short time steps
in order to resolve what's going on on very small spatial scales.
And that's very numerically challenging.
The viscosity is basically expressed in the dimensionless
Ekman number. And it's effectively orders of
magnitude too high in the numerical simulations that were
currently using. It's too hard to compute if we
use the correct numbers and what happens then is that there are
issues in converting the non dimensional numbers that are
used in these simulations in the equations and these simulations
to actual dimensional time scales for the Earth.
Usually we have to choose between scaling according to an
advection or a diffusion time scale depending on what we're
interested in and the relative importance of advection versus
diffusion in the processes. We're looking at so diffusion is
basically what happens as a result of ohmic heat losses in
the core. So electrical losses lose energy
in the magnetic field because of the finite conductivity of the
core the advection basically is a product of the convection that
drives the magnetic field in the core and with advection what
happens is that on short time scales and large length scales
the advection carries the field around with the fluid and we can
see this actually in maps of the modern-day field where you can
see some features of the field that are basically drifting
Westward or moving around the polar region, and the advection
is Anton short time scales, whereas on longer timescales the
diffusion becomes important and that's one of the things that
has to play into the loss of energy in the dipole part of the
field when you get an Excursion or a reversal just to come back
to the problem of getting the right viscosity into the models
roughly. What is the viscosity of the
core? Can you compare it to something?
We're familiar with the viscosity of the core is really
like that of water. And you might contrast that with
the silicate mantle of the earth, which has a viscosity
that is orders of magnitude higher and therefore the time
scales for convection in the mantle are much much longer than
they are in the core. If you think about the
convection advection timescales and the core there of the order
of hundreds of years versus We Believe millions of years for
most of the processes that are going on in the man.
And so we're trying to mimic the viscosity in the core the best
numerical simulations right now are using this dimensionless
Eggman number that I mentioned of the order of 10 to the minus
6 or 10 to the minus 7, and we believe that the correct number
for the core is somewhere between 10 to the minus 10 and
10 to the minus 15. Wow, not even close not even
close and that's why we have to be very careful about drawing
reliable physical conclusions about what's going on in the
core from Um these numerical simulations, it's not that
they're wrong in what they do. It's that they're just not
necessarily completely appropriate for what we want to
use them for when we're comparing with the geomagnetic
field. So we're mapping the field here
on the Earth's surface with the goal of improving our
understanding of what's going on in the core, but between us and
the core lie 29 hundred kilometers of mantle Is the
mantle completely transparent to the magnetic field?
No, it's not but it's is to a good approximation for what we
do the mantle is basically made of silicates and it has a finite
but low electrical conductivity the conductivity and the core is
much much higher. In fact studying.
The conductivity of the mantle is another field entirely in its
own. Right and that's done using
magnetic and electrical data as well.
So the effect of the mantle when we're looking at the signal
that's coming out of the corps is basically to smooth things
out. That's what we could call a
temporal damping in the signal so that everything that comes to
us is smoother over a period of a few months so we can't detect
very short-term variations that might be going on in the core
and that's the main effect and it doesn't matter for the
purpose of looking at paleo field structure because The
temporal resolution that we have in the best of circumstances
might be a decade to a hundred years.
What kind of things are. We looking to pin down better
using your spatial measurements? Can you constrain the values for
example of the various parameters that you mentioned
the viscosity in the Ekman number and there's also the
rally number which defines the amount of energy in the
convecting core. not directly the best estimates that we have
for these parameters basically come from mineral physics and
petrology and our understanding of the materials in the core and
the temperature of the court but in the spatial representations,
we had we can look at representative properties of the
Earth's field and we can compare them with the results from
simulations to decide if the simulations are doing a good job
and one issue that we Address, which is an interesting one is
whether there are long-term Geographic variations in the
spatial structure that reflects boundary conditions at the
core-mantle boundary. For example, there's some
evidence that the field appears to be more active in the
Southern Hemisphere and also more active in the Atlantic than
in the Pacific region and this Builds on a bigger picture that
we get from seismic tomography which supports differences in
the region around the core mantle.
A boundary. So that's something that we can
actually see the consequence of in the magnetic measurements and
we can then go back to the numerical simulations and say
well actually if you want to get a realistic representation of
the geomagnetic field, you need to include these spatially
varying boundary conditions in the numerical simulations, and
that's one part of it. And then there are other things
that are harder to deal with in terms of the constraints.
Which we don't really have a systematic way of addressing.
You said the actually doesn't generally conform to a simple
dipole. But can you describe what your
spatial-temporal map of the Earth field actually does look
like Yes, it depends on the time interval that we look at if we
look at long-term averages over say a million years.
It does look a lot like a simple dipole with Direction varying
around the earth as you might expect for a simple bar magnet
at the center of the Earth. But over time the direction and
the magnitude can vary a lot and it doesn't always look like a
dipole. So as I mentioned on average
there are indications that you see more activity.
In the field variations in the southern hemisphere than in the
north and also in the Atlantic hemisphere, the piece of the
earth that includes the Atlantic and the Indian Ocean versus the
Pacific Hemisphere and the Lush amp Excursion, which occurred
about 41,000 years ago is one example where the dipole and
overall field decays and strength and structures really
get quite complicated rates of change in the field can also be
quite variable over. Time and also with position and
we can actually estimate these from our time varying
representations. So we find the fastest
directional changes in Equatorial to mid-latitudes.
We find the fastest field strength changes at high
latitudes and the most rapid directional changes occur when
the field strength is lower. You mentioned the LaShawn
Excursion. What exactly is that?
Is that a very special occasion or was it just the most recent
such event? So we think that excursions are
basically failed reversals that they occur when the field
strength drops and in principle, the magnetic field could reverse
but something prevents that from happening and one possible
reason that might be is that the reverse field isn't able to
establish a hold in Earth's solid inner core.
And so it's just kind of moving around.
In the outer core and then the inner core which retains the
original polarity allows the original dipole polarity to
reassert itself over time. So there are lots of excursions
that have been found in the paleomagnetic record.
The Le sharp Excursion is the best documented one.
It's also one of the more recent ones are our suggestions of a
couple more that occurred since them, but we don't find a global
extent for these so maybe those are things that didn't quite
even make it. To the Excursion status I should
have said that the overall strength decays and once the
dipole strength decays, then the pieces of the field that I'm not
represented through the dipole become more dominant, you could
think of them as being background noise and they
basically make the field look much more complicated during an
Excursion. Do you have a qualitative
understanding of what's going on in the core that makes this
happen? The global picture that we have
no of excursions suggest that they're mostly related to the
loss of dipole strength the highest field strength occurs at
high latitudes. So we get rapid changes there as
a dipole strength drops, and the other thing that we think is
that excursions may be initiated at low latitudes by the growth
of reverse flux patches or reverse magnetic field at or
near the core surface and these And my great pole words and
result in a full-scale polarity reversal.
Are there any other qualitative findings you see in your Palio
spatial map? Sorry over time varying paleo
field models. I think is what we call of but
you know one person's model is somebody else's idea of reality.
So if we look over the past two million years one of the things
that we see is that on average the dipole decays more slowly
basically on diffusion time scales, which are of the order.
For 30,000 years then it bounces back and this is a fairly subtle
result in the sense that there's a lot of variability in the
dipole moment where it goes up and down anyway, but when you
look at it on long time scales, it's decaying more slowly than
it comes back which suggests that in bouncing back the
convection or the advection of the field to produce new field
is a more important property. The other thing that we see is
that if we look at the long-term decay of field strength going
into excursions or reversals that those Decay time scales are
of the order of 10 to 15 thousand years, but the
directional changes can be much more rapid and they can be as
large as one two, maybe even 10 degrees per year.
These are things that we would expect to be able to find in the
Core models the numerical models that people make about the core.
Do these observations sort out the models?
You said it wasn't specifically good enough to tell you what the
Ahmed number should be or what the rally number should be.
But does it nonetheless sift out some models from others infant
full? It does.
Yes, the simulations are numerically precise and they can
be used to predict anything that we could observe and make into
our paleomagnetic time-varying model.
But even though they're new. Actually precisely not yet,
universally successful. And of course paleo field data
are messy. So we need to be careful about
demanding precise agreement, but many simulations do not show
this asymmetric growth and Decay rate for the dipole.
We don't get understand a systemic reason for this in fact
because we would like to be able to find a family of simulations
that showed it and be able to say, ah, yes these simulations
have an Ahmed number that's small enough or energy
distribution. That's the right size.
We don't yet have a systemic reason for this in the numerical
simulations reversals are sometimes produced by just
turning up the Rayleigh number which is effectively a mechanism
for increasing the amount of energy in the system and tipping
the balance away from dipole dominance.
What we see in the real field at the earth's surface is a
decrease in the dipole strength, but not necessarily an in any
increase in energy in the rest of the field.
I think the short answer to your question is this is an area
where we can expect interesting developments as the numerical
model as find a path to more Earth-like conditions.
So the record that you found shows that there were periods
maybe even extended periods when the Earth's field more or less
vanished. Does that have an impact on
life? We might imagine that it would
because it's widely believed that a magnetic field is an
important ingredient for life. But actually it seems that we're
OK even during reversals development of the human species
has actually occurred over the past, you know few.
Use and it doesn't seem to have been detrimental to that
Evolution. What happens is that the Earth's
field creates the magnetosphere which holds off the solar wind
and cosmic rays, which would normally be considered damaging
and potentially carcinogenic and that's good for the pet that the
field existed is was good for the original development and
existence of the atmosphere which is essential to Life as we
know it if we Turn down the field today as we would during a
reversal the current atmosphere would still be present.
Even if the magnetosphere was very much compressed and smaller
and the atmosphere actually also plays a strong role now and
protecting us from cosmic rays. So that would survive certainly
for long enough that it would be.
Okay. It's not like on Mars for
example where the Dynamo died out long ago and there is
essentially no remaining atmosphere.
So you're saying the impact of a disappearing field might have
been more severe. At the outset when life was just
emerging then it would be now. Yes, I think that's true.
Do we know if the F already had a magnetic field billions of
years ago when in fact life was just getting going so the
earliest widely accepted magnetic paleomagnetic field
records are from about three and a half billion years ago, and
the field may have been in existence very early in Earth's
four and a half billion year history.
There are ongoing attempts to recover.
Our records from the world's oldest rocks.
It's widely believed that a magnetic field may be an
important ingredient for Life an interesting thing that is in
emerging just recently is that the sequencing of some
modern-day magnetic bacteria so bacteria that use the magnetic
field grow magnetite within their bodies and use the
magnetic field for navigation and things like this suggests
that their genes go back to the arc.
And so the archean is two-and-a-half to
three-and-a-half billion years ago.
So if they were using magnetite for the same purpose at that
time that suggests that studying of bacteria could actually also
provide a mechanism for studying the existence of the magnetic
field in the past or that's a provide supporting evidence for
the existence of the magnetic field in the past.
For my last question if you had unlimited funds for your
research, what would you use them for?
Okay, I'd like to mention three important issues that really
limit understanding of long-term field variations.
The number one thing I would like to deal with would be
improving chronological constraints.
How could we get a better measure of the age of these
paleomagnetic samples when they're recording the field so
that we could make that more accurate if we could do that?
Representations of the field would be better.
We wouldn't be putting a measurement of field strength or
Direction at the wrong time when we reconstruct the field.
So I would like to make everybody have to date every
paleomagnetic sample. They take before they record
what's going on and more importantly I would like them to
focus on samples that can be dated very accurately.
What would that mean in typical dating that we use?
We're relying on radiocarbon dating.
And for that to be a good measure of age, we need to know
what the production rate of radio carbon is in the
atmosphere. And that's a limiting factor one
way of getting around that might be to use something like
speleothems or vard records where in principle you can count
the years and establish a record going back in time in much the
same way as one would for a tree ring chronology.
What a speedier thumbs involves speleothems are basically
stalagmites or stalactites. Tights that grow in caves and if
you've ever been inside a cave you might have seen that they
have these annual layers related to the seasonal feeding of the
carbonate that forms a structure and that record if you can
establish the age and magnetic record within the stalagmite
could allow you to get a very fine scale temporal record of
what's going on in the magnetic field and principle.
It could be annual resolution. Now valve records come from
sediments with annual layering due to changing seasonal input
to the sediment and there are especially prevalent in glacial
or sometimes in tropical regions subtropical regions.
Basically, you can count the layers back in time and perhaps
get a very accurate record of the time evolution of the field
and pair that with the magnetic record which makes for a much
better kind of data than That is not accurately dated.
So that would be my first thing so my second thing and being
quite greedy here. Actually.
I also want to have better spatial temporal sampling and
uncertainty estimates for the records that people get so that
needs meat more field programs. So I want to send people out to
collect more stuff so that we have more records and better
Global and temporal coverage than we have at the moment and
the third thing I would put my money into would be more
concentrated. Aunt rated effort in the
numerical modeling to understand whether we can get close enough
to the right parameter regime to believe that we understand
what's going on with the dominant physical processes in
the court. First of all, we need to improve
the Paleo field records so that we can build a better view of
what we want to reproduce within numerical simulations and then
the numerical simulations have to figure out how to get across
Several orders of magnitude difficulty in not being able to
simulate the Ackman number Kathy Constable.
Thank you very much. Thank you.
It was a pleasure for more about geology bites as well as
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