This is Geology Bites with Oliver Strimpel.
Since almost all of our modern understanding of the Earth has
something to do with the theory of plate tectonics, it is
natural to ask when plate tectonics started.
This is a very challenging question, since so few rocks
from the early Earth survived to the present day, and many of
those that do have been heavily altered by intervening
metamorphic or hydrothermal events.
I discussed this topic in an earlier podcast episode with
Peter Kaywood. One key manifestation of plate
tectonics is the relative motion of tectonic plates.
My guest today recently succeeded in detecting such
motion occurring fully a billion years earlier than any previous
such detection. Alec Brenner studies Archaean
rocks using paleomagnetic and petrographic methods.
He is a post doctoral associate at Yale University.
Alec Brenner, welcome to geology bouts.
Thanks for having me, Oliver. Let's start with your results.
Exactly what motions did you measure and when did they occur?
We looked at a piece of Earth's crust that's been around for
over 3 1/2 billion years. Ancient pieces of crust like
that are called cratons, and we specifically were looking at the
Pilbara Craton. It's part of WA today, and by
making magnetic measurements of some of its rocks that are 3 1/2
billion years old, we measured the latitude where those rocks
formed over time. And those measurements showed us
that at the time, about 3.48 billion years ago, the Pilbara
was at about 53° latitude, give or take somewhat.
That's roughly the same place as Berlin, Germany is today.
But when we then did the same measurements on rocks that were
just a few million years younger than those, those slightly
younger rocks showed that the Pilbara was at quite a bit
higher of a latitude, about 77°, give or take.
That's about where Svalbard is today.
So a pretty big shift in just a few million years.
And that means that the Pilbara had moved rapidly from 1
latitude to another during that time, right?
And the rate turned out to be about several 10s of centimeters
a year, in line with the very fastest of modern plate motions.
And what's especially important here is that our colleagues
previously have measured rocks of the same ages in another
ancient piece of Earth's crust that's called the Koppal craton
in South Africa. And those measurements of those
South African rocks showed that that craton, the Koppal, was at
the equator at 3 1/2 billion years ago and was not moving at
the same time. It that's a big contrast to the
Pilbara part of Australia that was at higher latitudes and
moving around. And that means that we've found
two pieces of crust that were moving relative to each other
3.48 billion years ago. And fundamentally speaking, that
requires that there's a plate boundary in between those two
pieces of crust. That's differential motion, and
that's a hallmark of plate motions.
And you know that that was actual translation motion,
rather than just one giant rotation of a single plate.
It's a little more complex than that in the sense that all plate
motions are rotations, but yes, the fact that we see one piece
of crust moving in the other isn't allows us to constrain
that they aren't part of one global single plate, one global
shell that doesn't have plate boundaries in it, as has been
proposed pretty often for this time in Earth history.
You said that the one plate that you actually worked on the Ilbro
was moving fast, but the other plate was not really moving at
all. Is that actually something
different from what we see today, in the sense that we
wouldn't really find any plates that are static over the time
scale you described? It's quite similar today in that
respect. Actually.
There are plates that you can find that aren't really moving
all that quickly. A good example is South America.
It is moving in longitude somewhat, but paleomagnetically
using the technique that I used to study this problem, you can't
really see that it's moving because it's not moving in
latitude or it's, it's also not rotating, it's just moving in
longitude, if that makes sense. So that motion is invisible to
us. And there are also a few other
places on Earth where you can see no motion occurring.
But yes, that's roughly in line with what you might expect.
So as you said, you use paleomagnetic methods to measure
this motion and paleomagnetism has actually come up in previous
episodes of Geology Bites, in particular the one on super
continents with David Evans. Can you refresh us on how
paleomagnetic methods enable us to reconstruct past plate
motions? Yeah, sure.
Earth's got a liquid iron, nickel, metal outer core, and
that's electrically conductive and also convecting.
And so it generates a strong magnetic field that surrounds
Earth. And it's in the shape of a
dipole, right? The shape of a bar magnet with a
North Pole and a South Pole. And what's handy is that most
rocks forming on Earth's surface contain tiny grains of these
magnetic minerals like magnetite and hematite and puritite.
And those grains can act like microscopic compasses that when
they form, they can freeze the magnetic field where they form
in place and record it therefore for us to measure later.
And so, much like you can use a compass to navigate your way
around the world today, we can measure the directions of these
magnetic signals preserved by tiny magnetic mineral grains and
rocks to piece together how those rocks and the pieces of
crust they sit on have shifted over Earth's surface over time,
piercing together things like the supercontinent cycle and the
formation and closure of oceans. And for this study that we're
doing, we don't have very much context beyond just single
points in time for individual plates.
But what's important is that we can measure the latitude at
which those rocks formed. And that, it turns out, is very
straightforward to calculate from how steeply up or down the
magnetic field is pointing on Earth's surface.
If you're at the equator, the magnetic field is horizontal to
the Earth's surface, pointing north.
If you're at the North Pole, the magnetic field points straight
down into the Earth, and if you're at the South Pole, it
points straight up out of the Earth.
And it smoothly varies in between.
But how do you cope with the fact that the actual magnetic
pole is usually wandering around, even if it's not
reversing? Yeah, we cope with that with a
lot of samples. Yes, the magnetic field does
vary through what we would call in the business paleo secular
variation. I've heard it analogized to a
drunk person hanging on to a pole in a subway car, and they
kind of dance around it as they're unstable, but on
average, they're still standing up straight.
And the same thing applies to Earth's magnetic field.
It's chaotic and it varies quite a bit, but on average it's still
oriented along Earth's rotation axis.
And the reason that works is that Earth's rotation organizes
all of the convecting motions inside the core along that
vertical axis on average over 10s of thousands of years.
And so if you take a lot of samples that are preserving the
field at slightly different points in time over several 10s
of thousands of years in the year's past, you can average out
all of that variability and get at something a lot more
coherent, an average result. So we sample a lot of rocks.
In this case, we sampled several thousand samples for this study
to be able to average all of that out properly.
Wow, several thousand? And how much rock do you need to
drill out for each sample? Each sample is, in our case, a
core of rock that's about 2 1/2 centimeters an inch wide and a
few inches few like 10 centimeters long.
And we can also grab a block that we've oriented.
But the main thing that we have to do when we're sampling is
make sure that we're recording the orientation of the sample as
we found it, because we're measuring the directions of the
magnetization vector in these samples.
And so we have all these fancy oriented devices in the field
with us to measure that. But for any geologists or
geology enthusiasts who have been out in the field and
randomly seen, oh, that outcrop has a cluster of holes about
that wide of it, that's people like us.
That's paleomagnetists usually. And the number of samples there
reflects again our effort to try and reproduce our data and make
sure that we're getting that average field orientation.
Let's talk a bit about the actual kind of rock it is that
you're sampling. Are these rocks lava flows that
took place on an ocean floor and left kind of phonetic stripes
like we see today in either side of the Mid-Atlantic Ridge, for
example, and then using uranium, lead and zircons inside that
lava? Yes and no.
They are, yes, lava flows that erupted onto the ocean floor
about 3 1/2 billion years ago. They're mostly pillow lavas, so
exactly the sort of thing you might see in say Hawaii today.
They aren't necessarily in the same setting that would have
produced magnetic stripes or magnetic lineations on the sea
floor because these are forming in a setting that's not a mid
ocean Ridge that's spreading apart.
The setting would more look like maybe something like Hawaii or
parts of Iceland today. We don't have very much context,
but that's at least something that we do know.
And yes, the way that we date these rocks is using uranium
lead systematics and zircons. The lavas themselves are very
poor in zircon. So these are mostly basalt
lavas. And what's fortunate is that in
between some of these lava flows, you get sedimentary
horizons or Felzik volcanic horizons that are rich in
zircons that can be dated using uranium lead methods.
And so throughout this stack of lava flows that we've measured
throughout this stratigraphy, there are different points that
other of our colleagues have dated.
And so we can interpolate what time all of those different
rocks formed. So it seems like a huge question
is how do we know that the magnetic signal you're measuring
today was in fact printed so very long ago, and it's not the
result of some intervening effects that overwrote or in
some other way masked the original magnetic imprint.
And I wonder how intense such effects would have needed to be
in order to erase the original signal.
Right, as you've alluded to, magnetizations that are
preserved by rocks can get reset in a few different ways.
The main one is thermally. So if you heat a magnetic
material up to a high enough temperature, the magnetization
that it's holding onto gets overwritten with a new one.
You can actually try this at home by putting a fridge magnet
in your oven and baking it. And that will actually we neuter
the magnet. It will make it quite a bit
weaker for the mineral that we're measuring in nature most
often, that's magnetite. The temperature required to
completely reset any magnetite grain is about 585 Celsius.
That's almost 1100 Fahrenheit. And there's another issue also.
Thermal processes are not the only thing that can reset a
magnetization. You can also undergo chemical
reactions between minerals and fluids, for example, that might
dissolve or grow grains of magnetic minerals.
And so we have to be really careful that the magnetizations
that we are measuring date to when those rocks formed, or at
the very least that we can date when those magnetizations
happened in the first place. And that goes for anyone
studying the very earlier, the oldest rock, as you pointed out,
have all been heated and altered somewhat by billions of years of
later geological events. And so it doesn't matter if
you're a paleomagnetist like myself or a geochemist, you have
to make sure that the signals that you're measuring actually
date to when you think you're measuring them.
And we really have our work cut out for us.
We have developed a whole battery of tests in order to
establish the authenticity of that magnetic signal.
Can you describe what these tests are?
Sure, yes. So they're called field tests in
paleomagnetism. And as he said, they tell us
whether magnetizations in a rock predate or post date some
geological feature of interest and they take advantage of
traditional cross cutting relationships that geologists
use for mapping and relative dating of different units.
So one of those is a fold test. So let's say you measured rocks
in a bunch of different areas that are all the same age of
rock, but they've been folded or tilted in different ways after
they formed. And So what you should see if
the magnetization is older than the folding event, is that, you
know, two different rocks that originally had one coherent
magnetization that then got folded.
You should see that if you correct for that folding, their
magnetization should line back up again, roughly speaking.
And so therefore, again, the magnetization predates the
folding, which again you can date using other mapping and
uranium lead methods. Another one of these tests is a
baked contact test. This is something you can try
wherever there's a magma body, like a dike or a pluton that's
intruded into some older rock. And the trick there is to
measure the magnetization inside the intrusion, just outside of
the intrusion, and then gradually further and further
away from it in those surrounding older rocks.
And what you should expect to see if nothing has been
overprinted, is that the intrusion magnetization and the
magnetization immediately outside of it is One Direction.
And then as you work your way further and further away from
it, you should expect to see it gradually give way to some other
magnetization in the surrounding older rocks, which therefore
means that that intrusion magnetization dates to that time
of that intrusion. And the other magnetization in
the country rocks around it is older.
And there are more examples of tests than that, but those are
the two tests that we've used in this study are rocks fortunately
passed them with flying colors. That means that they're still
holding on to magnetizations that are older than a folding
event that's been dated to about 3.4 billion years ago and also
older than magma intrusions dikes that have been dated to
3.44 and 3.46 billion years ago. And again, just for reference,
the rocks themselves that we're measuring are 3.48 to 3.45
billion years old. So that's not much older in the
grand scheme of things. And so we can have confidence
that what we're looking at are truly ancient signals, rather
than some later noise superimposed on top of it.
And these tests rule out both sort of physical or metamorphic
events as well as chemical overprinting.
Yes and no actually. So yes, it doesn't matter where
the magnetization came from. These tests can still apply.
They're still useful for figuring out what the relative
ages of the magnetization and geological events are.
That said, we've actually done quite a bit of legwork on these
rocks to determine how they got magnetized in the 1st place, and
they're actually not holding on to the original thermal
magnetizations that these lavas would have picked up when they
cooled in the ocean floor. We've actually determined that
they got remagnetized in sea floor hydrothermal systems, and
this is where hot sea water percolates through the rocks,
makes them undergo chemical reactions, And those reactions,
as I mentioned earlier, both dissolve old magnetite grains
and grow new ones. So that's a little disquieting,
but what's fortunate here is that even though these
hydrothermal overprint magnetizations are overprints,
they're still incredibly old. And in fact, they actually
happened just after the rocks formed within a few million
years at most. And we've been able to determine
that used by mapping the hydrothermal deposits that are
formed by these systems with help from all sorts of different
data sets. A major one are regional scale
images, spectral images taken by planes and satellites of the
field area that we're in. And that all allows us to
pinpoint where in the stratigraphy, where in the stack
of lava flows and therefore when in the, you know, relative
timing of things. These hydrothermal circulating
fluids percolated through each of the rock units that we
sampled and reset them. So we're still able to use them.
We're leveraging that alteration as signal itself.
OK, let's get back to your results and what we can learn
from them. You show relative motion between
two plates at about 3 1/2 billion years ago.
Does this require full blown modern style plate tectonics
with subduction and so on? Or can we explain your
observations with alternative models?
No, it does not necessarily require modern style subduction.
So fundamentally, stepping back and looking at our results, all
that we can completely confidently say geodynamically
speaking, is that we see one piece of crust, the Pilbara,
moving relative to a different piece of crust, the Copvol.
And this rules out a situation, as I've mentioned before, that
has often been proposed for this time in Earth history where
Earth's whole surface is 1 giant global shell that's rendered
stagnant and unmoving by its lack of plate boundaries.
And that's because our data require a plate boundary of some
kind in between the two pieces of crust that we can see, again,
moving relative to each other, each on a separate plate 3.48
billion years ago. What we don't know is where or
how many or what kind of plate boundary that might have been.
There are different scenarios depending on what that might
have looked like that feature moving plates that are not
modern plate tectonics. So of course, modern plate
tectonics, as you mentioned, is possible.
So that's where plates actively move themselves as they're
pulled into the mantle by sinking at subduction zones.
But you can also maybe get plates moving as they're more
passively, sluggishly pulled along by the mantle as it drags
on their underside. And you can also envision an
Earth that might be stagnant most of the time, but
intermittently it might undergo these really brief but rapid
episodes of plate motion that resurface big portions of
Earth's crust. And our data can work with all
of those situations that I just mentioned.
So we can't necessarily with what we have in hand at the
moment say anything more than that.
We are working on additional data from a longer time span in
the pill borough. And those data may suggest that
what we're looking at is that last episodic intermittent case
that I just described. So that again, that'd be quick
bursts of plate motion punctuating these long 10s of
millions of year long periods of quiescence and static with no
motion periods. But just a work in progress.
But you can say pretty confidently that it wasn't just
a big stagnant lid all the way around the Earth, perhaps a bit
like Mars or Venus today. Correct.
Our data do require that there was some kind of plate boundary
to accommodate that differential, that relative
motion between two different pieces of Earth's lithosphere at
the time. Do your results enable us to say
anything about what else was happening in the Earth around 3
1/2 billion years ago? Yeah, I guess I'll make 2 points
there. One is that paleomagnetic data
are direct observations of what Earth's core is doing because
that's what makes the magnetic field in the first place.
And there are things that we can look for in our data that keep
tabs on that. And a great example of one would
be a geomagnetic reversal. This is where Earth's North and
South magnetic poles switch places.
And this happens pretty frequently on the modern Earth
and a few to a few 10s of times per million years or so, one to
10 times per million years. And excitingly, we actually
found a reversal in our data that happened about 3.46 billion
years ago. You can see its signature in the
Pilbara data and the data from the Kopfall Craton as well at
the same time. And that's actually the oldest
example of a reversal that's ever been identified.
And so you wouldn't be wrong to conclude that Earth's score
could circulate in a pattern that makes a modern like
magnetic field. And also could conveniently
enables us to do this geographic analysis that traces how things
moved around on Earth's surface. The other thing that I'd point
out here is that I'm working on another also unpublished data
set here, also from the Pilbara that likely tells us actually
about how the atmosphere circulated 3.4 billion years
ago. So that's the age of a bunch of
deposits that are in the Pilbara called evaporites.
Evaporites form inside of sediments that are exposed to
arid air and arid surface conditions.
And what we're finding is that those evaporites formed at
roughly the same subtropical latitudes, like 15 to 35° or so,
that most of Earth's deserts form in today.
And that latitude relationship is actually governed today by
how the atmosphere circulates from equator to pole.
And so it's a great illustration, I think, of how
powerful paleomagnetism can be as a technique, because it can
tell you about dynamical processes on Earth deep in the
past, all the way from the core to the mantle and the crust and
even the atmosphere. Wow, that's amazing.
So the subtropical desert regions are the result of a
certain kind of atmospheric circulation pattern that is of
the type that we have today. That's what it appears to be.
The type of circulation pattern here is called the Hadley
circulation, after I believe George Hadley in the 1700s came
up with it. But it's basically that warm air
rises and drops all of its moisture at the equator and then
makes its way northward at high altitude and eventually sinks
back down to Earth's surface at roughly about 30° latitude.
And that's responsible for the subtropical jet stream.
It's responsible for the trade winds at those latitudes.
And also, again, because all of that air had dropped its
moisture at the equator, it's dry.
And so when it gets back to Earth's surface, it dries out
that part of the Earth again. Hence why if you look at a map
of the Earth and look at where all the large deserts are, the
Sahara, the Kalahari in Africa, Australia, a match of the Middle
East, the desert southwest and and parts of northern Central
America. These are all deserts in that
latitude band for a reason. It's where that air is is coming
from high altitude and drying things out.
And it looks like we can see the same thing happening 3.4 billion
years ago. It's striking also that the time
period you're studying coincides with the time when we see
evidence in the rock record of the very first life forms.
I talked about this and about traumatolites in particular in
an earlier podcast episode with Martin Van Kraenndonk.
Have you seen any evidence for life associated with the the
rocks that you've been studying? Yeah, it's it's kind of perfect
actually that you mentioned Martin here, because it turns
out the stromatolites he's worked on for the last few
decades, among other things, are actually right in between the
rocks that I'm studying for this this paper.
So they're the oldest well established fossils on Earth,
and some of them even grew in a hot Springfield that Martin and
his colleagues were responsible for documenting that was fed by
one of the hydrothermal systems that gave our lavas their
magnetic signals. And so that means that now we
have a little bit of direct tectonic context for those life
forms. They grew on a tectonic plate
either in motion already or about to be.
And that's really intriguing because plate tectonics is
intimately linked with all sorts of processes that foster
habitable conditions. It plays a role in keeping
surface temperature stable for billions of years by modulating
them out of CO2 in the atmosphere.
It creates environments with sources and gradients of
chemical elements that living things can metabolize and build
from, for instance, in hybrothermal systems.
And to be clear, I should point out that the fossils that
Martin's worked on, the plate motion I found, they're probably
not the oldest examples of those things, full stop, right?
There are other even older examples almost certainly came
before them. But still, I find it profound
that in the endless quest of geologists to look further back
in geologic time, the oldest records of life, habitable
energy rich environments for that life, and the tectonic
plates that might have generated such environments all just
coincide pretty much perfectly. I I can't help but imagine that
early Earth might have dealt life a really great hand to get
started and develop. You spent quite a bit of time in
the Pilbara region of WA collecting all those thousands
of samples you talked about. What's it like to spend time in
the wilderness there? On a practical level, it's a
place what you might call ruggedly beautiful, I guess so
it's it's hot, it's dry. Pilbara comes from a local
Aboriginal word meaning dry country.
It's very hilly terrain that it's pockmarked by all these
spiky bushes of grass everywhere, scraggly desert
trees, the occasional kangaroo, sometimes an eagle if you're
lucky. And under all that desert is
like this deep sense of antiquity that you can't really
escape from. You're always aware of it.
The rocks, of course, are crazy old and they look like it too.
They've been out in the weather for billions of years and that
stained them all these weathered hues of red and orange, much
like a lot of the continent of Australia.
But you also get hints of a history that's a lot closer to
home. So during my field work, I've
had the great privilege of stumbling into prehistoric rock
art sites. Petroglyphs and chippings in the
rock, faces that depict people, human figures, interacting
animals. You'll see dingoes and Kangaroos
and emus all depicted there, parts of the landscape, maps of
river systems. And having had a chance to talk
with some of the modern descendants of those ancient
artists, they were clearly all trying to understand their place
in the world around them 10s of thousands of years ago.
And in a weird way, that's kind of what my colleagues and I do
out there too. We're trying to understand our
place in the world around us and how we got here, just using
different tools. And 10s of thousands of years
later and walking around the Pilbara really gives you an
immense sense of timelessness and heritage.
Some of the oldest art made by human beings to the oldest known
life forms that are right there next to each other.
And it makes you feel small in all the right ways.
Alec Brenner, thank you very much.
Thanks, Oliver. It's been my pleasure.
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