This is Geology Bites with Oliver Strimpel.
When the Earth formed, it was covered by a hot magma ocean.
At some point it cooled enough to form a solid surface thought
to be basaltic in composition, like today's oceanic crust.
Then at some point the surface began to differentiate into
different types of lithosphere, basaltic oceanic lithosphere,
and more silica rich continental lithosphere.
How did this process occur? Were the first ancient
continents similar to the present day continents and did
the continents form in a burst of activity at a certain point
when differentiation occurred, or was it a gradual build up
over Earth history? Renee Tamblyn studies the early
Earth and in particular the role of water in the formation of
continents. She's also exploring how the
production of hydrogen from geological processes could have
powered very early forms of life.
She's a postdoctoral researcher at the University of Bern.
Renee Tamblyn, welcome to Geology Bites.
Thank you very much, Oliver. It's an honour to be here.
Before we talk about how continents form, how confident
are we that the basic picture I laid out just now is correct,
namely that the Earth was born without any continents and that
these had to form at some later point in Earth history?
So although we don't have any rocks preserved from the
earliest part of the Earth, so particularly from about 4.56
billion years ago when the Earth was first accreted, until about
4 billion years ago, which is the oldest rock we have, we
don't really have any picture of what the Earth may have looked
like then because it's not preserved.
But I think we are quite confident that the Earth
certainly started as a magma ocean.
And we know that from this magma ocean we could not make the
continents, as you mentioned, these silica rich continents.
They need much more complex geological processes to form
rather than just forming from a simple magma ocean.
So we know that at some point, and when is the big question,
these continents first emerged, that there must have been more
difficult, more complex geological processes at play
that allowed these continents to differentiate.
And we know this because the chemical components which make
up the continental crust are very different from the oceanic
crust, or what would have been quite similar to this magma
ocean at the beginning. The continents today are quite
different from the continents in the past.
The continents today are very rich in elements as such as
silica as you mentioned, but also potassium, whereas the
earliest continents, the ones from the Archaean, so this is
the time frame from 4 billion years ago to 2.5 billion years
ago, tend to be more rich in elements such as sodium and
calcium. So this leads us to think that
the processes we see today making our continents, our
potassium rich continents must have been quite different to the
processes we saw in the Archaean.
Finally, the question of when the continental crust actually
formed is a huge question. And at Sciences, because as I
mentioned, we don't have a record from the 1st 500 million
years or so, but also it's a question between formation and
preservation. So we know we can form the
continents, but we also know they can be destroyed, they can
either be eroded or they can be buried back into the mantle and
lost to our geological record. So it's a balance between
creation and preservation. OK, so let's actually talk about
how continents form. Do any of the first continents
survive to the present day to give us some clues as to how
they formed? So we do have evidence of some
very, very ancient continents on the Earth, Luckily for us, and
as I mentioned, they're quite different to the younger
continents we see today. And this difference has spurred
a massive amount of research into these earliest continents.
We don't know if they were the first ones, but we certainly
hope they were some of the first ones.
And these give us a lot of clues.
But it's kind of like you have an end product and you have to
go back and work out how it was formed, What were the
ingredients, what was the recipe to make it.
And there's a few important things that we need to know.
We need to know the source rock because you can't create a rock
from nothing. There had to be some rock there
before that was affected to create the continent.
In this case probably melted or re melted.
And we also need to know what exact process did that, which is
that process modified, it re melted it to make the continents
that we see preserved now. And we think that either the
starting composition or the process, both or one of them at
least were different in the RTN to now.
Can we date the oldest continental cross fairly
confidently? Yes, we can.
So as I mentioned, the oldest rock we have is 4 billion years
old, but that's a small amount of rock.
But the first continents we can really find on Earth seem to be
around 3.8 billion years ago. And then we have them all
throughout the Archaeans. So we also have some nice ones
around 3.4 billion years ago, 3.2 billion years ago.
So we seem to have a good picture of the continents
throughout the Archaean. What can we learn about the
processes that took place at the beginning and formed the first
continents? Yes, If we go and look now, so
interestingly, these are key and bits of rock.
They only cover about 8% of the continents we have now.
So we find these are cratons, they're all over the world
amazingly, but some of the oldest ones and the ones which
have had the most study, the most well preserved are in North
America, the superior craton. We have very famous the couple
craton which is in South Africa. And there's also several cratons
in Australia, which are particularly old, many in India,
Central Asia, China and also in South America.
And they are 80% dominated by one series of rocks, one
magmatic series of rocks. And we call these TTGS, which
stands for Tonalite Trundumite granodiarides.
Say that a bit more slowly for me.
Way too fast. Tonalite trundumite
granodiaride. So these tonalite, trundumite
granodiarides, they're magmatic rocks, they're igneous, they're
melted and they crystallize in the crust.
And these are these very high silica rocks, very high in
sodium, very high in calcium, and importantly, they're not
very dense, they're very buoyant.
This is what actually makes the continent, the fact that they're
buoyant and thick and therefore are above the ocean.
So these TTGS, as we call them, we like to shorten it.
They seem to require sort of three things to form.
First of all, we need a starting rock, as I said, something that
they had created from, and in this case we know it's a basalt.
They come from the melting or remelting of basalts or mafic
rocks, as we also say. So these are rocks that are high
in magnesium and iron. They also require temperature.
To melt a basalt, you need to take it to fairly high
temperatures, to more than 708 hundred degrees Celsius, but
intriguingly, not that high temperature, so maybe less than
900°C. Prosteologist.
That's not too hot. And lastly, they seem to require
water, which is an interesting thing, free water in these
rocks. These cratons are mostly TTGS,
but they're not entirely TTGS, are they?
No, they're not. Around 80% seem to be the other
20%, a series of rocks which we refer to as greenstone belts.
And greenstone comes from the fact that these rocks tend to be
green, very inventive, but they're actually super
interesting. They're very diverse, unlike our
TTGS, and they seem to be primarily made of volcanic
rocks, so rocks which erupted. And these, unlike the TTGS, are
not so rich in silica. They're actually very, very
mafic. So they have high amounts of
magnesium and iron. And the rest of these TTGS,
where it's not volcanic, tend to be sedimentary.
So we have a lot of sedimentary rocks also preserved in these
greenstone belts. So mostly these TTGS, but a
certain number of these high magnesium volcanic rocks, those
are referred to as chromatiates, aren't they?
Yes, true, including cholateates.
That's my current area of work for the past four years is
looking at these rocks called Commadiites, which is a great
name named after a place called the Kamati River in South
Africa. And these are really strange
rocks because they are lavas and they're incredibly magnesium, so
20 plus weight percent magnesium, which is a lot.
And interestingly, they seem to have erupted at extremely high
temperatures, so maybe 1600°C, which is incredibly toasty.
And these incredibly high temperatures seem to come from
the fact that commodities were generated very deep in the
mantle, actually probably very close to the mantle core
boundary. And they then ascend through the
mantle as a mantle plume and erupt in the arcane to form
these commodities. And we don't find them really
post Archaean. There are some very rare
examples, but most of them in the Archaean.
So it seems to be something very special to the Archaean.
And that is because, simply, the Earth was a lot hotter back
then. Does the disappearance of
Comatia and TT GS I guess does that happen at a particular time
in Earth history then? Yes, absolutely.
So it's very intriguing that the TTGS, they seem to die out after
the Ikeans. So from 2 billion years is some
of the last big swaths of them, the Commadiites become
incredibly rare after 2.5 billion years.
So they seem to have really dominated the geological record
in the Ikean and then they suddenly start to really die out
and become less well preserved in the geological record.
And do we have a theory as to why that is?
For the commodities, the idea seems to be that the earth
cooled beyond some kind of tipping point.
So the earth at the beginning was very warm thanks to its
accretion, the kinetic force of accretion, and also from the
decay of radioactive elements which slowly died out over time.
So it seems around 2.5 to 2 billion years some sort of
switch went off. Suddenly the Earth cooled to
some point where we weren't making these commodities, these
ultra hot mantle plumes as much anymore.
Intriguingly, this also corresponds to some very
significant atmospheric events, but maybe we can touch on that
later. So I'm trying to form a picture
then. So we have these TTGS which were
platonic rocks. They're part of that ranatoid
family on the one hand. And then you have the volcanic
rocks that I guess spewed out over them, the high magnesium
comatiites, and then sedimentary rocks as well, which I guess are
secondary processes. And then after the end of the
Archaean, some new or slightly altered processes took place to
continue to build the continents.
Is that right? I think that's a very good way
of phrasing it. Some people might say that that
could be actually the emergence of plate tectonics.
And when I say plate tectonics, I mean modern plate tectonics.
So where we have very stiff rigid plates and very deep and
very steep subductions when the oceanic plate is thrust under
the continental plate and a lot of pupils align that with the
end of the reign of TTGS and commodities.
I personally think the picture is a little bit more
complicated. I think we have local examples
of subduction and some sort of horizontal tectonics, probably
in the Archaean, but there does some seem to be some major
change. As I mentioned, you're
especially interested in the role of water in the process of
forming continental crust. Can you tell us about that?
Yes. So water is a vital ingredient
in the continental crust, whether it be in the ancient
earth, in the Archaean or in the modern earth.
And the key thing is how do we get the water to high
temperatures? Because what we really need is
water to lower the solidus of rocks like basalts, IE make
melting of these rocks or re melting of these rocks easier
because that's how we form either our TTGS in the Archaean
or our granites in the modern Earth.
There's been a lot of research looking into the sorts of rocks
that might be able to carry this water capture in the crystal
structure and also carry this water to higher temperatures.
And by high temperatures I mean more than 700°.
So the temperatures of melting of these vessels in the modern
Earth. We understand this very well in
subduction zones where we have oceanic crust which is being
pushed down into the mantle, but in the Ikean this is a bit more
of an unknown. You mentioned the solidus, so
when rocks are wet, the solidus is lower, which means they melt
at much lower temperatures. Is that right?
Precisely, yes, and also they tend to melt in larger volumes.
It's a very simple equation, but the more water you put into a
rock, the more it melts and the more you can extract.
And of course what we're interested in is this
extraction, which is what forms Astorica rich, calcium rich and
sodium rich rocks. So I would say the best way to
make a continent is add water. So would it be fair to say that
if you didn't have the water a you wouldn't have the volume of
rocks formed that we see, and B they would have a different
composition because of the kind of partial melting that takes
place? Perfectly said.
Yes, If we didn't have water travelling into our crust and
travelling into our mantle, we would have a very different
world. We would probably have smaller
continents, as you said, because they'd be less volume and they
would have a very different composition because the melting
reactions, the reactions of different minerals to create
that would be radically different.
Is the water part of the story as to why we don't see TCGS
forming later? That just wasn't enough water to
form that kind of Celtic rock. Oh, that's a very interesting
question. So we know that water is very
important in making granites. Actually, this is only an idea
that's really come around in the past 25 years or so or 30 years.
We're understanding how important water in the crust is,
and most granites, especially the granites which we see being
made today, say in volcanic arcs, actually do have a lot of
water in their source region. The water just comes from
seduction zones. It comes from seduction of
oceanic crust. The switch between TTGS and
granites is, I think, a little bit more messy than that.
I think it could be looking at a change in thermal regime.
So the Archaean being hotter, it could be a source problem.
So the basalts forming the TTGS could have a very different
composition to the basalts and ultramipic rocks which granites
come from. And it could also be plate
tectonics. You go from modern, right now we
have this, as I said, this very steep cold subduction which
seems to release water which eventually creates a granite.
Whereas in the Archaean, perhaps we didn't have that, perhaps we
had more collision or shallow hot subduction.
So there's really all these different things which come into
play. So if you melt the bath salts
and you make TTGS early on, it's a very similar process that
produces the granites of today, but they have less magnesium in
them. Is that right?
Yeah, it's kind of really hard to put the finger on this
because there's like 3 or 4 different reasons why that's the
case. So it's like kind of all tangled
up. But yes, it's a similar process
to now. It just seems that these
different ingredients, the starting composition and the
processes seem to be different. But water seems to be the key
player all throughout the Earth's history.
It's just a question of how do you get the water to those
temperatures? What is the main difference then
between T2 GS and modern day granites?
It simply seems to be this geochemical difference.
It could be different starting compositions.
You could explain this all by having different basils in the
Ikean verses now and therefore making different platonic
magmatic rocks. Or you could have different
temperatures of melting which make these different protonic
rocks. Or you could have different
pressures of melting which use lutonic drugs.
So this is also one of the reasons why our field has been
tangled up for so long. But the fundamental difference
is that modern day granite is very potassium rich, so you have
a lot of K feldsparse, but TTGS it's much more sodium and
calcium feldsparse instead. Yes, exactly.
That is the main difference. Does that mean it's got more
plasoclase? Precisely, yes, much more pledgy
glaze versus K fell spot. It's a very simple switch.
The other thing this could all be driven by is differences in
fractional crystallization. So I've talked a lot about the
partial melting processes because that's what I work on.
But of course, once you make a magma, you can fractionally
crystallize it, so you can form crystals which format fallout
fall to the bottom of the magma chamber or magma system, and you
can differentiate your melt that way.
So that could also be one of the processes creating these
differences between modern day granites and Archaean TTGS.
There's a reason it's not solved yet.
Today, about 30% of the Earth's surface is covered by
continents, as I mentioned early on.
I'm wondering, do we know if this kind of continental volume
was formed in a burst early on in Earth history, for example
when the TTGS started to form? Or did the amount of continental
lithosphere accumulate steadily over geological time?
It is really one of the things that remains the most unknown.
And as I mentioned, there is like one big problem that
hinders our understanding of this, and that's the
preservation bias. So is what we have preserved now
on Earth, when we look back at our 8% of the rocks that are key
in does it actually represent what was on the surface at that
time or does it represent one type of rock which was able to
be preserved more easily? Or is it chance?
Is it luck? Because we're always looking at
this balance between creation of continental crust and
destruction of continental crust.
And those two things don't have to be the same throughout time.
They can change. And of course, because some of
this cross has been destroyed, it makes it very hard to say
something about about what we've actually lost from the
geological record. I would say that what we see in
the geological record is impacted by a preservation bias.
It does not represent the world at that time.
So we have to really take this into account when we're trying
to understand our crustal growth models, as you say, which is the
total volume of continental crust on the air.
I mentioned that you're interested in the possible role
of hydrogen in powering the earliest forms of life.
First of all, can life really be fueled by hydrogen?
Are we talking about molecular hydrogen?
And presumably this was well before the Earth's atmosphere
became oxygenated. That's exactly right.
So we're talking about molecular hydrogen or H2.
And yes, this is well before the atmosphere was oxygenated.
So the great Oxygen Asian event was around 2.4 to 2.1 billion
years ago, but we seem to have had evidence of life well before
that. So this is quite a fun thing
because these types of organisms are called Archaea and some of
them are hydrogen, some of them is methane.
They use all sorts of different compounds to get their energy.
And these particular type of methanogens, they're called,
they use molecular hydrogen, H2 and CO2 to form methane.
And this is their main energy source.
And incredibly, they're still around today.
So we believe that they evolved in the Archaean, but we can
still find them today. They're in our guts, they're in
our mouths, and they're living in the Atlantic Ocean in
hydrothermal vents where they get hydrogen from rocks.
So they seem to be quite prolific in the world.
So, but the key problem was in the Archaean for the Great
Oxygenation event, the atmosphere was really primarily
composed of nitrogen, carbon dioxide and perhaps methane or
ammonia. So this meant that the first
life forms had to be anaerobic. They couldn't use oxygen as an
energy source to create energy. So this group of Archaea, they
most likely evolved in the oceans or in shallow lakes in
the Archaean where they were using this molecular hydrogen.
And how would the hydrogen be produced in these Archaean
oceans or lakes? Yeah, it's a fantastic question
because we come back to these wonderful commodities I
mentioned earlier, these highly magnesium lavas.
So quite incredibly, the exact same reaction which tracks water
in these rocks. This hydration reaction is also
actually an oxidizing reaction. So during this hydration, the
iron 2 plus in the rock is converted to iron 3 plus.
And because of the unique chemistry of these rocks, this
in turn dissociates the water so it traps oxygen in the rocks and
releases hydrogen. And it's quite amazing.
It's all kind of the same chemical reaction, doing all of
these things, hydrating the rock, oxidizing the rock and
releasing the hydrogen. And because we think the oceans
then also had quite a lot of CO2 in them, we see evidence for
that in the sedimentary record. So these Archaea would have had
the CO2, the hydrogen to combine to make methane and survive.
And the intriguing thing is that this could also be a possible
way for the evolution of life on other planets where there's not
any oxygen, but there might be water available.
And also these oxidizing reactions taking place.
We can also go one step further, which is why I kind of mentioned
before is you brought up interestingly this kind of dying
off of the commodity eye about 2.5 to 2 billion years ago,
which actually directly correlates with the great
oxygenation event. So this big switch in how the
Earth, the solid Earth and also the atmosphere seem to work.
And there's been a few studies that have actually suggested
that these commodities produce so much hydrogen in the oceans
that are acted as a drawdown for oxygen from the atmosphere to
recombine to make H2O water. And that actually the
commodities, the Earth cooling and the commodities starting to
be erupted less and less. Therefore, producing less and
less hydrogen actually is what allowed the oxygen levels in the
atmosphere to finally take off, leading to the great Oxygenation
event. What are you working on at the
moment? So at the moment I'm finishing
up my work looking at the interactions between commodities
and the ocean and the atmosphere to understand some of the
chemical cycling that may have been taking place at that time.
And I'm starting to look more towards the TTG production.
So creating models to try to understand exactly how these
basalts may have melted and to constrain what we were
discussing before. Really, what were the greatest
effects? Temperature, pressure, water
availability, starting composition?
How did they all play a role in creating the TTGS?
Is the existence of the TTGS viewed as a puzzle, or do we
understand enough not to be surprised if they're there, but
you're trying to flesh out the details?
I would like to flesh out the details.
The details in this point matter.
It seems like there is a non unique solution to creating
TTGSI think I mentioned before. And there's also a non unique
solution from going from these TTGS to the more potassium rich
rocks we have in the more recent geological record.
So the focus of my next study is to work out what actually are
the most important factors. And also a lot of the studies
have done very broad scale studies, but I'm hoping to go
more into depth looking at very particular case studies and some
of these Arcan terrains to try to wiggle out.
What are the most important parts of this recipe?
Renee Tamblyn, thank you very much.
Thank you, Oliver, it's been very fun.
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