This is geology B with all of us trampled.
We know the Earth was born as a baking in finow.
So the presence of a relatively cool surface today suggests that
the Earth has cooled a lot since it formed but the very thin
crust on which we subsist is not representative of the Earth as a
whole. And although some heat must be
escaping from the earth. There are also ongoing sources
of heat. So the answer to the question as
to whether Earth is cooling down is not Forward.
Claude, your power is Professor of geophysics at the université
De Paris and Institute the physic do Globe, his research
aims to understand the physics of igneous processes.
In the earth, such as those occurring, in volcanic,
eruptions magma Chambers and the mantle, he's especially
interested in how heat flows out of the earth, particularly in
continents. He is measured heat flow in the
field. Mostly in remotest.
Canada, where he discovered surprisingly low heat flow.
Claude Joe, Parr welcome to geology B.
Thank you, Oliver. In my introduction.
I said that, we know the Earth was once a baking Inferno.
They can't be much. Direct evidence left from the
very beginning of the earth. So do we simply in furthest from
our knowledge of the sources of heat?
Yes, there are two main mechanisms that are involved.
First, you must gather material Outer space.
And all these material comes in through meteorites and fragments
of other planets that come crashing down and each one of
these impacts releases heat. So there's an enormous amount of
heat that's released this way. And there is another phenomenon
because we know that the Earth's has differentiated now there is
an iron core right in the middle.
And this core formed when the Earth was young and hot because
you have to separate the heavy iron-bearing minerals.
So that they could sink to the Center of the Earth.
And this very process, the sinking of these are enriched
material is also releasing heat because it's friction involved.
When a one material moves with respect to another.
So these two sources of heat up enough to have a limited Earth's
temperatures to above melting point.
What about the giant impact? That formed the moon?
There was an impact by a planet, which was about the size of
Mars, and that's the impact just Shred the bit of the earth out.
And this form the moon and this impact was so energetic, that it
also raised temperatures way above the melting point.
So at that point to this impact, in fact sets the clock back to
zero because there was enough it to remove everything that had
been happening since the earth was formed in the first place,
we believe that tidal forces are keeping some of the moons of
Jupiter such as IO and Europa and Saturn's moon Enceladus.
Halt is Is that something that keeps the earth warm as well?
Knowing the u.s. it's negligible, a lot of the title
for his come from the Moon. There's a bit from the Sun as
well, but these are small and that all of this gets dissipated
in the oceans, a bit of it gets distributed in the solid earth
because the solidus is also moving up and down just as the
ocean surface does. But there's a very tiny amount
of energy that's released this way.
Leaving aside the question as to whether the Earth is cooling
down for a moment. BF must be losing some heat
today, it can't be very much because apart from near
volcanoes or hot springs, the surface of the Earth is not
discernibly. Warm about how much heat are we
actually talking about if you average out all the heat that's
been released of the Earth because that varies from point
to point, the total average is about 18 million watts per
square meter. That is over an area of 1000 M2
your Using 80 watts and 80 watts is basically what it takes to
light up a light bulb. So you can see the the not a lot
of energy by our standards, but of course, geology doesn't work
on seconds, but human beings do. So in fact, you must not worry
about the rate of heat per second, but the rate of heat
release per million years. And if you scale up this number
that, I just caught it to million years.
That's an absolutely enormous amount of energy.
That's the energy. That's Go to generate volcanic
eruptions and that's also the energy that's able to drive
continental drift. It feels warm when you go down a
coal mine or something like that.
Presumably, we're feeling the heat flow from the Earth's
interior. When we feel that warmth, are
we? Yes, it was very clear to
earlier researchers that the years was hot inside.
Simply because as they went down in mines, the temperature kept
rising and the. So it was very clear that the
Earth was hot. Okay, let's talk about how we
measure the Heat flour. We use a basic law of heat
conduction. That is the mechanism that
carries heat through a solid and it's analogous to Ohm's law for
electric current in our case, the potential is the temperature
and when there's a temperature difference between two points
heat is Flowing from hot to cold and the heat flow is analogous
to the electrical current. So, what we do is we measure not
only the temperature difference between two points, but we
measure the temperature gradient and then we measure the Ability
of rocks to carry that heat which is measured by
conductivity which is the inverse of resistance.
So the heat flux then is simply the conductivity times the
temperature gradient. Now, conductivity is measured in
the lab. You take Rock samples from the
field and subject them to a known heat flux and measure the
temperature difference. So you can work out, what is the
conductivity? And then, in the field, you
measure the temperature. Gradient in a very deep bore
holes and this is done Drilling holes in the ground and measure
the temperature structure. When you go down, these
measurements are not easy because you don't control the
environment. And as we discussed in the
previous question, the D flow is very small so it can be
perturbed by many processes. So you have to go very deep to
have very large temperature difference so that you're sure
that you're really measuring the heat flow that's coming out of
the earth, is it very variable? I mean, does it vary on small
time scales and space scales when you go down the bore hole?
And in the shallow parts of balls, the rocks are permeable.
So there's water going through. The if water is flowing through
the rock, it carries heat. And that's something we don't
measure by the technique. We have been discussing and so
you must avoid this at all costs because that's not hit that you
can account for. So you must go deeper.
Then the parts where there's water circulation and basically
water circulates in pores and cracks that get close with
increasing confining pressure. So you just have to GP enough.
So that all these interstitial spaces through which water can
circulate get closed and in practice, you must go down to at
least 200 meter depth, suppose 600 feet and by then there's no
detectable water motion. We can check that because we
measure temperature with a very high accuracy, one thousandth of
a degree. So at this Precision, any small
water motion or any disturbance, gets recorded in temperature.
Ian. So, when we measure temperatures
in a bowl hole, we make sure the temperature is absolutely steady
over time. That is sufficiently long in
practice that's about 10 to 15 minutes and if temperature does
not change by one thousandths of a degree, were pretty sure that
nothing is happening there. Another factor that we need to
take into account, is that the Earth has been exchanging heat
with the atmosphere? And as we all know this been,
climatic changes in the past a One but also a much longer the
one associated with the glacial cycle so in the shallow part of
a bowl, the Earth is still equilibrating with the current
temperature and as a record of the past temperatures, so we
need to go sufficiently deep. So that we are not sensitive to
those changes which are not linked to the Earth, heat loss,
how deep do you go in fact, in some of these bottles?
Well for us, the deeper, the better because the deeper we
Ego. We can check that everything is
okay. What do I mean by everything is?
Okay, if you have a sufficiently very deep Bowl, you can measure
the heat flow through say, one part of the bowl and comparing
to the through, through another part of the borehole.
And if these two heat flows are the same, then you're sure
that's the heat that's being lost out of the interior of the
arrows. If there's a difference when
something has happened between these two different parts of the
borehole. So that's why when we go very
deep, then we have many, many different ways.
Checking, that's the Heathrow is basically the same, whether you
are at a kilometer depth or you had 500 meters depth in
practice. We measure balls that are
typically more than 600 meters deep and we are able to measure
the temperatures down. Bowls that could be as deep as
2.5 km. Wow.
Now I presume you're not drilling these boreholes
yourself. Oh no, a kilometer deep borehole
is about a million dollars. So we use bones that have been
drilled for other purposes. The main source of balls for us
is the mining industry. When the explore for mineral
deposits. They drill holes to make sure
that there is the deposit that they're looking for an
underground. You're basically piggybacking on
the exploratory, boreholes that other people have drilled for
prospecting purposes. Exactly.
Another reason, is that the very operation of drilling, a hole
will perturb the rocks, it heats up the rock because there's
friction between the drill bit and The Rock.
So we The bowl to cool down and they collaborate again with all
the Rocks around it. And so that takes something like
several months. So where we to drill our holes
have cells. We would have to go to the
field. Get permission to drill the hole
and drill it. And then with several months
before we can record it and that's clearly not very good for
an academic. Research team will use bones
that have been drilled a long time ago.
Sometimes we are able to penetrate holes that were as old
as 50 years. I know you've done a lot of
work. Work in the Canadian Shield and
to get good coverage. You go to extremely remote
places. How do you get there?
Well, in Canada, we go to remote places simply because we just
want to get the heat through map.
And that's absolutely independent of whether they are
roads or easy access to those places, so we get drop by drop
planes or helicopters. So we end up in places that
sometimes our completely Away From Any Road, the people who
drilled the holes go in the winter, So, everything is
frozen, so that trucks can go very easily, but we can't do
this ourselves because making measurements in the Canadian
cold winter. It's too complicated and would
require a lot of equipment. So by the time we go in
everything has sold. And so we're working through
marshes and wet areas and get dropped in the middle of
nowhere. So that leads to Cole,
interesting things. That's one of the things I
enjoy. Because you always get
surprises, we make measurements, for example, in the very
Northern tip of Canada near the Hudson strait There's no one
there, it's a beautiful and we set up our measuring gear.
It takes us about a day to log a ball.
So you're sitting here and waiting.
And suddenly, you see, an arctic fox that stayed with us for the
whole day, it was very curious and was apparently happy to have
some company. There's also disappointments
because in Canada, there's beavers and beavers are very bad
for us, because sometimes the Iraq dams and then they flood
all area. So, in several cases, we had
these fantastic balls that were just located where we wanted to.
The measurements. And by the time we get there,
they will just beneath a pool of water and we could not therefore
make the measurements. What about measuring the heat
flow under the oceans? Drilling a hole in the bottom of
the sea, is time-consuming and very costly.
So what we do is we use the fact that they're sediments and
sediments are Loosely Consolidated.
They can be penetrated. So, what you do is you drop a
probe, deep probe. It's just like the thin cylinder
that goes through the sediment. And in this scenario, you have
several Thermistors. There are thermometers that
record temperatures and then you can record the temperature
profile through the sediment. There must be quite a few
challenges to measuring the heat flow at the bottom of a deep
ocean. Well, it's a terribly difficult
Enterprise because you have to shove probe down to sediments.
Those sediments are lying at depths which might be four
kilometers or even more. So you have to shove the probe
you have to make sure that it's vertical because if it's Not
vertical, then you're not recording the vertical, gradient
of temperature. You recording the gradient at an
angle, which is not what you're looking for.
Furthermore, your probe cannot move.
Once you've shoved it into the sediments, it must remain
absolutely mobile because any small motion of the probe will
generate some heat because you have friction of the probe
against the sediments. Given that we are making
measurements are very small temperature differences.
This minor amount of sheer will generate heat that In a mess up
our measurements and you're standing on your ship, which is
four kilometers above in rough Seas.
Sometimes that might be huge waves and you have to make sure
that your program. It's totally mobile for about
one or two hours because it takes one or two hours for
temperatures to equilibrate and for you to be able to make a
reliable measurements. The old saying was never repeat,
he for measurement, twice for fear of having a new measurement
that deviates, from the previous, one, by more than a
factor of 10. You I've been measuring the heat
flow coming out of the earth directly.
Are there other observations that can yield an indirect
measurement of the heat flux? Yes, we use the depth of the sea
floor. What happens is that as the
oceanic lithosphere cools down, it contracts and this is what
explains the fact that we observe the seafloor to deepen
as we go away from a mid-ocean ridge.
That's a deepening is simply D to the fact that the little
circles down and Contracts and then gets deeper.
So the way contracts tells us how much heat is being lost and
therefore, what has been the heat loss?
Of course, when you measure the depth of the sea floor, you're
sensitive to very shallow structures.
That might be a seamount here. There might have been some
turbidity current with an extra segment recover there so it is a
roughness in the topography but by and large over a large
killed. The deepening of the sea floor
is just a record of the heat that's being lost out of the sea
fool. And that works on all oceans
that have spreading ridges which I guess all of them do right.
Yes. That's one of the beauty of the
physics is this has been done. The fact that there was a
mid-ocean ridge was known since the 19th century.
It was one of the big questions. What is the origin of?
The oceanic Mountain belts and it's quite different for what
happens in the continents. In the oceans.
What is happening is simply the cooling and we only realized
that in the 1960s 1970s. Geez.
But by then we knew that the deepening of the seafloor away
from a rich. We measured it as a function of
distance was absolutely the same.
In all oceans you could measure it in the Atlantic Ocean or in
the Pacific Ocean or in the Indian Ocean and you had exactly
the same deepening. So that was telling us that
there was a single physical mechanism at work there.
What do the results of all these heat flow measurements?
Tell us we Know how much heat is being lost by the Earth's?
That's one aspect and it's telling us two things.
One, is that larger fraction of the Earth's heat losses, don't
shoot the ocean and they're very big contributor for two reasons.
One is that it's the largest surface which is losing heat.
So, that's one aspect but also per unit surface.
It's also where the flow is largest.
Then if you go to countenance, it's a different story because
there are young continents, where you're sitting next to a
volcano. Clearly you're going to have a
large heat flow. Because the magma that's been in
place beneath the volcano is cooling down.
So that's extra eat. But when you go to very old
places, such as the Canadian Shield, the Canadian Shield on
average is more than 2.5 billion years old.
And we know that by this time, every transient heat loss due to
the formation of the continent has decayed away.
So we still measure differences of heat flow, simply because
coconut oil. Rocks are full of radioactive
elements and is radioactive elements release heat as they
DK. Depending on which work you're
standing on, there's more or less energy, that's being
produced, and that is reflected in the heat flow.
So incontinence. What we are measuring is by and
large the amount of heat that's released by radioactivity in
crustal rocks. Does the heat flow we measure
very on quite small, spatial scales.
You mentioned the presence of volcanoes do we see that kind of
variation elsewhere as well, equal varies on all sorts of
scale on a very large scale. New content, for example, North
America, if you measure heat flow close to the west, then you
have active volcanoes. You have also extension the
Basin and Range is the province where the Earth's lithosphere
and cross are being thinned. So, that's a very active region.
Where the flow is large. When you go to the Colorado
Plateau, the Colorado Plateau itself was a site of magnetic
activity about 100 million years ago.
And so there's a big jump in the Heathrow when you go from the
Basin and Range under Corrado Plateau.
Then when You go further east. Then you go to the older parts
of North American continent and you still have variations of
heat flow but on a smaller scale that is the scale of an
individual Rock. For example, from one pluton to
sedimentary Basin the change of geological setting will be
reflected in the change in heat flow.
So you have a large-scale operation which is due to the
story of the continent, a young continent, or an old continent
and within a content of a given age, then you have smaller scale
variations that are due. The juridical fabric of the
continent. Do you see much higher heat flow
in the tens or hundreds of kilometers around Yellowstone?
For example, Yellowstone is a place where the Earth is losing,
an enormous amount of heat and not by this mechanism, we were
talking about conduction. It's mostly releasing heat
because there's film roles, that's hot gases.
And as a gigantic amount of Magma's that has been in place
in the Earth's crust beneath Yellowstone.
So when we measure heat flow, we close to the axis of activity.
We still measure a high heat flow but we only measuring part
of it because most of it is actually being lost through
hydrothermal circulation in cracks but as we go away from
Yellowstone we measure decrease in the Heathrow simply because
we're getting away from the big heat source which is the magma
Reservoir that's beneath Yellowstone overall averaging
for the f is a whole are we currently losing more heat than
we're generating from radioactive sources?
The story of the heat sources and the Heathrow is a complex
one so we can measure this today and then we can infer what
happens in the past remodeled, but let's talk about what we do
today. So today I have all these
measurements in the oceans and the continents and I can't
figure out how much the Earth is losing heat.
We talked about on average was 80 million watts per square
meter and now I can calculate how much heat is being produced
by redirect ability because I have some idea about the total
amount Amount of radioactive elements that I've indiarose.
All the geochemical evidence and all the models for the arose
construction are such that the radioactivity can only
contribute half of what we're losing today.
So we're losing today twice as much heat as we're producing.
So we have a net loss of equal to 0.
So the Earth is cooling down. Does that translate into a
temperature loss rate of the earth?
Yes, if you For this to temperature with cooling down at
a rate of about 100 degrees per billion years.
Okay? So well over 1,000 degrees now,
so we've got a long way to go before we cool down.
In fact, it is a very small rate of cooling and this is why
geological activity has been sustained for so long.
Because a deer has been cool down much faster than everything
would be dead by. Now, as we go back in time,
there must have been more and more radioactive heat
generation. Ian.
What's that actually a time when the Earth was heating up before
it started cooling down? Well, that's the billion-dollar
question. There is some evidence that we
reach the temperature maximum sometimes in US history.
So it's clear that today we're losing mojito real producing.
It's clear also that because the radioactive decay we had to lose
more heat in the past because the radioactive elements are not
decayed as much as they've decayed today, but we also know
that heat flux should also go up as we go back in time.
Simply because as the earth cut after it became easier to
deform, then it could lose more heat it was more efficient that
mixing up its interior and shedding its eat so we have both
the heat loss and the heat production that go up
simultaneously with time. As of now we have clear evidence
that the Earth has been cooling down for between two and three
billion years. And we reach apparently some
Maximum at about 3 billion years ago.
Do we have any observations That we can use to corroborate these
conclusions that the F was hotter in the past.
Yes, we can use the composition of lavas, the Earth's mantle,
which is producing the Primitive lavas that eventually feed all
sorts of volcanic activity. It's the Earth's mantle that's
melting. Now, the Earth's mantle is not a
pure substance, so just does not melt at one temperature.
It melts over a temperature range.
Now, depending on the temperature at which you are
melting, therefore, you're melting different proportions of
the mental and hence you're generating liquids that have
different compositions. So the hotter, the mental, the
more primitive. The lava is so by looking at the
lava composition in terrains of different ages, we get a record
of the temperature at which the Earth's mantle got molten and so
we can transform composition of lava into mental temperature.
And when we do this, we observe that as we go back in time, the
mental temperature goes up and it goes up to about 3 billion
years as the calculations tend to indicate it's consistent with
what we observe these lava compositions but for ages that
are older than 3 billion years first, we don't have that many
samples as we go back in time. There's not a lot of lava flows
that are still around and they've been altered being
exposed to the winds and to the waters and they've been Sewed up
in a mountain belt. So for older than 3 billion
years, we're not quite sure. There's data that suggests that
the temperature doesn't change much and that's about it.
So you mentioned that the fact that the Earth is still very hot
explains that it's still very geologically active and it's
driving that activity. So, to our measurements, in our
understanding of heat flow help us understand how plate.
Onyx works pretty tunics is in fact the major agent for losing
heat. You cool, a plate at the surface
during Place material at the surface, it's coming from the
Earth's interior, it's hot, then it cools down and this is what
is responsible for. The deepening of the sea floor
with age and then by the time, you've cooled quite a lot of
material then you subtract it, you shove it in the Earth's
interior and that of course, is acting to cooling the Earth's
interior because you just carry cool stuff.
To the interior and just mix it with the surroundings.
So, plate tectonics is a major cooling agent.
So if we understand how the Earth is losing seats, there are
two aspects of the same problem. So if we understand the heat
loss, and we also understand the rate at which we've been
subducting things in the past. And so the rate at which
subtracting has operated in the past tells us the intensity of
mantle convection tells us how many plates we were generating
and other aspects like that. So, So yes, the E-class story is
actually providing us with a very strong constraint on the
operation of plate tectonics. In the past, the simple-minded
model of convection of the mantle driving the plate motions
has some major challenges such as the scale of the plates and
how subduction actually starts. If you look at any system that's
convecting, usually what you find is that the horizontal
scale of motion that is the distance.
Between an upwelling and downwelling is about equal to
the depth of the system that is if you have a cup of water which
is 5 cm thick, you will have upwelling and downwelling which
are separated by about 5 cm. Now on the earth, we don't have
this. The Earth's mantle is about
3,000 kilometers thick and we have this plate that I
extraordinarily, large the Pacific, Ocean extends, over
something like, 10,000 kilometers.
So, it is much wider than it is deep.
That's a major problem. We don't And this very well.
And the other problem is, we don't understand very well.
I wish abduct, we know it's happening.
We know it essential to cool viewers, we know it's bound to
happen, but why it happens in the way, it happens on the
Earth, where it is so difficult to subduct.
Why do we have to wait for 10,000 kilometers to see a
subduction zone in the Pacific Ocean?
This is something we don't understand very well.
And that's the key question. Because if you don't understand
subduction, you don't understand, convection.
I you don't understand heat loss.
Do you have any? Ideas as to how we might be able
to explain these puzzles this difficult because we don't have
that many records of Seduction, which is a starting we have
subduction that has been going on for a long time and so any
record of how it started is long gone.
So it's not an easy question. So the two kinds of theories one
is that the Earth has been deforming quite a bit from day
one. So you have zones of weakness
that are still lying about. For example, the collision
between Three, two continents and that will leave a scar and
the Zone which will remain weak for quite some time.
And by the time you have an ocean build up in the same area,
then this area is still weak and this is maybe how you can
generate subduction in this particular place.
Another concept is that we have continents and oceans continents
Yukon subduct provide us with the ankles to Anchor motion.
So you do have to make sure that the Motions that you're
generating, the oceans are consistent with the presence of
cut. So in some models, the contents
are actually those which force the subtraction, what are you
currently working on a new research?
Well, right now, I'm working on the your seat laws.
I'm interested in the way that he loves is connected to
Continental growth, continents today occupy something like 40%
of the external surface, but in the past, we started with
probably know conference. At also contents have been
growing and continents. Do two things.
One is that this They collect radioactive elements and those
radioactive elements will not participate in driving mental
motions, because they are just stuck in these rigid continents.
So that's one aspect, is that content will gross act to
deplete the Earth's mantle in radioactive elements.
The other aspect is that continents are thick and they
don't lose that much heat. So, you see, the contents are
doing two things. They reduce the area through
which you can release a lot of it back, Oceanic spreading it
and then they take real. Two lemons out of the mental and
hence out of the engine. That's preparing everything.
So these are two major aspects that are intimately linked to
the her stumble history. And that's what I'm working on
linking. The story of content or grows
with the story of mantle, convection and plate tectonics.
If you had unlimited funds for your research, how would you use
them? If I had a lot of money, I would
go and explore places that we don't know.
Yet understand quite well. What's going on in North
America. But there are large bits of
North America that we don't know very well geologically.
We don't know the Northern parts of Canada first because it's
under a lot of snow and ice and surprisingly enough.
This is also where the content is the oldest.
So we missing a big record of the early evolution of the North
American continent because we don't have a proper geological
map and we don't have samples from those areas.
Similarly, when you look at the oceans, we are very good.
Jewel of the ocean floor in the Pacific or the Atlantic.
When when you go to the Arctic Ocean we don't have that much
resolution. There's a mid-ocean ridge there.
We don't know exactly how its structure is.
And the last but not least, there's a big continent that's
lying in the South Pole Antarctica and we don't know it
George go structure very well. There's an active volcano Here.
Mount Erebus beautiful volcano. There's a mountain belt but we
don't know. It's me.
So we don't have access to a current parts of Earth's
history. So what I would do is go and
explore these unexplored places. Close your Park.
Thank you very much. Thank you Oliver for more about
geology b, as well as pictures and illustrations, that support
this podcast, you can go to geology B.com