This is geology B with all of us trampled.
When plate tectonics was first discovered some parts of the
planet quickly fell into place as textbook examples of plate,
tectonics in action. The best examples where the
oceanic plates of the Atlantic, and the Pacific with their rigid
plates, forming at mid-ocean spreading ridges, and being
consumed at subduction zones, But other parts of the world
were not so easily. Understood notably the
Mediterranean. Laurence Olivier is a professor
at The Institute of Earth Sciences at the sorbonne
University. He's interested in the processes
that drive the motion of the plates, as well as the motion in
the mantle below them, a field called Geo Dynamics.
He has unravelled, the geological history of the
Mediterranean, which to put it mildly is not quite as simple as
that of the Atlantic and the Pacific.
Let me suggest upfront that for this podcast in particular, it
would be helpful to look at the illustrations of a geology, by
its website long. As you leave.
A welcome to Georgie B in the introduction.
I said that the Mediterranean is not a normal ocean, but before
we talk about the Mediterranean, what do we mean?
When we talk about normal ocean, the best example would be the
Atlantic Ocean open between the Erasure.
AA on the Eastern side and North America on the Northern side or
Africa versus South America. So it's open, starting with the
rift within our continent. Continental crust is thin and
then it's replaced by oceanic crust.
And from there on the two continents, they move away from
each other and large ocean is Progressive reform during 100 or
200 million years. That's the case of the Atlantic
Ocean. So the bottom of the ocean is
everywhere. Oceanic crust made of mainly
basic rocks, like Basalt or Gabriel's Mediterranean is very
different from that used to be a very large ocean, it had a
compression or history that that modified it quite significantly.
So overall the crust of say the Atlantic is extending taking up
more space, on the surface of the Earth, but as you alluded to
the overall context of the Mediterranean tells us right
away that something quite different must be happening.
Okay. And that's because we know that
Africa is moving northwards and colliding with Europe, which
should in fact be reducing the space available between the two.
So there's a kind of paradox here.
There is a paradox if you think of the Mediterranean as a normal
ocean, but actually since the late Cretaceous say, around 84
million years ago, the Mediterranean has been shrinking
because Africa has been converging with respect.
Eurasia moving northward with respect to Europe and this
convergence is accommodated within several subduction zones
where the African lithosphere is diving into the mantle
underneath Europe and these abductions subduction zone, they
have a special behavior actually, not that special
because it's quite frequently, the surface of the Earth.
The fact that the subduction zone itself is the trench.
If you like where exactly the place, where the subducting
lithosphere A sphere is disappearing into the mantle is
retreating. What I mean by retreating is
that the trench is moving away from the continent under which
the lithosphere is subducting. And this causes extension within
a very wide domain that is shrinking.
So Africa gets closer to Europe. And in the middle bits of Europe
are moving away from Europe by this process that we call a
stab. So the slab is the lithosphere,
the rigid plate that rise over The Mantel.
And by Retreat, you mean that the place where the slabs?
Adducts, the hinge is moving backwards, what makes a slab
Retreat. So the idea is quite simple.
Let's imagine that. You have in your hand, a piece
of paper and you hold it with your right hand and then you you
fold the piece of paper with your left hand and you pull it.
Downward and you will see the hinge of that sheet of paper.
Moving closer to your right hand and this is exactly what
happens. They are in the Mediterranean,
the hinge is the trench. Most trenches are the deepest
part of the earth underneath the ocean.
This is the trench. This is the hinge of that big
fold and the subducting lithosphere sinks into the
mantle under its own weight. Just like the sheet of paper
that you are pulling downward. So the hinge retreats Respect to
the upper plate and this leads to extension India for played.
Oh, I see. So, as the hinge moves back, it
creates more space in front of the hinge, which creates an
extensional environment, just in front of the retreating slab.
Yes. Exactly.
So as that slab sinks down into the mantle.
Does it have an effect on the mantle?
Presumably the mantle has to make way for the slab.
Yes of course is a very important Part of the subduction
Dynamics because the slav is diving into the mantle sinking
into the mantle under its own weight.
And you can even imagine subduction without convergence
of the big plates at the boundary of your system.
So there is this component of subduction which is due only to
the weight of the slab. And there is another component
of subduction, which is due to convergence in the total amount
of Seduction is Convergence, plus slab, Retreat, so the
mantle Is underneath, of course, will be strongly affected by
sinking of the mantle and by slab Retreat.
Imagine that you have your hand in the water and you move it
toward you, the water above your hand, or below your hand.
With, of course, follow the displacement of your hand and
this is exactly what happens in the mantle.
And we can see this flow in the mantle using the tools of
seismology and especially what is called a seismic an isotropy
which It shows the orientation of minerals in the mantle and
disorientation depends upon this flow.
So you see that the flow is parallel to the direction of
retreat that you reconstruct from geological evidence.
Just to explain that seismic and isotropy, refers to seismic,
waves, such as those that are caused by earthquakes, whose
speed varies depending on what direction, they're traveling in.
We expect this to happen when crystals in the mantle are lined
up as Results of motion such as the motion induced by a sinking
slab. So we see the mantle flow moving
with this lab in front of and behind the slab.
What happens at the edges of the slab?
That's a good question, exactly. As when you move your hand in
the water, the base of Mount on the door, just above or below.
Your hand will follow exactly the motion of your hand, but we
are in an incompressible fluid. So as soon as you move a piece
of the mantle somewhere, you have to move it to create will
create Eight movement somewhere else.
And in that case, the motion of your hand will push the
mentality's below toward you then.
And then on either side of your hand, the water will return
making some sort of cells rotating cells with an axis of
rotation. That would be vertical at would
call the toroidal flow at the edges of each piece of slab.
So we have natural flow following the slab, in the
middle of the slab. And at the edges are And of
Eddie as you might see next to a paddle moving through water and
that's called toroidal flow. Perhaps we should mention that.
It's moving much, much slower of the order of centimeters a year
though that certainly adds up over millions of years.
Exactly. Does this flow exert an
influence on the overriding lithosphere?
In fact, we have shown recently that the overriding little
sphere and this is Mental Floss, strongly coupled.
We know from the And of safe, make an isotropy that gives you
the flow in the mantle and the direction of extension that has
been recorded by Roxy the crust. So the direction of extension
that you can see when you walk in the field.
For instance, we know that these directions are very often
parallel and numerical models also show that this coupling is
efficient and so it's flow of the mount or underneath the
crust is one very efficient driver of the deformation in the
cross. So, the discuss is And it is
also very strongly affected by the mantle flowing underneath.
It is sheared actually by the mantle flowing underneath and
this explains a lot of the features we see in the extending
crossed in the Aegean or indeed you're tearing and see for
instance in the Mediterranean region.
Is there a specific example where we can see the effect on
the lithosphere of the toroidal mantle flow going around?
The edge of a sinking slab? There is one very good example.
That is the Eastern Mediterranean, if you look at
the GPS videos, Citi Field which is the the active motion
recorded by space geodesy. So the GPS satellite
constellation, you see that the main body of Anatolia.
Anatolia is the plate that covers most of turkey is
rotating counterclockwise about a rotation pole.
That would be somewhere in the Eastern Mediterranean, say north
of see knife and this system is rotating further to the east.
Arabia is Moving toward the North and the Northwest.
And so there is rotating Anatolia and if you go further
to the west and to the South West, you have, then the motion
of the Aegean Sea of Crete toward the southwest.
And this draws an overall very large tidal flow with a rotation
about a pole in the Eastern Mediterranean, hmm, geometry,
seems quite complicated. I was sort of naively
visualizing a wide plate subducting along a trench
running roughly parallel to the northern coast of Africa.
Had they're in fact being such a large plate and did it then
break up. Yes.
Actually it all starts with the initial configuration of Africa
when it collided with Eurasia in the Jurassic, there was a sort
of promontory sticking out of Africa toward the north.
That we now called apulia or Adria, sometimes presented as a
micro content and this because Collided first with Europe way
before the main body of Africa. So, we had that time, we had
this Promontory of Africa, colliding, with Europe and this
started to form the Alps and the competence.
And then on the other side, we had the oceanic subduction of
the tethys, ocean to the East, and to the West.
So already the subduction zone were divided into Bart, the
Eastern subduction zone and the Western subduction zone, but it
was not yet the Mediterranean it was still open.
Open toward the East and open to our the west and then some 35.
30 million years ago, we had this Collision in the east, at
the longitude of Iran. And then the Zagros range
started to form and later further to the north, the
caucuses sensual. So the ocean was was somehow
locked in the East and the same thing happened in the west at
the longitude of Morocco and Spain.
So we had these two oceanic space subducting Underneath
Europe and separated by this Continental Promontory that we
call that Puglia, okay? To summarize in the Jurassic
about 150 million years ago, a former Promontory of Africa,
called apulia collided, with Europe, the Collision of two
continental plates, creates mountains.
And, in this case, the Collision started to form the Alps and the
carpathians, meanwhile on either side of this Collision, the
Duction of the tethys. Ocean under Europe.
Continued toward the north. This carried on until about 30
million years ago when two more collisions took place, one to
the east around, present-day, Iran, and one in the west around
Morocco and Spain. Yes, exactly.
In this very dramatic consequence because of the
Collision in the East and the West Northwood motion of Africa,
was considerably Sloan and this make the subduction Ray, Didn't
change completely because the seduction could not swallow
Africa at the same velocity because Africa, didn't move
northward at the same velocity. As, before subduction started to
retreat in the East and side and on the western side as well.
So, because if you return to my comparison, with a sheet of
paper, if you don't have this convergence, the only way for
the oceanic lithosphere to sink into the mantle at the same
velocity, because The engine is there.
It's the weight of the syncing data sphere.
You have this slab Retreat that increases the rate of
subduction, despite the slower convergence, and this 30 35
million euros. It's really the birth date of
the Mediterranean. When this peculiar way, of
subduction started to proceed. And during that process, the
subducting slab was torn in several bar.
And these slabs got very narrow, sometimes like below.
Ria Alba load, jibril dark toward the west, the narrower,
the slab, the faster, the retreat can imagine that it's
easier to move your hand in the water and to move a very large
Board of wood for instance because of the resistance of the
fluid. So there are several different
processes that you mentioned, there is slab Retreat.
There's mantle flow and slab breakup or tearing How do we
know these things are actually happening?
So you know, they are happening because of to well three main
reasons. The first one is the techniques.
We have to unravel the geometry of the the slabs that depth and
this comes from seismic tomography.
Make tomography is somehow analogous to echography when we
want to see what's inside the human body.
Except instead of using acoustic waves, we use face make waves.
We have 3D models of what's going on in depth.
So the geometry of stop where they are straight, where they
are curved, when there are detached tone and so on and the
geometry can be quite complex. Then we have the geological
record in the geological record shows very clearly that the arcs
like the Hellenic Arc or the collaboration Arcadian Arc and
so on are moving away from from Eurasia.
And we know the rates of this motion, then Can reconstructed
quite precisely independently of any other data set.
And finally we have numerical models and especially now 3D
numerical model with rather high resolution where you can put all
the material, you need the lithosphere, the mentor, the
asthenosphere and so on and you can calculate the displacement
and the forces and the thermal Evolution.
And you see that this process is feasible.
First an inevitable s it correlates very well with what
You see in nature, you mentioned arcs, an arc is a curved chain
of volcanic activity, that we usually see on the plate that
overrides a subduction zone, typically a few hundred
kilometers from the trench and parallel to it.
And if a slab is retreating the active parts of the arc will
move in step with the retreating slab.
So, to take the Hellenic Arc, for example, which includes
dormant volcanoes as well as active ones, like, Santorini
it's created by the Hellenic subduction zone to its South,
which is just North of Libya, which is subducting under the
Aegean Sea. And as it Retreats southwards
towards Africa, the volcanic activity of the Hellenic Arc
moves South with it. And that is in fact what we can
see in the geological record. Yes, exactly.
Okay. So I'd like to ask about some of
the geological events that many of us know about.
Let's start with the strong earthquakes in the apennines
such as the lethal one in. L'aquila in 2009.
The earthquakes in the Mediterranean they are related
to several different processes, some of the earthquakes, they
are related directly to subduction.
We do create or below the Adriatic Sea or not.
Along the northern coast of Africa, you have plenty of
earthquakes that are caused by convergence and compression, but
you also have extension realistic waves in the back Arc
region in the apennines. Yes, we have destructive
earthquakes every 10 years or so.
And these are earthquakes happen within the other nines, which is
the backbone range of an insular, Italy.
And these earthquakes are simply the consequence of slab Retreat
and the mountain belt that is formed during your Sometimes is
presently collapsing. And if you look at the map of
the earthquakes in that region, you will see a concentration of
earthquakes, shallow earthquakes.
When I say, shallow is shallower than 10, 15 km all.
Along the backbone of Italy. And these words correctly are
all extensional and their related to normal folk
extensional thoughts, that move rather continuously, and from
time to time, you have a magnitude 6.5 7 earthquake.
And as these are Earthquakes are quite shallow.
They can be rather destructive, like it was the case in in
l'aquila, as you mentioned. Yes they are really a
consequence of laboratory T, those Quakes.
So as you mentioned earlier because the lithospheric slab is
retreating as its abducts, the overriding plate in front of it,
has to fill the space being created by the retreat, which
causes it to stretch and hence the extensional nature of the
earthquakes you mentioned. I want to ask about another
geological event that many of us know about, which is the
explosive eruption of Santorini in 1600 BC that may have
destroyed the minoan civilization.
It is possible that the minoan civilization being already
severely damaged by a series of earthquakes.
This is not sure at all, but it seems that when the minoan
eruption the big eruption of Santorini had to endure minoan
civilization was already in a bad shape.
And there was not so many people left on the island at that time.
Also, there were apparently series of earthquakes that were
signaling, that something weird and dangerous was going to
happen at the difference. With what we see in Pompeii in
Italy, there was no discovery of bodies within the volcanic
ashes, so probably the people living on Santorini had left
already. But this eruption at the
consequences, all around the Mediterranean, Ukrainian with a
probable tsunamis and as far as Egypt.
Okay, let's talk about the volcanism in the central
Mediterranean such as Mount Etna in Sicily and Vesuvius near
Naples that destroyed Pompeii in 79 ad and the aeolian islands.
Just north of Sicily. Why do we have volcanism today?
Just concentrated in those specific locations in general.
First the subduction related volcanism is due to the
injection Of water into the mantle, by the subduction zone,
let me explain the subducting lithosphere carries sediments.
They contain a lot of water and also the upper part of the
oceanic crust underneath the sediments and the upper part of
the mantle and contain a lot of water when these rocks, go down
in depth of the subduction zone, reaching 100 or 130 km.
The pressure increases drastically and this increase of
the pressure will sort of Squeeze the Rocks like a sponge
and water will be invade the mantle below the overriding
lithosphere and this will lower the melting temperature of the
mantle rocks, and produce Magma's these magma, that will
then accumulate below the lithosphere.
And then they will go up inside Doritos fear, and, and then
quickly and up to the surface. So, that's the general cause of
subduction related for Canada men because there is Out of
water in these volcanic rocks, T.
There is a lot of fluids and lot of pressure accumulated below
the crust and this will lead to very explosive eruptions.
Then there is a precise location of these volcanoes.
So they can be like just a line like in the end is for instance,
a line above the subduction zone and this is quite simple.
But then when the slab is retreating and when the slab is
torn in several narrow pieces, the mantle fluid depth will be
more complex. As we said, we store it all flow
and so on and we'll have sort of Windows opened by these tears in
the slab. The magma will come up more
easily through those windows and you will see, for instance, in
the Southeast entire agency, north of Sicily, you will see
narrow bands of volcanic material.
Some of them are aligned like the aeolian islands like
Stromboli and would Cano and Is volcanoes.
Others are not visible at the surface.
There are only found at the seafloor and they are found with
the geophysical exploration and probably one of the most
spectacular example of this volcanism related to a slab
tearing is Mount Etna. It's probably located about just
above one of those tears. In the slab do there is still a
lot of work to head over to really prove this but yes, there
is a strong impact of The tearing of the slab on the
distribution and the evolution of volcanism in the areas that's
fascinating, that slab tears serve to produce the window that
shapes. The volcanism above the gap,
What is the long-term future of the Mediterranean inevitably?
The matron will disappear caught between the huge bodies of
Africa and Eurasia. And so the snow lotions that has
been formed during the cenozoic, these backup base, since we were
mentioning together with the old ocean across that is in the
eastern Mediterranean and even the old oceanic crust also, that
was formed in the Cretaceous in the Black Sea.
All those bits of ocean will disappear and be some Police
abducted, and some of these rocks will be integrated into
the future mountain belt. That will be forming there.
So, the geologist of the future within, I don't know, 20, 30
million years, they would have a very hard job making.
The difference between these poly Oceanic domains, they will
have very different ages from 200 million euros, 230 million
years only. And this makes our job at the
present very difficult when When we interpret, very old mountain
belts like the various compared to other caledonians Caledonian
because we don't know exactly the geometry of the system
before the complete collision was achieved.
So we hope that future geologist will be able to dig up the
papers. You and others are writing today
in order to find out what actually happened.
What are the main puzzles about the geology of the Mediterranean
that remain? Well, or a few questions?
Why did in the first place? Suction.
That's a big question. We know that somehow the motion
or relative motion of Africa and Eurasia.
I've changed some 84 million years ago, but it's a question
of hand. An egg did the motion of Africa
changed because of subduction initiated in the 30s, Ocean or
is it the other way around? The seduction is created because
the emotion of Africa will achieve to arrange your change.
So we don't really know why subduction initiated in general.
Oh then this is a very important question.
Not only in the Mediterranean but all over the world and then
there are several. Other questions may be smaller
scale, but quite as important, in terms of our society, is the
concentration of mineral resources.
We pretty well know that now that the concentration of gold
of iron of copper, and all these minerals that are very useful
for our Economic Development, these concentrations are
intimately related to You're a large-scale geodynamic
Evolution. This has been known for quite a
long time, right? After the Advent of plate
tectonics. But with the tools we have now
we can reconstruct very precisely the evolution of
Continental margins plate, boundaries and evolution through
time. We can model them numerically
and can play with all the parameters, the velocities, the
temperature, Etc, the nature of rocks, and can then study the
processes that lead to To the concentration of mineral
resources but also the concentration of heat geothermal
energy, is one very nice leader, toward a Greener energy in the
future. And if we understand better, why
heat is concentrated in that place or this place will be much
easier for prospecting for geothermal fields and the same
for mineral resources to better. We know the processes that lead
to the concentration, the easier.
It's to find new resources. For my last question, I'd like
to ask you about your current research.
There is one. Very exciting question, which is
the vertical motion of the floor of the Mediterranean Sea through
time, and not only the material and see that all the countries
that are around Venice, for instance, and the periodic
flooding of Kansas on Marco with seawater.
This is due to external processes climate and tides.
And And so on, but it's also due to the fact that Venice is
slowly subsiding, and it is subsiding because it is within a
crust that is loaded with the apps actually within the weight
of the arms. So, one important question is to
disentangle the effect of sea level rise, and with respect to
the part due to internal Dynamics, the mental Dynamics, I
was talking about. So with this complex, Joe
dynamic, Next of the Mediterranean but it's true for
other places in the world, just concentrate it in a small area
here. In the Mediterranean Joe
Dynamics leads to subsidence, Joe Dynamics leads to uplift and
you can have a place in one place and subsidence, 200
kilometers from there, and if you are not able to deconvolute
the signal and say which part is due to internal Dynamics and
which part is due to externally driven processes like climate
you, Cannot understand the system, and you cannot predict
anything and it's a very different topic.
But we are also working with archaeologist in the island of
delos. Where we have this huge
Sanctuary, they're the largest archaeological site of Greece,
and on this tiny Island, hundreds of temples.
And these temples are made of marble.
And there are very few marbles on the island that is made
mostly of granite and gneiss. So the idea is to find where
these marbles come from in order to reconstruct.
Strictly commercial system at that time and where the Rocks
were coming from. This is also quite exciting at
the interface between geology and archaeology and the only
show, leave a thank you very much.
Well, thank you very much for inviting me for more about
geology, b, as well as pictures and diagrams that illustrate.
This podcast, you can go to geology B.com