From 2 to 12 October 2020, BRGM, France's geological survey organisation, will be taking part in the 29th Fête de la Science, centred around a virtual media event aimed at the general public. This event’s highlights
13 November 2020

The Paris geological metronome

Do you know how the age of the rocks used to build Notre-Dame? Find out thanks to one of our geology PhDs, Nicolas Charles: In this live episode of Science, he will tell you incredible stories about the Paris subsurface: From the ground under the Rolland-Garros tennis courts to that under the Eiffel Tower, our clay-footed colossus.

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This is a live link-up with Nicolas Charles in Orleans. Hello.


You're a geologist at BRGM, the French Geological Survey. You study rocks, the subsurface, landscapes. You're going to talk about our geological heritage and tell some stories. Team, pay attention. At home, too. How thick is the crushed brick surface of Roland Garros tennis courts? 2mm, 8cm, 80cm or 142cm? You can discuss it, team. Nicolas, tell us about your work as a geologist.

Geology is not a well-known career. It's often associated with volcanoes and dinosaurs. Geologists work on the subsurface. It has many applications in everyday life, for natural resources. Water, materials, metals fulfil many human needs, and also energy transition, mobility and so on. We manage natural risks: seismic risks, volcanic risks and flood risks. Energy, too. Geothermal energy. That's geology too.

I hope we get the right answer. The team's done well so far. So what's your answer?


No, 8cm.

What's your answer?

2mm or 8cm?


Well done. It's only 2mm. And viewers at home, how did you do? Only 2mm. Explain, Nicolas.

The 2mm layer is the court's epidermis. Because the court is about 80cm thick. There's the 2mm layer of brick. Underneath it, there is an 80cm layer of other materials: porous limestone in different sizes, for good drainage, and more waterproof layers, to retain humidity and give some flexibility.

Great, thank you. My next question is about Notre Dame cathedral. How old is the stone used to build Notre Dame? 800 years, 2,000 years, 1 million years or 40 million years? I'll give you some time. Nicolas, don't answer, but tell us about Notre Dame's stone. Is it special?

It's called Paris stone. It was extracted from the ground in the Paris area. It's from the Lutetian stage. Lutetia was the old name for Paris. Geologists have to find building materials for cities. We often talk about monuments but rarely about their materials. It's the geologist's job. Some of my BRGM colleagues are helping to restore the cathedral. They gave me this stone from Notre Dame, the famous Lutetian stone, whose age we'll soon know. There are fossils in it because it's limestone formed in a shallow sea, 20 to 30 metres deep, in a hot, humid climate. Paris is on the same latitude as the Balearic Islands. I don't know if you can imagine it, but it was a calm, tropical sea, with sea snails, shells, microorganisms that died and accumulated at the bottom, over millions of years, to form this rock.

OK, I'll ask my team how old this rock is. Go for it.

40 million years.

Well done! Another correct answer. Now let's see what our viewers said. 40 million years. So this rock is pretty old. Tell us more.

It may sound old to you. For geologists, it's young. Earth is 4.5 billion years old. 40 million years ago was the Lutetian age, and at that time, if we study paleoclimatology, things were different. A hot, humid, tropical climate, as seen in the stone. There's a bit of the tropics in Notre Dame.

Can we see a picture of this Paris stone? Tell us more.

This is Lutetian limestone, which was used to build Notre Dame. You see the marks of seashells. Impressions left by the shells. They are known as cerites. This is typical Paris stone. If you walk around Paris, you will see it in walls and steps. Research has shown that the quarries used were upstream from Notre Dame, on the right bank, near Charenton-le-Pont. There were underground galleries. At each level, the quarrymen who mined this rock, knew each layer and how it could be used, for the walls, foundations and so on. A rock's mechanical properties depend on the layers.

Here's our next question. The Maison de la Radio is heated by... A nuclear reactor? Natural hot water? An underwater volcano? Or Jean's smouldering eyes? We'll give you some time to confer. It's nicknamed "the round house" and it was opened 1963. Tell us about its history, Nicolas.

It's a radio and TV studio. It was the 1950s, the height of the Cold War. As the state radio and TV HQ, it was strategic site. It needed its own energy supply, not dependent on power plants. So they used a type of renewable energy. It was the first building in France of its kind. The type of energy used at the Maison de la Radio is also used to heat Paris's airports.

OK, let's find out how it's heated. Team, what's your answer?

Hot water.

Well done. Natural hot water. Let's see what our viewers think. Natural hot water. Or rather geothermal energy. Tell us more.

Geothermal energy is the heat from the Earth. It's a natural phenomenon. And in France, if you go down about 1km, you gain about 30 degrees per kilometre. The natural water used for the Maison de la Radio is found at 27 degrees Celsius and at a depth of about 600m. This diagram shows the Dogger aquifer, which is even deeper. The water is even hotter. It's 55 to 80 degrees. This aquifer is used to heats Paris's airports.

You'd never guess! My next question is also amazing. Two of the Eiffel Tower's piers stand in water. True or false? I'll give you time to think about the answer. The Eiffel Tower is 324 metres tall, 7,300 tons of steel frames, over 18,000 pieces of iron and four piers. Tell us more about these piers.

What we see of the Eiffel Tower, like other Paris landmarks, is the tip of the iceberg. That's a clue. The piers go deep down. They're the foundations. It's part of a geologist's job to explain the structure of the subsurface. Such information is useful when erecting buildings. In the Greater Paris area, a lot of work is being done digging new metro lines. Colleagues are working with the constructors to better understand the subsurface so they know where to lay the tracks without any risk of collapse.

Let's see if our team thinks if it's true or false. Two of the Eiffel Tower's piers stand in water. True or false?


Correct. It's true. But how come? Two piers stand in water?

The tower is not in water strictly speaking. But the subsurface, the flip side of a city, is very important. As for the tower's foundations, the two piers near the Seine, when the workmen did the test drilling... - this is a cross-section from the 1890s - they discovered that the aquifer connected to the Seine continued. It isn't just the water you see. There is water in the subsoil too. This posed a problem when laying the foundations. The builders worked in caissons filled with compressed air. So they could dig underwater without getting wet. The Eiffel Tower's piers rest about 16 metres underground on rock that is 55 million years old.

Next time we see the Eiffel Tower, we'll know about it. Thank you, Nicolas Charles, for joining us. Let's see who our winners are. Of everyone playing at home, let's see who did best. Well done to you all! Thank you for joining us.

Thank you, Nicolas. He has lots more stories to tell us about geology. He's a great storyteller! Thank you for watching. 

In what way can the subsurface help to reduce the amount of greenhouse gases?

Didier Bonijoly, BRGM’s Deputy Director for georesources, will tell you about a surprising solution to cut down on greenhouse gases in the atmosphere.

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Can the subsurface be used to store carbon dioxide? Jean and his guest will tell us.

In this last linkup, I'm not taking you underground but to Orleans, to meet Didier Bonijoly.


You're deputy director of the georesources division at BRGM, the French Geological Survey. You're going to tell us about an amazing way to reduce greenhouse gas concentrations in the atmosphere. My first question, so everyone understands: what is a greenhouse gas?

A greenhouse gas is any gas that traps solar radiation in the Earth's lower atmosphere. This radiation is reflected back towards Earth, causing temperatures to rise. Among these gases are those on the screen. There is methane, one of the most powerful greenhouse gases, then carbon dioxide, which has a much weaker greenhouse effect but is very abundant. Gigatons of CO2 are redistributed in the atmosphere.

What is the largest emitter of greenhouse gases, or CO2?


That's astonishing.

Volcanoes emit the most concentrated CO2. But humans have added to these emissions, which were natural and compensated until now by the absorption of CO2 by the ocean and by plants for photosynthesis. Industrial and human activity have created an imbalance. And CO2 emissions from human activities... We see factory fumes here. But it isn't just industry. There's transport, farming... All these human activities have added CO2 to the atmosphere, which the Earth cannot absorb. And this has amplified the greenhouse effect.

France and the European Union have set targets for 2050. What are they?

Europe is following the 2005 Paris Agreement. The European Union and other countries around the world have decided to act and set the ambitious target of carbon neutrality by 2050.

Zero carbon emissions by 2050. So... Yes?

Sorry. It's a balance sheet. That is, countries, in particular European countries, should only emit as much CO2 as natural and manmade systems can actually capture and store.

Right. This brings us to the innovative solution you are studying at BRGM: to store excess CO2 underground. Tell us how it works.

There are various ways to reduce CO2 emissions. This method of capturing, conveying and storing CO2 underground only concerns the largest CO2 emitters, industrial emitters. It isn't suitable for the transport or construction sectors, which need other solutions. For these emitters of concentrated CO2, the idea is to divert the gas released by combustion engines or production units and pass it through large columns containing solvents, which trap the CO2 molecules.


These solvents are then heated in order to recover these CO2 molecules, which we compress and convey in pipes, tanks or ships to the storage location.

Where is that? Deep underground?

The demonstration my colleague gave earlier was very enlightening. She showed an equivalent... She injected water into an aquifer which showed up as blue. What we intend to do is to inject CO2 in a state... It's almost liquid. It's called a supercritical state. It's extremely dense. It will be injected at great depths, over 1000 metres underground, into a deep aquifer, and will create a sort of bubble. It isn't a real bubble but a liquid that separates from the water in the aquifer, and contains this CO2 for as long as necessary.

But won't putting CO2 in the ground pollute the soil?

No, because CO2 is not a pollutant gas in itself. It's part of the air we breathe. We take in something like 450ppm every time we breathe in. CO2 is emitted naturally by Earth, by volcanoes, as I said earlier. We will simply store it underground so it won't return to the atmosphere.

OK. So how much CO2 can we sequester, and for how long?

The quantities... Currently, the amount of CO2 that can be stored in aquifers is much greater than the amount of CO2 we could capture from industrial units. So the problem of capacity is not an issue at present.

Right. Are there already industries, factories using this process to trap CO2?

There are industrial pilots around the world today using this technology. Most are in the US and Canada, some are in China...

And France?

In France, a major pilot study was conducted by EDF in Le Havre, at the coal-fired power plant.

In Le Havre?

Yes. It ran for about a year. This is a great photo. On the right, you can see the CO2 capture facility. During this pilot study, some 1,900 tons of CO2 were captured. There are other experiments that should result, economic and legal conditions permitting, in commercial CO2 capture facilities. ArcelorMittal in Dunkirk is testing a new solution developed by IFPEN, using a new molecule that is more effective at capturing CO2.


This carbon dioxide can either be used, reinjected into the industrial cycle to manufacture products, for example...

Or enhanced?

Yes, enhanced to make biofuels or plastics, or aggregates, for example.


And the rest, what cannot be used, will be stored underground. For example, the Sleipner gas field in Norway has been used as a CCS facility since the 90s.

Great. Thank you, Didier Bonijoly, for telling us about this innovative solution. We see that CO2 can be given a new life.

A quiz about aquifers

According to you, how much of our planet’s freshwater comes from groundwater?

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This is our big quiz on groundwater tables. 


It's time for our team to play. Join us using the code on the screen, to play the quiz with Géraldine Picot. 


You are a hydrogeologist at BRGM, the French geological survey. Great to have you here. What if I were to tell you that drinking water, our tap water, is running out? We'll see that here. First question. Of the water present on our planet, 97% is salt water, 3% is fresh water found in glaciers, ice caps, surface water and groundwater. How much is groundwater? Four choices: 1%, 31%, 61% or 91%? Join us on Kahoot to take part. So, Géraldine, where is this groundwater? In spaces between rocks, right? 

Yes. It's what we don't see, what's beneath our feet. An aquifer is like a sponge. 

May we see the model? 

Of course. This is the subsurface, what we don't see. There are different facies, different rocks... 

Beneath our feet? 

Yes. There are gaps between the grains, and water accumulates in these spaces. We can't see very well here, but imagine there's water everywhere in these three different layers. The whole thing is an aquifer. It holds groundwater. The top of this zone is called the water table. 

OK. Team, what is your answer? Go on. 

Please repeat the choices. 

Sure. 1%, 31%, 61% or 91%? How much is groundwater? 

I'll let you answer. 

We think it's 61%. 

Are you sure? 


Here's the answer: 31%. 1/3 of fresh water is groundwater. Tell us more. 

That's right. When we look at the planet, what we see is the ocean, which is salt water. 97% of the Earth's water. Of the 3% that is fresh water, most of this is found, two-thirds of it, in the Earth's glaciers, as ice. Surface water is also found in rivers and lakes, but that's less than 1%. The remaining one-third, which we don't see, is beneath our feet, in the spaces between rocks. 

Let's see our viewers' answers. What did you think at home? It's very close. But it isn't 61%, it's 31%. Now for the second question. Some groundwater is used for food. Our tap water. In metropolitan France, how much groundwater is used for food? Is it 2%? Is it 22%? Is it 42%? Or 62%? I'll give the team time to confer. Géraldine, you said this water is known as groundwater. Right? 


It's used for our food, drinking water and farming? Yes. It's vital for life. We drink it, wash with it, wash things, grow our food, and use it for industry. We need water to make paper, to make clothes... We have to extract this water from somewhere. This fresh water is surface water or groundwater, which we extract in France. I won't answer the question. 

We must take care of this groundwater. 

Absolutely. Team, what's your answer? 
We think it's 62%. 

Well done, it is 62%. We use almost two-thirds of groundwater for food, don't we? 

That's right. It depends where we live. We're not equal around the world, as regards what's beneath our feet. In France, we're fortunate to have many aquifers of different rock types, but not everyone does. There are different types of rocks, and these rocks vary in how they can be exploited, so we may choose surface water over groundwater. 

Surface water? 

Surface water is found in rivers, lakes and so on. There is plenty of groundwater beneath our feet. France has some huge aquifers. We use this groundwater and have done for ages. We need to understand the geology beneath our feet to know what groundwater there is, so we can exploit it. 

Let's look at our viewers' answers. It's very close again. But the correct answer is 62%. So two-thirds are used for food. Now for the third question. What is the instrument used to monitor groundwater levels? Four choices: a rain gauge, a piezometer, a flowmeter or a hole punch. I repeat: a rain gauge, a piezometer, a flowmeter or a hole punch. I'll give the team a few minutes. Géraldine, when it rains, the water seeps into cracks in the ground. 

Yes, water landing on the surface seeps into the ground and collects in these underground pockets. Water is everywhere in our aquifer. If enough water seeps through, the water level will rise. When we extract this water, the water level drops. I really like the choices. I'll explain why later. 

Let's see if our team has an answer. 

We think it's a piezometer. 

Are you sure? 

Yes, positive. 

Well done! A piezometer is used to monitor groundwater levels. There are 5,000 piezometers in France, aren't there? 

Yes, 5,000 piezometers in the groundwater monitoring network. That's why I found the choices amusing. A hole punch? Some piezometers do punch holes. 

Can you explain how it works? 

A piezometer is a hole drilled in the ground to reach the groundwater. Here we can see a water level marked by my thread. It's a high-tech probe to measure and record fluctuations in the piezometric level of groundwater. At the surface, we have scientists and others who check, measure, and collect all the data. 

Like this woman. 

Yes, with her computer. You can also use a tape measure. When it reaches the water, it beeps and the level is noted. And there are probe systems which connect to the data collection centre. This piezometric network monitors the health of our groundwater. And the BRGM gathers all this information and informs the public about the state of our groundwater. 

And we know all this thanks to the piezometers. 


Piezometers for the water level and qualitometers for water quality. 

OK, great. Now for the fourth question. Listen carefully, team. What is the process that contributes to groundwater recharge? Is it runoff? Is it evaporation? Is it distillation? Or is it recharge? I repeat: is it runoff, evaporation, distillation or recharge? I'll give you time to think about the answer. Fred, do you know? 

No, but I have a question for Maxime Morin. If a falling water droplet is contaminated, it enters the groundwater and stays contaminated for ages. Can you check that? 

Radioactivity gets into the soil. And it can spread. Some radionuclides are carried in water. To check the groundwater near nuclear facilities, we have piezometers monitoring the water. France has strict groundwater pollution regulations. We are not allowed to pollute groundwater. Piezometers check water quality. On nuclear sites, there are frequent checks, every day in some cases. The water samples are analysed for tritium content. 

Great. Thank you for your answer. Back to our team. What's your answer? 

We think it's recharge. Runoff is on the surface. 

Well done! Recharge is the correct answer. Explain, Géraldine. There's natural recharge and another type. How does it work? 

Natural recharge is the drop of water that falls to the ground. It has several choices depending on the season and where it falls. This drop of water can flow along the land surface to rivers, for example, or it can soak into the soil and be absorbed by plants, or it can be heated by the sun and evaporate. If there is enough of it, it seeps into the unsaturated zone, where there's no water. And it can be held there. But if it goes further, it reaches the groundwater table. That's the natural recharge process. 

Right. Let's just see what you answered at home. It's not runoff, but recharge. Natural recharge. But with rising temperatures, there is a decrease in groundwater recharge. So tell us about artificial recharge or managed recharge. Over to you! 

Yes. With climate change and global warming, we'll get less natural recharge, to balance our groundwater tables. There will be an imbalance. We will always need water for drinking, farming, industry, but we'll have less and less. So we really need to find a number of different solutions: reduce our consumption, change our lifestyle. But as well as doing this, another solution is to recharge groundwater artificially. 

Could you show us? 

This is managed aquifer recharge. I'll show you. 

Using your model. Talk us through it. 

I pour water into an infiltration basin on the surface, to recharge my aquifer. You see right away the piezometric levels, with our threads, rising and rising and the water is recharged. It's forced, it isn't rain. It's artificial. I'm controlling the flow of water and recharging it. We need to know where it goes, in what quantities, how it moves... It's a really fascinating topic. Natural recharge only occurs a few months a year, in the winter, when plants go dormant and don't need water and there is less sunlight. We only have winter to recharge our aquifer. 

Where do you get the water for this? 

There are many sources. Depending on the country and regulations. It has been done in France for ages. Water is taken from surface water bodies and cleaned before the recharge. It's filtered. Managed recharge enables us to provide drinking water for our wellfields. 

Right. Quickly, what about the quality of this water, how do you ensure it? 

We use all kinds of measuring and analysis instruments: piezometers, qualitometers... We can also add or create others if need be. Analyses are conducted to check the artificial recharge has not altered the water. We try to understand what is going on and predict it with mathematical models. It involves different fields of geosciences. It's multidisciplinary. That's what we do. 

Now we know all about managed aquifer recharge. Thank you, Géraldine Picot. Let's look at our winners' podium, to see the three people who gave the best answers. Well done to you. You're a hydrogeologist. Maybe you'll inspire others. Thank you for joining us. Well done to our team. Fred, over to you. 

Thank you. Bravo, Géraldine, for the model. I know someone who'll be envious! 

News scenarios concerning the rise in sea level

Since the 2019 IPCC account, new scenarios were established about the rise of the sea level based on recent data about the melting icecaps.

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We're going to talk about sea level rise. Gonéri le Cozannet. For a while we've been working on this. For a while you've worked on this. But what's new is, the forecasts have changed, new scenarios arisen. Tell us more.

Yes, in every IPCC report, there are new sea level rise projections. In 10 years, we've learned more about the melting of polar ice caps. The sea level rise in 2100 will be due to thermal expansion, if it follows the most likely scenarios on melting ice caps. The risk of exceeding these scenarios comes from Antarctica and Greenland.

We'll come back to that. What are the scenarios? Until now, we were talking about 2050 and then 2100. What rise? And now, what are the new numbers?

The 2019 report says 30 to 60cm if there is a significant reduction in greenhouse gas emissions. The two degrees goal, the Paris Agreement's target. And between 60cm and 1.10m if emissions don't fall much, if we continue to emit greenhouse gases.

Some say 1.50m, others even say 2m.

The big issue is the role of Antarctica and Greenland. That's the big question. The CNRS has a major project, a European project, PROTECT, on rising sea levels, the risk of exceeding the IPCC scenarios. The scientific community is mobilised.

It is mobilised, but there isn't a perfect consensus.

We've learned more about sea level rise, hence the upwards revision of predictions since the last IPCC report in 2014. In 2019, we added 10cm to the 2014 projections, because we know more about melting. But we still have questions.

So the phenomenon... Elodie, go on.

I have a question. You explained why this was happening. Apart from the objective of reducing the greenhouse effect, are there other ways to try and reduce this phenomenon today?

Some researchers have asked: "Can we halt "sea level rise without stopping climate change? "Can we take water from the ice cap edges "and put it in the centre?" It requires a lot of energy, it isn't feasible. Others have said: "Can we close the Mediterranean with a dam..." I think we have to be very careful. The impact on the Mediterranean ecosystems is unknown, and probably very harmful.

The best solution is to reduce greenhouse gases. Explain this new phenomenon. Let's see a picture from ESA, the European Space Agency. What's happening on those ice caps?

On the screen, you see the marine caps, so called because... Excuse me. They're in contact with the ocean. The ocean is warming. It's not the air that warms and causes melting, it's the ocean that warms and melts the cap. These sea caps are retrograde, i.e. the nearer the centre, the lower the ground. Once instability starts, we can't stop it.

It goes on.

It continues. Like a marble on the top of a sphere, it falls. It's been observed on two large glaciers in Antarctica. This led to the revised 2019 scenarios. And there's worse.


The worst-case scenario is in the 2019 report, but we think it unlikely. According to this scenario, the floating ice shelves that extend from land glaciers will break off, forming cliffs that are too high to be stable. Surface melting will cause cracks. And the glaciers will soon break up.


Predictions of over 1.10m are based on this.


It's just a hypothesis for now. We and the CNRS are working on it to see if it might happen now, in the 22nd century or never.

Yes, we set dates: "This will stop in 2100." No. The phenomenon can...

The sea level rise will not stop in 2100, it'll continue for centuries.

Besides rising sea levels, there are also storms. Are they becoming more and more frequent and fiercer?

They aren't more frequent and fiercer because of global weather change. It's because of rising sea levels. The 20cm rise already observed, with storms that used to occur every 100 years, a century ago, makes these storms fiercer.

We see this in your model. The storm, combined with higher sea level, means the dykes are... Well, overwhelmed.

This is Gâvres. Storm Johanna, in 2008. A reanalysis of all storms occurring in Gâvres showed that climate change had already caused storms to become more frequent. This doesn't necessarily mean more severe flooding, as dykes have been improved. During Storm Johanna, the football pitch was flooded because the dykes broke. But we can see that the risk is already aggravated by climate change. And this will continue.

We have coastal risk prevention plans in France, to protect residential areas. In the UK, they hypothesized a 3m rise. They built a structure to protect the Thames estuary.

Here's a picture.

Spanning the Thames, there is a barrier to protect London from flooding if storms combine with higher sea levels, low pressure and wind. Their approach was the opposite of ours. We asked: "What scenario shall we assume?" Our rules suggested around 60cm. That's our current estimate for plans in France. In the UK, they asked: "What can we do "to the Thames Barrier, for these scenarios?" For a rise of up to 3m. They can work on the barrier itself or upstream, creating spaces for the Thames to flow more slowly, so less water reaches the barrier and the flood risk is reduced.

So they are already working to limit the effects of the phenomenon. A quick point: we keep hearing that islands will disappear. In the Pacific, there's French Polynesia. Let's start there.

They've worked on small islands. The landscape is very varied there, especially on the Tuamotu Islands. Their base is a conglomerate about 60cm above sea level. 2,000 or 3,000 years ago, the sea level was higher than today in this region. So the atolls of Polynesia, the Tuamotus, may be less vulnerable to sea level rise than atolls in the Maldives, which don't have this conglomerate. There's also a trend for building new towns.

In the Maldives, the Indian Ocean, they're building a lot. Even new islands.

Finally, the ocean is warming. On the Maldives, that affects the coral, which constitutes the islands. The sand there is the debris of coral reefs. And so... We think Polynesia will still be inhabited in the 22nd century, provided sea level rise remains moderate and global warming is limited.

Live from the Marion Dufresne ship off the coast of Mayotte

Isabelle Thinon, a marine geologist and geophysicist at BRGM and Emmanuel Rinnert, a researcher in geochemistry at Ifremer will take you on their ship, the Marion Dufresne, which is off the coast of Mayotte.

These two scientists are working at the same time on an oceanographic campaign to study the earthquake swarm and the new underwater volcano which emerged at the bottom of the ocean a few months before.

© L'Esprit Sorcier


They're waiting for us, far from home. Two scientists. One from BRGM and one from Ifremer. It's great timing. They're waiting for us on an oceanographic research ship. Jean.

Yes, Fred. I too have a BRGM/Ifremer duo. We're off on a trip to the Indian Ocean, off the island of Mayotte, to join my two guests live on the exploration vessel Marion Dufresne. Hello.

Good evening.

Welcome aboard.

Thank you. Glad to join you. Isabelle Thinon, you're a geologist and marine geophysicist at BRGM. And Emmanuel Rinnert, you're a researcher in geochemistry at Ifremer. Good to see you. It's as if we're on board too. We should see the ship on our screen now. This is the Marion Dufresne. Emmanuel, where are you right now?

We're on the back deck, where most work is done. You can see behind us, right at the back, the white rear gantries that can launch heavy equipment from the stern of the vessel. In front of them is a submersible, an autonomous underwater vessel. It's nicknamed IdefX. It's an autonomous submarine which can take measurements at a depth of 3000m. The other thing you can see behind us is a pulley to drop cables on the starboard side, to take core samples or launch our underwater camera. So equipment to explore the oceans.

You're on this boat today thanks to an amazing story. It all started in May 2018, when Mayotte was rocked by many earthquakes. Isabelle, why did those earthquakes astonish scientists?

Because this region is considered a moderate risk area. But in May 2018 what happened was a seismic swarm: several earthquakes occurred within a short period of time. And that surprised scientists.

Is that unusual in this region?

Yes, it is a region with few earthquakes. Only isolated earthquakes. But there were several earthquakes. It was a seismic swarm.

Right. The earthquake swarm went on for months. The island moved 18cm east and the ground sank about 15cm. This endangered the residents of Mayotte, so you and other scientists came to see what was going on. Isabelle, what can explain this ground deformation and the movement of Mayotte?

At the time, in July 2018... this sinking phenomenon, seen by GPS, was explained as being due to a magma chamber deep beneath the surface, that emptied. This caused the island to move.

Right. What is a magma chamber?

A magma chamber is a place where liquid rock, or magma, converges. Some of this magma will be stored here. Some fluids will settle and others will come out, and gas too. It is a storage space.

Right. As soon as you got there, you began probing to see what was going on deep down. Emmanuel, how did you study the ocean floor?

We didn't know what to look for or what we would find. We were looking for traces on the seabed. At first, we used a sounder, a tool on the ship's hull, a multibeam echosounder. It enabled us to examine the ocean floor and what was going on in the water column. We could see the topography and emissions of fluid, which might be gases, particles, heated water, and search for activity on the seabed. The tool allowed us to quickly scan a large area of seabed. And since we had no idea where to look, it helped us find phenomenon on the bottom.

Right. I think we have images showing how you scan the ocean floor. A year after the earthquakes began, on May 9, 2019, you all gazed at a computer screen as you'd found a significant clue for your research: an underwater volcano. Emmanuel, could you describe this volcano to us?

What we discovered... The most recent maps we had were from 2004. And of an area that was different then. In May 2019, we discovered a new structure that is 800m high, with a diameter of 5km. Today it is... That's what's on the screen. The red line is the ocean floor. You can see the new volcano and, escaping from it, what we call plumes. We measure these anomalies with the same sounder. They are due to emissions of gas, fluids and particles.

Right. So you found a huge underwater volcano hitherto undetected by radar. Don't go away. We'll be back to talk some more. Fred, don't you want to travel?

Yes, I'd love to be with the team over there. Any remarks, Gonéri? The island is sinking.

The link with the sea-level rise is that Mayotte is sinking in parts between 10 and 20cm. Roads and quays are flooded at high tide. We'll see this in 20 or 30 years' in all France's overseas territories, such as Martinique.

That's already a problem in Mayotte, isn't it?

That's right. Flooded roads make infrastructures temporarily inaccessible for people.

OK. We'll go back to Isabelle and Emmanuel now. You're live from the Marion Dufresne, just above the volcano. Good to see you!

Yes. We still have lots to say. Since the discovery of this volcano, you've followed it closely. What's going on, Emmanuel?

We've followed its activity since May 2019, on several expeditions, notably on the Marion Dufresne. We also came in June, July and August on a navy ship operated by the SHOM. Then we got the latest news in May 2020. And now it's October. We've just arrived and are starting work.

Do you have any idea, can you estimate or predict, when the volcano will emerge from the water?

A volcano, on a geological scale, can be said to grow relatively fast. But in our lifetime, we won't see the volcano emerge, especially as it stopped growing taller quite quickly. We haven't seen change at the top since last year. But the latest measurements, in May, showed emission activity, though far less than before. It's growing laterally, due to lava flows, but no new reliefs are emerging.

To learn more about this underwater volcano, you go deep down, not you but a dredger that scrapes the bottom and recovers volcanic rocks. When you bring them up, they explode. It's amazing. We'll watch that right away.

It's falling apart!

Why is that?

We hear a crackling noise. The rocks crackle and explode when they emerge.

Isabelle. How do you explain this?

The clip you saw showed the first dredge carried out on the active volcano. The dredger recovered the rocks. These rocks are filled with gas. When they come to the surface, the gas expands. So fragile rocks, such as volcanic glass, explode when the gas expands.

The rocks are compressed by the high pressure, but when they emerge, they explode. What can you learn from these rocks, Isabelle?

We can discover their origins, where they were formed, their depth, if they formed under pressure, at what temperature. Studying their minerals tells us the path the lava takes to the surface. And its age too. And also its behaviour.

Right. So a monitoring of the volcano. You went back on October 1st. Emmanuel, what operations are planned?

There are many planned operations. The 1st phase is maintaining the waterfront stations, which measure danger, to keep them working. We recovered stations from the seabed. We extracted the data then reconditioned them, so they'll work for another three to six months. Next, we have many operations, using the submersible you can see behind us, which has a multibeam echosounder, which can scan the seabed very clearly.

Show us. Where is it exactly? The submersible with its echosounder? That's it? OK.

The yellow thing.

And from there, it's launched into the water?

Yes. The gantry lifts it above the bridge, and it tilts above the water, and the winch lowers it into the water. The submersible, which is an AUV, is released from its cage and dives up to a depth of 3000m, usually staying 30 or so metres from the floor, to measure from close up.

So no one goes inside the submersible? It's operated remotely?

Yes, it's unoccupied, autonomous, with batteries that last up to 12 hours, and we stay connected with it remotely.

Right. Isabelle, you had a piece of rock. Can we see it again? It's pretty impressive, a big chunk. What is this volcanic rock made of?

Volcanic rock, and this is a typical specimen, is made of elements... This is volcanic glass... I can't really show you, it's on top... Lava freezes in water, and it vitrifies. A vitrified layer. And here we have some crystals, invisible to the naked eye. We use a microscope.

Right. So all the rocks you collect are brought to the lab to be studied. How much longer will you be there?

We'll stay in the Mayotte area until... the 22nd or 23rd of October. We return to Reunion on the 26th of October.

So you have good days ahead on this boat. There are many of you. It's a team effort. Many people are on board. Who's in the team?

There's the crew, of course. They're very important. We couldn't do anything without them. The technical team of engineers and technicians, who help with the instruments and the Internet connection. Then there's the scientific team. We couldn't do without them either. This expedition is part of... or rather operated by REVOSIMA, the network...

The Mayotte Volcanological and Seismological Monitoring Network. So it's a TAAFS ship, operated by the shipowner Louis Dreyfus and chartered by Ifremer. On board, we have scientists from various institutes, as well as BRGM and Ifremer.

Excellent. Thank you for sharing the discovery of the volcano. Good luck with your mission.

2020 Festival of Science Ambassadors

Presentations by the 2020 Festival of Science ambassadors from BRGM, Gonéri Le Cozannet and from Ifremer, Elodie Fleury. Learn more about climate change and the rise in sea level.

© L'Esprit Sorcier


Ifremer and BRGM together. Nice to see you. Ladies first for the introductions, if you don't mind, Gonéri. Right. Let me introduce Elodie. You are a marine biologist at Ifremer, the French Institute for Ocean Science. You work in the invertebrate physiology lab, studying the effects of the environment and climate change on the physiology of bivalves, and in particular of...


Oysters, yes. As an ambassador for Ifremer, could you tell us about it in a few words?

Or a few figures. We have about 1,500 permanent staff, and five centres in 25 locations on the coast. We have a large oceanographic fleet and submarines.

How lucky!

Yes. We have several missions. Research, of course, but also expertise, innovation and public policy support at national level. And, as you can imagine, we work on the ocean and its biological resources. That's it in a nutshell.

Thank you. You have something in common. You both work on climate change issues. It must be fascinating. Gonéri le Cozannet, you're a researcher at BRGM, the French Geological Survey. You are working on the impact of sea-level rise on coastal erosion and also marine submersion. You're working on the 6th IPCC report, the Intergovernmental Panel on Climate Change. It will be published in 2021. We can't discuss it yet. This year, you're an ambassador too, for BRGM. Tell us more about it.

It's the Ifremer of the subsurface!

You dive into the earth?

Not quite. It's the French Geological Survey. We work on geology, in France and overseas. BRGM also works on subsurface risks, hydrogeology and the coastline. The coastline is sand, cliffs, mud. We need to know the subsurface to evaluate coastal risks.

Right. We won't say more about climate change for now. We have a question for our viewers and our team in Nantes. Clemence, over to you.

Everyone at home, ready for your first question? Team, I'll ask you. Melting ice cap raise sea levels. True or false? Think about it. You can take part at home. Fred, could our guest tell us more about rising sea levels?

Gonéri, tell us what's happened over the past thirty years. How much have sea levels risen?

Sea level remained stable for 6,000 years, the current interglacial period. The rise began in the 19th century. 1.4mm a year in the early 20th century, 2.5mm a year towards the end, 3.2mm since 1990 and 3.6mm since 2005. So the rate of sea level rise is accelerating.

It's definitely rising?

It's confirmed by observations, tide gauges and altimetry satellites.

No doubt about it. Now where's that answer?

Team, tell me. True or false?

It's true.

Are you sure?


Well, the right answer is "false". You can test it at home: if you melt an ice cube in a glass of water, you'll see that the water level doesn't rise. Right, Fred?

I think so. I have an expert to explain.

It was a trick question. Continental glaciers and ice caps on bedrock contribute to sea level rise, but floating sea ice doesn't.

Tell us more.

It floats. Yes.

So it doesn't raise the level. Let's recall. What makes the sea level rise? You mentioned melting glaciers, but there too...

There are two phenomena: the melting of mountain glaciers and continental ice caps, and ocean thermal expansion. So thermal expansion...

What is it?

The sea warms, expands, and the result is a rise in sea level.

Yes, because when things warm, they take more space.

The expansion is small but enough to raise sea levels. It contributes one-third of the change.

But is it the same in every ocean, since some spots are warmer than others?

There are variations depending on the region. The areas with most warming have the highest sea level rise. Then there's oceanic circulation. For example, the Pacific Ocean, the equatorial Pacific Ocean, around Indonesia and Micronesia, has seen dramatic increases. Over the past 50 years, the rise has been twice the global average.

We'll come back to rising sea levels later. Now, Elodie. Let's talk about oysters. Is it true that you've worked on them for 15 years, but never eat any?

No, I love them too much.

In your own way.


Exactly. You say the oyster absorbs global warming. What do you mean?

Like blotting paper. It's a sentinel of climate variability, for three simple reasons. The adult oyster is attached. It can't move. It can't escape from danger. It's also an ectothermic animal.


It can't control its body temperature. The ambient temperature influences its metabolism, so it's subject to climatic variations. It can't control them. The third point is... it filters a lot. An oyster filters four or five litres per hour.

Four or five litres of water?

Yes. Per day, excuse me. It filters everything around it.

Good and bad?

Yes. If there's pollution, it will filter... That's why we use oysters like laboratory mice, like mice in medical research, drosophila or other model organisms. We get results with them and extrapolate them to other species.