Hélène Le Mével: To Understand How Magma Moves and Evolves in the Crust, Use Gravity
For centuries, scientists have used gravity to study the shape of our planet. But it wasn’t until recently that instrumentation caught up with them to precisely measure aspects of Earth such as the magmatic processes that lead to an eruption. DTM’s Hélène Le Mével, a volcanologist who studies how volcanoes deform, is one of the few scientists in the world using gravity data to understand what to expect before an eruption. The idea, she says, is to find patterns between different volcanoes and understand how much magma injection into the crust is needed to trigger an eruption. It’s the link between magma dynamics and—in the long run—prediction of eruptions.
In a prelude to her April 25 Neighborhood Lecture, Le Mével spoke about her work, the importance of gravity for volcanology, and the questions she is tackling through her theoretical and field work on magmatic systems. The interview was edited for length and clarity.
DTM: What can we learn from data on the shape and gravity of Earth?
Hélène Le Mével: It can tell us a lot about many different processes, but for Earth sciences it can tell us about what’s happening inside the Earth. For example, I’m interested in what’s happening in the crust, but some other people are interested in water, glacier, or mantle dynamics. Gravity anomalies happen at different scales, so if you have large anomalies you can look down really deep into mantle processes. With small anomalies, we can see shallower processes.
DTM: How exactly can gravity give us this information?
HLM: Basically, you measure the relative values of gravity from space or from the ground, and you can identify anomalies and model them in terms of changes in density underground. If you have a gravity low, it might indicate the presence of magma underground rather than rock. We have an idea of different densities for different materials, so that’s how we can get an idea of the processes happening underground: by measuring the values of gravity and seeing how they change with time.
DTM: Why is all of this work on gravity and magma so exciting to you?
HLM: In geophysics in general there are many different techniques to image magma reservoirs. Gravity is really unique because it’s the only method that can give you an indication of changes of mass and density. For example, something easier to imagine is if you have an ore deposit. That is very massive, so it has a very large density—much larger than the crust around it. If you go with your gravimeter to make a survey above these ore deposits, there can be a huge anomaly because of the higher density. If someone injects water underground, then more mass is added and a gravity anomaly would also arise.Hélène Le Mével testing her gravimeter during a DTM-led field campaign at Stromboli volcano in summer 2018. Photo courtesy Hélène Le Mével, DTM.
DTM: How exactly do you measure these anomalies?
HLM: There are different instruments. Here at DTM we have a relative gravimeter, or gravity meter. It’s a very, very precise device measuring gravity with a precision of 10-9 g . The general principle is that you have a very tiny mass attached to a spring, and when you go somewhere and the gravity is higher, the mass is attracted, so then the spring is elongated. This instrument is calibrated, so we can measure the difference in the length of the spring, how it elongates, and then relate it to the change in gravity. This mass and spring are kept in a vacuum so there is no friction, and they are kept at a constant temperature thanks to electronics. It’s a complex and very expensive instrument. Each one is around $120, 000.
DTM: What other methods and instruments do you use?
HLM: I use two different geodetic techniques in my research. I’m starting to do gravimetry (the study of the gravity field) and I’ve done it in the past, but my whole Ph.D. was on Interferometric Synthetic Aperture Radar (or InSAR) and GPS, so measuring how the ground deforms. Both are really powerful together. Gravity tells you about density and mass underground. The deformation tells you how the ground moves. Anywhere I go that I measure gravity, I also measure the precise positon with the GPS. The reason for that is that you need to know if the ground has moved because if it has moved up, then the gravity has changed. However, we are not interested in this change in gravity. We are only interested in knowing the change related to the source, so you measure it every time with very precise position.
DTM: How exactly do you use these pieces of information to infer things about the magmatic processes underground?
HLM: This is through modeling. I work to develop new ways of modeling the pressurization of the magma chamber at depth. The idea is to see how the crust responds when you inject more magma in it. This way I can calculate how the ground would move in relation to this process. I can compare to actual measurements that I take on the field or satellite data.
DTM: What does “imaging” actually mean?
HLM: Every piece of data we get is data at the surface. We only know what’s happening at the surface. For example, I get values in centimeters or millimeters of ground displacement and the direction it moves. But my models simulate changes in pressure at depth, and through equations you can relate the two of them. Using geodesy is not really an imaging technique. It’s imaging changes based on my surface data. I can tell where the change is in pressure and where that pressure change happened.
DTM: You work extensively with models. Can you tell us what are the most challenging aspects of using these models?
HLM: It’s choosing what process you want to model, or what physics to include. A model is a simplification of reality, so you cannot model all the physics or the chemistry that’s happening in a volcano. You have to choose what matters for your measurements. For me, I’m most interested in things that create gravity change or deformation change, but a geochemist might be interested in phase changes in minerals. The challenge is to balance how complicated you want to make the model (in terms of physics), and then if you want it to be used by other people. If I develop an extremely complicated model here and nobody else can run it—well, you also want it to be useful for other people and volcanoes.
DTM: What’s keeping you busy lately?
HLM: I’m doing a lot of work related to theoretical modeling. I’m working on a different modeling framework to model magma in the crust in different ways. What I’m doing is called poroelasticity. It’s used in hydrology usually, like how water flows in rocks. But I’m applying that for magma flowing through rocks. Using these types of models to understand the deformation of magma reservoir is a new application of this modeling concept.
I’m also really heavily focused on my upcoming project, which will start in the Pacific Island nation of Vanuatu next month. It’s going to take a lot of my time in the next three years. We are going to measure gravity at this very active volcano called Ambrym in Vanuatu. We are going to measure gravity in three different ways at this system. Gravity mapping is one type. Another type is microgravity changes. We are going to measure some sites and then go back the next year and see how they changed. These are very small changes to see where magma moves. The third type is continuous gravimetry. We’re going to leave the gravimeter by the crater or the lava lake and see how gravity changes. This is going to record continuously for two weeks, instead of just making one measurement. At the same time, we are going to measure volcanic gas, and DTM volcanologist Diana Roman is installing seismometers. We really are going to try to understand the system with different geophysical methods.A clear view of Ambrym Volcano in the Pacific Island of Vanuatu shows the active vents and lava lake. Credit: Roberto Molar Candanosa, DTM, with satellite image taken by NASA’s EO-1 satellite on August 9, 2013. Photo of lava lake at Ambrym’s Marum crater on December 1, 2014 by Claire Cousergue, via CC license.
DTM: Why that particular volcano?
HLM: This system hasn’t been studied by really any kind of geophysics. There have been some geochemistry and some gas studies. It’s very remote and very hard to get to. But it’s also a very active and very open system. And up to last December, there was an active persistent lava lake. There are only 7 of these on Earth. Ironically, the lava lake at Ambrym drained in December, when the volcano erupted and all the lava went to the east rift in a similar way to what happened in Hawaii last year.
This project is going to be very interesting because we are expecting to measure the magma coming back to the lava lake. The reason I’m into lava lakes is that you can really see the magma, so it’s like an open window directly linked to the reservoir. We can really measure gravity changes even just with video to see if the lake’s levels are changing and relate that to the change in gravity. The last thing about Ambrym is that it is one of the largest producers of volcanic gas (including sulfur dioxide and carbon dioxide) in the world. That’s important because since I’m measuring changes in density, if we measure the degassing occurring at the crater and I measure the changes in gravity underground, we can understand what’s happening between the magma in the reservoir and the degassing at the surface. It’s another type of data to constrain our models.
DTM: Now can you tell us why you are so into volcanoes?
HLM: I was always interested in volcanoes since I was a kid, but I don’t know why. My parents didn’t know why either. We have a long history of volcanology in France, and I watched a lot of documentaries about French volcanologists. One of the them is Haroun Tazieff. And then there was Maurice and Katia Krafft. I also traveled a lot, so I had seen a couple of volcanoes when I was young.
DTM: How many volcanoes have you been to, and do you have a favorite?
HLM: So in my work I tend to go back to the same volcano, but maybe I have been to a dozen. I really like Hawaii. I spent some time at the Hawaiian Volcano Observatory, and I really didn’t get tired of seeing the activity there.
DTM: Is there something in particular you wish people knew more about volcanoes?
HLM: Maybe that some people think there is a huge ocean of magma under the crust, but there isn’t. It’s mostly solid rock, and then sometimes it melts in different parts and evolves. But it’s never a huge amount of magma.
Also, I’m mostly interested in uplift—how a volcano inflates—and most volcanoes inflate and deflate at different moments. Most people think or associate uplift with impending eruption. But actually most studies show that inflation doesn’t always mean eruption, and we are still trying to understand how all that works. We don’t know everything yet. But yes: uplift does not equal impending eruption.