A Conversation with Jacques Malavieille

Part of the Earth Inside and Out Curriculum Collection.

Dr. Jacques Malavieille is the research director at the Université de Montpellier-2, where he runs the Geophysics and Tectonics Laboratory. He studies the formation of mountains through fieldwork and by building models in sand to understand geological processes like folding and faulting. These processes, which occur over many millennia, are replicated by the sand model within a matter of minutes. “There is absolutely no substitute for actually seeing a dynamic process happen,” explains Dr. Heather Sloan, Exhibit Coordinator of the Hall of Planet Earth. “Unlike looking at static models, where how all of it works together may never dawn on you, people grasp these mechanisms intuitively when they actually see them at work.” One of Malavieille’s models is on display in the Hall of Planet Earth.

American Museum of Natural History: What is your field of study?

JM: I study tectonics, the formation of the Earth’s crust, particularly places where continental plates move together. I study mainly mountain building, so I’ve done fieldwork in Tibet, in the French Alps, and the Rocky Mountains in the western United States and Canada.

AMNH: Why did you become a geologist?

JM: I was born in France, in a small city in the Massif Central, which is part of a very old mountain belt, about three hundred million years old. As a small boy I loved to look around the mountains for fossils and rocks, so maybe that’s where my interest came from. Geology is an interesting science, because it combines many kinds of disciplines, physics, and mathematics, and also biology. Mountain building generally results from the interaction between two continental plates. In particular I’m studying compressional tectonics: what happens where two plates converge. To better understand what I have seen in the field, I need a simple device which reproduces the structures seen in nature—on a small scale, of course.

AMNH: How did you get the idea to make a model with sand?

JM: I wasn’t the first or the only one. Many people in different countries now consider models an important tool to study geology. The first model I know of was built during the eighteenth century, by a Scottish geologist who observed that rocks were folded and deformed. So he built a small box, filled it with clay, and pushed on the layers. The structures he obtained were similar to what he had observed in the field, so he demonstrated that rocks could be deformed by a significant force. At the time, nothing was known about plate tectonics and (the forces that cause crustal) movements, so it was an important discovery.

AMNH: So generally a scientist uses a model to simulate a natural feature?


JM: Yes, that’s generally the way it works. We call these analog models because we make a realistic model—an analog—of the natural object. We take care to use materials which are scaled relative to the behavior, to the time, to the size of the particular feature. For the model in the Museum I used mainly granular materials, sand and powders, because these materials are close in behavior to the rocks of the continental crust in nature at the estimated rate of deformation. For example, one centimeter of the model represented one kilometer of thickness of rocks, and one minute of running time about a hundred thousand years.

AMNH: How big is the actual model in the Museum?

JM: It is about two meters in length and maybe sixty centimeters high.

AMNH: And it represents what place on the Earth?

JM: It represents the continental crust and lithosphere, the rigid part of the Earth.

AMNH: A particular place, or the general process of mountain building?

JM: The process, because at the end of the experiment I need to have a simple structure which clearly explains what happens when two continents collide. It’s a good illustration for people who don’t know anything about geology.

Using computers to develop numerical models is another very popular approach. The risk here is that some people tend to use parameters which are far from the natural ones, which can have bad effects on science. You choose different parameters—the age of rocks, your measurements from the field, your observations, and so on—and run them through the computer. You must be critical of the result because it depends on what you put into the computer. Of course it’s a useful and interesting tool, too.

AMNH: How is it different with your model? You also choose the parameters.

JM: The main difference is that the analog model takes into account an important fact: natural processes are not really clean. Instabilities have an important role in geology, in generating faults for example, and in the numerical model it’s difficult to introduce instabilities. In the analog experiments, faults develop in materials because our natural materials mimic the behavior of rocks.

AMNH: So you have a box, you have your different grades of granular material, you have a motor. How do they come together?

JM: First, I establish special boundary conditions which imitate a subduction zone, where two continents collide and one plate goes down below the other. As you probably know, in nature, the continental crust is lighter than the oceanic lithosphere, so it cannot go down very deep. Instead it will deform, thicken, and shorten to form a mountain belt. One end of the box serves as a rigid backstop and the other is pulled at a rate that I determine. I set the motor at that rate to compress the contents of the box and simulate the actual process.

AMNH: What purposes have the models served?

JM: There are different ways to use these models. For example, through the two glass side walls, you can directly observe the dynamic of how faults and other geological features develop, the geometry of structures. Also, the model can be operated at different scales—the scale of a simple fault, for example, or the scale of an entire mountain belt—and it’s easy to change a parameter. This is important because it’s not always easy to decide which parameter is important. When you get an unexpected result, you have to think, why is it different? What induced the change? Probably a parameter that turned out to be more important than you thought. I discover a lot of interesting things this way.

We also tape and photograph all the models as they are run, because sometimes we have to go back to old ones using new information and compare results. For example, in the Museum you have a lot of fossils, and if you discover a new one you have to compare it to the ones you’ve previously collected.

AMNH: So there’s computer modeling, and there’s analog modeling, and then there are people out there making observations, measuring mountains and so on. How do those three areas complement each other?

JM: For me it’s very simple. I am first a field geologist: the most important thing is to make good observations and measurements in the field of the geological objects that you want to study. I also do physical modeling. I combine this with computer modeling. I combine approaches and compare the results with other people, even if it’s not in the same university.

Geology is not an exact science like mathematics. You must be curious, you must ask other people from different disciplines for their view of the object you are studying—which is the Earth, a very, very complex subject.

AMNH: What’s the one new thing that you want students to learn from seeing your physical model?

JM: From an outside view, geology seems static; we only have a snapshot of processes which may have taken thirty million years to achieve. The model establishes the relation between the geological time scale and the human one. Immediately the student can imagine that mountains are not fixed structures, that everything is moving on the Earth’s surface. It shows a dynamic process.

This is an excerpt from EARTH: INSIDE AND OUT, edited by Edmond A. Mathez, a publication of the New Press. © 2000 American Museum of Natural History.