Carol Stein (BS '78), Geophysicist

As an undergraduate following the transition of Caltech's original Seismology Laboratory located in a mansion in the San Rafael hills, to its current location, Carol Stein witnessed seismology in its early modern incarnation on campus. She remembers working with Hiroo Kanamori and learning the fundamentals of earthquake measurement, and she had opportunity, like all Caltech undergraduates, to conduct original research—all one needs is the interest and the initiative.

Following Caltech, Stein completed her PhD at Columbia. Her research has always pursued basic questions in plate tectonics, and specifically, the thermal and mechanical evolution of the lithosphere. This research has focused on heat flow at the sea floor, and she has studied differences in the heat flow at the transition of plates located near Iceland. Stein has also examined the mid-Continental Rift System, which she has presented in video form as a "billion year old story," and her TicoFlux research near Costa Rica has yielded important insights on the thermal state of oceanic crust.

Stein is a fellow of the Geological Society of America and she has served in several leadership positions at the American Geophysical Union.

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It's Monday, April 18, 2022. I am delighted to be here with Professor Carol Stein. Carol, great to be with you. Thank you for joining me today.

CAROL STEIN: Thank you for inviting me.

ZIERLER: To start, would you tell me your title and affiliation?

STEIN: I'm a Professor in the Department of Earth & Environmental Sciences at the University of Illinois Chicago.

ZIERLER: What is the best term to describe your field of research? There are seismology, plate tectonics, geophysics. Is there one that works better than the others?

STEIN: I guess I would choose plate tectonics slightly over geophysics. But that general area.

ZIERLER: And what do you consider a subset of what? In other words, what is the umbrella discipline, and then what are the levels of specificity under that as they relate to what you work on?

STEIN: I think plate tectonics would be the top one because there are so many different techniques you can use to study plate tectonics.

ZIERLER: Are you more on the theoretical or observational side?

STEIN: More the observational.

ZIERLER: Are there any theories in particular that are important for your work that provide guidance for data collection and analysis?

STEIN: I work in the context of plate tectonics, and the Wilson cycle of the evolution of continents and oceans. My first approach is usually to go out and get a bunch of data, then try to understand what the data are telling us.

ZIERLER: What are the instruments you use that are important for capturing this data?

STEIN: In the first half of my career, I was looking at how the oceanic lithosphere thermally evolves when it first is formed at mid-ocean ridges, then moves away as it ages and cools. I was mainly looking at oceanic seafloor depth and heat flow data. Now, most of the data I've been using to try to understand continental rifting and passive margins, formed after the continent rifts and successfully starts seafloor spreading, are a combination of seismic reflection data and, to a lesser extent, gravity and magnetics.

ZIERLER: For you, when is it important to do field research, to go out there and see for yourself, and where is remote data collection perfectly sufficient for the kinds of questions you're after?

STEIN: If I have an interesting problem and I can convince the National Science Foundation to fund me to collect new data, then I go out and collect new data. Most of the time, it's looking at preexisting data.

ZIERLER: The default for you is, you want to get out there if you can find the funds to do it?

STEIN: Yeah. Unfortunately, for the kinds of problems I'm interested in, I can't just pick up and do it on a shoestring.

ZIERLER: I wonder if you can explain. What is the value exactly of being there on hand?

STEIN: If you can be there, then you can design your own experiment to gather data to fill in the knowledge gaps, to get the data in the areas that you think are critical for understanding your feature. Certainly, if it's on land, and you can get there and see what's going on, then even if you can't do experiments or make new measurements, you have a better feel for the feature and what some of the issues and problems are, and why there are problems trying to interpret certain features. Otherwise, there's been a lot of data collected over the years, and a lot of sites and issues have not been reexamined in terms of what we know today about how geological processes work compared to the 1970s and early 80s when plate tectonics was being developed. It's a chance to look at older problems with a new eye.

ZIERLER: 40, 50 years out, do you consider plate tectonics to be a mature field?

STEIN: That's a tricky issue, and no one wants to say so. Certainly, one could argue that mineralogy is a mature field because we know most of the minerals that are important but they're developing new ideas, new ways of looking at mineralogical processes, new questions, especially for environmental issues. I think for plate tectonics, in most places around the world, we sort of understand to the first order what's happened. I think that there are still a lot of interesting questions. I don't think we really understand details of how it works. But it certainly is a more mature field than 30, 40 years ago in the sense that we are confident of the basic model and have answered a lot of questions.

Plate Boundaries and Plate Motion

ZIERLER: One way to get at the question of maturity is, what do you consider settled science now? What is no longer debated that might've been a big question in the 70s?

STEIN: Two big issues are the plate boundaries and the motion of plates. In the 70s, all we could do was determine three-million-year averages from the magnetic anomalies on the seafloor, earthquakes, motion directions, and transform faults. And we could only outline the big plates, which there were about 14. Now, we're probably somewhere around 100. And we didn't really understand the difference between what was going on a plate boundary that we thought was a really narrow, 100-kilometer feature. I think with GPS today and overall better understanding of how deformation is distributed when we have plates, for example, colliding, we're starting to separate out this issue of inter-plate, what's happening within the plate boundary zone, and intra-plate, in the middle of a big plate. We now, with GPS especially, can start sorting out what's really a motion of a smaller area that's acting like a rigid plate. Another big issue is how continents break apart to form new ocean basins.

ZIERLER: To flip the question around, what are some of the biggest ongoing debates in the field, despite all of the advances over the past 40, 50 years, that remain?

STEIN: I think the biggest issue, really, is how the plates move. At least to me, it's kind of what really is going on, the interaction between the rigid plate above and the asthenosphere, the more fluid-like material below. Of course, how to forecast when the earth finally fails, and you get the earthquake is unknown. It doesn't help the average person, but we've made huge progress understanding why earthquakes occur and where they do, which was a big mystery, certainly, before plate tectonics. But I think one of the big issues is just trying to figure out when earthquakes will occur, and it requires better forecasting. There, I don't think we've made much progress, other than we know there have been earthquakes before. However, we've made a lot of progress with the paleoseismic record, when these earthquakes have occurred in the past, but it's clearly a very irregular process.

ZIERLER: On the question of helping the average person, do you feel that your research contributes to that broad debate about whether earthquakes can even be predicted? In other words, not it being a limitation of our science, but even understanding if the earth itself knows when an earthquake is going to happen?

STEIN: Those are good questions. No, unfortunately, I don't think my research is anything to do with that. And we may be debating it 100 years from now.

ZIERLER: Just as a snapshot in time, what are you currently working on?

STEIN: I'm currently working on passive continental margins, margins that have rifted when continents split to form new oceans. One of the interesting questions is that, for about half of them, they're very volcanic-rich. Under all the sediment that's accumulated after the rifting, there's often a very thick accumulation of basalt. Some of the questions are, how much has come out before seafloor spreading starts, how thick is it and how the volume relates to the kinematics of rifting.

ZIERLER: Now, when you talk about rifting and margins, I wonder if you can translate those technical terms for me. What are you referring to?

STEIN: The land that we stand on, continents, moves around a lot. It's not just that we get more frequent flyer miles between us and Europe because of seafloor spreading and new material coming up and widening the Atlantic Ocean, but that continents sometimes collide with others, like India's colliding with Asia and also rift apart. We see relatively young rifting today. For example, that the Arabian Peninsula is moving away from the rest of the African continent along the Red Sea and the Gulf of Aden. It probably starting about 30 million years ago when the Arabian Peninsula started to stretch away from Africa. As it did, the continental crust between what's eventually these two fragments got thinner and the block of Arabia successfully broke off, rifted apart from Africa. Now we have two passive margins on either side of the Red Sea and the Gulf of Aden. In most parts, seafloor spreading has started.

ZIERLER: Methodologically or intellectually, how do you separate out your studies of seafloor plate tectonics and continental plate tectonics? What are those divisions that are relevant for you? In other words, are they totally separate fields of study? Or where are the connections that you see?

STEIN: I think that a lot of the break has to do with where you're looking at the data. There are loads of geologists who are mainly on land, going out to areas and looking at them. There are marine geophysicists, and that's kind of where I started in my career in grad school, who are actually looking at the data from sea. Relating the geological history and rocks on land to that in the adjacent oceans can be difficult. Hopefully, we'll get enough data from both to try to connect the two.

ZIERLER: What would it take to get there? What's the best case scenario?

STEIN: I think a lot of it is, in the end, deep drilling of wells between on land and sea, then trying to connect the rocks on either side. Also, doing seismic reflection data, where you send out sound signals, they bounce off, and you try to map what you see on land to what you see in the margins underwater. It is a difficult one to try to link up.

ZIERLER: Either by accident or on purpose, has any of your research led to applications or translations in a commercial sense?

STEIN: Not that I'm aware of, unfortunately. [Laugh] It would be nice, but no.

ZIERLER: What about computers? All of the growth in computational power over the decades, how has that changed your research or not?

STEIN: I use a lot of seismic data from my current project and have used it before. The computer power now makes it easy to handle a lot of seismic data, and to be able to image what we're seeing at depth much better. The ability to handle large datasets with computers has greatly changed our ability to understand what we're observing. And I think that in general, computational power has gotten much easier. When I started out, if you wanted a computer, you either bought an expensive little desktop computer that was mainly for drawing simple diagrams and editing manuscripts, or you went to the big computer center. Obviously, there's a lot more power in our phones than we had back then. For me, other than the handling of datasets that others have largely put together, I have not gone into the computational aspect of our field. But it has changed the way we can look at problems and do much better data analysis.

ZIERLER: A service question as it relates to the main scientific societies you've been involved in. Where is the overlap between the Geological Society of America and the American Geophysical Union?

STEIN: When I started out in this field, GSA was considered sort of the old stodgy one, hard to break in, and the one that most people went to, a place where a lot of the early plate tectonics papers were actually published. You had to submit your abstract months in advance, and it may or may not be accepted. At least, the rumor was that there was a 50% acceptance rate. AGU was so much easier. It was a smaller group at the time for the meetings, but it was so much simpler submitting it, and you knew that if you submitted it, you'd be accepted, unless what you did was completely unscientific. But for any grad student, that wasn't an issue. The meetings were small, more intimate, and then, the management of AGU tried to get into every field, including what GSA was doing. Back when I started, AGU was about geophysics. Since then AGU has expanded to cover all of the geosciences and solid-earth geophysics has become a minor component. As a result, AGU is much bigger with a much larger professional staff and budget and is rooted in the Washington D.C. culture. As a result I've become increasingly involved with GSA, which now is a much more personal-scaled organization where one feels less anonymous.

ZIERLER: To set the stage, going back to your time at Caltech, when you were in high school, were you specifically interested in geophysics and seismology?

STEIN: Sort of. I had sort of a little unusual path in that my older brother had already gotten interested in geophysics and was at college. Being the little sister six grades back, it was kind of a natural thing to be interested in, read about, and want to follow in the footsteps.

ZIERLER: Were you attracted to Caltech because you were aware of their reputation with the Seismo Lab?

STEIN: Although I knew about the Seismo Lab, that really wasn't my interest when I went there. I was really interested in astronomy and wanted to go in that direction. When I got to Caltech, I realized that at that time to succeed in astronomy you had to build a better widget for the telescope. And that wasn't my strength. As a Caltech freshman, you had to take a certain number of so-called lab courses as part of your distribution, and one of them I took was GE 1 with Hiroo Kanamori, a seismologist. That's how I ended up switching my interest to geophysics.

ZIERLER: You entered Caltech in 1974?

STEIN: Yes.

ZIERLER: What was the class that Hiroo was teaching? Do you remember?

STEIN: GE or Geology 1.

ZIERLER: This is just an intro to the field?

STEIN: A number of different faculty have taught GE 1 over the years. Like a lot of these courses, what you taught boiled down to what you were interested in. If you had a geologist, it would be more geology-oriented, a geochemist, more chemistry-oriented. It was sort of an introduction to geophysics course, actually. [Laugh] I really liked it. It was a lot of fun.

ZIERLER: Do you remember what Hiroo was working on at that point? Or more broadly, as an undergraduate, would you have a window into that kind of thing?

STEIN: I actually worked for about a year doing some data analysis for him, looking at earthquakes and measuring magnitude of earthquakes from seismic records. At that time, I think it was using surface waves to try to improve earthquake focal mechanisms, which is a fancy word for how the fault slipped in an earthquake, using seismic waves to figure out the sense of motion in that earthquake.

ZIERLER: As an undergraduate, was your sense that, at the Seismo Lab, there was opportunity to do real research on your own? Or were you mostly shadowing graduate students, that kind of a thing?

STEIN: I think that any undergrad who wanted to get involved with research could. I'm not sure at the time there were many geophysics undergraduate majors who were doing that much research. And the Seismo Lab was an interesting place because I think everyone except for maybe one faculty member was really earthquake-oriented.

ZIERLER: How many women were in the program as an undergraduate when you were there, roughly speaking? Would you be the only woman in most classes, or was it not that bad?

STEIN: My first class I walked into was a history class. At most colleges, they have these little writing classes or smaller seminar-type classes. This was a history class, and the professor looked at me and said my name, and I was the only girl out of 15. It was 10% women in my entering class. [Laugh] The undergraduate program had only become coed a short time ago, and I was in the fifth class that had accepted women. But after a while, you just kind of accepted it. It wasn't a big deal. In fact, I remember at lunch sometimes, some students who had been accepted and were thinking about going to Caltech as undergrads, what we would call pre-frosh would visit. A friend sitting next to me said, "Going to Caltech is sort of like being in a monastery." [Laugh] It wasn't the way I was thinking, but there were a few more women in geology. Not that many geophysics undergrads, but I think there were maybe three or four other women in some of the beginning geology classes.

ZIERLER: Were there any women faculty?

STEIN: Not in the Seismo Lab. I think the only female professor I had was actually in a humanities course. Part of it is that there had been sort of a wave of some women going into science, but maybe starting ten years before I went to school. But there were hardly any in earth sciences at the time.

ZIERLER: Now, you were in the academic generation after the move from the mansion. Was your sense, from talking to people who had been there longer, that it made the Seismo Lab less of an island? In other words, did it become more integrated with GPS and the campus generally as a result of moving into Mudd?

STEIN: I think moving into New Mudd made it a little bit more linked to the rest of campus. At the time, I really didn't have any feel for how much more connected they were to the rest of the division. What I really remember is that the building just had that new fresh scent. [Laugh] Everything was clean, it was big. They had huge offices and lots of room for grad students, even undergrads who wanted to work, they had nice little cubicles for them. And I think the faculty and the grad students were really enjoying the extra space. It was kind of like, "Wow."

ZIERLER: What were the big debates at the time? What were people really excited about?

STEIN: I don't really have that much of a sense. I think the big excitement was the new methods to analyze seismic waves from earthquakes recorded by seismometers. With new algorithms, seismic records of ground motion could be used to determine earthquake parameters such as the motion on the fault, the length of the fault that moved, and the time history of fault movement.

ZIERLER: Did you benefit from visiting scholars? Would be come to the Lab from elsewhere to present research, interact with the faculty there? Was that part of your experience?

STEIN: I did not interacted much with visiting scholars. A little bit, but not much. But what was really nice is that undergrads were encouraged to go to seminars. I remember, as a freshman who had just decided to change into geophysics, being very puzzled by these terms like Cretaceous and Jurassic, long before the Jurassic Park movie. I know now that a lot of these are basic geological terms. But it was really fun to be able to feel that you could just come in, and go to seminars, and hear what people were doing and saying.

ZIERLER: Was your sense that the Seismo Lab had proprietary instrumentation, or at least instruments that no one else had, at that time?

STEIN: That wasn't on my radar. They obviously had their own instruments and their own network for Southern California, but that wasn't something that I was involved with. I was very aware that when you had a big earthquake, they were always on TV or being called up. [Laugh] But that was not something that, as an undergrad, I wondered or worried about.

ZIERLER: What about the proprietary nature of data? In other words, today, we see that data is democratic, it's worldwide, it doesn't really matter where you are as long as you can gain access. Was the Seismo Lab at that point an archive of data that simply wasn't available elsewhere?

STEIN: I think some of the data I worked with was available elsewhere, but you had to know how to get it and how to go about asking.

ZIERLER: Did you stay on campus during the summers? Was that an opportunity for you to do more research?

STEIN: I did a little bit of research over the summer, yes.

Measuring Earthquake Magnitude

ZIERLER: What faculty did you work with? Who were mentors over your undergraduate experience?

STEIN: Hiroo Kanamori was my undergrad advisor.

ZIERLER: What was his style as an advisor? Would he give you problems to work on? Would he let you explore on your own? How did that work out?

STEIN: It was mainly that I was doing some data-gathering for him, measuring some material. I wasn't actually involved with a paper itself. It was just making some measurements that would then be useful ultimately for what he was doing.

ZIERLER: Which was what? What were the big questions he was after?

STEIN: I think the question he was after at the time was simply how different magnitude scales measure earthquake size. Initially, when people were looking at the size of earthquakes, they used a magnitude scale (like the Richter) based on a seismometer that was very sensitive to relatively short periods of the earth motion. As they had seismometers that could also look at longer wavelengths, it was realized as you got to the really big earthquakes, the Richter scale wasn't as good at representing the amount of energy as the surface waves that traveled mainly on the upper surface of the earth, the uppermost 500 kilometers, and had longer wavelengths. The really big earthquakes had more energy in these waves, so magnitude scales were developed to better representation of the amount of energy released. I think a lot of the interest in that was in which scale to use. In addition to the shorter-wavelength body wave energy that would come in or the later surface waves, Hiroo Kanamori was one of those working on something that's now called moment magnitude, based on trying to estimate the total energy, which is related to the amount of motion on the fault during the earthquake, whether it's a centimeter or ten meters, and the surface area of the fault that moved. How to represent the amount of energy allows you to start looking at the significance of the earthquakes and try to separate the magnitude-7s from the 8s and 6s.

ZIERLER: Was your sense that Hiroo was representing a particular school of thought, and was there an opposing school of thought at that point?

STEIN: That was not on my radar.

ZIERLER: Above your pay grade.

STEIN: No, it's not even that. It's just that as an undergrad, you're still taking most of the basic courses. And at least among the faculty that I had classes with, I never detected any difference of opinion.

ZIERLER: Do you have a specific memory or a-ha moment when you realized you were good at this, you liked it, and it was something that you might want to pursue professionally? Did that happen at a particular juncture for you?

STEIN: No, there wasn't any bright, shining flash or anything like that. It was kind of gradual.

ZIERLER: When did you start to think seriously about graduate programs?

STEIN: I don't recall. It was a good time and science was expanding, departments were expanding, and geophysics and plate tectonics, and the various aspects of studying them, were the big things. And of course, there was the possibility of going to oil companies or similar things. That never really occurred to me. I just kind of decided that the grad school path was one that I was going to take. There wasn't any great angst about it, I just assumed that was what I would do.

ZIERLER: What kind of advice did you receive about programs to apply to, and specifically whether or not to stay at Caltech for graduate school?

STEIN: I don't think I wanted to stay at Caltech. It was a great place, but I think I felt that I wanted to get more variety. I spoke to some of the grad students and other people and decided where to apply. I think a lot of undergraduate students, even today, ask a few people, and decide. You don't necessarily do a great search on it. At the time, since everything had really been oriented toward seismology, that was what I was interested in. I didn't stay in seismology in grad school. [Laugh] But that's what my focus was for looking around at places.

ZIERLER: Ultimately, why Columbia? What was attractive about Columbia?

STEIN: I think of the places that I looked at, I just liked the friendly environment, and it seemed to have a lot to offer in a lot of different research areas.

ZIERLER: Going in, what was the game plan? What did you think you were going to be focusing on?

STEIN: I thought I'd be a seismologist. I liked the people in the seismology group, that wasn't the issue. But I realized that my strength wasn't working on theoretical seismology. They also had a big program with a network of seismometers near Lamont, but a number of graduate students were working on the project. However, throughout Lamont there were many other people doing research on other aspects of plate tectonics and I became intrigued with marine geophysics. I ended up switching to marine heat flow.

ZIERLER: Was it Lamont-Doherty that you were originally?

STEIN: Yeah, it was then Lamont-Doherty Geological Observatory, but I moved up the hill from the seismology department to the Marine Geology and Geophysics group.

ZIERLER: What was the point of entree? Was it a professor, a class? What caused you to switch?

STEIN: I think the big thing was that at Caltech, it was almost exclusively seismology. Looking at the other tools you could use to understand plate tectonics was a real eye-opener.

ZIERLER: For people who don't understand these nuances, where it all sounds like it's the same field, how big of a transition was that for you?

STEIN: It didn't seem like much of a transition.

ZIERLER: It was just a different building, essentially.

STEIN: It was different problems we were looking at, but it was all trying to understand how the surface of the earth was moving around. And it was just a different tool and different data to understand the earth.

ZIERLER: Who ended up being your advisor?

STEIN: The main one was Roger Anderson.

ZIERLER: What was Roger known for?

STEIN: He had also started off in marine heat flow, and by the end of my grad student career, he'd actually gotten involved with the Ocean Drilling Program and was building up a group to do experiments in the drilled borehole. It was sort of like what an oil company would do on-land, but here it was at sea. For example, a goal was to understand the permeability of the rock down the hole, how easily fluid sent through the rocks, and other characteristics.

ZIERLER: What was your thesis research on? What did you ultimately focus on?

STEIN: I went out to the Central Indian Ocean, 1,000 kilometers south of India, and I was looking at an area that, at the time, was considered an inter-plate, that is within a plate deformation zone, where the oceanic crust had been deformed in these great folds. We now know it is a very diffuse plate boundary between India and Australia. The other half of my research was looking at an area off of Arabia, to figure out whether we were dealing with oceanic crust or thin continental crust and how the Gulf of Aden extensional spreading had extended out into the Indian Ocean.

ZIERLER: What was the vessel? What was it like conducting this research?

STEIN: We ended up going on a ship called the R/V (Research Vessel) Vema. The big thing for oceanic institutions was having a ship to go out and do research at sea. A major plate tectonic question was the age of the seafloor around the globe. Scientists have been able to determine this, in part, with information about the earth's magnetic field. The earth's magnetic field direction "randomly" reverses over time. While the average time is about 50,000 years or so, sometimes the inverval is shorter and sometimes much longer. As new oceanic crust forms it is "imprinted" with the direction of the then present field. Examining the magnetic anomalies (measured less the expected one) shows the reversal history of the earth's magnetic field. A magnetometer is an instrument that is used to measure the earth's magnetic field. Determining the seafloor's magnetic anomalies and knowing the earth's reversal history, one can figure out how old the oceanic crust was and how fast the oceans were spreading. We also gathered data about the seafloor depth. We used equipment to image the geologic structures below the seafloor using seismic reflection techniques. In addition, they would take a core, going down five meters at most, to understand the sediment and sedimentary history, and from that, you could get some other interesting aspects. There was also measuring gravity. To be in the game, you needed a ship. The Vema actually started out as a luxury sailing yacht that the folks at Lamont purchased and put an engine and the equipment on. It was sort of getting up in years is the kindest way I could say it. When I was on it although a huge amount of science had been done. I was actually on the third and fourth last science legs, each about a month long. The equipment was relatively old compared to other institutions and their ships. Before I went on the Vema, I took a month-long science cruise on another ship, just helping out with folks measuring heat flow. I remember they looked at all the new equipment in amazement. But it was a lot of fun, and we got a lot of good data, answered some questions but not others.

ZIERLER: Why a sailing ship? Why does it have to be a sailing ship?

STEIN: It didn't. That was the ship they could get that was big enough for their needs. I think it was relatively cheap. Of course, the sails were gone by the time I was there.

ZIERLER: Was this NSF-supported?

STEIN: Yes.

ZIERLER: What's the history of NSF supporting this subfield? Does it go back a long time?

STEIN: That's something you'd have to ask someone else. But I think certainly after World War II, and the start of the Cold War there was a realization that they needed to know more about the seafloor. With the development of the idea of plate tectonics, the only way to get the data to better understand what was going on was to go out to sea and make measurements.

ZIERLER: What were some of the conclusions of your thesis?

STEIN: One was that, at least in some places, there was somewhat higher heat flow than expected.. Another was that the earthquakes were showing us compression in the area, and that everything was still actively deforming. We took sediment cores and we asked others at Lamont–this was not our specialty–to date the oldest sediment in them. It indicated that the deformation had been going on for about eight million years. That date was something that was really useful, too.

ZIERLER: What were some of the bigger debates in the field at that point, and how might your thesis research have contributed to them?

STEIN: I think there was an overall debate as to what was causing the inter-plate deformation, what was going on with the crust. I don't think our results really solved anything, but we did show just how widespread these folds were over the huge length of area and, at some level, how the directions of the folds changed. Later on, those directions were used by others to look at the forces involved in the plates and to realize that the orientation of the folding and where it was located was, in fact, what one would expect with a lot of stress building up in the area due to the plate motions, the seafloor spreading pushing stuff north in the Indian Ocean and the compression from India colliding with Asia.

ZIERLER: After you defended, what opportunities were available to you?

STEIN: I was already involved with another earth scientist, and we had gotten married. I was very fortunate that there was a post-doc where my husband was working. Also, at the time, in 1984, a lot of universities were hiring geophysics people, really building up that aspect the way colleges now are building up and getting more environmental people. I was very lucky that relative to where I lived, two faculty jobs became available in geophysics in the fall after I'd gotten there. I applied for both, got offered both, but chose the one closest to my house [Laugh] We didn't have to go to the same faculty meetings.

ZIERLER: That's a great way to resolve the two-body problem.

STEIN: Absolutely, we were very pleased.

Heat Flow Measurements and Superswells

ZIERLER: Did you take on new research as a result of your faculty appointment? Or were you looking to continue what you had done previously?

STEIN: I carried over some of the work I'd been doing as a post-doc, and I largely continued using heat flow measurements that had been already made and were available in the database, to understand other problems. Initially, it was working largely in the same area. I did expand areas that I was using heat flow to try to understand what was going on. One of the big areas that I expanded into, and I don't know if you've heard of this, it's not trendy anymore, was called the South Pacific superswell. There's a shallower than typical area around Tahiti with lots of little islands, and a lot of vulcanism in what we call hot spot tracks like the Hawaiian islands. One of the big debates about five years or so after I graduated was what was really causing this area. At the time, there were two competing ideas about what was going on beneath the crust when you had a hot spot like Hawaii. One idea was that the area around Hawaii or these other features, like Iceland, was elevated because most of the area down to maybe about 100 kilometers had much higher temperatures. The other was that most of the uplift was due to dynamic forces of the mantle pushing up and raising the area. The problem was that when you looked at the elevation at Hawaii and the others where the hot spot had passed, the subsidence of these island chains could be viewed as either a thermal process like cooling as the oceanic lithosphere moves away from the ridge crest, and everything cools, or this dynamic push-up by forces, so as the crust moves away, everything kind of subsides down. I had attended a talk about this at AGU, proposing that the South Pacific Superswell area had been reheated such that the oceanic lithosphere, the strong part of the oceanic plate, had been thinned. I came away thinking, "No one's looked at the heat flow data for that area. Is it really hotter?" A colleague and I looked at the data, and found that it wasn't any hotter than similar age crust elsewhere. People asked how it compared to the theoretical cooling curve then used at the time giving the best reference curve for how heat flow should vary with age. We did that, and it looked sort of OK. Someone said, "That's great, but you've got to look at the depth and heat flow of the same age crust elsewhere to see how this area compares", so we did that. We concluded that the Superswell area's heat flow was not unusual. However, to explain the shallower seafloor depths, would require much higher temperatures, so we should've seen super high heat flow if it was a thermal process, So obviously a thermal process was not producing this anomalous area. This led me to look at global heat flow data with age and a new reference model that better matched the observations.

ZIERLER: You mentioned Iceland. When did you start to get interested in Iceland?

STEIN: That was really after I had done a lot of work on other hot spots, mainly in the Pacific and Indian Oceans. It was much later. [Laugh]

ZIERLER: What's unique about Iceland? Are there things that happen there where the rules simply don't apply elsewhere?

STEIN: There are very few places today where the mid-ocean ridge comes up above sea level,. One of the fascinating things about Iceland has been that others have put GPS stations on Iceland, which you can easily do on land, and determined the spreading rate. The spreading rate is the same as what the magnetic anomalies are showing in the ocean seafloor surrounding it. It's a really neat, fascinating place, besides just a great place to visit.

ZIERLER: More broadly, what have been some of the big takeaways for you in thermal evolution of the seafloor? What do we know now that we didn't when you started this work?

STEIN: I think a lot of the change has been understanding the seafloor at older ages, mainly about 80 million years to the oldest stuff at about 200 million years. When I started out, the assumption was that old seafloor of that age had been so affected by hot spots, thermal anomalies coming up, and vulcanism, that it wasn't really that useful to study. At younger ages, you've got a very rapid increases in depths, and it sort of slow, as you get to older stuff. But the assumption was that most of the old seafloor was hotter and shallower because of the hot spots. A lot of the work I and colleagues did showed that it wasn't hotter and shallower because of hot spots, but rather because people had used the wrong reference model. With our better reference model–and others have improved since what we've done–that you can explain the average heat flow and depths of old oceanic seafloor with the same model that explains the younger ages. It was, in some ways, a data issue. When the first reference model was done, it was a lot harder to get data from everywhere. The maps weren't as good, and because they thought so much seafloor was affected by hot spots, they mostly used depths for the old seafloor from areas that were really deep because they thought they were avoiding the volcanic features, especially areas with submerged volcanoes. This biased their model at a time when we didn't have much data from old seafloor because a lot of the old seafloor, the ages weren't known. This biased the model to deeper areas, for creating an artifact that 90% of the old seafloor had depths and heat flow that were way too shallow and heat flow too high to relative to that reference model. With more data, we showed that the misfit was a reference model issue, rather than a hot spot issue.

ZIERLER: Your work on hydrothermal circulation, is that separate from the broader effort in thermal evolution, or is that part of that study?

STEIN: It's part of it because I was interested in how much heat has been removed from the ocean seafloor. To answer that, you first have to have confidence in the reference model. To assess the amount of fluid flow, you need a reference model that works.

ZIERLER: I'm curious if your work on the mid-continental rift ever inspired broader thinking about just how that influenced human history.

STEIN: I'm not an anthropologist, but I think humans have only been in North America probably 20,000 years at most, maybe a little less. They weren't around 1.1 billion years when the Midcontinent rift formed. It's not like the East African rift currently extending in Africa, where there's a lot of work that examines how that feature influenced human evolution. The Midcontinent rift is only exposed in a relatively small area, in the shores of Lake Superior and going a little bit towards Minnesota. It's certainly true that the copper in the rift can be mined–and Native Americans did mine it–and also, the glaciers moving over the area carried the copper far south. That certainly had an impact on Native Americans societies. The other thing is that the shape of Lake Superior is controlled by the Midcontinent rift structure. The lake was an important area for fishing and transportation.

ZIERLER: I asked about the uniqueness of Iceland. What about Costa Rica? What have you learned from the Tico Flux Research Project?

STEIN: There is a really interesting heat flow discrepancy between parts of the oceanic crust there, with the crust more or less the same age. One area has super low heat flow and the other adjacent area has super high heat flow. These were found with relatively old equipment in the 1960s. But the first folks who went out and made marine heat flow knew what they were doing and got good data, even though it was old equipment and only a few temperature measurements into the seafloor. When we got out there, we found a sharp divide between the area that had very low heat flow for its age and the area that had essentially normal heat flow for its age. A lot of that was because of the seafloor irregularities from its formation. When formed, the cold area had a seafloor with many highs and lows. The hot area was relatively smooth when formed. With time, about 300 m of sediments covered the seafloor. Today, the cold area has a lot of seamounts that above the surrounding seafloor, so fluid flow could get out of it, especially where the sediments are relatively thin or the basalt is exposed. In the other area there are essentially no seamounts extending above the surrounding seafloor. This was not a tectonic issue, but an issue of what was controlling the water flow. Oceanic seafloor from both areas is subducting beneath Central America. Others found the depth of earthquakes in this subduction zone shows differences relative to the hot versus the cold areas subducting. We collected a lot of water samples from the areas where there were seamounts. It was an eye-opener to understanding that even in 25-million-year-old crust, you could have fairly vigorous circulation affecting the chemistry of the seawater going in, exchanging in the rocks, and coming out. And I think in subsequent cruises they later found microbial communities down there, although I don't know the details.

ZIERLER: On the teaching and mentorship side, for your department, is there a large graduate program? Or are you primarily focused on teaching undergraduates?

STEIN: We've got a program that's had between about 15 to 20 graduate students each year. Although, we teach a lot more undergraduates than that. When I started out, the graduate program was mainly masters students. I'm not sure how it works at a private institution, but at a state institution like Illinois, you can't just declare, "This is our PhD program, or this is this, this is that." You have to go through the boards to get approved. When UIC started in the 60s, we had a small department. Unfortunately, we still do. But early on we got a PhD program by working with the Department of Civil Engineering. We could put our PhD students through This program. About maybe 10 or 15 years after I got to UIC that we finally got our own PhD program. [Laugh] Now, we have more PhD students than masters. Most of our teaching is at the undergrad level, and it's a really diverse population. Many of the students are first-generation college students. It's a real challenge teaching them, not in a bad way, but because most of them are living at home, trying to balance family issues, and working to afford to be in school, and doing schoolwork. They spent so much time on these other issues that they hadn't even gone to the forest preserves in Chicago. It was a lot of fun really expanding their horizons about volcanoes, earthquakes, and via the little field trips we do locally.

ZIERLER: Bringing our conversation back to Caltech and the Seismo Lab, have you kept up with the Lab over the years? Is there an alumni association or anything like that?

STEIN: As far as I know, not for the Seismo Lab, and no, I haven't kept up. I still have my Seismo Lab cup.

ZIERLER: For the last part of our talk, a few retrospective questions, and we'll end looking to the future. I wonder if you can reflect on the opportunities you received being part of the Seismo Lab in terms of connections, in terms of a way of approaching problems. What were you able to achieve, do you think, as a result of being there for your undergraduate program?

STEIN: I think the biggest thing was the openness of people around me, faculty and grad students, to treat an undergrad as an equal, answer questions and see their excitement about science. And, even now, in most places, the number of geology majors is relatively small. You have a connection to not only your advisor but your other instructors that you don't have elsewhere, where you're taking classes with hundreds of other people. It was just kind of openness, ease of communication, and all questions being considered respectfully. [Laugh]

ZIERLER: The counterfactual question, had you stayed at the Seismo Lab for graduate school, do you think your field of research would've changed much differently than what it actually was?

STEIN: I don't think I ever would've gotten involved in marine geophysics. I think I would still be doing seismology.

ZIERLER: To go back to those big questions about debates and unanswered questions, where are you most confident that the things that are currently still not understood–what's the lowest-hanging fruit? In other words, what's the frontier for which there's real possibility for the coming generation of seismologists and geophysicists?

STEIN: I'm not sure I know how to answer that question relative to what you're asking. At least relative to what I do now, we still really don't understand a lot about how the Central US formed. We say that there were a series of continental collisions that have built up–and I'm talking about long before Pangaea, in the pre-Cambrian time before half a billion years ago–the central part of the US. I don't think we really know much about it, I don't think we know the structural weaknesses, I don't think we know very much what is under a kilometer of sediment, what the potential resources are, if it's cost-effective to drill down to get minerals and stuff. And I think that until money is forthcoming to really do a lot of high-quality seismic reflection experiments and drilling through the center of our country, we're still not going to know really how it formed over time.

ZIERLER: Last question, looking to the future, what are the most important research projects you haven't yet taken up?

STEIN: There are loads, but I'm too close to retirement. I'll leave them to the next generation.

ZIERLER: [Laugh] Well, I won't let you get off that easily. What should the next generation focus on, if not you?

STEIN: I think that it's really too hard to tell. I think they're going to have to define the problems. When I started grad school we thought continents just kind of split apart, and that was it. No one really thought much about how you got from a continent to seafloor spreading and the transition zone that remained. It was just, "There's this continental-ocean transition zone, and we'll leave it at that." Over the years, work has really shown how complicated it is. We know now about the different ways that area can be stretched, how it stretches, how it subsides. I think there will be new problems and challenges, and a lot won't be what we anticipate today.

ZIERLER: That's always a safe bet in science. Carol, it's been a great pleasure spending this time with you. I'm so glad we were able to do this. I'd like to thank you so much.

STEIN: Thank you.

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