As a teenager in Toronto in the 1950s, Paul Hoffman would spend hours in the Royal Ontario Museum studying its collection of rocks and minerals. He became a passionate collector, trading rocks with friends and exploring abandoned mines in search of crystals.
During his freshman year at McMaster University in Hamilton, Ontario, Hoffman landed a summer job with the Ontario Department of Mines, which dispatched him on a four-month journey to map rocks in northern Ontario. It was 1961 and his first field season.
The medal citation begins, “Paul Hoffman is one of the most inspiring and charismatic geologists of our time, inspiring a wealth of new research fields but also a generation of younger geologists who have worked with him as research and postdoctoral students.”
Hoffman had prepared remarks for the occasion brimming with his characteristic humor and enthusiasm, thanking those who have supported and inspired him throughout his long career, including his parents, for making him play outside; Harvard University, for a second career in field geology and the opportunity to learn by teaching; his students; and his wife, Erica Westbrook, “for never asking why I spent 120-odd months feeding mosquitoes in northern Canada and dodging thorn bushes in southern Africa.”
Through a career that spans almost half a century, Hoffman has worked like a detective solving the mysteries of the Earth’s ancient past. He has traveled to almost every continent searching for clues, piecing together a puzzle that tells the surprising story of how the Earth and its climate changed during Precambrian times.
“It’s an amazing thing that by looking at piles of rocks in different places you can actually work out and share with others the amazing history of our planet’s development,” said Hoffman. “That, to me, provides the rationale and the reward for a lifetime of hard work.”
Hoffman, Sturgis Hooper Professor of Geology, Emeritus, is perhaps best known for his work on the “snowball Earth” theory. According to the theory, about 700 million years ago glaciers covered the Earth from the poles to the equator. It didn’t rain. There were no flowing rivers. Just ice. Ice on the oceans. Ice on the land. Volcanoes erupted from mountaintops, spewing carbon dioxide into the atmosphere, eventually setting off a sweltering greenhouse effect that thawed the world. And then, the world froze over again and the cycle repeated itself. The second thaw may have paved the way for the origins of multicellular life.
If snowball Earth sounds controversial, it is. But the theory has endured. It’s been 11 years since Hoffman and Harvard colleague Dan Schrag, the current Sturgis Hooper Professor of Geology and professor of Environmental Science and Engineering, first published “A Neoproterozoic Snowball Earth” in the journal Science. Since then, the theory has ignited animated debate, generated grants, and precipitated a blizzard of papers.
“The snowball earth work is the back end of a very impressive double-header,” said Harvard colleague Andy Knoll, Fisher Professor of Natural History and professor of Earth and planetary sciences, and author of “Life on a Young Planet: The First 3 Billion Years of Evolution on Earth.”
United Plates of America
InLink 1969, Hoffman began work as a research scientist for the Geological Survey of Canada. Plate tectonics was the exciting, new theory in Earth science. Plate tectonics were known to have formed mountains and oceans in the past 300 million years, but Earth is 4 billion years old.
“Plate tectonics was a revelatory idea because it made sense of all the great problems in geology that previously had no explanation – the origins of mountain ranges, volcanoes, great faults and so on,” said Hoffman. “From having worked in the Precambrian shield of Canada, which contains very old rocks, I was really driven to try to see whether the great concepts of plate tectonics could be applied to the older parts of Earth history.”
Hoffman’s exhaustive research for the Geological Survey of Canada culminated in an influential 1988 paper titled “United Plates of America.” He demonstrated that the same processes that occurred during more recent plate movements had also been occurring in the ancient past. Hoffman referred to the ongoing process as the “dance of the continents.”
“That early work showing how you could determine the workings of plate tectonics as much as 2 billion years ago was very influential on me and everybody else looking at early rocks,” said Knoll. “Paul documented the rifting stage, where ocean basins are opening, and then the drift stage, where you develop the continental stage, and then the closure stage when you have collisions and mountain building. It’s all there laid out in these 2 billion-year-old rocks. It was hugely important to the development of all work on early Earth history.”
A great field geologist
“United Plates of America” was the result of a quarter century of meticulous and insightful fieldwork. Francis Macdonald, Hoffman’s graduate student who was recently hired as an assistant professor by the Department of Earth and Planetary Sciences, spent several field seasons with Hoffman.
“A great field geologist,” said Macdonald, “requires great powers of observation, the experience to know what you are looking at, a great spatial and visual memory, and mental and physical stamina. It doesn’t hurt that he was a marathon runner.”
Hoffman has spent 47 seasons in the field. It is in the field that he seems to be most truly at home.
“Paul is about as much in his natural environment in Namibia as I’ve seen him anywhere,” observed Sara Pruss, a paleontologist from Smith College who spent last summer in Africa studying the fossils of microbes. “He is really comfortable and he emanates an absolute love for that country. He has such an enjoyment of the landscape, not just the rocks, but the birds and animals and plants.”
Photographs of Hoffman in the field clearly show a man at home in the landscape as he points out streaks of red iron in a rock formation, perches on a ledge below a glacial boulder, or brews coffee by a campfire. In his own photography, Hoffman captures patterns, colors, and angles of rocks that convey a sense of the Earth’s turbulent movements.
“It seems kind of ironic that I would go to subtropical Africa to study glacial deposits,” said Hoffman. “But there is very good evidence there that ice sheets existed very close to the equator. Around 600 or 700 million years ago, there seem to have been ice sheets on all of the continents and possibly the ocean as well. There was gathering evidence for extensive glaciation at that time and tantalizing evidence that this was very close to the time of the first appearance of multicellular animal life.”
Building the snowball theory
Glacial deposits in tropical and subtropical regions had puzzled scientists for years. A 1964 paper by W. Brian Harland presented solid evidence that explained a global ice age but most people didn’t take it seriously.
In the early 1960s, computers allowed physicists and mathematicians to churn out sophisticated climate simulations. Physicist Mikhail Budyko of the Leningrad Geophysical Observatory formulated models that measured how dark oceans absorb heat from the sun and warm the planet, while bright surfaces like ice reflect sunlight and cool the planet. Known as the albedo effect, Budyko’s model demonstrated an alarming feedback loop that could run out of control and trigger a global deep freeze. But no one, even Budyko, thought it could actually happen. Once the Earth had reached a tipping point, how could it recover?
Then, in 1992, Joe Kirschvink, a geologist at the California Institute of Technology, suggested a way out of the condition he dubbed “Snowball Earth.” Volcanoes would continue to erupt, emitting gases loaded with carbon dioxide into the Earth’s atmosphere. Carbon dioxide would accumulate to levels that would activate a greenhouse effect, warm the planet, melt the glaciers, and reverse the feedback loop.
The brief paper was largely ignored, but Kirschvink’s proposal excited Hoffman’s curiosity and he pursued it like a crystal in an abandoned mine.
“When I am confronted with a new idea or new theory I like to try to test it by accepting it tentatively to see where it takes me,” said Hoffman. “A good indication that a theory has merit is that it not only explains the thing that you originally thought or hoped it would, but that it starts to explain all sorts of other things as well.”
Through the 1990s, Hoffman spent six field seasons in Africa gathering evidence to support the snowball theory. Peter Huybers, oceanographer in the Department of Earth and Planetary Sciences, describes the patience and persistence with which Hoffman tackled the problem. “He spent years mapping in the field, trying to understand what the deposits are indicating without publishing any results, but quietly mulling over what he was observing and being aware of Kirschvink’s idea and being aware of Budyko.”
Hoffman also brought his extraordinary knowledge of plate movement to the theory. When Rodinia broke up over 700 million years ago, something unusual happened. The dance of the continents seemed to have been choreographed by Busby Berkeley: the plates circled the equator like a chorus line. This formation had a dramatic effect on the carbon cycle.
Rain turns carbon dioxide in the atmosphere into carbonic acid. The carbonic acid erodes rock, converting carbon dioxide into calcium carbonate, essentially burying the carbon dioxide, a greenhouse gas. Weathering occurs more rapidly at the equator than at higher latitudes. So, with all the continental surfaces exposed to maximum weathering, massive amounts of carbon dioxide were rapidly consumed through the process of erosion, causing the Earth’s temperatures to drop. Ice fields expanded. Budyko’s albedo effect kicked in and glaciers covered the Earth. Kirschvink’s volcanoes kept cranking out carbon dioxide. With weathering turned off by the freeze, carbon dioxide built up and eventually prompted a greenhouse effect and a thaw.
Piecing the puzzle together
One piece of geological evidence perplexed Hoffman – the presence of thick layers of carbonate rocks above the glacial deposits. These rocks contained surprising features such as huge crystal fans, tubular structures, and an unusual pattern in the ratio of carbon isotopes.
“I knew that somehow they were linked to the buildup of carbon dioxide,” said Hoffman. “But carbon dioxide is an acid. Normally, acidic conditions increase the solubility of calcium carbonate. You would expect that, under high carbon dioxide conditions, the calcium carbonates would dissolve rather than precipitate.”
Hoffman showed a draft of a paper he’d been working on to colleague Dan Schrag, who had a background in geochemistry. Documented in the book “Snowball Earth” by Gabrielle Walker, the two engaged in an intense period of creativity, spending late nights brainstorming, sharing research, and arguing.
Schrag suggested that under the high temperatures and highly acidic conditions after the greenhouse period began, weathering reactions would take place much faster than they normally do. He envisioned a scenario in which rising temperatures turned weathering back on with a fury. Violent rainstorms washed vast amounts of carbon dioxide out of the atmosphere and onto the rocks, eroding them like Alka-Seltzer and flooding the oceans with calcium carbonate. Under these conditions, the carbonates precipitated on the ocean floor rapidly and caused the strange crystals and tubular features to form.
The chemical reactions explained the cap carbonates and galvanized the pieces of evidence so that at last, Hoffman had a big picture theory.
“It was very exciting to have a theory that could explain for the first time so many different observations that previously had not had any explanation,” said Hoffman. “And so it’s like what Darwin once said in a letter to Asa Gray, ‘I can’t believe that a false hypothesis would explain so many different classes of facts.’”
To build the theory, Hoffman and Schrag used climate models created by Kenneth Caldeira of the Carnegie Institution for Science and James F. Kasting of Pennsylvania State University. Their work showed that the snowball period would have lasted tens of millions of years before the greenhouse effect would have kicked in. Climate models by Raymond Pierrehumbert of the University of Chicago estimated how the greenhouse effect would elevate temperatures to 50 degrees C (122 degrees F).
“What is so wonderful about Paul,” said Schrag, “is that although he wasn’t trained in atmospheric science, he realized that if you want to understand the Earth – and that is his passion – then you have to understand all the components.”
Hoffman’s passion for learning outside his discipline impressed those around him.
“At Harvard, Paul got into fields that may not be considered traditional geology,” said colleague Macdonald. “Fields like climate modeling and chemistry and oceanography. The further you move away from your expertise, the more you don’t know. To read papers in a field you’re not familiar with and really understand them takes longer.”
Almost every year before he retired in August 2008, Hoffman would choose a topic he wanted to learn about and then offer a course on it. That way, his enthusiasm and wonder were always genuine, and he shared the excitement of discovery with his students.
Rolling the snowball
In 1998, Dorian Abbot was in high school when he saw a television program about snowball Earth. He was hooked. A few years later, he decided to contact Hoffman. “I had been reading the papers and just e-mailed him out of the blue,” said Abbot. “He got excited and we met. He was very encouraging.” Today, Abbot creates computer simulations that model climate change as a postdoctoral fellow in Earth and planetary sciences at Harvard.
Sara Pruss is looking forward to returning to Africa with Hoffman. Like Abbot, she too admires Hoffman’s willingness to share his knowledge.
“One of the funny things I’ve seen in scientists is their protective nature,” said Pruss. “They don’t want people to visit their field sites. And Paul is the exact opposite. Paul brings everybody to his sites. He wants to show them exactly why he arrived at a conclusion and he wants to hear what other people have to say. If you can make a good argument for why you think what you think, then he will listen to you.”
Pruss, Hoffman, and an undergraduate from Smith will continue to study the fossils of microbial mats that formed after the first glaciation.
“What were they doing? What were they eating? What were they using to respire?” asks Pruss. “Certain microorganisms require oxygen. Is it possible that the things that were making our microbial mats didn’t require oxygen? That would put very different constraints on the types of environmental conditions that would have been around.”
Scientists are just beginning to discover how life survived and adapted during and after the glaciations. Recent chemical evidence shows that sponges, the most primitive multicellular animal, were present during the last of the big snowball glaciations. All these data add to the tantalizing evidence of multicellular life that has intrigued Hoffman from the beginning.
“I don’t think we have much of an idea of how a causal relationship might work,” said Hoffman. “But I would like to think that if this very tight relation in timing proves to be true, then the biggest impact of the snowball theory will be not so much on climate science but on evolutionary biology.”
Throughout an extraordinary career, Hoffman has made broad contributions to science by greatly expanding our understanding of plate tectonics, Precambrian geology, glaciation, the carbon cycle, climate change, and biology.
“He has been inspiring to me and others,” said Macdonald, “because he has shown the value of being a true scholar.”