Plate Tectonics: Are Modern Geologists Treading on Solid Ground?
Plate tectonics is the reigning theory of earth’s dynamic crust. Laymen may not realize that acceptance of plate tectonic theory came quite suddenly in the 1960s. It was like a revolution. For decades, the consensus of geological scientists was adamantly against the notion of shifting plates moving horizontally in various directions. Those who taught such things were considered on the fringe and were shunned at conferences. The consensus view, however, was quite rapidly subducted under heat and pressure till now it’s hard to find any academically trained geologist who does not accept at least some form of plate tectonics. Yet many details remain controversial. Though taught as fact in schools and on national park signs, how strong is the evidence? And how do geologists arrive at theories of the earth?
Alexandra Witze, American correspondent for Nature, traveled with a large group of geologists in Wyoming last month. They had gathered near the Wind River mountains for conferences and field trips to try to anchor the shifting consensus about plate tectonics theory. Though all are believers in the ruling paradigm, groups and individuals among them differ sharply on the details, particularly whether tectonic movements began early or late in earth’s history. Some feel it began 3.5 billion years ago, soon after the earth cooled. Others feel it began a billion years ago or even less.
Witze’s report in Nature1 puts a human face to scientific inquiry, as we follow a group of geologists working out their differences as both personalities and experts. Her diary allows us to tag along and see how science is done – before the textbook writers set the conclusions in stone. In discussing several stubborn controversies that continue to cause sharp disagreement, however, she unwittingly raises some larger questions about what geologists know, or can know.
Undoubtedly it was not Witze’s intention to suggest that any of these scientists doubt the general picture of earth history and its age, nor their ability to find answers and come to agreement. After all, plate tectonics has achieved the status of the “grand unified theory of geology” as she dubs it. But to the perceptive reader outside the guild, certain statements she makes stand out as quite startling. They hint that the theoretical ground on which modern geology stands is more social than solid. Historians of science know that geological science has already endured numerous revolutions since the 18th century. Who can tell whether today’s ideas are closer to “the” truth? Though the context deals primarily with the debate over an early vs late onset of plate tectonics, the following excerpts feel like tremors of deeper issues:
- Murmurs of earth: Witze follows the group as Kevin Chamberlain (U of Wyoming) calmly displays a rare kind of rock he claims is komatiite, 2.7 billion years old. “But as the other geologists chip off fresh layers and scrutinize them through hand lenses, murmurs of dissent start to grow,” she reports, saying “few are convinced” of his claim. The lesson: “The scene brings home the difficulties of trying to study the early Earth – there aren’t many old rocks to look at, and those that are around are often so altered, chemically and physically, as to be nearly indecipherable.”
- Solar system context: Why here and not out there?
On other Earth-like planets there’s no evidence for today’s plate tectonics. Planets do not have to work this way, and there was probably a time when this one didn’t. “You don’t just make a silicate planet and plate tectonics starts,” says Robert Stern, a geologist at the University of Texas, Dallas. “Something special has to happen.”
- Heart surgery: Invoking “something special” in earth history is a painful operation.
The nature of that special something cuts to the discipline’s philosophical heart. Since the early nineteenth century, geology has been ruled by the principle of uniformitarianism – that the planet operates on unchanging laws, and that the present can be used as a key to the past. But how can that approach hold up when a science from a world where plate tectonics explains more or less everything is applied to a world that may have lacked it? How can you understand ancient rocks when you do not know what processes put them there?
- Word games: To understand one another, experts need to talk the same language:
Scant and difficult-to-interpret evidence presents one set of problems; slippery definitions present another. Plate tectonics has lots of constituent parts. It’s not just a theory of how things move, but of how they are made and from what. For example, explanations for different sorts of volcanism in different settings also explain why the mineral make-up of continental crust and the crust beneath the oceans is so different.
Working out which attributes are essential to the theory, and which incidental, is not easy. The 65 attendees at the Wyoming conference came up with 18 different definitions of plate tectonics.
In fact, the only points of agreement in the definitions were that the plates are rigid, they move apart due to seafloor spreading, and they dive under one another at subduction zones. But that leads to another problem:
- Alternate explanations: “The problem is that Earth could display one or even two of these properties without necessarily having a system like that described by modern plate tectonics.” Polar ice floes, for instance, fit some of these characteristics. Witze found one point of agreement; most of the geologists considered subduction as the diagnostic process of plate tectonics.
- Collateral damage: A late onset of plate tectonics, as argued by Stern, would have had catastrophic effects on earth’s atmosphere and biosphere, such as extreme glaciations enveloping the earth – “It was a wild time of change,” says Stern. “The biosphere was out of control.” Yet those who disagree with his late-onset view have tectonic motions occurring for billions of years.
- Dramatic effects demand dramatic explanations. Stern claimed that those who need to explain “snowball earth” scenarios need a cause big enough, and that could be the onset of plate tectonics about a billion years ago. As support, he says that portions of ocean crust that looked mashed up, called ophiolites, and metamorphic rocks called blueschists, diagnostic of subduction, are rarely found earlier than a billion years according to standard dating methods. A critic, Alfred Kröner of Germany, disagrees, Witze points out. He thinks there are other markers pointing to plate tectonics over three billion years ago.
- Silent treatment: Apparently a conference like this a first – or at least rare.
The exchange of papers led to the Wyoming conference. “It was overdue,” says Kröner. “Nobody ever talks to one another.” In Wyoming, they did: palaeomagnetists clustered around a white board with field geologists; geophysicists sat down for a beer with geochemists.
Hopefully it was not to drown their sorrows. Though new friendships were struck and some altered their views, no strong consensus appeared to be forthcoming.
- Whoops, we were wrong: Witze tells about how one ophiolite from China was reported in 2001 as being 2.5 billion years old. “Now Guochun Zhao, of the University of Hong Kong, has re-dated those rocks, giving them an age of just 300 million years.” That’s an 830% difference. Some at the conference criticized the new date, but others found Zhao’s result convincing. With corrections and disputes that large, some outsiders may not feel comfortable that rock daters know what they are doing.
- Can’t get a date: Another dispute arose over claims that Australian zircons were 4.4 billion years old, as dated by hafnium ratios.
Simon Wilde of the Curtin University of Technology in Perth, Australia, isn’t so sure. “You have to be very careful with these rocks,” he says. Measuring one spot on a crystal, as opposed to another, can yield very different hafnium values that lead to very different interpretations, he says….
Such differences of interpretation make the problem of solving when plate tectonics began extremely difficult. In many cases, data can be interpreted in several completely different ways – all of which may seem valid.
A follow-up question arises at how such measurements can be calibrated, and whether all the interference factors are taken into account.
- Battle of the stories: Witze tells about contradictory explanations for the Pilbara formation in Australia. One Aussie geologist, Hugh Smithies, presented “seemingly convincing evidence” based on geochemical signatures that two sections of the formation date from different times, one from before the onset of plate tectonics, and one from after.
But then along came Julian Pearce of Cardiff University in Wales, who argued that each of the geochemical markers in the western Pilbara can be explained by other phenomena, such as magmas with an unusual amount of water in them, or crustal material from different places getting mixed up. The various researchers are hoping to settle the matter with a field trip.
Alas, “field trips don’t always resolve things,” Witze laments, telling how in the Wind River mountains, “the meeting attendees continued to argue about plate tectonics as they hiked from outcrop to outcrop.”
- Social consensus: By the end of the conference, most of the geologists tended to converge on the earlier date for the onset of plate tectonics, with Stern remaining a stern defender of the later date. “It’s not a simple question,” he maintained. Witze adds, “And on that, at least, others agree.” Then there’s Michael Brown (U of Maryland), who came up with a compromise solution. He suggested that plate tectonics may have started early but changed around the time of Stern’s favorite date. An even more complex idea was put forth by Paul Silver (Carnegie Institute) who thinks tectonics started and stopped several times during earth’s history. Apparently Stern would drink to that.
In summary, despite their disagreements, they all seemed to get along and enjoy the conference and the hikes in the mountains. This might make for an interesting case study in scientific sociology, but what about the true history of the earth? Witze ends with a disturbing side note to the art of consensus building:
An ‘intermittent approach’ would be a wonderful way to reconcile things – but it takes geology even further from the comforting realm of uniformitarianism, into a world where the most basic principles come and go in fits and starts.
Could Charles Lyell have attended, he might have shed a tear for the subduction of his comfort zone.
1Alexandra Witze, “Geology: The start of the world as we know it,” Nature 442, 128-131(13 July 2006) | doi:10.1038/442128a; Published online 12 July 2006.
2Mars, for instance, shows no sign of plate tectonics. As for non-uniformitarian “special somethings,” consider the case of Venus. Planetary scientists are convinced that 90% of its surface history has been obliterated by some unknown global catastrophe; see 08/16/2004 and 11/26/2003.
3On uniformitarianism, see 11/05/2003, 11/04/2003, 05/22/2003, 07/02/2002, 02/02/2004, and 06/10/2002
Alert readers will notice more than just surface disputes about the details of a scientific theory. We supplied lengthy quotes to give an adequate feel for what Alexandra Witze revealed: there is almost nothing that modern historical geology can stand on and claim is factually true. To stave off charges of quoting out of context, we provided ample disclaimers that we don’t intend to portray any of these geologists as doubting plate tectonics, uniformitarianism, billions of years, or any of the standard secular evolutionary theory of the earth.
The question in the title, was, however, are modern geologists on solid ground when they come up with theories about the early earth? They may believe they are, but they are not the ones to tell. They are too close to their craft, too much inside the guild, too familiar with reigning paradigms to break ranks very far and risk thinking boldly outside the box. They are too partial to the validity of their vocation to be able to fairly evaluate whether anything they claim they know they really do know.
This is not to suggest that currently observable processes and measurements are in serious doubt, such as the current rate of seafloor spreading, the current rate of continental separation, the chemistry of this or that crystal and its hafnium ratios, the strength and orientation of a magnetic signature in a rock in Utah, and the like. But as ideas are built (by humans) about how things got this way and when, observation and interpretation diverge, complexity increases, and assumptions crowd in. It’s no longer possible to have a single, simple answer that will explain everything to everyone’s satisfaction. Worse, the best ideas can never be adequately tested without a time machine and an observer.
Notice that there was not one piece of hard evidence they agreed on. Data do not speak for themselves; they must be interpreted by fallible humans. Interpreting geological data from supposed billions of years ago when there were no human observers is fraught with problems, both technical and philosophical. The less a process can be observed or repeated, the more assumptions must be made, and the more one’s world view determines what questions are interesting, and even what qualifies as evidence.
From outcrop to outcrop these geologists wandered (whether in Wyoming or Australia doesn’t matter), murmuring among themselves about how old this formation is and what the earth was doing at the time it was deposited. To a positivist or progressivist, this is wonderful. Scientists get together, swap ideas, share data, air their differences, and come to at least a partial consensus. This is how science is done. Whether a consensus has anything to do with the truth of what really happened in earth history is a completely separate question.
If you doubt it, look at what was taught as fact (or the “best theory” of the day – see best-in-field fallacy) through the centuries. Look at what the consensus of scientists was in 1800 compared to 1900, and 1900 compared to 2000. Current ideas of earth history are radically different than they were 100 or 200 years ago, and there is no reason to believe they won’t be radically different 100 years from now if science continues. If even the hard sciences of physics and chemistry have undergone complete overhauls since 1900 (relativity and quantum theory), how much more vulnerable are sciences where unobservable history must be inferred from evidence in the present?
Consensus science may provide a comfort zone to those in the guild, but they have no guarantee it is not the twilight zone. As was plate tectonics before the revolution in the 1960s, today’s fringe idea could become tomorrow’s orthodoxy. Wait a few more decades and the orthodoxy could switch back, or to some completely unforeseen new heresy. Most people don’t care as long as the national park diagrams look nice. If your kid reads one and asks, “but how do they know that?” you should respond, “That is a very good question.”*