Why science needs philosophy (cont.), and why it matters, with examples from geology
No one, so far as I know, has any religious objection to the Periodic Table and the unifying concepts of chemistry. But some people do have religious objections to the geological record, and to the unifying concepts of geology, because these don’t agree with what the most learned men of their time and place wrote down some two and a half thousand years ago.1 And as I argued in my last post, such people will seize on real or imagined anomalies as evidence that the entire intellectual structure is unsound. By contrast, the scientist’s response to such anomalies is to regard them as a potential source of new knowledge, far more likely to extend the framework in a mature discipline than to destroy it.
Example 1: Superposition and overthrusts
It is more than 350 years since Steno (who eventually became a bishop) proposed that strata consisted of layers of rock laid down one on top of another, newest on top. We have known for over two hundred years that both the London and Paris basins are filled with relatively recent sediment, on top of marine deposits (chalk or limestone) that emerge in hills to the North and South, that these in turn rest on an older basement, and that the more recent sediments were laid down in layers. The familiar geological sequence, Precambrian upwards (click to enlarge), was established in something like its present form before 1860, by merging the overlapping but incomplete local rock columns, although it was not until the 20th Century that it was recognised that the Precambrian occupied far more of the Earth’s history than everything since that time.
One major disruption of the usual order occurs in the northwest of Scotland, where older rocks lie above younger along a 200 km front. The resulting confusion (the “Highlands Controversy“, fuller account here) was not resolved until the 1880s, with the recognition of what is now known as the Moine Thrust.
The Moine Thrust and others nearby were the first established examples of an overthrust, where compression has forced one layer of rock over the top of another, as in the diagram to the left. In time, no doubt, the uppermost yellow and brown layers will be eroded, leaving the orange directly above the much younger brown, to the confusion of geologists. The Moine Thrust belt provides extreme examples of this. Thus at the location illustrated below the upper rock is Precambrian gneiss (igneous rock that has undergone extensive change when buried at depth), while the well-bedded rock beneath it is Cambrian quartzite (compressed sandstone), at least a billion years younger.
R: The Glencoul Thrust at Aird da Loch, Assynt in Scotland, part of the Moine Thrust belt; the irregular grey mass of rock is formed of Archaean or Paleoproterozoic Lewisian gneisses thrust over well-bedded Cambrian quartzite. Click to enlarge.
The Moine Thrust region was described, and interpreted invoking thrust faults, between 1888 and 1907. At that time, there was no known mechanism to generate the relevant forces, but this illustrates the general thesis that historical science trumps physical science. We know what happened, even if we don’t know how it could have. On current thinking, the thrust zone was generated during the process that gave rise to the mountains of Northwest Scotland, when Baltica (now part of Eurasia) drove west into Laurentia (roughly, North America and Greenland), 425 – 400 million years ago. But when the overthrusts in this region were first recognised, plate tectonics was still decades in the future.
I would also mention here another famous overthrust; the Lewis Overthrust in Montana, where Precambrian limestone rests on top of Cretaceous shales. Chief Mountain (illustrated L; click to enlarge), one of North America’s most photographed, is a product of this thrust. where the upper layers of eastward-moving island arcs were forced over the existing continental crust. The mountain itsef what is known as a klippe; a surviving portion of the overthrust material, left isolated where neighbouring material has been eroded away.
Example 2: The Grand Canyon date discordancies
From Permian to Cambrian
One of the clearest displays of superposition is provided by the Grand Canyon. The strata there have been described many times, for example in Prothero’s Evolution: What the Fossils Say, in David Montgomery’s The Rocks Don’t Lie, as chief witness in Grand Canyon, Monument to an Ancient Earth, and at length in the beautiful Geological Society of America 2012 Special Paper collection, Grand Canyon Geology.
The Grand Canyon runs across the Colorado Plateau, which was lifted up as a block, without tilting, during the mountain-forming processes that gave rise to the Rockies. Thus the uppermost exposures give an unusually rich collection of more or less horizontal strata, including limestones, desert and marine sandstones, and shales, deposited in a range of environments and dating (with some gaps) from the Permian to the Cambrian (270 to 525 million years before present).
So far, everything is quite straightforward. We have unambiguous radiometric dates, a record of changing depositional environments, and fossils in the appropriate sequence.
The Great Unconformity and the Grand Canyon Supergroup
Throughout much of the Grand Canyon, there is what is known as the Great Unconformity, below which lie a complex and much distorted mixture of sedimentary and igneous rocks, known as the Vishnu Formation. The Vishnu formation rocks are 1,700 million years old or more (this date will be important later), so the gap between them and the Cambrian rocks correspond to more than twice the length of time between those Cambrian rocks, and the present. During that interval, the Vishnu rocks were buried to depths of 25 kilometres, before being forced upwards again during the formation of the continent of Laurentia, and then weathered down to sea level before deposition of the Cambrian rocks described above. The heat and pressure of burial has done much to transform the rocks, but we can still detect cross-bedding in the sandstones, and the accumulation of coarser material at the bottom of the separate layers.
Exposed at some places in the Grand Canyon, in between the Vishnu formation and the Cambrian rocks, we have a succession of rocks known collectively as the Grand Canyon Supergroup. These lie parallel to each other, but tilted relative to the more or less horizontal rocks that lie above them. The upper formations of this supergroup (Chuar group and Nankoweap formation) span from 740 to around 840 Mybp (Million years before present), and were laid down on the edge of a then continental shelf, not far from the equator (as shown by magnetic measurements on the rocks), with sea level rising and falling in response to a changing climate. They are rich in microfossils, and display features such as ripple marks, cross-bedding, and filled-in cracks caused by exposed mud layers drying out.
The lower rocks, known as the Unkar group, are a total of 2 km thick, and record river and shallow sea sediments, fed by mountain-forming during the assembly of the long lost supercontinent of Rodinia, and subsequent erosion. It spans a period from around 1200 to 1100 Mybp, and lies on top of older granites and schists. It too contains microfossils, and includes such features as mud cracks and ripple marks.
In between we have a layer of igneous rock (the Cárdenas basalt), and there lies the anomaly. Early estimates gave an age, based on potassium-argon (K/Ar) dating , around 800 Mybp. But rubidium-strontium dates were around 1100 Mybp, and the theory of radiometric dating tells us that dates obtained using different clocks should agree.
The ancillary hypotheses associated with this theory were spelt out over a century ago. Essentially, they are three in number. The first is that radioactive decay rates had not changed over time. Initially, this was an assumption, albeit a very plausible one (why, after all, should there have been a change?) But in 1928 George Gamow showed that radioactive decay was a quantum mechanical process, whose rate depended on the fundamental laws of physics and underlying values of physical constants. Had these been different in the past, so would the laws of physics and, especially, chemistry, and the composition of the rocks (if rocks had formed at all) would be completely different to what is observed. The second ancillary hypothesis, in the early days, was that none of the decay product was initially present. But since the 1940s, we have had methods for estimating initial amounts, using a non-radioactive isotope as a kind of internal calibration. The third ancillary hypothesis is that we are looking at a closed system. To these, as we shall see, we need to add a fourth; that we are agreed on what event the date refers to.
In the case of the Cárdenas basalt, the weakest ancillary hypothesis is that of a closed system. Argon, after all, is a gas that can readily escape, giving spuriously young apparent ages. Moreover, the argon age varied with the composition (and hence, perhaps, historical porosity) of the exact sample being examined. What was really needed, was a method to preferentially sample the least porous parts of the rock.
We now have such a method, argon-argon (Ar/Ar) dating combined with thermal desorption. The rock is bombarded with neutrons, which transforms a fraction (we measure what fraction, by including a reference sample) of the 40K originally present to 39Ar.2 We then heat the rock, measure the ratio of 39Ar to 40Ar in the evolved gas, and infer from this the 40K to 40Ar originally in the rock, which is what we need to know. The beauty of this method is that more prolonged heating is selective, both for the most tightly held 40Ar, and also for the 39Ar derived from 40K in that same part of the rock.
Applying this method to the Cárdenas basalt, we find that this ratio in the gas given off by gentle heating (i.e. from the regions where it is most loosely held) corresponds to an age of some 700 Mybp, but that incremental heating soon leads to a consistent age of 1104 Mybp, in excellent agreement with the strontium-rubidium.
We can then ask further interesting questions. Like what led to the loss of argon. Not heating, because that would have erased tracks made by radioactive decays, so presumably some kind of chemical alteration. What kind, and does this tell us anything about the rocks (now missing) that initially covered the basalts? These remain questions for further investigation.
There is another, seemingly much more dramatic, dating anomaly in the Grand Canyon region. To the north of the canyon lie a string of recently active volcanoes, K/Ar age at most a few million years. Yet lead/lead (Pb/Pb) ages3 are some 2.3 billion years, making the rock even older than the Vishnu formations. Why are geologists not worried by this? Because the two different dates refer to two completely different processes.
The K/Ar date refers to the eruption of the lava flow, which (in the simplest case) resets the clock by outgassing any argon present in the source rock. The Pb/Pb date refers to the separation from the convecting mantle of the semi-molten lithospheric material at the base of the crust, an ancient process and one that must have taken place even before the basement rocks were formed.
This distinction is clearly drawn in the original scientific literature, so much so that I wondered whether or not to regard the distinction as an anomaly at all. However, it is paraded as such in the creationist literature (see e.g. Grand Canyon, Monument to Catastrophe, and many other creationist compilations). So the appearance of anomaly has been created by suppression of actual scientific content. Unfortunately, there are many examples of this kind of thing in the creationist literature; for a particularly egregious case, see the Genesis Flood discussion of the Lewis Overthrust fault.4 However, I consider that they can be used as learning opportunities; see here.
Afterword: the nature of science
When I set out to write this essay, I was firmly of the opinion that there is no intrinsic difference between scientific knowledge, and the other knowledge that we have about the world. Now I am not so sure, and there are two reasons for this.
One reason is that in each of the examples I have given here an anomaly was resolved within the context of a prolonged, often multi-generational, research programme. Such evidence-guided persistence is rare outside science. The other is the degree of inter-connectedness between topics. The reinvestigation of Prout’s conjecture led directly to the discovery of argon. But where did all that argon come from? From radioactive decay of potassium, and subsequent escape from the rocks, the kind of escape that gave rise to the apparent dating anomalies in the Grand Canyon. We can use radiometric dating to quantify the geological timescale, but the results can only be reconciled with the general order derived from superposition, if we recognise that under certain special circumstances this order will be disrupted by overthrusts. Or, to take two further examples, our understanding of evolution is linked to our knowledge of the geological record, and our concerns about global warming arose directly from the analysis of Ice Age climate feedbacks.
1] I have as little patience with those who dismiss Genesis as the work of ignorant bronze age goatherds as I do with those who, equally unhistorically, regard it as the literal Word of God, unconstrained by the limits of contemporary understanding.
2] 39K, the major isotope, is struck by a neutron and ejects a proton, giving 39Ar. A co-irradiated sample of known age is used as calibrant.
3] This depends on the ratio of 206Pb (from the decay of 238U) to 207Pb (from 235U), with 204Pb as non-radiogenic referent.
4] http://www.talkorigins.org/indexcc/CD/CD102_1.html; I quote with minor editorial additions:
Whitcomb and Morris, The Genesis Flood, 1961,, 187, Footnote) quoted a description of the Lewis Overthrust out of context to give the impression that rocks along the fault are undisturbed. They quoted Ross and Rezak (1959),
Most visitors, especially those who stay on the roads, get the impression that the Belt strata are undisturbed and lie almost as flat today as they did when deposited in the sea which vanished so many [million] years ago.
[The Genesis Flood is a foundational document for 20th-century Young Earth creationism. Henry Morris was a key figure in the Institute for Creation research untl his death in 2006, and was succeeded in its presidency by his son.]
Whitcomb and Morris silently omit the word “million,” and the actual passage in the original paper (Ross and Rezak 1959, 420) continues directly without interruption:
Actually, they are folded, and in certain zones they are intensely so. From points on and near the trails in the park it is possible to observe places where the beds of the Belt series, as revealed in outcrops on ridges, cliffs, and canyon walls, are folded and crumpled almost as intricately as the soft younger strata in the mountains south of the park and in the Great Plains adjoining the park to the east.
Geological record cartoon retrieved via The Oldspeak Journal; would happily acknowledge original provenance. Overthrust diagram, Myrna Martin in Kids Fun Science. Aird da Loch image, Andrew (ARG_Flickr on Flickr) via Wikipedia. Chief Mountan, public domain (US National Parks Service ), via Wikipedia. Grand Canyon stratigraphy, US National Parks Service, public domain, via Wikipedia.
I thank Ken Wolgemuth, Michael Roberts, Martin Rudwick, and Mike Timmons for helpful correspondence. The responsibility, however, for the errors, faults, and unresolved anomalies that no doubt persist in this piece is entirely my own. An earlier version of this material appeared in 3 Quarks Daily
Posted on March 9, 2018, in Evolution, Fossil record, Science and tagged Argon-argon dating, Assynt, Cardenas basalt, Chief Mountain, Flood geology, geology, Grand Canyon, Isochron, Lewis Thrust, Moine Thrust, Over thrust, Radiometric dating, Rodinia, Superposition. Bookmark the permalink. 11 Comments.