Where rocks touch: geologic contacts
Another delightful posting from my friend Marli Miller.Thanks, Marli. I’ve blogged here earlier myself about the famous unconformity at Siccar Point, and the depositional contact at the Giants Causeway between a later lava flow, and the paleosol formed by weathering of the one before it.
Geologic contacts are the surfaces where two different rocks touch each other –where they make contact. And there are only three types: depositional, intrusive, or fault. Contacts are one of the basic concerns in field geology and in creating geologic maps –and geologic maps are critical to comprehending the geology of a given area. For those of you out there who already know this stuff, I’ll do my best to spice it up with some nice photos. For those of you who don’t? This post is for you!
Depositional contacts are those where a sedimentary or volcanic rock was deposited on an older rock (of any type). Intrusive contacts are those where igneous rocks intrude older rock (of any type). Fault contacts are… faults! –surfaces where two rocks of any type have moved into their current positions next to each other along a fault.
In a cross-sectional sketch they may…
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Roll over Nessie – dinosaur alive and well in Scottish Parliament
Post featured in this Sunday’s Herald, here
If you thought (like the books handed out to the children in Kirktonholme Primary) that dinosaurs were almost wiped out in the Flood and then used as beasts of burden until finished off by Nimrod the Mighty Hunter, I have news for you. We have a real live dinosaur sitting in the Scottish Parliament.
John Mason, MSP for Shettleston and a member of the governing Scottish National Party, has just tabled Motion S4M-12149, asking the Scottish Parliament to resolve that
… some people believe that God created the world in six days, some people believe that God created the world over a longer period of time and some people believe that the world came about without anyone creating it; considers that none of these positions can be proved or disproved by science and all are valid beliefs for people to hold, and further considers that children in Scotland’s schools should be aware of all of these different belief systems.
STV story here. Herald story here.
John Mason’s challenge to me and pretty well every other scientist on the planet: prove the world was not created in six days. Now here’s my challenge to Mr Mason: prove you are not a dinosaur.
What is the evidence that you are not a dinosaur? Why is it not a valid belief for people to hold that you really are a dinosaur? How does the evidence compare with the evidence against a 6-day creation from geology, physics, astronomy and cosmology, geography, and I haven’t even started on the fossil record or molecular biology. I have decided that I believe, as a matter of faith, that you really are a dinosaur, and I maintain that this is a valid belief for people to hold, and further consider that children in Scotland’s schools should be aware of this different belief system.
Off the top of my head, the evidence from geology presented by the geologist (later Bishop) Nicholas Steno in 1669, who established the science of stratigraphy. The evidence of Siccar Point, one of the most famous locations in Scotland, where rocks have been laid down, tilted, eroded, and covered by more rocks, as described over 200 years ago by the geologist (and theist) James Hutton. The evidence of the geological column, established during the 19th century by clergyman-geologists like William Buckland and Adam Sedgwick. Other evidence for lengthy processes between geological events, like the weathering at the Giants’ Causeway.
Physics: radiometric dating, first used by Rutherford and colleagues over 100 years ago, and checked and cross-checked many thousands of times since, using different isotope pairs. (And how do we know the clock runs true? Because, since the work of George Gamow in 1928, we have known that decay rates depend on fundamental physical constants, and if they had been different, so would the laws of physics and chemistry and we wouldn’t have the rocks anyway.)
Astronomy and cosmology: around 1920, the Jesuit priest Georges Lemaitre had worked out (although Edwin Hubble gets most of the public credit) that the further away galaxies are from us, the faster they are receding. So running the film backwards, we infer an initial Big Bang (or, as Lemaitre called it, “Primal atom”) where all the matter in our Universe was aggregated at a single point. And according to the best curreent estimate, that took place 13.8 billion years ago, with a further 9 billion years needed to get from there to the processes forming our own Solar System. Making 6 days wrong by a factor of around half a trillion.
How about helioseismology? We can observe vibrations spreading throughout the Sun, use the data to infer how much of the Sun’s original hydrogen has been converted to helium, and calculate how long this would have taken. The answer comes out at 4.5 billion years, which is also the age of the Solar System as inferred from radiometric dating.
As to how far we know the galaxies to be, we can work that out by looking for supernova explosions. Simplifying slightly [I have to say that, because if they find any detail out of place the Creationist shout “Gotcha!”] all supernova explosions of a certain type release the same amount of energy. And apparent energy depends on distance. And we know that the speed of light hasn’t changed, because we can make sense of the patterns of light emitted by the most distant stars, so the laws of physics that dictate the speed of light haven’t changed since the light left these galaxies.
So we know that we have seen galaxies 12 billion light-years away. Which means that they’ve been receding from us for 12 billion years. Not 6 days.
Geography: we have the facts of plate tectonics, as shown by the movement of continents (which we can now follow directly by satellite), and confirmed by palaeomagnetism, showing how the earth’s plates have moved relative to its magnetic field. The reasoning is so simple that a child can follow it, and in fact I’ve reviewed a children’s book that explains it.
And remember, we haven’t even started on the fossil record, or the life sciences.
Now the evidence that John Mason is not a dinosaur. Pretty thin by comparison. Comparative anatomy? If you accept that, you will also have to accept that comparative anatomy shows we’re chimps, with some twenty or so known extinct species more or less intermediate between us and our last common ancestor with the other two surviving chimp species. Physiology and molecular biology? Tush! Go with that, the same kind of evidence as we use in our law courts to establish family relationships, and you end up second cousin to a monkey and fourth cousin to a mushroom. Not what you have in mind.
Intelligent behaviour? On present showing, pit you against a velociraptor and my money’s on the ‘raptor, every time.
And so I think all Scottish schoolchildren, or at a very minimum all schoolchildren in Mr Mason’s constituency, be made aware of the theory that Mr Mason is an Intelligently Designed dinosaur. Otherwise we risk bringing them up with closed minds; minds closed against what we know from overwhelming evidence to be utter absurdities. And that would never do.
Time turned to stone, Part 2: The Giants’ Causeway; time as process
An earlier post here described Siccar Point, where an 80,000,000 year time gap is present between near-vertical tilted strata, and their roughly horizontal overlay. This gap corresponds to the formation and subsequent erosion of fold mountains thrown up when Iapetus, precursor to the modern North Atlantic, closed. Today’s post is (mainly) about the Giants’ Causeway, part of the enormous lava field first produced when the modern North Atlantic began to open, and still growing at the Mid-Atlantic Ridge and, most spectacularly, in Iceland. Fragments of the initial outpouring were separated as the Eurasian and North American plates moved away from each other, and now can be found as far apart as Greenland and Denmark.
The Antrim Lava Field, of which the Causeway is part, was formed in three separate phases each consisting of many individual episodes. The most spectacular feature of the Causeway is provided by the second of these. Here, the lava cooled slowly, to generate a solid layer which stressed as it cooled, finally fracturing to give a complex array of columns, up to 10 metres high, and showing in places an almost regular hexagonal pattern. The lava of this second phase shows subtle chemical differences from the first, evidence of changes in the hot lava plume feeding the outflow. But what most excited me at the site was the existence of a band around 5 metres thick, between these columns and the lava beneath them. This layer is not a sediment, but a palaeosol, an ancient soil formed by in situ weathering of the top of the lavas deposited in the first phase. Its nature is confirmed by the presence of occasional unweathered lumps, and there are occasional round scars (“Giants’ eyes”) in the exposed surface where these lumps have come away. Humid conditions are confirmed by the presence of valleys carved by streams, and filled in by the later lava flows. The chemical composition is like that of tropical soils, which have undergone extensive prolonged leeching under warm and wet conditions, with the most insoluble materials, iron and aluminium oxides, predominating towards the top, and there are traces of charred plant roots in the topmost layer. So here we have direct evidence of an extended interval, variously estimated at between 100,000 years and 3 million years, between the first and second phase of eruptions. After my visit, I discovered that this interbasaltic layer is found across the whole area of the Antrim Lava Field, and that there is another such layer between the middle and upper lavas. There are also extensive dikes, penetrating all the lower levels, caused by the eruption of the lava layers above. The entire coastline has been extensively reshaped and eroded over the intervening millions of years, and most dramatically during the Ice Ages, and subsequent exposure to the storms of the Atlantic. For more extensive descriptions, see here, p. 30, or here, and references therein.
Right: “The Chimneys,” columnar structures on skyline. Note additional columnar sructures to right and beneath.
Below: Interbasaltic layer beneath The Chimneys. Click and click again to magnify: note uneven upper contact surface between weathered layer and basalt, due to erosion of palaeosol, and holes (“giants’ eyes”) in the layer where incompletely weathered basalt chunks have been dislodged.
To summarise the sequence of events, we have
- The formation of the lower basalts in 11 separate episodes
- A pause of at least 100,000 years, during which the first interbasaltic layer formed by weathering, this weathering was accompanied by the erosion of stream valleys, and there were changes in chemical composition beneath the crust in the lavas feeding the eruptions
- The formation of the middle basalts
- Their slow cooling to give regular columns
- More weathering, to give the second interbasaltic layer
- Formation of the upper basalts, and finally
- Further gradual processes of weathering, erosion, and exposure.
One would expect any politician to be proud to have within his constituency so dramatic a statement of the Earth’s history. Not so. Northern Ireland Assembly member Mervyn Storey, who is currently Chair of the Northern Ireland Assembly Education Committee, is vice-chairman of the Caleb Foundation. This body rejects the whole of modern geology as well as evolutionary biology, and claimed credit (if that is the correct word) for temporarily persuading the National Trust exhibit on the formation of the Causeway to give Young Earth creationism parity of treatment with scientific geology. The resulting outrage led to a letter writing campaign with its own Facebook page (which survives as a discussion forum), and eventual removal of the offending language.
I was amazed when I learnt what Storey considered to be the real cause of the Causeway; Noah’s Flood. Surely, I thought, even he would be aware that basalts had an igneous rather than an aqueous origin. It was only later that I learnt of the Flood Geology version of creationism. According to this, the year of Noah’s Flood was accompanied by a catastrophic remoulding of the entire planet, including dizzying motions of the continents, and massive outpourings of lava. (No matter that there is absolutely nothing in the Bible to suggest anything of the kind.) This is why the creationist literature is full of attempts to revive the long-dead conflict between catastrophism and uniformitarianism. As I mentioned in my last post, T. H. Huxley had pronounced the obituary on this conflict in 1869, but I do not think that people like Melvyn Storey pay much attention to Huxley.
Storey also managed to grab a few headlines in 2009, the year of the bicentenary of Darwin’s birth and of the 150th anniversary of the publication of On the Origin of Species. The Ulster Museum mounted a special exhibition celebrating this, and Storey publicly threatened the Museum with litigation, for violating equality laws. As he put it in an interview with the Guardian,
In the past, when I have written to the museum about necessity to show the public an alternative to Darwin’s theory (and let’s stress it is still only a theory), they have been quite dismissive.
They could be subject to a legal challenge under equality legislation within Northern Ireland if they chose to ignore alternative views that many people here in the Province believe in.
This takes us to the heart of the matter. Equality legislation is about how people are treated as a result of group membership, rather than as individuals. Denial such as Storey’s is based not on reason or religion, but on his own group loyalty. Northern Ireland, notoriously, is inhabited by two rival tribes. One tribe is overwhelmingly Catholic, accepts the facts of evolution and an ancient Earth, and has traditionally sought to be ruled from Dublin. The other, to which Storey belongs, is biblical infallibilist Presbyterian, believes that the world must have been made in six days some 6000 years ago because God said so, and is fiercely insistent on being ruled from Westminster. If Storey were to accept the scientific evidence, that would mean letting the requirements of truth override the requirements of tribe, which is unacceptable.
And finally, time as sequence of events. I show here a photograph of a pebble, collected on the beach at Benalmadena, in southern Spain. This is a geologically interesting area because that is where the African plate moving northwards presses against the Eurasian plate. Around the rim of the pebble, especially at 2 o’clock and 6 o’clock, you can see a hint of separate layers. These are not strata, but caused by the lining up of tiny crystals in the rock under pressure. The pebble is crisscrossed with white lines (quartz, carried there in superheated water) and pinkish lines (the same, but contaminated with iron) in various directions. The thickest of these pinkish lines runs more or less from top to bottom of the image, and it is easy to see that this line defines a fault, with the rock material having yielded and been pushed upwards on the left-hand side. Thinking about how the lines cross each other, it is clear that the pebble has undergone at least two separate episodes of intrusion by white quartz, and two by pink quartz, the more recent of these corresponding to the fault formation. That’s four separate events, involving at least two separate sources of quartz-bearing fluid, after the rock had already formed and then been subjected to enormous tectonic pressure, but before this particular piece was detached, to be worn down in the water and eventually deposited on the beach.
As Yogi Berra said, you can see a lot just by looking. And one thing you can see, if you are prepared to look, is that it takes millions of years to make a pebble.
I thank the Giants Causeway Visitors Centre. Photographs by the author. An earlier version of this post appeared on 3 Quarks Daily