Volcanoes and mass extinctions – tracking a killer

Posted on 29 October, 2017 by Metageologist

Look in a bookshop and see how many shelves are taken up with murder mysteries. There’s little that is as compelling as the idea of a dead body on the ground and a search to find the culprit. I’m going to try out the genre here today. I can promise you the deaths of entire species, a glamorous prime suspect with spectacular methods and an overlooked serial killer who has poisoned many different victims. I can’t promise you detectives who are troubled mavericks who break the rules, but there are geologists who sometimes feel like they are underdogs.

The dinosaurs were killed by a giant impact. There’s little debate in the public mind about that and the role of extraterrestrial impacts on earth’s history is now inarguable (I’ve written about it myself). Sometimes though it irritates me how much focus is put on speculation about extra-terrestrial causes for mass extinctions. The worst example I’ve seen is speculation that dark matter (that we don’t understand) has caused past extinctions. Glamorous ideas about Death From Space, which (with the exception of the Cretaceous-Palaeogene) event have little supporting geological evidence always seem to get attention.

This makes me grumpy because geologists have a perfectly good explanation already. A serial killer stalks earth’s history. It doesn’t kill by flaming impact (or however dark matter is meant to work) but by poisoning, choking the life out of countless plants and animals. Death From Above is spectacular but Death From Below, a murderous force rising slowly and unstoppably from the Earth’s core is much creepier.

Forensic evidence

Mass extinctions leave an unusual sort of murder scene. Instead of there being a single dead body, there is a sudden lack of them, as fossils of particular species disappear from the geological record. For a normal murder, you would study the body for any clues, any evidence of what killed it. Same with a mass extinction, only you look in the layers of rock round about where the fossils run out.

These need to be places with a continuous sedimentary record, where we have sediments from the age of the extinction. Often these are marine sediments, which can contain relatively large volumes of small fossils. Chemistry is the best form of forensic evidence as it gives us an insight into the state of the ocean over time. Carbon isotopes track the ebb and flow of the Carbon cycle and often the extinction horizon is associated with a sudden change in them. This means that rocks from the time of the extinction event can be found even in layers with few fossils.

The K-Pg event (RIP non-avian dinosaurs, plesiosaurs, ammonites), is famously associated with a layer rich in Iridium, an element rare on the earth’s surface but much more common in material in space. Similar connections are found between other extinctions and Zinc. The P-Tr event (RIP trilobites, nearly everything else) shows a spike of Zinc in marine sediments immediately before the extinction. A recent study (Liu et. al 2017) also shows how the isotopes of Zinc change over time. Zinc is an important nutrient for marine phytoplanktons, meaning their growth changes the isotopic ratio of Zinc in marine sediments. Using this they demonstrate not only that more Zinc is found, but that it came from volcanic or igneous material entering the ocean. This happened abruptly around 35 thousand years before the extinction event. Soon after, the ratio shifts back in a way consistent with phytoplankton activity returning to normal within 360 thousand years.

Figure 2b from Liu et al. Showing changes in Zinc concentrations and isotopic ratios immediately before the Carbon isotope changes associated with the extinction event.

Other studies show anomalous peaks of Nickel abundance just before the P-Tr event in many sections across the world. Once again the source is inferred to be volcanic activity. Different sets of forensic evidence point to an obvious suspect – the Siberian Traps – an enormous area of volcanic rocks covering a huge area of Russia that was formed across the P-Tr boundary.

The Murder weapon

Volcanic eruptions are dangerous to be near. It’s obvious why life suddenly swamped by lava will not survive, but a mass extinction is a global phenomena. How can a volcanic area kill animals or plants on the other side of the world?

Figure 1 from Jerram et al. showing extent of Siberian Traps, highlighting sill intrusions, coal and explosion pipes.

One clue comes from odd structures found around the Siberian Traps, for example within the Tunguska Basin1. These structures are pipes called diatremes, formed by gaseous explosions.

Some diatremes are formed by gas ready mixed within the magma, but with these Siberian pipes the gas came from heating of the sedimentary rocks that were already there. Buried below the many lava flows, are flat sheets of rock called sills that pushed between existing sedimentary layers. These sills heat up the surrounding rocks, which in the Tunguska basin include much coal and evaporite rocks. This heating produced vast volumes of CO2 and CH4 that poured out of the pipes into the atmosphere.

Figure 2 from Polozov et al. Showing portions of basaltic pipes, exposed within mining works.

These gases of course affect the climate. A huge outpouring of CO2 and methane, plus also nasty gases such as SO2 represent a pretty convincing murder weapon. A brand new paper demonstrates malformed parts of terrestrial plants about this time, attributed to pollution. Sudden ocean acidification and climate change followed by a collapse in planktonic growth leading to the the death of the dependent food webs is a uncontroversial story. It may be a story we are beginning to retell as gas forms mysterious holes in the ground in Siberia once more.

Killed and killed and will kill again

The Siberian Traps are just one of many Large Igneous Provinces (LIPs). Other ones are also associated with extinctions. The pleasingly named CAMP province, found in Atlantic facing areas of Africa, Europe and the Americas, overlaps in time with end-Triassic mass extinction event (RIP conodonts and various reptiles & amphibians). An open-access paper from mid 2017 demonstrates a link between sills intruding into organic-rich sediments and the extinction event – exactly as proposed for the P-Tr event.

Figure 1 from Davies et al

The bodies are piling up. The Late Ordovician extinction event (RIP 85% marine species, no big groups) has been linked to Mercury enrichment in marine sediments. The authors link this to a LIP, even though one of this age has not yet been found.

What do we know about this serial killer? LIPs are thought to be formed by huge plumes of rock, rising from the edges of odd features on the very floor of the mantle. Their ability to kill may depend on the nature of the crust they rise into. Both the P-Tr and Tr-J events see sills intruded into sediments. The Deccan Traps, active across the extinction of the dinosaurs (K-Pg) rose through basement rocks, so there were no sediments to heat, meaning they pumped out only volcanic gases. Maybe this is why the extinction event required an impact to finish the job.

Case for the prosecution

“So, ladies and gentlemen of the jury, I suggest to you that the accused is a serial killer. The mighty plesiosaur, the ever-busy scuttling trilobite, even the wriggly conodont, all were killed by the monster sitting before you. It killed, not by showy eruptions and square miles of lava, but by the silent injection of sheets of magma deep underground. This devilish act then poured huge quantities of poison into the air, bringing the very earth to its knees.”

We’re not quite ready for a trial. Some of the evidence is circumstantial and we certainly don’t have a full roster of victims. But LIPs should be high on the list of anyone’s list of suspects for the greatest murders the world has ever seen.

Scars, acne and others: circles on the ground

Posted on 5 April, 2015 by Metageologist

Looking is never just looking. When we gaze at something, we are not passive recipients of an image, instead our brain is constantly looking for patterns. If you are drifting over the earth, whether as an astronaut or via Google Maps then a simple shape such as a circle will ‘jump out’ at you. It turns out there are many different types of circle on the earth. They can be born in seconds or slowly, be mysterious, sinister, large or small. All are round.

Volcanoes

A classic volcano has an elegant cone1 and round crater at the top. If you’re lucky you’ll get multiple circles, like on Mount Kilimanjaro.

Or here in Italy, Mount Vesuvius gives us three circles. The first is the volcano itself, its steep sides rising above the towns surrounding it. The highest portion is relatively bare of vegetation and makes a smaller grey circle for the crater to sit in.

Craters form in different ways. Some are holes made by volcanic explosions. Crater Lake in the US is a glamorous example of this – what was once a volcano is now just a hole in the ground.

Calderas are round structures formed when a magma chamber (the big pool of molten rock deep under the volcano) empties. The rocks above collapse down into the now empty space.
Circular structures under volcanoes too. A volcano is typically a single point source of magma. With no real reason to be asymmetrical, any structures that form tend to be circular. For example here in Ardnamurchan in Scotland.

The volcano here was active 60 million years ago, so we are looking at the eroded roots of it. The rock structure used to be interpreted as a ‘ring dyke’ a circle of rock that filled the vertical cracks within a caldera. It’s now thought instead to be a saucer-shaped sheet of rock called a lopolith.

Bullet-holes

Volcanic craters are common on the earth, so scientists long assumed that the many craters on the moon were volcanic in origin. In fact they are impact craters, not planetary acne as volcanoes are, but bullet holes – scars that show we are living in a dangerous neighbourhood.

The most elegant scar is in Quebec in Canada, lake Manicouagan.

Like Ardamurchan above, this is not a fresh structure, but the eroded roots of an ancient impact. With it’s old rocks and ancient surface, Canada is rich in impact craters. Australia, another craton is good too, such as here at Gosses Bluff.

The biggest impact crater know is the Vredefort structure in South Africa.

Over 2 billion years old, this was once a huge hole in the ground – up to 9 km of rock were instantaneously removed when a huge rock from space hit the ground.

The Meteor/Barringer Crater in Arizona is small but much fresher, a mere 50,000 years. Here we are looking at the original hole in the ground, rather than deep structures now brought to the surface by erosion.

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Fakes

Once we saw impact craters on the moon and thought they were volcanoes. Now we see round structures on the earth and assume they are impacts. Here are a few that formed in other ways.

The Richat Structure “eye of the Sahara” is formed of domed sedimentary rocks, perhaps uplifted by volcanic activity (volcanic rocks sit in the middle). Scientists have searched for ‘shocked’ minerals and other signs of impact and found none.

The Kondyor massif in eastern Russia is beautifully circular, but is not a crater. It’s an eroded volcanic structure.

Into the Anthropocene

Let’s end with a type of crater scoured by a massive fireball created by the same process that powers the sun. They appeared all over the earth for a few brief decades but (hopefully) they’ve stopped forming for good.

This sinister pair of holes were caused by nuclear testing on Enewetak Atoll in the Pacific. The left hand one was an island called Elugelab, until the explosion of the world’s first hydrogen bomb on the 1st November 1952.

A world without subduction

Posted on 6 May, 2014 by Metageologist

The greatest achievement of the generation of Earth Scientists now retiring is the concept of plate tectonics. The insight that the earth’s surface is made up of rigid plates that move has shed light on all aspects of Earth Science, from palaeontology to geophysics to the study of ancient climates. What’s less well known is that the way the plates interact has changed over time. Key plate tectonic features such as subduction, didn’t happen for large periods of earth history.

Cut-away diagram showing modern convection from computer modelling by Fabio Crameri. White is hot rising plumes, black cold sinking plates.

Cut-away diagram showing modern convection from computer modelling. White is hot rising plumes, black cold sinking plates . Image used with permission of Fabio Crameri.

Earth scientists have a pretty good idea of the details of how modern plate tectonics works. This has required the integration of indirect observation of modern subduction zones (using geophysical techniques) with direct study of rocks that have been inside subduction zones (such as eclogites) plus the creation of subduction zones ‘in silico’ (with computer modelling).

Of these 3 methods of study, only the first (direct observation) cannot be used on the ancient earth. So what do the rocks and computer models tell us?

Old rocks are odd

We’ve known for a while that ancient rocks (Eoarchean–Mesoarchean, older than 2.5 Ga1) are very different from modern ones. Often they consist of greenstone belts – containing an unusual lava called komatiite – surrounded by large areas of granitic gneiss. The pattern of metamorphism in these rocks shows high temperatures, even at shallow depths.

The chemistry of the igneous rocks tells a similar tale. Komatiites only melt at temperatures of around 1600°C – 400 degrees hotter than modern basalt lava. Granitic rocks have tonalite–trondhjemite–granodiorite compositions and are thought to have formed from direct melting of basaltic rock – unlike granites formed above subduction zones today.

Rocks characteristic of modern subduction – blueschists and eclogites – are not found in rocks this age. There is a pretty good consensus, based on field evidence and model modelling, that subduction did not happen in the early earth. The earth’s mantle was much hotter and more heat was flowing up through the crust. Hot rocks are weak rocks – forcing a slab of rock into the deep mantle requires it to be cold and hard. Hotter rocks act not as rigid slabs but as soft blobs.

Computer modelling confirms the importance of temperature, both of the crust and the underlying mantle. Models are our best hope of understanding what a hot planet without subduction looked like. More like a bubbling pan of porridge perhaps, with tectonics dominated by hot upwelling plumes and lithospheric delamination, with blobs dripping-off down again. Some studies of mantle mixing suggest a ‘stagnant-lid’ model where the earth’s surface layer doesn’t move at all.

Subduction starts

At some point in time between 3.2–2.5 Ga, subduction started. The planet had cooled enough that a lithospheric plate stayed rigid enough to sink down into the mantle. Evidence for this is found in ‘paired metamorphic belts’. Rocks within the subduction zone remain cool at depth (as they are pushed down before they can get as hot as the surrounding rocks) and form eclogites or high-pressure granulite rocks. Rocks nearby in the overriding plate are much hotter and enjoyed granulite–ultrahigh temperature metamorphism.

Mathematical modelling of the earth suggests subduction started because the earth cooled below a particular threshold. As an explanation, this is a little dull. Much more excitingly, coverage of a recent paper suggests massive meteorite impacts about 3.2 Ga could have broken up the surface and somehow kickstarted plate tectonics. Scientists who study impacts are always really keen to use them to explain events or features on earth, whereas other scientists are sceptical, preferring to explain them via things that they study. We’ll need to wait to see who is right about this one (but my money is on the dull explanation).

Cut-away diagram showing modern convection from computer modelling by Fabio Crameri. Red is rising plumes, blue sinking plates.

Cut-away diagram showing modern convection from computer modelling. Red is hot rising plumes, blue cold sinking plates. Image used with permission of Fabio Crameri.

Subduction as a cure for boredom

When subduction first started, mantle temperatures were still 175–250 °C hotter than today. Hotter, softer slabs are more likely to break off, perhaps making subduction something that stopped and started.

Blueschists and low-temperature eclogites, high-pressure & low-temperature rocks that are found in modern subduction zones are not found until the the Neoproterozoic at 600–800 Ma. Mantle temperatures by then were less than 100 °C greater than today – this marks the wide spread development of modern-style (cold) subduction on Earth. Cold slabs of oceanic lithosphere break-off deep, allowing large volumes of dense oceanic crust to pull continental lithosphere down, creating the first ultra-high pressure metamorphic complexes.

The Neoproterozoic is the end of what is known as the ‘boring billion’ – a time of tedious environmental and evolutionary stability. A recent open acess paper in Geology suggests a link between the exciting changes that followed (glaciations! Cambrian explosion!) and the onset of subduction. The boring billion was stable in part because most continental crust was part of a supercontinent called Rodinia. The paper argues that the disruptive effects of the onset of cold subduction broke Rodinia apart, setting off a chain of events that transformed the world.

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The early earth was a very different planet. Understanding it better informs the general subject of planetology. As we get more and more data about other planets (both within and beyond our solar system) it’s natural to speculate on their tectonic activity. Why does Venus not have subduction? Does subduction here exist because of life and its role in moderating climate and creating the earth’s oceans? Ancient rocks and computer models may help us answer these questions as much as probes and telescopes.

REFERENCES

Brown M. (2014). The contribution of metamorphic petrology to understanding lithosphere evolution and geodynamics, Geoscience Frontiers, DOI: 10.1016/j.gsf.2014.02.005
Available here

Cawood P.A. & Hawkesworth C.J. Earth’s middle age, Geology, DOI: 10.1130/G35402.1
Available here

Gerya T. (2014). Precambrian geodynamics: Concepts and models, Gondwana Research, 25 (2) 442-463. DOI: 10.1016/j.gr.2012.11.008
Available here

Radioactivity and the earth (and moon?)

Posted on 23 December, 2013 by Metageologist

“Castle Romeo” atmospheric nuclear test – March 1954. From CTBTO

We tend to think of radioactivity as an artificial thing; some argue that the first nuclear explosions in 1945 should mark the start of a new human-dominated geological epoch called the Anthropocene. These man-made explosions have left distinctive radioactive traces that may well outlive us all. It turns out that natural radioactivity, even fission reactions, played an interesting role in Earth’s history long before we came along.

A little background

Our sun is a nuclear fusion reactor, taking simple atoms such as Helium and Hydrogen and squeezing them together to create new elements, plus energy. This normal activity, along with dramatic events in a star’s history such as supernovae, have created virtually all the atoms you see around you. Radioactive decay is where large unstable atoms break-up, creating new smaller atoms plus various left-over bits, such as alpha, beta or gamma particles. Sometimes these particles hit other unstable atoms and cause them, in turn to break up. Put enough radioactive atoms of the right sort together and a nuclear fission reaction starts. When nuclear fission is used to generate electricity, the reaction is controlled. If used to kill people, a chain reaction is created to generate as much energy as possible.

Too much radioactivity is dangerous, damaging cells and DNA whether the source is natural radon gas or a nuclear weapon. But it’s not all bad. Some people regard plate tectonics as a pre-requisite for life on earth. It certainly makes things more interesting. Plates move because the mantle convects because it needs to release heat to the surface. This heat comes partly from radioactive decay within the earth – without it this planet would be a cold and dull lump by now.

Fossil fission

Radioactive decay is massively useful to geologists as a dating tool. Rates of decay, usually expressed in terms of half-lives, are constant. If you can work out that a grain of zircon started out with twice as much Uranium-235 as it now has, then you know it formed 703.8 million years ago.

Let’s turn that round: 703.8 million years ago there was twice as much Uranium-235 around as there is now and therefore four times as much 147 million years ago. This means that the earth used to be hotter (more radioactive decay), which is why Archean geology is so weird (odd komatiite lavas, crust that dripped back into the mantle). It also means that fission reactions were easier in the past.

Much of the hard work of a nuclear weapons program involves enriching Uranium. From the Manhattan Project through to the Iranians today the most laborious job is taking natural Uranium (a mixture of Uranium-235 and Uranium-238) and increasing the proportion of Uranium-235. This is important because U-238 is more stable, with a longer half-life and less interest in breaking up. Humans increase the proportion of U-235 using centrifuges, or lasers, but a time-machine would do the same job.

Around 2 billion years ago, a Uranium-rich deposit in modern day Gabon was the site of seventeen natural nuclear fission reactors. Self-sustaining nuclear reactions, moderated by groundwater, lasted for about a million years. There are two excellent blog posts that cover the site in more detail.

Such natural reactions are extremely unlikely now, since much more U-235 has decayed into lead over the intervening 2 billion years. But what about the 2 billion years of earth history before the Gabon reactors started up? Were fission reactions active in that time frame? Some argue that they were, with explosive consequences.

Huge explosions and the moon

The deep Earth is a mysterious place. We know that the crust is relatively rich in radioactive elements but we don’t know much about their distribution in the mantle. One day Neutrino detectors may help map out the modern day distribution. How they were distributed earlier in the earth’s history is anyone’s guess.

Some people’s guesses (informed by computer modelling) suggest that heavy radioactive elements such as Uranium, Thorium and Plutonium, sank to the bottom on the mantle, near the core-mantle boundary. Plutonium is now regarded as a man-made element, but it would have existed in the early earth, as it would have had less time to decay since being created in a supernova. Geochemical models suggest that while substantially enriched, the average concentrations would still be too low to cause fission reactions.

Dutch scientists (R.J. de Meijer and W. van Westrenen) have suggested an amazing thing. Their theory is that concentrations of radioactive elements were higher in some areas than others (not unreasonable). They suggest that, just as human nuclear bombs are triggered by using conventional explosives to pressurise the radioactive material, a major impact on the earth would send shock waves into the inner earth and compress the material enough to initiate a nuclear reaction.

This reaction would take place in a large volume of rock and so would be create a huge explosion. Big enough, their modelling suggests, to fragment the earth and send lots of material into space. In time, some of this material formed a large moon orbiting the earth – the one we see today.

The moon? Really?

I suspect you are feeling a little sceptical right now, which I think is the right reaction. But bear in mind that we don’t really know how the moon formed. The best available theory is based on the idea of a massive collision with another large body. This has big problems because of the many isotopic similarities between the earth and moon. Any other body coming in would be expected to have had a different composition, traces of which would be present in the moon today.

The giant impact model is still the best. A recent conference on the moon’s origins discussed many ways in which the similarities between earth and moon could be reconciled with the model. The impact could have thoroughly mixed the material, or maybe the impactor had the same composition. Perhaps the moon originally came from Venus. We don’t know anything about the composition of Venus – it may be very similar to earth.

As far as I can tell, nobody discussed the nuclear explosion model at this conference. This may be because there is no actual evidence for it, just inference from modelling. In their latest paper R.J. de Meijer and W. van Westrenen predict distinctive patterns in Xenon and Helium isotopes in lunar material. Measurements of these elements on our current Apollo samples are contaminated by the solar wind, so samples of deeply buried lunar material would be needed to test it fully.

We’ll have to wait then. Perhaps some future lunar rover will dig up the required samples. If it does, it is likely like the Chang’e rover currently on the moon to be powered by Plutonium. Useful stuff, radioactivity.