Radioactivity and the earth (and moon?)

"Castle Romeo" atmospheric nuclear test - March 1954. From CTBTO under CC

“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.

Categories: geochemistry, impacts

Where on Google Earth #416

I happened to recognise Brian’s picture of a section of the Richat Structure in Mauritania straight away, so here I am hosting this excellent geological competition.

Woge 416

Where on earth is this place? To find out, get to Google Earth and get searching. Brian talked about how recent WoGEs have been found quickly, so I’ve zoomed down quite a lot.

As always, the goal here is to name the location and describe the geology. The winner gets to host the next WoGE. See complete rules here, hosted by Felix on his blog. I do not invoke the Schott rule. Get looking!

Categories: Where on Google Earth

Story of an atom: diamond

This is the third part of a story told to me by a Carbon atom in my brain. It started with her tale of how she ended up on earth, followed by an inside view of the Carbon cycle.

So like I said, I’ve fallen into this pattern of cycling around different places, occasionally going underground for a bit. One time I got buried was really special. Just… just a really *deep* experience, you know?

It started off as normal, I was in a bunch of organic matter that’d settled onto the bottom of the sea. Things got slowly hotter and more squashed, but this time it just didn’t stop. Things started falling apart – everyone got antsy and wanted to move around, some atoms badly wanted to escape. The organic gunk I was in broke down. Lots of water started coming out and disappearing upwards, eventually there was just us carbon atoms left. Lots of other atoms around us too, of course, lots of silicate minerals – big structures dominated by Silicon and Oxygen, all in rigid ranks. It got so hot and pressured that even they couldn’t cope and had to start rearranging themselves to get more comfortable. Some minerals just disintegrated and the atoms had to find other minerals to join1. I’d never experienced this before – I’d never been  buried so deep.

You know when you’ve drunk too much coffee and you’re stressed? You feel like you’re vibrating really fast but you’re quite uncomfortable and you feel stuck. Same with us atoms in the deep earth. One way to cope is to try and help each other. Take us Carbon atoms in that organic lump. At first we were just jumbled up in a heap, but eventually we sorted ourselves out a more comfortable arrangement – we got together in lots of groups of six, all joined up but flat. If we stacked up like this then things weren’t too bad 2. But it just kept getting more extreme. Eventually every single mineral changed, even quartz – I’ve never seen that before or since 3.

Things kept getting more extreme. All around us atoms kept shuffling around into new configurations that were more comfortable. Hydrogen was very uncomfortable, it became harder and harder for it to find a place in these new arrangements of Silicon and Oxygen and more and more it joined with Oxygen and disappeared upwards. Eventually, some of us Carbon atoms broke up and starting moving upwards in a liquid4.

We travelled a long way like this (but were still very deep) when suddenly: it happened. There was this crystal of carbon – the most amazing thing. You call it a diamond. I’d always been a bit snooty about atoms all locked together in a mineral – fancied myself as a free spirit. But I so wanted to join in this crystal of just carbon. The other Carbons beckoned me in and at first I couldn’t see how it would work. When I’ve been with carbon before, I was joining with 3 other carbons in a flat plane. In this crystal all 4 of my bonds were joined, each with another Carbon. Not just flat too. I only managed to squeeze myself in because we were so tightly squashed together and buzzing around so much 5.

Once I was properly in there, I didn’t mind the conditions. We felt so strong, all together, bound so tightly. After a while I started to lose myself – no more ‘me’, only ‘us’. I was merging myself into a greater thing. One great collective of Carbon, perfectly happy in one eternal unchanging moment….

Sorry, drifting off there. Amazing times, so special. It couldn’t last though: diamonds aren’t forever, not really. The first sign of trouble was when the rock around us started to melt. We ignored it, but suddenly the whole area around us started shooting up through a big crack! The pressure dropped incredibly quickly – we were in a panic because it felt like we might start breaking up – could we stick together in these new conditions? Luckily we quickly cooled as well, making it easier for us to stay together6.

We soon got used to the new conditions – we were still underground after all – and we remained strong, ready for anything. It was a rude surprise when the rock around us got crushed up and we saw daylight for the first time. We were having a lovely time bouncing those photons about through us when this human hand grabbed us and put us in the dark again.

We were still together through all of this, which made what happened such a sudden shock. We were whipped out of the bag and put in a funny metal box. All the air around us disappeared and then ZAP! A huge beam of light hit us. Hit me! There was so much energy that a bunch of us got blown apart, all our bonds broken. There I am, alone and floating in space, just I was in my first memory. I soon hit some weird thing and ended up back here on the surface, going through the same old cycles7. For a while it all seemed so shallow, so temporary, so lonely. I’ve talked to other Carbon atoms about it, but none of them know what I’m talking about.

Still it’s been nice talking to you about it. It looks like you’re about to break up this molecule I’m in, so I’ll be off soon, back out into the atmosphere. Who knows where I’ll end up next!

No atoms were harmed in the making of this story.

Categories: diamonds, excessive anthropomorphism, imaginary conversations, subduction

The edge of Cheshire. Part 3 – abandoned

This is the third part of a set of posts describing a walk I took across Cheshire. My goal was to find out everything that was interesting about the places I visited. Previously I’ve seen traces of apocalypse and traced the layers of the landscape.

Dropping off the ridge, the wind suddenly stops and everything feels very different. I’m walking down towards a deep river valley, quite unlike the surrounding area. Rivers hereabouts tend to feel rather incidental, hemmed in by ridges of sandstone, man-made dams, or simply too small to matter. Shell Brook is an exception (brook is one of the many English words for stream). Sitting on an unusually large expanse of soft shale, overlaid by softer glacial boulder clay it is a proper dendritic stream that has made a big hole in the ground.

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View of the top of Shell Brook. The peak is Shutlingsloe and the clump of trees contains Cleulow Cross, site of an ancient barrow. Note that the sheep are staring at me.

Shell Brook is also unusual for being so full of trees. Apart from the horrid industrial evergreens of Macclesfield Forest – planted to make pit-props in coal mines that have since been shut – woodland is rare in these parts.

If you look at a geological map of the area and toggle the geological overlay, there is a clear pattern. The gentle upper slopes of the valley, covered in glacial sediment and free of trees. Closer to the river it has cut down to the shale beneath and this area is mostly wooded.

On the way down into the valley, there is a ruined farmhouse, Mareknowles. Summer 2013 181It’s an old building, started in local sandstone and extended in rough bricks. The roof tiles are local flag stones. If the house had been built in the 20th or even late 19th century it would almost certainly have been built with thinner lighter slates from North Wales.

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You can trace the change in use. Windows lose glass and are bricked up. A layer of ancient dung indicates it was used to house livestock. Vivid white streaks down the inner wall suggest local buzzards live here now.

It is genuinely surprising to see a wrecked house here. High demand for housing combined with planning restrictions on rural areas mean that old buildings in nice bits of  England are usually converted into expensive dwellings. I would be very happy to live with a view like this.

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After a steep descent, my first glimpse of the river is a little surprising. Did such a small thing make such a big hole? It seems it did.

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Once I reached the stony stream bed I look for fossils (of course). The second piece I pick up has this beauty on it: a ridged bivalve, a fossil shell – what else would you expect from Shell Brook?

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I cheated a little, making sure I met the stream where the geological map marks a ‘marine band’. These rocks were deposited during times (like today) when distant Ice Caps caused periodic fluctuations in sea level (more here). Times of high sea level brought sea water into this area, leaving muds rich in fossils.

This is a quiet spot. Nearly 200 years ago, it was affected by the world’s first phase of industrialisation and the introduction of an important transportation technology: canals.

Macclesfield Canal was brought in to link the industrial towns of Macclesfield and Congleton to the wider world. Road transport was inefficient; although Macclesfield was linked to the turnpike system, there were cheaper ways of shifting goods than a horse and cart. Places with navigable rivers were more fortunate – a boat can carry a big load. So in 1826 they started to build a man-made river, a canal.

Britain’s canals are waterways that join up the country. They are narrow shallow waterways with long flat sections and little flow of water. Long thin shallow boats can be heavily laden and yet pulled by horses that walk along the tow-path. Some are triumphs of engineering. Keeping canals flat, yet tracing them across a bumpy landscape may require tunnels (no horses here, men would lie on the roof of the boat and push against the tunnel roof to move), aqueducts or locks (to link two sections of canal at different heights).

Locks lift boats up by flowing water down. All of this water must be managed somehow and this is where Shell Brook comes in. When the canal was built in To feed the canal, a new reservoir was built near Bosley. To feed the reservoir, they weren’t allowed to use the main river in the area, the Dane1 so they turned to its tributaries. They built a small channel from Shell Brook, that travels 5km to Bosley Reservoir. It is fairly flat, so it follows the contours right round to the other side of Wincle Minn ridge where it joins a network of channels (marked as ‘conduit’ on Ordnance Survey maps). Neatly, one of these conduits follows the trace of a glacial meltwater channel – a line scooped out of the hillside by water flowing underneath or alongside the major Ice Sheet that covered this land around 10,000 years ago.

Engineers built things to last then, lots of traces remain.

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Where the conduit meets the stream

No doubt water in the canal is managed by pumps these days – the channel is somewhat neglected. But I disturbed a heron while walking along here, so someone likes it.

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This is barely a path, mostly used by sheep, but it still deserved a little bridge over the conduit.

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This tracery of industry, wriggling up from the plain into the edge of Cheshire is rather neglected now. I kept seeing traces of neglect.

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The lower horizontal branches of this straggly bush show that it was once part of a managed hedge. The technique of hedge-laying, part cutting of  branches and laying them flat to get a thick barrier to livestock, is little practised these days.

After a pleasant walk along the Dane Valley I arrived in Wincle, passing the place where last night’s pint came from.

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Note the colour of the stone. In Britain this red-pinkness is associated with the desert sandstones of the New Red Sandstone. Yet these rocks are the older Carboniferous ones. New Red Sandstone is found nearby sitting on top, the old desert surface was not far above us. These 300 million year old sandstone were nearly exposed 250 million years ago and stained by the desert conditions.

Beyond Wincle we move into Gawain and Green Knight territory. A classic of english Medieval literature, it is written in a dialect from this area. The climactic scene takes place in an eerie Green Chapel. Many believe it is based on a real place – a nearby chasm formed by landslip, Ludchurch. Reading it makes you see the place with new eyes. The depictions of hunting are a reminder of why nearby “Wildbercluff” is written Wildboarclough. Viewing the crags from the route Gawain would have taken to the Green Chapel, its tempting to see them as the “ru3e knokled knarrez with knorned stonez” (rough, rugged rocks) of the poem.

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ru3e knokled knarrez with knorned stonez

One of the mysteries of the poem is the meaning of the word “wodwo”. From the context it is some type of wild thing. Crossing this stile there is a funny wooden post that can be lifted up to let your small dog (or your wodwo) through.

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The Roaches are part of the High Peak, a browner more desolate landscape. I’ve crossed the edge of Cheshire now and reached wilder lands (Derbyshire!). My journey is over.

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Categories: England, landscape