30 November, 2005

Lava lake tectonics

Part of the duties of my new job is some lecturing on behalf of my supervisor (now boss, I suppose). I'm finding this a bit daunting, even if I have been given Powerpoint presentations for each lecture; this means at least I don't have to compile things from scratch, although it does take a while to remove all the instances of text zooming in with a swooshing sound (another crime against humanity perpetrated by Microsoft).

My first lecture started with a discussion of why we have tectonic plates, and I got to present a really cool example of a situation where similar processes are occurring over timescales much easier for us to grasp conceptually. All the pictures in this post come from a lava lake in the crater of Erte Arle volcano in Ethiopia. You get lots of the sorts of activity you’d associate with such an environment (such as fountain eruptions and bubbling molten lava), all occurring as the lava is heated from below and the hotter, less dense, material forces its way to the surface. However, what's even more interesting is that a lot of the time the surface of the lake is formed by a solid crust of cooled magma. Vigorous convection is still occurring beneath this crust, and in response to this the crust breaks up into a number of discrete slabs.

Sound familiar? The relationship between the lava crust and the convecting molten lava beneath it is precisely the same as that between the lithosphere and the asthenosphere - the convecting and non-convecting parts of the Earth's mantle. In both cases the boundary is controlled not by composition but by temperature - contact with the air has cooled the upper part of the lava lake so that it solidifies and begins to act rigidly; heat loss through the Earth's surface causes the same thing to happen to the upper 100 km or so of the mantle.

The internal strength of the lava 'plates' compared to the molten material beneath them causes them to move coherently over the surface of the crater, with little frictional drag at their base to slow them down, or dissipate any forces which are applied to them. In such a situation the forces exerted at the boundaries of the slabs become particularly important. Again, the same situation seems to apply for the tectonic plates on the Earth's surface, so it is no surprise that you can see analogues of different types of tectonic plate boundaries at the borders of the lava slabs. Here's a rift zone, where two slabs are moving apart:

The orange crack shows where lava is welling up to the surface of the lake. It cools and solidifies and is itself pushed away from the rift by yet more upwelling magma. Notice the symmetrical pattern of light and dark grey bands moving away from the crack - this is caused by slight variations in the composition of the upwelling magma over time (and is very reminiscent of the striped magnetic anomalies seen either side of a mid-ocean ridge).

The force pushing the slabs apart at this boundary will be transmitted to the opposite boundaries of the two slabs, where they will be pushed against their neighbours. In such a situation one plate is forced beneath the other, forming a 'subduction zone' (in the picture below, the lava slab is being pushed against the crater wall, but the principle remains the the same):

The best thing about this example, though, is that molten lava has a much lower viscosity than asthenospheric mantle, flowing at centimetres per second rather than centimetres per year. This means that you can see the equivalent of hundreds of millions of years of tectonic processes occurring in the space of a few hours. This movie (Quicktime) was created using time-lapse photography: as evening draws in, plates on the surface of the lava lake are created and die, move apart and push together. It's really cool. Watch it.

Probe update

Not as dodgy as it sounds, I promise!

  • Following a successful launch, ESA scientists have begun checking out the instruments on the Venus Express probe, by pointing them back at the Earth-Moon system and taking some pictures with the VIRTIS spectrometer. VIRTIS stands for 'Visible and Infrared Thermal Imaging Spectrometer' (it can apparently measure at ultraviolet wavelengths too but that would have mucked up the acronym...) and is designed to measure the composition of the lower atmosphere by measuring in spectral windows which are not blocked out by clouds in the upper atmosphere - according to this site you might also be able to make some surface measurements at near-infrared frequencies, which would be pretty cool.

    The probe is already 3.5 million kilometres away, so the data collected are not particularly ground-breaking, but they confirm that the instrument is in good shape for the real observations, starting when the probe enters Venus orbit in April next year, and are useful for calibration purposes.

  • The MARSIS radar on the Mars Express has now been operating for four months, as is summarised here. The orbiter has a highly eccentric orbit, and during most of the observation time so far the time of closest approach has been in the daytime. This means that the best measurements are being made when the upper atmosphere (ionosphere) is excited by solar radiation, making it more opaque to the lower radar frequencies required to penetrate the sub-surface. We're still getting lots of data about the structure of the Martian atmosphere and surface, but the real fun begins next month, when the closest approach will be at night; the search for underground water can then start in earnest. This delay in sub-surface measurements was not intentional, but a conseqeunce of the delay in deploying the radar booms last year. I'm glad the mission has been extended to compensate

  • Meanwhile, New Scientist reports that the Japanese Hayabusa probe, which managed to land on an asteroid and collect surface samples over the weekend (on the second attempt - on the first landing attempt last week the pellet gun designed to dislodge material from the surface of the asteroid failed to fire) is suffering from a thruster malfunction.
    Problems with the thrusters and control systems have plagued this mission from the start, so even if they get it working it's not clear if there's enough fuel to return the samples to Earth. Fingers crossed.

  • Finally, another in the seemingly non-ending stream of fabulous images from Cassini:

    This image of Enceladus is backlit by the Sun, and illuminates plumes of water vapour rising up from its south pole. The cause of this geological activity is not yet understood, but it's still pretty cool!

    A larger image can be found here,and an enhanced false colour image here.

28 November, 2005

Fission back in fashion?

The nuclear debate has reared its ugly head again, with Our Glorious Leader apparently 'convinced' (which sounds awfully familiar; does he never learn?) that we must turn to nuclear power to meet emissions targets and ensure energy security.

My views on the nuclear question are complex. I'm aware of the disadvantages, of course: the production of long-lived, highly radioactive waste products (and, more importantly, still no clue about how to provide safe long-term storage for them), the reliance on expensive technology which is difficult to maintain and dismantle, and the horrific consequences if something goes wrong. There are, however, potential benefits: the cuts in CO2 emissions resulting from less use of fossil fuels, and the reduced dependence on diminishing supplies of oil and gas, most of which comes from areas of questionable political stability (and that's before we stick our oar in). So the question is, do these benefits outweigh the risks?

When this first hit the news last week, I took what I thought to be a pragmatic 'lesser of two evils' line: it's not ideal, and in a perfect world we wouldn't touch nuclear fission with a barge pole, but given the likely gap between our energy use and that which is probably going to be supplied by renewables (in the near future at least), if we're serious about cutting greenhouse gas emissions then we might well have to consider it.

Then I read this op-ed by Magnus Linklater in The Times, which is mainly based on the work of Jan Willem Storm van Leeuwen and Philip Bartlett Smith. Unfortunately, I can't find any of their stuff on-line as a primary source, but their biographies (pdf) indicate that you can't just write them off has raving eco-hippies; they have both spent large parts of their careers involved with the nuclear industry.

According to Linklater:

What they have done is look at the entire life cycle of a nuclear power station, from the mining of the uranium to the storage of the resulting nuclear waste. Their conclusions make grim reading for any nuclear advocate.

They say that at the present rate of use, worldwide supplies of rich uranium ore will soon become exhausted, perhaps within the next decade. Nuclear power stations of the future will have to reply on second-grade ore, which requires huge amounts of conventional energy to refine it. For each tonne of poor-quality uranium, some 5,000 tonnes of granite that contains it will have to be mined, milled and then disposed of. This could rise to 10,000 tonnes if the quality deteriorates further. At some point, and it could happen soon, the nuclear industry will be emitting as much carbon dioxide from mining and treating its ore as it saves from the 'clean'” power it produces thanks to nuclear fission.

And that's before you've even considered the enrichment stage, where you separate U-238 from the non-fissile U-235, which also requires energy. We will of course meet this point even sooner if a new generation of reactors increases the global appetite for enriched uranium; after which the CO2 emissions 'saved' by nuclear electricity generation will be insufficient to offset that used to produce the fuel for it. Worse still, as demand for uranium ore increases, where will it all come from?

Not friendly Canada, which produces most of it at present, but places like Kazakhstan, hardly the most stable of democracies. So much for 'secure'” sources of energy. We would find ourselves out of the oil-producing frying pan, right in the middle of the ore-manufacturing fire.

Hmmm...suddenly I'm not so sure about that cost-benefit ratio. Especially when you consider that electricity generation as a whole only accounts for about a quarter of the total UK energy usage - getting on for 50% of the fossil fuels we burn are presently consumed by transport and domestic gas (The DTI provides some only slightly impenetrable figures here). Switching some gas-power stations off and replacing them with nuclear isn't going to affect those sectors at all.

So, could nuclear be an intermediate option in a long-term strategy - buying us time to switch over to more sustainable energy sources and usage? Possibly, but building nuclear power stations is a long and drawn out process (even the optimistic projections of proponents seem to reckon if we started right now it would be at least 10 years before we could add new nuclear capacity to the National Grid. And the cost will be fearsome. I wonder what you could do if you invested those billions in renewables (still scandalously cash-starved despite the high-minded rhetoric)? Or used it to every house in Britain loft insulation and solar heating systems?

I won't deny I have environmental sympathies, but I'm willing to be convinced that nuclear has a role in providing for our future energy needs. But I do have severe doubts which I'd like to be addressed. In detail. Unfortunately, if you'll forgive my cynicism, I have a feeling that this 'debate' which is currently being called for might well consist of 'this is right, anyone who disagrees is out of touch with the issues'. Don't know where I could have got that idea from...