The Palaeomagician’s bane

lightning.jpg
During last week’s field trip, we were treated to several spectacular evening thunderstorms of the kind that you very rarely get to see back in the UK. The massive forks of lightning arcing down from the sky during these storms were certainly awe-inspiring, but as I sit back in my office analysing preliminary demagnetisation data from my KwaZulu Natal samples, I’m starting to realise that this spectacle comes at a price.


It shouldn’t be difficult to see why lightning might be a problem for us palaeomagicians: a lightning strike on exposed rocks will briefly cause currents of 10,000 amps or more to course through them. Strong currents generate strong magnetic fields. Strong magnetic fields will reorient the magnetic grains to a new (and, in the case of a lightning strike, completely random) direction. This can easily wipe out any trace of the ancient signals I’m searching for.
Unfortunately for me, this is much more of a problem here in South Africa than it ever was in New Zealand. The map below shows the global density of lightning flashes (note that not all of these will strike the ground) over the average year, courtesy of the Earth Observatory. There is a strong latitudinal dependence, with equatorial regions experiencing many more strikes per square kilometre per year than higher latitudes, and although it doesn’t fare quite as badly in the electrocution stakes as equatorial Africa, South Africa still experiences a fair amount of activity – especially in that rather disturbing red patch on the west coast, displeasingly centred on my field area.

global_lightning.jpg

The problem is exacerbated by the fact that the South African landscape is pretty old – the last uplift event of any significance occurred 20 million years ago, and that only slightly changed the topography produced by a more major bout of uplift and erosion associated with plume-related rifting back in the Cretaceous. Thus, many of the rocks I’m sampling may have been sitting under lightning-filled skies for a good long time, greatly increasing the chances of them being struck and magnetically scrambled.
I’ve done all I can to avoid this problem – taking samples from valleys rather than hills, and targeting exposures freshly exposed by rivers – but I’m seeing an awful lot of random directions emerging from my data, so random (most overprints will at least show some consistency between different samples) that I’m starting to suspect that lightning-induced remanences are contaminating what was already going to be a complicated dataset.
Hopefully, with careful analysis I can identify most of the lightning-affected samples (they will often have a much stronger magnetization than the surrounding rocks, for example), and in some cases the demagnetization techniques I’m using might remove the overprint to reveal a more useful signal. But quite frankly, photogenic as lightning is, I could do without quite so much of it harassing my poor Archean volcanics.

Categories: fieldwork, geophysics, in the lab

Comments (8)

  1. Bob O'H says:

    Things will be so much easier when you get yourself a minion to act as a lightning conductor.
    Bob

  2. eohippus says:

    When I read this post – all I could think of was “Archean paleomag, you poor bastard”. As a paleontologist (and on mammals on top of that) I am a consumer of paleomag rather than a producer and all of the relevant paleomag is much younger with less complicated overprints. Good Luck
    David

  3. Kim says:

    I work on metamorphic rocks, so I’m amazed that you’ve got rocks that made it all the way from the Archean with any original magnetization intact. (Though it’s kind of sad, really… they survived the hazards of burial, low-grade metamorphism, hydrothermal alteration, intrusion and heating by those plume-related magmas, for billions of years.. only to be messed up by lightning.)
    Maybe, as a side-effect, you could test the various ways of avoiding getting struck by lightning. Is it true that it’s safer to be in a valley than on a hilltop, for instance? I’ve heard conflicting claims (mostly from wilderness medicine instructors).

  4. I had no idea lightning could effect palaeo magnetics! You learn something every day…
    Both a cool and understandable uncool phenomenon.
    Just discovered Scienceblogs this month, and love your site. What part of New Zealand were you working in? (I’ve just moved down here to take an ed degree, but looking for some fun geo and palaeo sites to explore)
    Cheers

  5. Billy (A Liberal Disabled Vet) says:

    Just curious — are you working on trying to date the great Triassic extinction? I read the book “Gorgon” (author not remembered, book currently at my father’s house) and found the accounts of trying to get accurate cores both amusing and sad.
    (Sorry if I am not naming the extinction correctly, I think it was the one that wiped out the mammal-like reptiles and allowed for the proliferation of the archosaurs (I majored in history and paleontology and geology are minor hobbies).)

  6. dmonte says:

    Spent several days on a field trip to the Book Cliffs of Utah, USA last month where there is abundant evidence of lightning strikes. It locally started coal seams on fire which further baked the rocks. It discolors the sandstones a deeper red color and I believe they call it clinker. Found a website that describes occurrences in North Dakota.
    https://www.dmr.nd.gov/ndgs/ndnotes/ndn13_h.htm

  7. Alberta Canada experiences the same lighting strike burning of coal seams phenomenon. It is indeed called clinker (though I’ve heard clunker too).
    They can be occasionally hazardous when working around old coal mining sites (I was looking for fossils) as the old mine shafts expose less eroded coal deposits to the air. Normally the surface coal only burns slow due to erosion contaimination (from bentinite rich mudstone in Alberta’s horseshoe formation anyway) which prevents oxygen from properly getting into the combustion. In the mine shafts if not properly sealed (use of dynamite doesn’t gurantee air tight) can cause explosive ingnitions. The explosion isn’t the coal exactly, but rather something to do with the air pressure of the trapped gas in these improperly sealed shafts. I understood the geo part of the explanation of the upcoming story, but most of us weren’t exactly sure on the physics end of it.
    Long story short we were checking out a nice patch of hadrosaur bonebed when suddenly about 8 metres away part of the hillside blew up!… Not crazy explosive hollywood style mind you. More like a big hammer in the hill hit it from the inside and sent some rocks flying wicked fast. Had someone been directly in front of it at less than 2m they might have been seriously hurt.
    We knew their were old mine shafts, but that exact shaft was new to everyone in the last 50 yrs (wasn’t on the old mining companies maps). The erosion of the area had covered the surface features so we couldn’t see it, and there’d been a lighting storm 2 days previous (we think we even found the strike point for the lightening that caused it, but aren’t sure of course). The supervising geologist explained to us the cause when we got back to the museum rather wigged out.
    Moral of the story don’t be too scarred around old coal mine sites, just be a little cautious around them right after a lightening storm…

  8. Badis Abdou says:

    Hi there,
    I would like to ask you a question, and I would really appriciate if I get an answer from you. I`m a field engineer, I work in the algerian desert. Recently I`ve been in a place called IN AMINAS in the algerian-Lybian borders, I have found a fossile of a sea Escargot. Do this means that this region has been emerged under sea or a lake.
    Regards
    A.BADIS