Annoying misconceptions in Geology

For various reasons mostly beyond my control, I haven’t really had time to write an entry for for this month’s Accretionary Wedge, the theme for which is “(Un)Favorite Geological Misconceptions”. Fortunately, this very subject was the theme of a post from way back in the early days of ye olde blog. Since this was also a time when my readership wasn’t much larger than me (and I half expected it to remain that way), I feel quite justified in dusting it off, giving it some polish (since I have revisted some of these themes in later essays) and reposting my list of pet peeves. As for the ‘pie’ sub-theme, I’m with Julia: most good pies are savoury pies, and steak and stilton pies are the best of the lot. But I digress.

Tectonic plates move around on top of a sea of molten lava. Many people mistakenly believe that the beneath the Earth’s crust lies a seething mass of molten lava, ready to erupt at any minute. Not true. As the figure below shows, although the temperature of a rock does increase with depth (as shown by the red line, the geotherm), so does its melting point due to the immense pressure exerted by tens of km of rock piled on top of it.


As you can see, at no point does the geotherm cross the solidus (the temperature at which melting would start), so the rocks remain solid even at great depths. However, below about 150-200km, the geotherm gets quite close to the melting point, which allows rocks to more easily deform and flow (a process known as creep). Think less molten lava, more Silly Putty. Mantle convection can therefore occur below this depth, even though the rocks are not melted. The more rigid, non convecting rocks above this boundary form the lithosphere, which forms the major part of tectonic plates (the crust is just a skin on top).
Melting to form magma, and hence volcanism, only occurs when something happens to disrupt this situation; three common ways that this happens are shown below. The geotherm can cross the solidus by being shifted upwards by rifting (which thins the lithosphere and allows upwelling of hotter mantle to shallower depths), or across by a mantle plume (an upwelling of hotter material from deeper within the Earth). The melting point of the mantle rocks can also be reduced by a change in composition; at subduction zones, this is caused by the addition of water from the downgoing plate.


This is the reason why volcanoes only occur in particular geological environments on the Earth’s surface, rather than everywhere.
What earthquake and volcano ‘prediction’ actually means. Sorry folks, but it’s never going to get to the stage where we can point to a particular volcano or fault and say, “this will erupt/rupture next Thursday at 11:00″. For both volcanoes and faults, geologists can try and reconstruct the past history of eruptions and earthquakes, to see how regularly they happen. They can also watch for signs of imminent activity: the filling of a volcano’s magma chamber might be accompanied by small earthquakes and ground uplift; the build-up of strain across a fault can be monitored by GPS. This gives some clues as the the likelihood of something happening, but it’s just that – a probability, not a certainty. Volcanoes and faults are complex systems which exhibit chaotic behaviour – large changes in outcome are caused by tiny variations in the initial conditions. In the case of a fault, sometimes when a small section of the fault fails, the rupture remains very localised and you get a small earthquake, only detectable by instruments; sometimes that small rupture will trigger failure over a much larger section of the fault and cause a big earthquake. So although you can easily measure when a lot of strain has built up across a fault, predicting which of the many tiny earthquakes, that happen all the time, is going to turn into the big one which releases it all is probably impossible (as is explained in much more detail here).
So, unfortunately, the most precise forecast you’re ever going to get is, “there’s a high (or low) likilihood of rupture or eruption in the immediate future”. And, particularly for earthquakes, “immediate future” could well mean “in the next decade or two”…
There is one ‘geological column’ for the whole Earth. The geological timescale divides the 4.5 billion years of earth history into distinct chunks, based on changes in environment or dominant forms of life over time. However, this does not mean that every rock of a particular age is the same. Just like today, in these periods many different processes were leaving their own unique record; marine deposits formed in shallow seas, deltas built out from coastlines, deserts formed vast dune fields. In other places, uplift of ancient mountain ranges led to no deposition, and erosion of rocks that had been laid down in earlier times. So there is no one geological column; each area has its own unique sequence of rock types, often with large time gaps in between different units. Correlating them all with each other is one of the major tasks of the geologist (see also last year’s more detailed debunking of this misconception).
All radiometric dating is ‘carbon dating’. You wouldn’t believe how many times this has come up in undergraduate exam papers (hopefully not as a result of anything I said in lectures), let alone amongst the wider public: the notion that all radiometric dating involves 14C. It’s the isotope most people have heard of, because it is so widely used in archaeology. However, trying to ‘carbon date’ rocks of any sort except for very young sediments is generally pointless; 14C has a half-life of 5,730 years, which means after about 50,000 years it has virtually all gone. Assuming there was anything to begin with, of course; there’s not generally much carbon in volcanic rocks. This doesn’t matter, because there’s a whole array of different isotopes with half-lives much better suited to dating rocks tens of millions of years old.
Reversals of the Earth’s magnetic field are instantaneous.
Because it is generated by vigorously convecting molten iron in the outer core, the Earth’s magnetic field is quite dynamic by geological standards: the magnetic poles shift their position by around 10 kilometres every year. A more extreme type of behaviour is the periodic reversals of the field’s polarity: every million years or so, north becomes south and south becomes north. However, this does not mean that we could wake up tomorrow and discover all our compasses pointing in the wrong direction. Although detailed and well-dated records of a field reversal are quite hard to come by, those that we have found indicate that the reversal process actually takes several thousand years to complete. The magnetic dipole field gradually weakens, and then there is a period where the magnetic field goes a little haywire: there may be more than one magnetic north pole, meaning that your compass will point in entirely different directions depending on where you are on the Earth’s surface, and they can move further away from the geographic poles than they currently do, even to the equator or the opposite hemisphere (more technically, the magnetic field energy that is presently found in the dipolar field is probably instead used to generate quadrapolar or octapolar fields). Eventually the dipole field begins to strengthen again, and since both a ‘normal’ and ‘reversed’ polarity field are equally stable, it will often regenerate in the opposite polarity to the one that preceded the collapse.
It all sounds quite wild, but in reality these changes all happen at pretty much the same rate as the secular variations in the field that we’ve been seeing for the last few hundred years, meaning that it would take us at least that long again to notice ‘reversal-like’ behaviour, and it’s certainly not anything which is going to crash ships, screw up computer systems or fry us all with cosmic rays a la The Core.
A geologist can tell you the life story of a random pebble you’ve picked up on the beach. Don’t get me wrong, pebbles can be useful – they tell you about local rock types along that section of coast, for one thing, which is very handy when you’re mapping. Just don’t be disappointed when the most information you get from me is ‘sandstone’. The most interesting geological information often comes from features which are only found at larger scales than a pebble – where a particular rock type occurs in a sequence, for example, or the sedimentary structures or deformation that are found in large outcrops.
Blowing up Earth-bound comets and asteroids at the last minute will save the day.. Nope, we’d still be doomed (OK, this is tenuous geologically, but it does annoy me).

Categories: basics, geology

Comments (16)

  1. divalent says:

    nice concise description of the melting conditions. Isn’t it true that melting generally results in a decrease in density, so that once started, the tendency is for the molten rock to rise, taking it farther into the melting domain? (so its a race between cooling and rising that determines whether there will be a surface eruption?)

  2. Laelaps says:

    Well done, Chris. This actually reminded me that I had promised to write something up about uniformitarianism and catastrophism, although I don’t know if I’ll be able to get it finished in time. Still, I’ll be looking forward to see what else shows up in the carnival.

  3. Ian says:

    I loved this. Thank you!

  4. Julia says:

    Nice one Chris. So you get the plaintive cry along a beach of “Chriiiiiiis? What’s this rock?” then…
    As for the carbon-14 issues, my father-in-law cites the inaccuracies of carbon dating as evidence that the earth isn’t as old as us geologists “think” it is (he’s not quite YEC – he watches Walking With Dinosaurs and says “225 million years? I don’t buy that. I could be happy with 127 million years…”). I like to ask him if he’d measure the dimensions of the back garden using a 30cm ruler, and try to explain that THAT is why carbon dating can’t be used on fossils older than about 50ka. It’s a bit simplistic, but I’ve started to see a glimmer of understanding…

  5. chezjake says:

    Thanks for another excellent post.
    I’m a Yank, but I agree with you on savoury pies.
    I’m also going to suggest to John Wilkins that he add this post to the collection of “basics” posts.

  6. bigTom says:

    Regarding melting. It would help to provide some indication of the adiabatic lapse rate, i.e. if I move a rock sample up/down say 1KM don’t allow heat to flow in/out its temperature will change with the pressure. A solid rock which cools slower than the solididitus as it rises is potential magma -it it can the convected up.
    Blowing up comets & asteroids in the last hour. I’ve long felt this might marginally help. Under the theory that the amount of long-lived debris left in the upper atmosphere is a nonlinear function of the impacter size. If all the pieces are small enough, and well enough dispersed you might avoid the climate impact.

  7. rob says:

    Thanks for the article! I was a geology major in college though now I work as an environmental scientist. I think that one of the biggest myths in geology is simply that all geologists do is look at rocks. most people dont realize how vast and varied geology is as a science. I always tell them of my structural geology final that took me 8 hours and was actually technically only one problem. i didnt look at any rocks at all during that test

  8. Ijon Tichy says:

    It’s likelihood, not “liklihood”.

  9. Lab Lemming says:

    A good pie shop will do excellent breakfast, lunch, dinner, and dessert pies.

  10. Chris Rowan says:

    divalent – indeed so, this is something which comes into play when looking at mid ocean ridges with slow and fast spreading rates.
    Laelaps – you should definitely write that article, it’s a subject I’ve been wanting to address myself at some point in the next few months and I’d be fascinated to hear your thoughts.
    Julia – oh, yes. Sometimes it’s really tempting just to make stuff up.

  11. Dave Briggs says:

    Blowing up Earth-bound comets and asteroids at the last minute will save the day.. Nope, we’d still be doomed (OK, this is tenuous geologically, but it does annoy me).
    Glad you mentioned this! The more I study the science involved the more likely it seems one could show up at our door with just enough time to send quick good bye cards to all your friends via e-mail! LOL
    Dave Briggs :~)

  12. Jez Rowan says:

    I’ve missed your writing style Chris. And strange as this may sound you seem to be the only man alive that actually hates The Core more than I do because you actually know why it’s wrong, whereas I just presume so going by Hollywood’s dutiful care of science and historical fact… Favourite (what annoys me as I write that word is that Firefox is telling me to drop the ‘u’!) bit of The Core has to be in the meeting where he demonstrates what’s going to happen by setting a tennis ball alight with a can of deodorant. You know, because you can just imagine that convincing a room full of ‘experts’ and world leaders that you’re right. Hmmm….maybe I should have adopted that technique at my Oxford interview…

  13. Kim says:

    Ok, Chris, I’ve got one for you.
    My local search and rescue guys don’t understand the cause of Earth’s magnetic field. One of them explained magnetic north as the result of “the magnetic blob” somewhere “up there,” about eleven degrees from the north pole (from our perspective). He knows that the magnetic declination changes, but he thinks that’s because the magnetic blob floats around.
    My husband wants me to explain it, but when I try to explain the magnetic field, I end up waving my arms and saying “and the outer core is iron and it convects and electicity and magnetism are the same thing and that makes a magnetic field.” But I know it’s more complicated than that.
    Also, whenever I type “magnetic blob” it comes out “magnetic blog.” And I figured this is the (paleo)magnetic blog, and you can probably do a better job explaining it than I can.

  14. Chris Rowan says:

    Well I did write this a while back…
    Is it that the search and rescue people think the ‘magnetic blob’ is actually in the crust?

  15. Mike says:

    You might want to point out that there IS a “seething mass of molten lava” within the earth – the outer core. This also means that there’s a fourth way to “melt” the earth, where the composition of the earth changes from rocky mantle material to an iron/nickel alloy, with a much lower melting point. Of course, the stuff is so dense it will never erupt, but pointing out the existence of a molten outer core helps you resolve the apparent contradiction between only localized melting in your first major topic, and the need for a molten layer in the explanation on the magnetic field

  16. Ana Rod says:

    Can someone help please?
    When people go down in caves sometimes it gets really warm. Is it because you are closer to the core or is that irrelevant at this kind of deep? If that is, why does it get warm them?
    Many thanks!