The far-travelled ground

A post by Chris RowanBe honest: when Evelyn asked the geoblogosphere, ‘what’s your favourite geology word?’, you all knew which word I was going to pick, didn’t you? Allochthonous. Allochthonous, the word that no-one can spell. Allochthonous, the word no-one can pronounce, it seems, given that I am asked how you do whenever I meet a reader of this blog. I personally think it trips off the tongue quite nicely. It is also a word with heft: it sounds like it means something, something important, interesting, monumental. And it does.

‘Allochthonous’ is derived from two greek words:

  • αλλος or allos, meaning other, or different;
  • κθονος or kthonos, meaning earth.

So, literally, allochthonous means ‘different earth’; or ‘stuff that’s not from around these parts…’ In tectonics, it is generally used to describe a sequence that has been moved a long distance from its original location by faulting (most usually, thrust faulting). For example, a sequence of sediments which were originally deposited in the deep sea, and have then been thrust over shallow marine or continental deposits of a similar age.

The formation of an allochthonous sequence by motion along a large thrust fault. The 'autochon' is the stuff that stays put, relatively. Colour coding represents continental (red) shallow marine (yellow) and deep marine (green) rocks.

Ophiolites would be another good example: the ocean crust itself thrust up from the depths onto the shore.

Part of the lower crustal sequence, Oman ophiolite. Photo taken by C. Rowan, 2010

In a wonderfully evocative turn of phrase, John Challinor’s Dictionary of Geology describes an allochthon as ‘the far-travelled ground’, and I think this gets to the heart of why I’ve always loved the word: it embodies the central idea of plate tectonics, the notion that the earth is continuously being reshaped and transformed by the large-scale movement of bits of crust across the Earth’s surface. Allochthonous thrust sheet, or nappes? You’re in a continental collision zone, where the opposite shores of a once large ocean – and bits of the ocean itself – are all mashed up against each other. Allochthonous terranes? Basically a larger scale version of the same thing, with island arcs and microcontinents once spread across half the planet getting welded together above a subduction zone. Allochthonous conjures up all of the things that I find most fascinating about geology. Plus, it has lots of syllables, something I also have an unhealthy fascination with…

As it turns out, the word allochthonous does also pop up in other branches of earth science, most particularly within the watery realms of my co-blogger. There, it is used to describe material, particularly organic matter such as leaves, which fall or get washed into a stream or lake from the surrounding land (Anne has now provided a more detailed explanation). I don’t think Anne thinks it’s quite as cool a word as I do, but it’s nice that the name of our blog does have a meaning in both of our areas of research.

Categories: basics, geology, hydrology, tectonics
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Comments (9)

  1. Gnarlodious says:

    You are wrong, anyone who ha a working knowledge of ancient Greek knows what the word means from its parts and can even spell it.

    “Allo” is a common prefix; allopathic (medical), allotropic (physics), allosaurus (paleontology)… etc. Any educated student from a public school would have known this stuff 100 years ago. But nowadays…

    “Chthonic” is a common word used in anthropology, myth study, witchcraft and shamanism. It means the spiritual forces of the underworld, seduction, death, decay and rebirth. Of course there is none of that icky stuff going on in western civilization.

    Thanks for an interesting read.

    • Chris Rowan says:

      I almost deleted your comment for its misplaced pendantry. Are you being serious? Allo is a common prefix which generally means ‘other’ or ‘different’. All of the examples you use are, well, examples of this (even if in the case of allopathic medicine, it’s used in a derogatory sense).

      And chthonic also seems consistent with its Greek root if it’s associated with the underworld. Just because it’s got more exotic meaning in modern usage says nothing about the original meaning of the word in ancient Greek.

      Ironically, of course, you are criticising someone who actually did study ancient Greek at school….

      • Suw says:

        I believe what Gnarlodious may have been driving at is that, as ‘allo’ is a prefix which means ‘other/different’, as you point out, and ‘chthonic’ means ‘of or inhabiting the underworld’, then ‘allochthonous’ must mean ‘The Other Cthulhu’.

        Ph’nglui mglw’nafh Cthulhu R’lyeh wgah’nagl fhtagn.

        Sheesh, and you say you studied Greek…

      • Gnarlodious says:

        Not being critical, just stating that knowledge of Greek used to be standard educational material. If people think a word like allochthonous is a mouthful they didn’t study the basics of science, literature and language.

        Also, Suw amusingly brought up a good insight I did not notice before. Even though the word Cthulhu is not literally the same, phonetically it has association to the underworld. And succeeds in triggering a subconscious sense of terror at the unknown.

  2. Lyle says:

    How about autochthonous which is allochthonous first cousin, but is a stay at home, while allo is a wander. Actually that might be a good way to explain the difference. Ones a stay at home and the other a wanderer.

  3. Ryan Brown says:

    @gnaroldius- What is plate tectonics if not the death, decay and rebirth of the earth’s plates? If a plate is subducting, it eventually melts, becomes magma and returns to the surface of the earth through volcanism. It seems to me that it’s not Chris’s greek at fault here, but your own understanding of basic geology.

  4. Chris Rowan says:

    Note to all: this is a post about geology, not linguistics. I referred back to the greek roots of allochthonous because that provides some insight into its geological meaning. It’s part of an explanation. Explaining being what I like to do on this blog.

    Now stop making me cranky.

  5. Susanna says:

    I am put in mind of a happy field trip evening in Mull, playing charades. The word we were trying to guess was “ferrostilpnomelane”. It took a while.

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