1/03/2010

Geological terminology I hate.

Geological terminology is always evolving. The meanings of old terms can change through time. Early suggested terms may turn out to be inaccurate and may be replaced, or may become more or less specific in their usage. Nevertheless, old habits die hard. Certainly the use of certain terms help us establish or test identity or affinities - scientific jargon at large plays important roles in social relationships.

4. Basement
"Basement" is the ultimate relativistic term. It describes the rocks one is not interested in talking about. If you write a paper about soils, the recent sediments underneath may be the "basement". If you're writing about those sediments, the granite underneath those may be the basement. Basically it means whatever is older or under or around or near the rocks one is actually interested in. My friend Mike, who loves ice and lichen and moss and squirrels and everything else more than he loves rocks, gave me another variation on "basement" the other day when I accidentally picked him up on the Golden Gate Bridge: "underburden". Nice.

3. Subduction channel
Apologies to my good friend Åke, but this one has to die. The "subduction channel" refers to the area between the subducting lithosphere and the overriding plate in a subduction plate boundary, where sediments are subducted and variably deformed and metamorphosed and de-watered. It is generally seen as a tabular region - not linear - so it isn't the shape of a channel. The term also causes confusion due to some people suggesting models where material flows only downward through this "channel", while others invoke backflow within the "channel" to help uplift metamorphic rocks which record very deep conditions. Both camps use the term "channel" and neither one is describing a 1D feature. I'm just confused by this. Finally, in a channel, the flowing media flows in one direction relative to the walls of the channel, right? But in a "subduction channel" the flowing stuff (subducting sediments) moves in a direction and rate intermediate between the upper and lower walls. It's a shear zone. Not a channel.

2. Pseudotachylyte (or pseudotachylite)
Aside from having two spellings (-yte is older and therefore preferred although it is counter-intuitive most of the time and looks weird with 2 "y"'s so close to each other), the term "pseudotachylyte" is an example of defining something by what it is not, instead of what it is. Pseudotachylyte is a glassy rock formed by either seismic or impact-related melting of any rock (but in practice is restricted to silicates). It does not therefore include a whole suite of other glassy rocks (igneous, melt cortices on meteorites, etc), so it is not a good descriptive term, but requires an interpretation of how the thing formed. Finally, tachylyte is an igneous glassy rock similar to obsidian. So pseudotachylyte is something that could be mistaken for tachylyte but is not. Oh yah, and there are other very fine grained, dark coloured fault rocks (e.g. ultramylonite, ultracataclasite) that can't be distinguished from pseudotachylyte without some serious microscopy. So it's not useful as a field term either. Fail.

1. Pan-African
As far as I can devise from the literature, the term Pan-African refers to nearly any geological event (mostly magmatic but also metamorphic, deformational, etc) occurring during a period of approximately 250 million years (roughly 750-500Ma) anywhere in Africa or continents formerly associated with Africa. My dear colleagues who advocate the use of this term tell me the exact meaning can be deduced from the context of the specific location or events being discussed which makes this term actually less useful than not using any term at all. I can think of no good reason to use "Pan-African" at all unless one is trying to obscure the problem of massive dating errors or giant uncertainty about tectonic events. THIS ACTUALLY HAPPENS.

ok just to make me sound a little less cranky, here's a comment left by somebody called NJ on Kim's blog a while ago that makes me totally happy:

"You'd better wait. My desk is totally Franciscan right now and I have no idea where to start looking.

Or:

"He completely Franciscaned his first draft and his advisor wouldn't even read it."

17 comments:

CJR said...

I am so with you on the 'Pan-African'. It is a zombie event that will not die.

Mary said...

I'm going to have to start referring to my laundry as franciscan. I'm not sure I've ever heard a more relevant and descriptive term in this context.

NJ said...

(blushes)

I am so going to steal the "underburden" thing...

enadin said...

i'm going to be Miss Cranky Two-Comments.
on your point 1) since i'm a "hard rocker" I always think of basement as met or ig. I wrote in a paper "Cretaceous granitic basement" and a reviewer asked me to clarify, mentioning the terms "acoustic basement," "economic basement," and "magnetic basement." Since I'm cranky today, I would say that if you look it up in the Dictionary of Geologic Terms, there is a pretty specific definition!
On your point 2) I think of a channel as being simply descriptive. And NOT 1D at all! Think of the Chunnel, with the trains going back and forth. I think of an artery or any tubular geometry. Why does the subduction channel have to be tabular? What the difference between a tab and a tube?
But I was convinced by your anti-pseudotachylite argument that it is a genetic term and therefore needs to be used with care.

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