30 November 2011

Continental Drift vs. continental drift

Posted by Callan

I got this e-mail this morning from a geology colleague. I’ve despecified it and made it gender-neutral:

I have a silly question about word usage and I’d love to hear your opinion.

Today I overhead [a biologist colleague] talking about continental drift, and I cringed. S/he used it in context with continents moving apart and how that can alter gene populations. I shudder whenever I hear my students use conti drift [sic], because I make the association of Conti Drift, proper, as explained be Wegener and replaced by seafloor spreading and Plate Tect.

After talking with [him/her], s/he said that continental drift is in common usage in bio and oceanography circles, and that s/he was specifically talking about conti drift, the verb (continents moving apart) rather than Conti Drift, the proper noun – the explanation by Wegener. I suspect “we” (geologists) use the phrase ‘plate motion’ where “they” (biologists) use ‘conti drift’, but to me, conti drift really isn’t quite right and “they” should be using ‘plate motion’, like “us”. Accordingly, I think that conti drift should really only be used in reference to Wegener and his notions.

Do you have a take on which is more proper?

Here’s what I think:

Continents move over time. They drift. They do this not by plowing through the oceanic crust, but with temporary “skirts” of oceanic crust forming and falling apart fore and aft of the more buoyant continental crust. Alfred Wegener’s ideas about this were partially right (continents are not fixed in their position on the earth) and partially wrong (in his notions about the relationship between continents and oceanic crust). We retain part of his Theory of Continental Drift (all caps: the proper noun) and reject part of it. With the advent of a deeper (pun intended) understanding of the seafloor in the decades following World War II, we came up with a new notion, that of seafloor spreading. New oceanic crust was generated at mid-ocean ridge systems, and extant oceanic crust was destroyed/distilled/recycled at subduction zones. This got added to the good stuff from Wegener’s legacy, and the result was plate tectonics. It’s additive:

continental drift + seafloor spreading = plate tectonics

In other words, plate tectonics is a theoretical explanation that covers both the motion of the continents and the structure and genesis of the ocean floor. So, continental drift (lower case) is a symptom of plate tectonics. Seafloor spreading is another symptom, or manifestation, or consequence, or expression, of plate tectonics. The continents are the long-lived portions of the plates; the oceanic “skirt” is ephemeral. As the continents drift, new “skirt” is knitted, and old “skirt” is unraveled.

So I think it’s fine to talk about continents drifting, especially if you’re a terrestrial biologist whose focus is limited to the continents. To be complete, I think it’s therefore also fine to talk about seafloor spreading without referencing the continental crust that also may be moving along as part of that same plate. For me, neither term carries the connotation that it has to be Considered in Capital Letters, and therefore independently of a fuller understanding of the lithospheric system.

I think this may be a pet peeve of word usage from my esteemed correspondent, but it’s not necessarily all that significant in the grand scheme of things. I’ve got my pet peeve words too, as listed in previous posts like these:

Words’ worth I
Words’ worth II
Words’ worth III
Words’ worth IV

What do Mountain Beltway readers think? Should the term “continental drift” (in lower-case letters) be junked in favor of “plate motion” or some other phrase?