[books] rant of the day: NO IT ISN'T

[kaberett]

The most recent part of Otherlands that I stalled on:

It is not a clean process, and the continents crumple, throwing land skywards under its own momentum and down into the mantle, the crust becoming nearly twice as thick as under the average continental plate. The principle is exactly that of a buckling car bonnet in a crash test, where mountains and valleys emerge from the previously flat metal sheet.

Because, per title: NO IT ISN'T.

You are getting the rant copy-pasted from elsenet rather than something freshly made, but I am laughing a bit because I indignantly made Adam read it & his reaction was almost entirely "... seems fair enough?" and to be fair I had asked because I suspected this was a quartz situation (see also the associated explainxkcd) but even so.

The problem, to be clear, is specifically the phrase "The principle is exactly that of..." (emphasis mine). Herewith the rant:

ITEM THE FIRST: buoyancy is an absolutely crucial component of tectonic collisions. These plates float on top of the mantle because they are lighter than the mantle. (This is why oceanic plates subduct, eventually: they're light enough to float on top of the mantle when they are still hot and therefore less dense. As they cool they get denser. Sooner or later Something trips them, they stop being able to float, and they start subducting. This is why we have billion(s)-year old continental crust but the oldest oceanic crust is 250 million years old.)

A rigid sheet of metal suspended in air by its own strength does not have buoyancy going for it.

Specifically, buoyancy is some of why mountains are tall. If you smoosh a bunch of continental crust together so you wind up with a double-thickness layer, instead of sinking IT FLOATS! Like The Iceberg! Mountains are tall because they are Like The Iceberg not just! because they've been. folded up. (And then because it's sticking up it causes Weather to happen to it, which starts eroding it down again. The taller things are the faster they erode. Valleys are caused not so much by Cromple as by THE FUCKING PRECIPITATION.)

[This is the point at which I stopped editing for readability except for putting the italics back in...]

SECONDLY collision is not simply mechanical deformation by, like, pushing on something's nose. and folding it up neatly concertina-style. you are layering shit. it's sort of like trying to shove a baking sheet underneath a part-assembled jigsaw puzzle. the heat and pressure cause widespread chemical AND MECHANICAL changes, with some degree of mixing, INCLUDING VOLCANISM that is identifiably full of shit FROM THE LOWER PLATE, which is essentially Where Metamorphic Rock Comes From. (igneous rocks are also produced in these settings but the significant majority of igneous rock is produced along mid-ocean ridges. which are underwater. so we don't notice them much most of the time. whereas the shit that happens on land fucks us up more!)

ITEM THE THIRD, not unrelated: A CAR BONNET IS A SHEET OF UNIFORM CHEMICAL COMPOSITION AND FAIRLY UNIFORM PHYSICAL AND MECHANICAL PROPERTIES (don't @ me crumple zones ARE a thing I'm acknowledging their existence) if he'd even!!!! said "you can think of it like"!!! I'd be grumpy but not so cross I'd had to stop reading for 24 hours and had expunged The Reason For Same from my consciousness!!!

but NO he has just fucking BLITHELY forgotten everything he EVER learned in geophys at undergrad because frankly he does not care because metamorphism fucks your fossils right up.

and! I'm Cross!!!

Comments

[shanaqui]

♥ I love your explanations.

[redbird]

That is a fine explanation. Also, using "exactly" for that sort of comparison is practially asking for trouble.

[fyreharper]

“The principle is a bit like…” would have worked FINE. BUT NO. :grumpily: he’s probably the sort of person who uses ‘literally’ to mean ‘figuratively’, too, isn’t he >:-(

However!
(This is why oceanic plates subduct, eventually: they're light enough to float on top of the mantle when they are still hot and therefore less dense. As they cool they get denser. Sooner or later Something trips them, they stop being able to float, and they start subducting. This is why we have billion(s)-year old continental crust but the oldest oceanic crust is 250 million years old.)
I did not know this! I am delighted! I have learned a thing!

(Also your rants are great :D )

[kaberett]

<3!!!

[fyreharper]

Okay also I have a followup question: so why does *continental* crust float then? Since it’s evidently doing A Different Thing?

[ysobel]

I love this rant!

[silveradept]

That is an excellent explanation, and I have not generally thought of our things that we walk upon as floating objects instead, but it makes sense to think of them that way, compared to what they are floating on.

[azurelunatic]

"exactly" and metaphors-to-aid-understanding DO NOT MIX WELL! One, one, eleventy, one.

[hairyears]

As always, a clear explanation of complexity is far, far more interesting than an oversimplification.