Sunday, March 24, 2013

Geologists 'R Us

Geology  Rules - you bet!



Fig.1. Students contemplate a basalt dyke. And appear to be expressing manifest disinterest. But I don't think they are. I think they're being very diplomatic. 


Being centre-stage, the subject of this photo is almost certainly the dyke. but I do wonder at the apparent disinterest, .. and why that might be.

It must be the first or second field trip when students get taken to look at a dyke, because a dyke is one of the most interesting things in geology for a number of reasons.  Firstly, it would have to be the closest igneous equivalent of stratigraphy in the Principle of Stratigraphic Superposition, which says one sedimentary layer on top another means that the top one is younger than the bottom one.  In the above case the dyke is cutting, and therefore younger than,  the granite.  Usually at any rate - unless there is some structural dislocation between the two that can contrive to put an older unit on top of a younger one, as is common in thrust belts, .. or in the case of the dyke here, contrives some faulted margin that juxtaposes something that rightly shouldn't be there, .. so complicating the issue.  So properly speaking we should refer to The Principle of Stratigraphic, Structural, and Magmatic Superposition. This is the triumvirate of Earth processes operating at different scales and different levels in the crust that allows proper time-sequencing of geological events and interpretation of Earth history. The other reasons relate to larger questions of global significance as sketched below.

What seems interesting to me about this picture is why somebody is taking it, .. and that that somebody is probably the group leader who has decided to take a picture in order to save himself the bother of saying a thousand words about something to do with this particular pit-stop, and which is self-evident.  And maybe something too about the apparent disinterest.  For that dyke epitomises a fundamental point of logic that Plate Tectonics ignores.

So let's consider this by asking some leading questions.

Did the country rock, which is granite in this case move sideways to let the dyke in, or did the dyke (being magma - an incompressible fluid) intrude and forcefully heave the host rock aside. Ostensibly the students appear to be looking for an answer.  One at least seems to be convinced it lies underfoot, whilst the others seem to think it lies somewhere off to the side.  [Well, .. at least they're looking at the right side.] [If they look at it my way, that is.]

So, .. pit stop, ..questions, .. thinking, .. photograph.   Here's my take on it (it goes  like this) :-

"Here's a dyke, .. etc., etc., .. and a chilled margin (both sides) (indicating cooling) (etc.,  etc.)," .. and .. (Applying best teaching practice and leading with a question they can answer themselves, .. "Which was first and which was second, the dyke or the margin?"  Class responds, "..Well obviously the granite (light-coloured in the figure) is first, and the dyke was second (that is, since the outcrop is good and we have already established it *is* a dyke cutting through a mass of granite) (it needn't always be quite so obvious, if the outcrop is poor - in fact like everything else it often isn't.  (A sense of scale is a good thing to have, when approaching the conundrums of geology.)

Then the next question follows :- "So where did the granite come from?"  To which the answer is, well, .. it's coarse grained, .. cooled slowly, and there's a great mass of it, so it must have cooled at depth.
<  ... >
"So what's the answer?"
"Deep."
"And where did the dyke come from then?"
"Deeper."
"But the dyke is fine-grained and chilled quickly, so where did it crystallise in relation to the granite, and how did it lose its heat?"
"The intruding dyke lost its heat to the granite, which must have therefore been cooler than the magma."
"So how did the granite get from being in a hot place to being in a cold place?"
"It must have been uplifted."
"Right? Who thinks that's right? ... How much granite have we got here?"
< .. The whole country .. >
"So how did the whole country get uplifted from a hot place to a cold place?"

You can see here the students beginning to shuffle a bit.  This is not what was expected from a simple dyke in a simple granite. It was, after all, supposed to be about the dyke, not the granite.

"And what about the dyke?  Where did it come from, what was it feeding? And where is all of that feed now?  And if the granite cooled at depth and got uplifted so we can stand on it, what happened to what was on top of *it*?  Who said Erosion?   And how did *that* erosion, relate to the dyke's erosion?  And how do we think this uplift happened exactly, given that it applies to the whole country, and the dykes (and that little sill over there - out of the picture) are not folded?  And what do you think uplift means for the 'sideways' aspect of this intrusion; how much 'crumpling do you think happened? And where did the *granite* come from in relation to the basalt, if it was at depth long enough to cool down and be coarse grained?  And if the basalt was below the granite in the first place why didn't it just stay down there and likewise be coarse-grained?  And since it 'decided' to come up, why did it come up in such skittery bits, instead of in a big mass like the granite - as big as the country?  Fracture?  How deep was it, and what sort of lateral extent might it have been?  What was the spatial relationship of the granite (which must have been originally a melt) to the basaltic melt, and which do you think was under the greater pressure to come up? "  Why did the granite 'come up' on a scale of the whole country while the basalt is just coming up what is essentially a hairline fracture or less?

This is the bit where the students begin to look right and left, and realise that what he's going to say ... is ...

" And let me have your thoughts by Monday."
















Fig.2.  Filaments of NW-striking dolerite dykes intrude a diapiric granite pluton.  (Pilbara region, Western Australia.)  [ Google Fly-to =  22°49'35.74"S, 117°16'46.88"E ]


[*Footnote :- The "nice point" being the importance of 'up' (and down) (rather than 'sideways') in the scheme of Earth forces, bearing in mind the lunatic view that 5cm of dyke heaving the Indian Ocean floor aside could cause India to crumple Asia and build the Himalayas - 3,000km away!   Truly, we live on a flat ('sideways') Earth.  This, I think, could well be what is occupying the collective minds of the students in the picture, which is why they're shuffling and contemplating their boots.  But how are they going to weasel such idiocy into their class exercise on which they'll be judged at the end of term?  It's all very well for the group leader to be asking suchlike questions implying 'up', but they know perfectly well that 'im indoors, .. their professor, waxes lyrical about sideways Plate Tectonics being the best thing since sliced bread and deserving a Nobel prize, with five centimetres of dyke three thousand kilometres away heaving up the Himalayas and all..  Why are three thousand sideways kilometres needed anyway, if up-and-down according to convection (whether it be lithospheric 'skin' or mantle cells) is the issue?  Why can't it all just happen on the spot according to the up-and-down of lithospheric stretching - like Earth expansion says?  Why the need for all the hyperactivity?

(That's just a punt for stretching versus convection in the scale stakes - taking into account the rest of global geology.)

Let's hope that's what they're doing anyway (contemplating).  We need a whole new crop of geologists apparently.  Otherwise another generation is (well and truly) screwed., and screwed up.

Geology.  It's all a question of scale. Observation.  And logic.  And not getting prematurely carried away by anthropomorphic-homoeccentric fantasy and speculation. ["India came running full speed at Asia and boom, they collided," MIT geology Professor Oliver Jagoutz said.]  Communicating to the public on the level of three-year-olds is one thing, but precisely what is being communicated could be arguable. A mindset is a mindset.  And the mind of Plate Tectonics is both zombie-like and naive.  (And a few other adjectives as well.)
 

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