Sunday, March 24, 2013

Geologists 'R Us

Geology  Rules - you bet!



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 and is instructive for a number of reasons.  We won't cover them all here, .. just to say that it would have to be the most representative expression of the Principle of Structural Superposition in that it is clearly cutting through the host rock, which in this case is granite.  The dyke, which is probably of dolerite (slightly coarser than basalt but of the same composition) is cutting, and therefore younger than, the granite.  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.  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 (among many) that Plate Tectonics ignores.

So let's consider what it is by asking some leading questions.

Did the country rock (granite) 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 - looking 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., .. with a chilled basaltic margin (both sides) (indicating cooling) (etc.,  etc.)," .. and .. (applying best teaching practice by using leading questions the class can answer themselves, .. "Which was first and which was second, the dyke or the margin?"  Class responds, "..Well obviously the granite is first, and the dyke is second.

Then the next question follows :- "So where did the granite come from?"  To which the answer is, well, .. it's coarse grained, .. it cooled slowly, and there's a great mass of it, so it must have originated at depth and cooled slowly. .. .. 
<  ... >
"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 dyke intruded, .. lost its heat to the granite as it came up, which must have therefore been cooler than the magma of the dyke."
"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 'feeded stuff'  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'?   Did anyone say erosion?   And how did *that* erosion, relate to the dyke's erosion - and what it was feeding?  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 shown by the separation of the walls of the dyke?  Did this 'sideways behaviour crumple anything?  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 the basalt 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 as this dyke, instead of in a big country-wide mass like the granite.  Fracture?  Who said fracture?   How deep was it, and how long might it have been?  What was the spatial relationship of the melts of granite (which must have been a melt originally to the basalt?  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.)

[*Footnote :- The point here being the importance of fracturing as indicative of scale of penetration of brittle failure of the crust, and the likely importance of slight 'upwards' movement as a means of creating a sideways space, rather than (as Plate Tectonics has it) 5cm of dyke a year heaving the Indian Ocean floor aside and crumpling Asia to build the Himalayas - 3,000km away - and further, deform the Russian Peninsula ["far-field tectonics"] Truly, we live on a flat ('sideways') Earth (according to Plate Tectonics).  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 cleverness 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..  The students are no doubt wondering why three thousand sideways kilometres are needed if up-and-down on the spot will do the job anyway, even if this is due to convection.  Or even better, due to lithospheric stretching - like Earth expansion says?  Why the need for all the hyperactivity?

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 / Logic /'Science'. And not getting 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.]  (Hi Oliver)  Communicating to the public on the level of three-year-olds is one thing, but precisely what is being communicated is arguable. A mindset is a mindset.  And the mind of Plate Tectonics is both naive and zombie-like.  (And a few other adjectives as well.) (if you ask me.)
 

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