Geologists 'R Us

Geology rules
( .. you bet! .. )

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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.

[This image was taken from the web a while back; can't remember where from.  If anybody objects to me using it I'll remove it, but in view of the subject of these posts it seems to illustrate a nice point (see footnote*)]

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 like mountains (and valleys) a dyke is one of the simplest, yet most interesting things to contemplate in geology and is instructive for a number of reasons.  We won't cover them all here, .. just to say that when looked at properly (i.e., from the viewpoint of the most incisive question - being, "How the Hell did it get there?"), one look is all it takes to dismiss Plate Tectonics in an instant.  Less incisively perhaps are some others that are more 'circumlocutory' steps along the way to an answer, and are therefore equally valid.

It would have to be the most representative expression of the Principle of Structural Superposition in that it is clearly cutting through, and is therefore later than, the host rock, which in this case looks like 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 on show, for that dyke epitomises a fundamental point of logic that the "outsiders" of Plate Tectonics ("without a geological clue") ignore.

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

Did the granite body 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) :-

(Team leader) :-
"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 host rock?"

"..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 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 at this point.  This is not what was expected from a simple dyke in a simple granite. Anyway, it was 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*?  Erosion?  Who said 'erosion'? Did someone say erosion?  And how did *that* erosion, relate to the dyke's erosion that we see now - and what it (the dyke) was feeding?  And how do we think this uplift happened exactly, if 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 did it come up?  Why didn't it just stay down there and likewise be coarse-grained?  And since it did 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 laterally extensive might it have been? Can we map it and find out?  What was the spatial and temporal relationship of the respective melts?  And which do you think was under the greater pressure to come up? "  Why did the granite 'come up' on the 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 next 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.)  [GoogleEarth Location :-   -22.822591°, 117.341177°]

[*Footnote :- The point here being (apart from the obvious one of erosion) the importance of fracturing as indicative of scale of crustal penetration, and the likely importance of incremental upwards movement as a means of creating sideways space rather than (as Plate Tectonics has it) ~3,000km of (sideways) movement such as are said to build the Himalayas and, further away, 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 at all if just a few centimeters of on-the-spot 'up' will do the job anyway.  Or even better if the global distribution of dykes in general are the issue, just lithospheric stretching - as the crust adjusts to outwards movement from the centre - like Earth expansion says?  Why the need for all the sideways 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-homo/eccentric fantasy and speculation (/'models'). ["India came running full speed at Asia and boom, they collided," - said.]  Communicating to the public on the level of three-year-olds is one thing, but precisely what is being communicated is another.  A mindset is a mindset.  And the mind of Plate Tectonics is set in a both naive and zombie-like cast.  (And a few other adjectives as well, i.m.o.,  when it comes to questions relating to the biggest 'dyke' intrusions of all - those  (as Plate Tectonics would have it) constituting the sheeted dykes of the ocean floors.)