Okay, so –
yes – I DID say that my last effort (Triple Junctions) was la ultima; the last. But
then I got to thinking about how I had completely ignored one aspect of RRR
triple junctions that has some fairly important geological significance, and moreover
provides a neat name to drop at cocktail parties – aulacogen.
In plain
English, aulacogen translates into “failed arm”.
Consider the
Afar TJ, featured in Plates K. I mentioned
that its southern arm, the East African Rift Zone, might someday separate east
Africa from the rest of the continent.
But what if it doesn’t? What if
the Afar RRR TJ runs out of steam before rifting is complete? Then we will be left with a long, linear valley
that after a suitable period of time will fill with sediment. Also, while filling up it may, probably would,
be, the locus of a major river system.
So, are
there any now? You bet.
Aulacogen was first used by a Soviet scientist,
S. N. Shatski, as a distinctive term for long, linear accumulations of
sedimentary rocks; as such of much interest in petroleum geology. This was about 1960, and Shatski hadn’t a
clue about plate tectonics, much less about RRR triple junctions. Subsequently, however, geological mapping has
revealed many such linear features; Google “aulacogen” and you will see. Most are inactive; prominent modern examples
include the East African rift zone and, perhaps, the Congo River basin.
As an aside –
why do RRR triple junctions form? Well,
pretty clearly, they are a response to crustal stretching, sometimes possibly caused by initiation
of a new mantle hotspot. As to why such
stretching results in three cracks – not four, or more – better ask a
physicist. I conjecture that it has
something to do with minimum work; it is “harder”, in an energy sense, to form
more cracks than just three – and a single crack won’t do the trick. Consider cooling lava flows and ask yourself “why
are so many basalt columns six-sided”? Same
reason, I suspect.
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