Saturday, July 17, 2010

Magnetic Termite Mounds?

A typical Pilbara termite mound

Termite mounds are ubiquitous in arid Australia.  I’ve looked at them with variations of awe and boredom for twenty five years – mostly from trucks.  For some reason, they are not very abundant in the areas I’ve studied the geology.  So most of my observations below are passing, literally.  Termite mounds aren’t anthills; I’ve seen one over four meters – seventeen feet - high and with a volume of at least three couple cubic meters.  They’re commonly at least a meter high.  Whatever the size, mounds are truly edifices.  They have vaguely columnar shapes with rounded tops, and rise abruptly from the surrounding landscape, whether it’s a grassy plain or a rocky slope.  They seem anomalous and clearly organic.  Texturally, the surfaces of the mounds are composed of several thousand oval blobs the size of a finger accreted together to form a single mass.  Imagine making a sculpture out of toothpaste squeezed from tubes, and you’ll get the idea.  Internally, they are a honeycomb of passageways, based on the abandoned and decayed mounds I’ve come across. 

Close up of a Pilbara termite mound

Termite mounds seem to vary geographically in two ways: color and shape.  Color certainly reflects the underlying soil.  In the Pilbara they’re most frequently deep red-brown.  In the Red Centre, they are a lighter orange-red.  In the East Kimberley and NT, they are tan to brown.  Mounds seem to be biggest in the NT and smallest in the Red Centre.  In terms of shape, Pilbara mounds are massive, and often seem to have a point on top.  Red Centre mounds are more truly columnar in shape.  NT mounds are particularly elaborate; many feature multiple columns amalgamated into a single mass.  I wonder if this more complex structure reflects erosion caused by higher annual rainfall.

I should also say that in all three areas, termite mounds are either loners or joiners.  The joiners occur in fields of up to several dozen mounds, each a unique shape and size.  I plead etymological ignorance: I don’t know how connected they are underground, nor how mounds propagate within a field.  On the other hand, in the Pilbara at least, the really big mounds are isolated.  It’s possible that these variations reflect different termite species.

Pilbara - a field of termite mounds

So what about the so called Magnetic Termite Mounds?  When I read that these things existed in Litchfield National Park in the NT, I filed them away as something to go see if time allowed.  The MTMs, as I’ll call them, sounded like a variation on my categories of termite moundness.  I figured that the termites either incorporated magnetic minerals into their mounds, or that they were magnetically oriented in some way.  Intriguing, but seeing them was a back burner plan: termite mounds are interesting, but hey, they are not rocks.  Time allowed, so as I wrote in an earlier post, Paul and I headed to Litchfield in hope of a final night outdoors in the NT.  We were thwarted by the heat and crowds, but not before seeing the MTMs.
The access road into Litchfield wound through outcrops of red sandstone and conglomerate.  Our progress was slowed by any number of slug-like vehicles; cars towing trailers, locals in old cars, and distracted tourists.  I passed them when I could.  Eventually the MTM pullout appeared.  It was large and full of vehicles.  There is not much else to do on the way into Litchfield except this stop, other than sweat and pass slow vehicles.  Or maybe drive one. 

We got out and geared up.  A short stroll along a boardwalk ended at the edge a flood plain.  The large, crescent-shaped area looked like it got pretty damp in the Wet.  It was full of termite mounds.  I was dumbfounded.  They had huge sheet-like shapes, like giant cooling fins.  I had brought my GPS unit along to see about the magnetic business.  Using its compass app, it was soon clear that the MTMs weren’t oriented with any relation to the Earth’s magnetic field.  This had been my assumption.  Instead, they formed a long arc that followed the broad curve of the stream drainage where they occurred.  What to make of this? 
I thought the MTMs were brilliant.  What whimsical shapes to see, what a mystery on a hot winter day.  As with much of my experience in the NT, they challenged my mental model of termite mound-ness.  They were worth the drive to Litchfield. 


More MTMs, note "normal" termite mound in rear.

The park signage offered an explanation for the MTMs.  Their environment is unusual; most mounds form in dry settings, while the MTMs have been built in a stream channel.  There was direct evidence of this at the MTM site.  A “normal” termite sat just off the stream drainage.  Anyway, the wet setting means that when it gets hot, the termites can’t go underground, as they apparently do in normal environments.  To avoid sauna like conditions the MTMers built these giant platy mounds, which have more surface area and thus stay cooler.  This seems plausible to me; if not it’s a good hypothesis.

One final comment: termite mounds of any shape and origin are hard.  I kicked one, and my foot hurt for several days afterward.  I doubt I could knock one over with a truck.

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