Weathering it out

Puyehue Cordon Caulle, September 17th 2011.

Last night the NASA Earth Observatory posted up a natural colour satellite image of Puyehue Cordon Caulle – the volcano in Chile which has been erupting since June (and which I wrote about here).  The eruption itself started on the 3rd of June, and has not really stopped since.  It’s going to take a while before we manage to get a proper handle on how much material has been ejected, but the plume has caused widespread disruption in the Southern hemisphere throughout that period.

The thing I most wanted to draw attention to from this image is actually how little expression this eruption has had on the immediate surrounding area in this time.  Bearing in mind that there has been widespread fall of pumice (check out the video below which shows pumice rafting on a local lake) one might expect a widespread blanket of fall to be obvious in the region around the volcano.  While there is certainly an effect within 10km of the vent, the outlying area is almost completely unaffected at this scale.

Pumice rafting, June 20th. Click for bigger.

That’s not to say material has not fallen here; what is quite impressive is the capacity the area has shown for dispersing these tephra.  The June 20th images (right, top) show very obvious and large pumice rafts accumulating on lakes 40km or so East South East of the volcano.  The more proximal deposit is more clearly demonstrated in these false-colour ASTER images from June 11th (red is vegetation, grey is ash accumulation).  It is clear that large areas have had significant build up of material, yet which have very little observable footprint in the most recent images – the pumice rafts are gone, and the snow appears to have little change in albedo.

False-colour ASTER image, June 11th. Click for bigger

Perhaps most strikingly, east of the mountain range we find a very significant dispersal and deposition of ash. The image below shows a natural colour satellite image from 13th June, in which tens of thousands of square kilometers of Argentina are blanketed by ash.  This ash blanket is still visible in the 17th September image.  while it is true that the dominant weather systems have had the plume travelling East of the volcano, there have also been significant periods where the plume has been transported West, over Chile, yet here we observe no ash blanket.

13th June ash dispersal and deposition. Click for more info.

My assumption is that this is a rain shadow effect; the wet Western side of South America is washed clean regularly, whereas the dry Argentinian side does not see enough rainfall to clear the blanket of tephra.

As far as geology goes, this may have some interesting lessons for us.  Volcanostratigraphers tend to map tephra dispersal in terms of isopachs – essentially contours of at what distance certain thicknesses of ash are achieved. These are strongly affected by dominant wind patterns, but I wonder how often people looking for tephra from particular eruptions consider the environment in which they are looking?

It is generally assumed that fall deposits from volcanoes (i.e. pumice and ash) simply drape the topography, while flow deposits (pyroclastic flows, lahars and so on) fill valleys. The image below is a composite photo I took near Rotorua in New Zealand, showing a fairly ideal drape of fall material with almost constant thickness lying over a what had been an outcrop of rock.

If we instead begin to consider that large volumes of fall deposit can be transported quickly and efficiently in wet environments, then several issues become apparent.  Tephra thickness measurements will be directly effected by where in the palaeotopography they are found; topographic highs will be depleted of material, while the palaeovalleys may be relatively thick (or, to the other extreme – stripped completely).  Workers looking to tephra for stratigraphic correlation would encounter similar problems, with apparent thickness differences (or just absence) of material potentially leading to miscorrelation.  The palaeoenvironment is often considered in terms of being erosional or depositional, but – and I admit I’m exposing myself as a stereotypical Englishman here – how often do we talk about the weather?