Geology with altitude: the road to ALMA

The road to ALMA cuts through solidified pyroclastic flow

One of the things I didn’t have space to go into detail on in my article “Telescopes with Altitude” (November 2011 issue of Astronomy Now) was the amazing geology, flora and fauna of the area, particularly on the drive up to the ALMA high site. (If you’re new here, ALMA is the Atacama Large Millimeter/submillimeter Array, which will eventually comprise 66 antennas that will work together to track down some of the mysteries of the cold and distant Universe. See previous blog posts Telescopes with Altitude and Telescopes with Altitude: the movies!). 

To set the scene we’re starting this particular story at around 3,000 metres altitude, at the ALMA Operations Support Facility, the base camp for the observatory where the antennas are built and the scientists control the telescopes and process and analyse the data that is collected by the current twenty operational dishes. We’d just been checked out at the medical station, stowed some portable oxygen canisters in the van, and set out with 5,000 metres altitude in site. 

Desert life: a vicuna

Shortly after, at about 3,300 metres, our guide, ALMA project scientist Richard Hill, pointed out an old farm, which was occupied until quite recently by a family and their donkeys. How anyone could farm anything in this arid desert is beyond me but apparently they did just fine!

After that, there was very little sign of life, except for one vicuna (a little bit like a llama) which, with its orangey-brown fur took a moment to spot against the similarly coloured desert background. We also learnt that the silky wool from these animals is extremely expensive – up to thousands of dollars for a single garment made from the precious material – and as a result, and to prevent the creatures becoming extinct, it is illegal to kill them. Instead, gatherers must collect the fur from cactus or other plants that the vicuna fur has caught against.

With every 500 metre ascent there was a change in vegetation. At around 3500m there’s towering cactus plants and higher still these give way to ground-hugging bushes. Richard pointed out short spiky bushes which are somewhat tongue in cheek nicknamed “mother in laws cushion” – the group were pretty amused this humour applied even half way round the world. 

Cactus give way to ground-hugging bushes at high altitude

Over in the distance the volcano Mount Lascar was steaming away, which last erupted in 2006. We then learned that the road we were driving on cut right through solidified pyroclastic flows – the once fast-moving surge of superheated gas and debris that hugs the ground as it hurtles out of the volcano at hundreds of miles per hour. One of the group asked about the threat to ALMA by volcanic eruptions; Richard said that a problem would only arise if the wind blows south, which hardly ever happens, and that earthquakes would actually pose more of a problem. He told us that there are seismometers at the high site and the scopes are fitted with the second highest standard of quake protection, but this far from the coast the earthquakes are deep down and vibrations in the mountain are much less than would be felt on the coast.

As we continued up we looked out over boulder strewn fields, similar to the ones we’d battled through on our way to the E-ELT site. These are typically products of a once glaciated terrain – the boulders dumped in text-book U-shaped valleys as the ice retreated. But our other local chaperone had another idea – that the boulders where “shaken up” by seismic activity, causing them to rub against each other to give them their smooth characteristic, and roll downhill. It turns out that this isn’t quite as an absurd idea as I first thought, as this recent news report explains. And it certainly explains how the rocks were transported – after all, there’s not a drop of water in sight and that would be the most usual transport agent! 

Person height penitentes 

Finally, at 5,000 metres there were just vast expanses of brown with pockets of curious blade-like ice structures called penitentes, which I’d never seen before (but probably because they only exist above 4,000m!). They point in the direction of the Sun and form because the air is so dry, heat from the Sun can turn ice directly into water vapor (a process called sublimation). The ice would have once been totally smooth but depressions will result as some regions randomly sublimate faster than others. The depressions then receive more focused solar energy and the process is speeded up in a positive feedback effect, leaving the spikes towering over the deepening hollows. 

Looking down on some smaller penitentes 

And with each increment of altitude gained, the more difficult it became to breathe – atmospheric pressure is about half what it is at sea level at 5,000 metres, which gives the Everest Base Camp a good run for its money. We took blood oxygen saturation monitors with us to check how well our bodies were dealing with the altitude. Back home in the doctors surgery my reading was a healthy 99%. At 4,000 metres it was 85% and approaching 5,000m was 78%, recoverable to 80% and above with a few deep breaths. But at its worst, my reading plummeted to 74% and I wobbled with a pounding head and feeling of severe exhaustion to the medical room onsite at the ALMA high site, for ten minutes of relief via means of an oxygen tank. The best way to describe how I felt for the reminder of the day was as if I was suffering the most terrible hangover imaginable!

It was incredible to experience such extreme conditions and to please the planetary scientist in me by learning about the interesting geology of unfamiliar territory and appreciating that there is more to the desert than just brown – every different shade of brown in fact! – and, as I conclude in my feature article in the November issue of AN, I know that every time a new astronomical image lands in my inbox I will look beyond the initial wow-factor and nod in the direction of the engineers and astronomers living and working in such unforgiving, remote sites, in order for me to enjoy these images from a location where I don’t have to remind myself to breathe!