#Popo2016: down the rabbit hole | Volcanology

The storm is over. All that is left are some patches of snow on Paso de Cortes, which are being exploited to the very last snowflake by hundreds of people from Mexico City and Puebla – their children have probably talked them into coming here to witness the rare, exciting snow and build small, dirty snowmen on top of their car’s windshields (!). When we pass by Paso de Cortes at nine in the morning, the scenery already resembles a small fun fair, with improvised food stalls, barbecues, policemen with machine guns (they are there to protect the people from bandidos), and snow-capped Popo as a side attraction:

And this is only the beginning of the day. Many more will arrive in the next few hours, but we won’t be around to witness the chaos. We need to dig a hole…

One of our main goals of this field season is to clarify the temporal and spatial relationships of the different lava flows and the deposits of explosive eruptions, the pumices. In other words, we are trying to reconstruct an order of volcanic events.

When we were collecting samples from about a dozen different lava flows last year, we started establishing such a stratigraphy, but our main focus back then was to get our hands on the lavas themselves. This year, with a clear picture of the geochemical characteristics of the lava flows in mind, we need to get a better idea about the relative ages of the lava flows.

Having good constraints on the volcanic stratigraphy is crucial, because this is the only way we can find any trends in the geochemical evolution of the volcano. And if we have trends, this might be a piece in the puzzle about the changes in Popo’s eruption styles.

Photo showing a hillside with trees on top, with the stratified pumice exposed beneath.
Pumices from the most recent Plinian eruption ca. 1,000 years ago, on top of the Capula lava flow. This outcrop gives us a minimum age of 1,000 years for the Capula lava.

So we go into the field, asking ourselves questions like: is the lava flow we are standing on older than the explosive eruption 2,000 years ago, or younger? We would then look for the deposit of the explosive eruption 2,000 years ago, and see if it is above or below the lava flow. So far, so easy.

The problem is that most of Popo’s lava flows are covered by dense vegetation and sometimes tens of metres of soil and volcanic sediments. If finding fresh lava samples was time-consuming last year, finding out about the relative age of the lava flows is even more intense. In some cases, this means simply one thing: we need to dig downwards until we find the lava flow and then have a look at what we find on top of it.

Let me guide you through our most sophisticated work, which took us down to almost three metres before we succeeded in finding the lava beneath the pumice cover:

Photo showing one of the team digging a hole in the ground using a spade, with pumice exposed below the soil surface
Digging in the dirt for one of our lava flows. We don’t know yet how far down we would have to go.
Photo showing the whole team taking part in the digging, with the hole now a few feet deeper and wider
One for all, all for one.

I am not sure how many of you, dear readers, got seriously involved in the digging business in their lives. Digging is an elaborate craft: you are not simply digging downwards, no!

From very early on, you are mostly busy widening the hole so there is enough space to safely continue digging. And of course, if you intend to get out of your hole again, you should also think of an access trench, which also becomes wider and longer as you proceed.

All in all, there is a lot of planning to do, material to move, and sweat to be sweated. A work for body, mind, and soul; a true catharsis, especially for scientists who spend large parts of their lives sitting in front of a computer in an office at the Museum.

Photo showing two members of the team going further down into the ground and expanding the access trench to the side
Our trench slowly evolving

At this time, we have almost forgotten why we are doing this. The digging is just too hypnotising. Luckily we have Popo in the background, which has become more active in the last few days, and every so often bestows a small explosion upon us:

Photo showing a tree line with the peak of Popo just visible. Rising above the peak, a plume of ash drifting leftwards in the wind.
Meanwhile at the volcano, a small explosion…

We are digging through consolidated, firm ground, which is good and bad. Good, because the walls of the trench are very stable, which keeps the shoveler in the pit reasonably safe (ultimately – as much fun digging and hunting for lava flows is – none of us wants to be buried by pumice). But bad, because it is really hard to dig into the ground! But our blisters and sore muscles tell a story of persistence (five hours of digging!), scientific rigour (this! Lava flow! Needs! To! Be! Found!!) – and success!

Photo of Guillem standing at the bottom of the hole, holding a piece of the lava flow in his hands and smiling
Guillem proudly presents the excavated lava flow

Of course, sometimes manpower has its limits when it comes to removing soil and sediments. Here is an example: in order to examine a series of explosive deposits, we had started digging a bit last year at one outcrop. Soon, we reached solid volcanic bombs and rubbish, the combination of which made us decide to stop and let a proper machine do the work. A year later, the time has finally come: Excavator time!

Photo showing the excavator living a pile of soil away from the face of a hillside to create a hole.
The excavator gets ready to embarrass us and our shovelling skills…

We have rented an excavator including a person to handle it from a nearby village for 6 hours. After 15 minutes, the hole is approximately as deep as our man-made hole after 5 hours, and even though we haven’t found the units of interest yet, we are sure: this is going to be a quick(er!) job. Another 20 minutes later, we have reached more than 5 metres, and a yellow layer catches our eyes – our target units!

Photo showing the excavator having dug much deeper into the ground
Somewhere between the shadows and the machine, the presence of pumice makes us happy – for a short while

Now, the only thing left to do for the excavator is to make the hole larger and create steps to safely access the hole. But alas, this turns out to be pretty much impossible for the machine and its operator: too short an excavator shovel, too restricted the manoeuvring space at the outcrop. Three hours later and the hole has not become wider or deeper at all; on the contrary, our pumice layers have vanished under debris again. Yes, sometimes even machines reach their limits!

Photo showing the excavator at the hillside with soil piled to left and right but the hole no bigger
Despite large amounts of material moved from one place to another, the hole is neither deeper nor safer. Score: hole 2, machine 1.

Even though we couldn’t have a closer look at the deposits, confirming their presence has been an important piece of information for us, as it completes our picture of the pyroclastic stratigraphy at this and many other outcrops. And thus we can drive home satisfied, passing dozens of cars that got stuck in the snow on their way up to Paso de Cortes, waiting for it to melt in the sun. At least some of them have happy snowmen on their windshields.