Doctor Dahrén: "there's a few tonnes of Indonesia in our basement"

2016-01-27

Börje Dahrén in action at the Indonesian volcano Kelud. Photo: David Budd.

Doctor Dahrén is an accomplished mandola player, has mastered the art of analogue photography, and wears the bow-tie at parties. Occassionally, he also climbs volcanoes.

​- This one is from Kelud, and that one is from Katla. Börje Dahrén opens drawer after drawer in the cabinet that contains rocks from all four corners of the world. He holds up rocks for me to see. Black, white, grey. Some light enough to float on water, others are heavy as lead. In the rock archive at Geocentrum, there are thousands of kilograms of rocks, sampled from all corners of the world during geological excursions. Börje himself has visited active and extinct volcanoes on the Canary Islands and the Faroe Islands, as well as Indonesia, one of the most volcanically active regions on the globe.  

On the summit of Merapi, with a view over the neighboring volcano Merbabu. Photo: Börje Dahrén. 

He has just been awarded the doctoral degree, and his dissertation, containing no less than eight papers, is adorned with an Indonesian volcano on the cover. The thesis deals with explosive volcanism, and about how shallow magma reservoirs influence the character of volcanic eruptions.

- The most striking result of this research is that we can see a number of common characteristics in the subvolcanic magma plumbing systems in all of the different volcanoes we studied for this thesis. Notably, all of the studied volcanoes appear to host shallow magma storage regions, which was previously thought to be a rather rare phenomenon.

- The aim of the research has been to describe how the magma is transported and stored on its journey from the earth’s mantle to the surface in a volcanic eruption. To improve our understanding of how the magmas traverse the crust is fundamental for our ability to describe why some volcanoes are so aggressive, explosive and unpredictable.

Professor Valentin Troll samples volcanic gases at  Kelud (Indonesia) together with Dr. David Budd and Dr. Börje Dahrén. Photo: Privat. 

The science of petrology is the study of rocks and minerals down to crystal scale. The rocks Börje brought with him home to Uppsala have been studied carefully to decipher the sequence of magmatic processes that took place prior to eruption.

- The rocks, that are solidified magma, carries a lot of information about what happened in the magmatic system leading up to the eruption. Before reaching the surface, these solidified lavas have travelled from the hot mantle, through the cooler crust. This is where the magic happens, he says. Initially, the magma is rather homogenous, but during the ascent, it differentiates and interacts with the surrounding material in the crust.

The magma cools and crystallises while assimilating portions of the bedrock. All of theses processes leave chemical and textural traces, and that is what we are looking for in our research, with the aim to reconstruct subvolcanic magma plumbing systems, he explains.

- The fact that different minerals form under specific pressures and temperatures allows us to calculate these temperatures and pressures by studying the exact composition of the individual minerals.

He illustrates his point by drawing a diagram of a mineral as it grows during crystallisation. When we look at a mineral in the electron microscope, we are able to resolve it down to nanometer scale, he says. If you look at this mineral in cross section, you’ll find that it is zoned, a bit like the growth rings in a tree. The whole crystal is made up of the same mineral, but its composition varies in the different zones, or “tree rings”. By looking inside these crystals, we could see that parts of the minerals crystallised at shallow depths in the upper crust, within the upper five to ten kilometres. This could help explain some of the explosive and sometimes unpredictable eruptions of some volcanoes, he exclaims.

This is of particular relevance to the Icelandic volcano Hekla, which gives very short warning times before eruptions, which in turn makes it difficult to evacuate the public in time.

- Recent geophysical studies have indicated that the main magma reservoirs are located in the lower crust below Hekla. This makes it difficult to explain the very short precursor times before eruptions at Hekla, as the magma would have to travel extremely fast to cover the distance from the deep storage to the eruption at the surface in very little time, he explains. Our results, on the other hand points towards shallow magma storage, which makes it much easier to explain the short warning times at Hekla.

The geophysical surveys are sometimes unable to resolve small magma pockets, that are typical for the upper crustal magma storage. Our petrological study of Hekla, in turn, was unable to find any evidence for deep magma storage, which could mean that none of the crystals that formed in the deep reservoirs made it to the surface, he says.

- Both geophysical and petrological approaches have their respective limitations, he explains, but they complement each other extraordinarily well. Petrologists and geophysicists really should work more together, which would enable us to describe subvolcanic magma plumbing systems to a much higher degree of certainty, he concludes.

Check out Börjes PhD thesis here

/Katarina Sundberg.  

News archive 2016