The right temperature

Materials World magazine
1 Jun 2018

The temperature at which volcanic rock cracks to create columnar formations, such as those seen at the Giant’s Causeway, has been identified to be between 840–890oC, giving new understanding to geothermal energy systems. Ellis Davies reports.

Geometric columns are impressive formations that occur in volcanic rocks, creating such spectacles as the Giant’s Causeway, Northern Ireland. The formation is caused by the cooling and contracting of the rock, but the temperature at which this occurs was previously unknown. Now, researchers at the University of Liverpool, UK, have identified that the cracks occurred between 840–890oC, which could have an unforeseen influence on the capture of geothermal energy. 

Professor Yan Lavallee, leader of the study, told Materials World, ‘The question first came to me during my undergraduate degree. I went on a trip to Ecuador in my first year in 1999, and we saw some columnar joints – I was very curious. I asked the professor at what temperature do these form, and he said nobody knows. I parked this question for quite some time. But, when I was designing a lab to conduct experiments on hot rock and magma at Liverpool, I started thinking about the problem again.’ 

Lavallee designed a press device and experiment to allow the cooled magma to contract, crack, and form a column. ‘If you imagine two columns next to another, the centres’ distance from each other never really changes. It is the material around the centre that contracts inwards. This is what forms the joints. Between columns there is one joint that is created by the contraction between the two centres,’ he said. The experiment involved a 20x1.5cm basaltic core, taken from the Eyjafjallajökull volcano, Iceland, which is heated in the press to expand. When the sample reaches 990–1,100oC, it is left to equilibrate before the extremities of the core are locked in place. 

‘By doing this, we set the distance between the columns. Then we allow the furnace to cool under controlled conditions so that the centre of the long core is trying to contract while the ends are gripped and cannot move,’ explained Lavallee. ‘This induces tensile stress and tension, which builds up until it cracks to form a columnar joint.’

Geothermal interest

‘When I started this work, geothermal application was not what I had in mind,’ said Lavallee. However, the work has piqued the interest of the geothermal energy industry because it improves the understanding of the stability of volcanic constructs and how heat is transferred in the Earth. Knowing the temperature at which rock cracks allows engineers to gain a better understanding of fluid circulation in a fracture network. This is important because fluid flow controls heat in volcanic systems, which is what is used to create energy. ‘We now know how the rocks are contracting, which is very important as that is the root of geothermal systems,’ added Lavallee.

As an example, Lavallee highlighted the Icelandic deep drilling project that began in 2009, in which a 5.5km deep hole was attempted in search of high-energy fluids. The study could explain the coolant loss experienced, once poured into the hole, during this project. This was not anticipated, and was somewhat of a mystery. However, the study suggests that the substantial contraction of hot rocks would have opened wide fractures, draining away the cooling slurry from the borehole. 

Lavallee believes that the information gained from the study can allow projects to revisit drilling strategies, and that sites previously thought to be unusable could now be developed into magma energy sources.