Journey to the centre of the Earth (in BS8)

When asked to explain what I actually do in my PhD, my preferred answer is inevitably “making pretend volcanoes”. No, not the Blue Peter-style baking soda kind, but actually trying to simulate the conditions found beneath active volcanoes, in the laboratory. If you are wondering how this is done, then why not take a blog-tour of our labs?

Bristol Experimental Earth STudies (BEEST), of which I am a member, is one of seven research groups in the Earth Sciences department at the University of Bristol. Its primary activities involve conducting experiments to probe deep into the Earth’s interior. The BEEST labs consist of a series of experimental apparata that are capable of replicating the conditions at which igneous rocks are created, from lava flows at the surface down to the core-mantle boundary and everything in between. Experiments can either be conducted on natural rocks or synthetic mixes of chemicals. The latter option has the advantage of allowing us to simplify complex natural systems and control the amount of volatile species (such as CO2 and H2O) in the sample.

Lava flows = 1 atmosphere furnace
A 1 atm furnace is in effect a gloried oven, though it is a little hotter than you might use for your Sunday roast! Samples are lowered on a wire into the hotspot of the furnace via a vertical pipe and are heated to temperatures of up to 1700ºC at room pressure. If desired, the amount of oxygen that is delivered to the sample can be controlled by pumping in a mixture of CO and CO2. The 1 atm furnace is primarily used for studying the changes in mineralogy of lava erupted onto the Earth’s surface.

1 atm furnace
Lava flows can be costly volcanic hazards, destroying property and agricultural land; however, certain types of lava can be economic assets. Dr Richard Brooker is using a 1 atmosphere furnace to look at the properties of kimberlites – lavas that are famous for their propensity to carry diamonds.

 

Shallow magma chambers = cold seal pressure vessel
The cold seal pressure vessel allows us to simulate conditions where magma is stored beneath active volcanoes. Samples are enclosed in small metal capsules and inserted into a metal tube, called a bomb. The bomb is then flooded with water, creating a pressure on the capsule corresponding to being at a depth of ≤6km below the surface of the Earth. Finally, temperatures are elevated to ≤900ºC using an electric current. The decrease of pressure and temperature can be controlled such that we can accurately simulate the eruption of a volcano.

Cold seal pressure vessel
A cold seal pressure vessel being operated by Bristol PhD student, Jenny Riker. Jenny is investigating how decompression drives crystallisation in erupting magma at Mt St Helens (pictured left).

 

Lower crust and mantle = piston cylinder and multi-anvil apparatus
As we descend deeper into the Earth, the amount of pressure we need to apply to a sample increases dramatically. Both the piston cylinder and multi-anvil apparatus do this by squeezing a metal capsule between differently shaped blocks of metal; the greater the pressure needed, the smaller the sample must be. The capsules used are typically <5mm in length and must be analysed using high magnification electron microscopy. The piston cylinder can simulate conditions equivalent to ≤120km depth and ≤1400ºC; this makes it an excellent device for studying how magmas are generated in subduction zones. The multi-anvil can probe much deeper (up to the core-mantle boundary) and is used to investigate the state of the Earth’s molten mantle.

Piston cylinder
The piston cylinder apparatus can be used to simulate conditions where magma is generated at island arcs: pressure is applied manually using a lever. This is my patch – here I’m pressurising an experiment to lower crustal depths. With the resulting data, I hope to gain insights into the magma chamber beneath Grenada, Lesser Antilles.
Carbonatites are carbonate-rich igneous rocks. Oldoinyo Lengai in Tanzania (above left) is currently the world's only active carbonatite volcano although pyroclastic carbonatite deposits have also been discovered in Europe. Sorcha McMahon's PhD is focussed on how these strange magmas form, using the multi-anvil apparatus (above right). High pressures are achieved placing samples within an octahedral ceramic pressure cell, at the centre of 8 carbide cubes (see inset)
Carbonatites are carbonate-rich igneous rocks. Oldoinyo Lengai in Tanzania (above left) is currently the world’s only active carbonatite volcano although pyroclastic carbonatite deposits have also been discovered in Europe. Sorcha McMahon’s PhD is focussed on how these strange magmas form, using the multi-anvil apparatus (above right). High pressures are achieved placing samples within an octahedral ceramic pressure cell, at the centre of 8 carbide cubes (see inset)

 

Core = diamond anvil cell (DAC)
In order to replicate conditions at the centre of the Earth, we need to employ the hardest substance known to man-kind – diamond. The most astonishing aspect of this equipment is that the diamond anvil cell is small enough to fit into the palm of your hand! Sample powder is placed between the two diamond tips and heated using a laser. The samples are so small that analysis must be carried out at a synchrotron, such as CERN, using high-energy x-rays generated by electrons which are accelerated close to the speed of light. Applications of this technique include testing the hypothesis that the Earth’s iron core contains small amounts of a light element, such a carbon or silicon.

DAC
A diamond anvil cell (DAC) can replicate the pressures and temperatures found deep within the Earth. Andrew Thomson and others at Bristol are using a DAC to study the chemistry of the Earth’s core

All this equipment can be found in three rooms in the basement of the Wills Memorial Building. It just goes to show you can take a journey to the centre of the Earth without leaving Bristol!

This post has been adapted from an Outcrop article, which you can read here.

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About Charly Stamper

I’m an ex-experimental petrologist.
I used to make pretend volcanoes; now I work in renewable energy

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