Compressing and heating rocks for science

Using large instruments, the researchers simulate extremely high pressures and temperatures deep within the Earth. New research using such an advanced “stone press” shows how talc deforms and changes in the deep faults from which earthquakes originate.

By far, most rocks on Earth are deep beneath our feet, at temperatures and pressures much higher than those at the surface. In the continental crust – the Earth’s subterranean crust – the temperature rises by 30 degrees Celsius for every kilometer of depth. So after four kilometers you’re already over 100 degrees, Geologists’ emails Charles Hornfrom the University of Washington, USA.

Horn and her colleagues studied the behavior of talc subduction zones, which are areas where an oceanic tectonic plate slides below another oceanic or continental plate, to a depth of several tens of kilometers. They posted About their research in Geophysical Research Letters.

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“We expose rocks – such as talc – to similar pressures and temperatures in the laboratory,” Horn writes. This is to understand what happens to rocks under these conditions, and what impact this has on processes in and on our planet, such as plate tectonics and earthquakes.

Rock trembles

“Intense and deep earthquakes can occur in subduction zones, which can lead to tsunamis,” says Horn. Scientists want to understand the processes that play a role in these potentially dangerous earthquakes.

The powdered mineral talc occurs at fault surfaces, such as those at subduction zones, and affects how quickly the fault moves. This is related: the fast-moving bug is the earthquake. “There’s been a lot of interest in the conversation lately,” says Horn. “Scientific publications have indicated that even small amounts of talc can significantly weaken fractures.”

To find out how talc behaves at such depths, the researchers subjected it to pressures of more than a gigapascal and a temperature of 500 degrees. When they removed a sample of talc from the device, it turned out to have a brittle, porous structure. Horn: “That was surprising, because most materials don’t remain brittle at such high pressures and temperatures and close their pores.”

This means that liquids such as water can flow through the pressurized talc. As a result, matter reacts differently to shifts, as it does during earthquakes. What exactly this means for earthquake risk is still under investigation.

obnoxious device

In this experiment, the researchers used a beast of a machine: the so-called LVT device, which stands for LVT Big torque. This device can produce pressures of about three gigapascals, as found at a depth of about 100 kilometers in the Earth, and temperatures in excess of 1,500 degrees Celsius. It is also a torsion device. This means that you fix the piece of rock at the bottom and flip it over at the top, as if you were operating a tap. This mimics major deformations that occur underground, for example during displacements.

Horn says there are devices that can generate higher pressure or torque, but they work with much smaller rock samples. “The LVT device can work with cylindrical pieces of stone that are 4.2 mm wide and 5.2 mm long.” This may seem small, but some other experiments work with pieces less than half a millimeter in size.

This is what makes the LVT device unique, says the geologist Hans de Presser from Utrecht University, which was not involved in the LVT study. “The great thing is that this device can also achieve significant deformations at high pressures and temperatures using relatively large pieces of rock. With the small samples that some of the devices work with, you can ask yourself how representative the deformation is of the large amount of rock in the Earth’s interior.

High pressure coefficient and temperature

In the High pressure and temperature lab In Utrecht, De Bresser and his colleagues are also working with various devices with which they can reach high pressures and temperatures. They even work with 10 to 20 mm rock samples. De Bresser: “The pressure we achieve is lower than the pressure we achieve in the LVT, but we are therefore more focused on the rocks at the top of the crust.”

Using the so-called medium gas apparatus, the researchers in Utrecht can, for example, subject 10 to 20 millimeters of stone samples to pressures of 0.6 gigapascals and temperatures of up to 1,200 degrees Celsius, which corresponds to depths of 20 to 40 kilometres. . Using a torsion device, they can induce significant deformations in powdered materials at pressures up to 0.3 GPa and a maximum of 700°C.

Utrecht Research is mainly fundamental, but it also has a relevant social touch. For example, researchers are studying the behavior of porous rocks such as sandstone, which once contained natural gas. They investigate the consequences and risks that come with using carbon dioxide2 in conservation.

They are also looking for salt. Underground salt caverns can be used to store hydrogen, which is an energy carrier for energy transition. For this application, it is important to know how the salt interacts with it.

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