The pores of this one-millimetre-sized rock sample are highlighted in red.
Photo by: Dr Florian Fusseis, University of Western Australia
CSIRO Australia reports that scientists have discovered a natural pump deep within the Earth that is important in the formation of earthquakes and ore deposits. The process – called ‘creep cavitation’ – involves fluid being pumped through pores in rock.
The process is thought to occur 15 kilometres below the Earth’s surface, in the mid-crustal shear zones. Mid-crustal shear zones are areas in the middle of the Earth’s hard outer shell that are more likely to deform or fold.
To observe this process, the researchers examined one-millimetre-sized cubes of rock from Alice Springs. About 320 million years ago, this rock was in mid-crustal shear zones at a temperature of about 400 degrees Celsius. It was pushed to the surface, during a period of natural mountain formation.
The scientists used a number of technologies to analyse the rock, including high-resolution synchrotron x-ray microtomography. This is where x-rays are used to create cross sections, which are then used to make a 3D model without destroying the original sample.
To find out more, a scanning electron microscope was used to observe the rock at the nanoscale. Advanced computer visualisation was also carried out at iVEC in Western Australia. iVEC is a facility specially designed for high performance computing and visualisations.
Research leader, Dr Florian Fusseis, from the University of Western Australia says that the discovery could reveal more about how some earthquakes are triggered. “While we understand reasonably well why earthquakes happen in general, due to stress build-up caused by motions of tectonic plates, the triggering of earthquakes is much more complex.”
Under pressure, grains in rock slide past each other. This causes small cavities to open up. As cavities appear in one place and disappear in another, high pressure fluids move through the rock. The fluids that move through the rock include water, methane and carbon dioxide. There are also chemical interactions between the fluids and rock, which can result in rock dissolving or reforming.
As cavities develop and reach a specific density, they can interact and increase the stress in the rock, leading to fractures that can trigger an earthquake.
Dr Rob Hough is a member of the research team from CSIRO’s Exploration and Mining. Rob points out that creep cavitations can also explain ore deposits. “We are seeing the direct evidence for one of the processes that got ore-forming fluids moving up from the mantle to the shallow crust to form the ore deposits we mine today,” says Rob.