An earthquake potentially turns water into gold, according to Australian scientists who have studied earthquake dynamics. Using a simple model, these scientists have shown that mountain-building earthquakes deep below earth pull apart rocks so quickly that the high-pressure fluids they contain instantly vaporise. This process leaves behind residues rich in minerals including gold.

The study conducted by Richard Henley of of the Australian National University in Canberra and Dion Wetherley from University of Queensland in Brisbane, has been published in ‘Nature Geoscience’ journal. The process takes place along ‘fault jogs’, which are sideways zigzag cracks that connect the main fault lines in rock, says first author Dion Weatherley. When an earthquake hits, the sides of the main fault lines slip along the direction of the fault, rubbing against each other. But the fault jogs simply open up. The study authors wondered as to what happens to fluids circulating through these fault jogs at the time of the earthquake.

The scientists found that a rapid depressurisation that sees the normal high-pressure conditions deep within Earth drop to pressures close to those we experience at the surface. For example, a magnitude-4 earthquake at a depth of 11 kilometres would cause the pressure in a suddenly opening fault jog to drop from 290 megapascals (MPa) to 0.2 MPa. (By comparison, air pressure at sea level is 0.1 MPa.) When mineral-laden water at around 390 °C is subjected to that kind of pressure drop, the liquid rapidly vaporises and the minerals in the now-supersaturated water crystallise almost instantly - a process that engineers call flash vaporisation or flash deposition. The effect is sufficiently large that quartz and any of its associated minerals and metals will fall out of solution, Dion Wetherley said.

Eventually, more fluid percolates out of the surrounding rocks into the gap, restoring the initial pressure. But that doesn’t occur immediately, and so in the interim a single earthquake can produce an instant, although tiny, gold vein. Furthermore, the scientists found that bigger earthquakes produce bigger pressure drops but smaller earthquakes produce surprisingly big pressure drops along fault jogs.