Rainwater can penetrate many kilometres deep into the earth – further than geologists had realised. Researchers from Southampton University, working with colleagues in New Zealand, have found evidence of fluids derived originally from rainfall below the “ductile crust”, where high temperatures and pressures cause rocks to flex and flow rather than fracture in response to tectonic movements.
The discovery, published in Earth and Planetary Science Letters, has implications for understanding earthquakes, which may be triggered by fluids in the crust. It may also provide clues to the formation of mineral deposits.
Researchers studied geothermal fluids and rocks from New Zealand’s Southern Alps, where the collision of tectonic plates forces deep layers of the earth close to the surface. “When fluids flow through the crust they leave behind deposits of minerals that contain a small amount of water trapped within them,” says Southampton’s Catriona Menzies. “We have analysed these waters and minerals to identify where the fluids deep in the crust came from.”
Water that has percolated down from rainfall can be distinguished from water that originated in chemical reactions in rocks deep below the surface, because the two sources have different ratios of oxygen isotopes. “We wanted to test the limits of where rainwater may flow in the crust,” says Menzies. “Our data shows… rainwater does penetrate into rocks that are too deep and hot to fracture.”