Underwater lava flows reveal that seawater gets cycled to depths within the Earth as great as 2,900 kilometers (1,800 miles), the boundary between the mantle and the outer core. The discovery of seawater trapped in basalt not only overturns theories about what happens when tectonic plates are forced beneath each other, but provides evidence the oceans and atmosphere that make life possible come from comets or icy meteorites.
“There are two different classes of basalt,” Dr Mark Kendrick of the Australian National University told IFLScience. One arises when mid-oceanic ridges pull apart, creating a gap in the Earth's crust through which the uppermost parts of the mantle spills before turning to rock when exposed to the ocean. The other sort occurs around oceanic islands, most famously Hawaii, where magma rises up from near the core/mantle boundary to be released by volcanoes.
Kendrick compared examples of each in Nature Geoscience and found evidence that water and other molecules trapped in oceanic island basalt comes from deep in the mantle. “It's very important that the lava studied come from the bottom of the oceans,” he told IFLScience. “The water pressure keeps the volatile materials, other than CO2, trapped inside.” If the lava is released into the atmosphere in a surface eruption these substances are released as gases.
“The combination of water and halogens found in the volcanic glasses enables us to preclude local seawater contamination and conclusively prove the water in the samples was derived from the mantle,” Kendrick said in a statement.
Dr Mark Kendrick with a sample of volcanic glass, with tiny amounts of water and gasses trapped inside Stuart Hay, ANU