Mysterious rumbling deep in the Earth finally has an explanation

23 Nov 2017

Image: Vadim Sadovski/Shutterstock

New research on oxygen and iron chemistry deep inside the Earth may have solved a longstanding seismic mystery.

While seismic activity presents itself as earthquakes up here on the surface, deep within the Earth, rumblings known as ultra-low velocity zones (UVZ) have mystified geologists and chemists – until now.

UVZs sit at the boundary between the lower mantle and the core, nearly 3,000km beneath the surface, and have been visualised using technology, similar to how ultrasound is used to look inside our bodies.

Knowing they exist and how they occur are two very different things but, by analysing chemical reactions beneath the Earth, a solution was found and published in the journal Nature.

Iron-rich reservoirs

It turns out that water contained in some minerals that get pulled down into the Earth due to plate tectonic activity could split under extreme pressures and temperatures.

This ‘liberates’ hydrogen, enabling the residual oxygen to combine with iron metal from the core to create a novel high-pressure mineral called iron dioxide.

The team estimates that as much as 300m tonnes of water could be carried down into Earth’s interior every year, creating massive reservoirs of iron dioxide.

These underground lakes could be the source of UVZs that slow down seismic waves at the core-mantle boundary.

Far-reaching implications

Given the obvious challenges of getting samples 3,000km beneath the Earth, the researchers recreated the conditions with help from a laser-heated diamond anvil cell.

The team found that a mixture of normal mantle rock with 40 to 50pc iron dioxide had the same seismic signature as the enigmatic UVZs.

The discovery has impacted our understanding of Earth’s geological history, including life-altering events such as the Great Oxygenation Event, which occurred 2.4bn years ago.

“Finding the existence of a giant internal oxygen reservoir has many far-reaching implications,” said the study’s lead, Ho-kwang Mao.

“Now, we should reconsider the consequences of sporadic oxygen outbursts and their correlations to other major events in the Earth’s history, such as the banded-iron formation, snowball Earth, mass extinctions, flood basalts and supercontinent rifts.”

Updated, 2.11pm, 23 November 2017: This article was updated to remove references to iron peroxide and replace them with iron dioxide.

Colm Gorey was a senior journalist with Silicon Republic