Geology: Chemical leftovers from Earth's formation 4.5 billion years ago may ...

Geology: Chemical leftovers from Earth's formation 4.5 billion years ago may ...
Geology: Chemical leftovers from Earth's formation 4.5 billion years ago may ...

Like 'clumps of flour in the bottom of a bowl of batter', chemical leftovers from the formation of the Earth 4.5 billion years ago may reside near the planet's core.

This is the conclusion of a study led by researchers led from the University of Utah, who studied some of the 'ultra-low velocity zones' on the core–mantle boundary.

Geologists use the passage of seismic waves through the Earth to plumb its interior — extrapolating features based on how the waves are reflected and refracted. 

Ultra-low velocity zones are so named because the seismic waves that pass through them slow by up to half, while the density of matter increases by up to a third.

Modelling the formation of some of these zones, the team concluded they represent the denser fractions of a magma ocean that formed after the moon-forming impact.

This material settled to the bottom of the mantle, forming a layered structure. Over time, mantle motion caused this material to be pushed into small patches.

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Like 'clumps of flour in the bottom of a bowl of batter', chemical leftovers from the formation of the Earth 4.5 billion years ago may reside near the planet's core. Pictured: a map of the Earth's interior, showing the locations of the ultra-low velocity zones (yellow) in question above the outer core, as well as sinking crustal material (blue) and so-called 'superplumes' (red)

Like 'clumps of flour in the bottom of a bowl of batter', chemical leftovers from the formation of the Earth 4.5 billion years ago may reside near the planet's core. Pictured: a map of the Earth's interior, showing the locations of the ultra-low velocity zones (yellow) in question above the outer core, as well as sinking crustal material (blue) and so-called 'superplumes' (red)

Geologists use the passage of seismic waves through the Earth to plumb its interior — extrapolating features based on how the waves are reflected and refracted (as pictured). Ultra-low velocity zones are so named because the seismic waves that pass through them slow by up to half, while the density of matter increases by up to a third

Geologists use the passage of seismic waves through the Earth to plumb its interior — extrapolating features based on how the waves are reflected and refracted (as pictured). Ultra-low velocity zones are so named because the seismic waves that pass through them slow by up to half, while the density of matter increases by up to a third

OTHER THEORIES TO EXPLAIN THE ULTRA-LOW VELOCITY ZONES 

In their study, Professor Thorne and colleagues have concluded that some ultra-low velocity zones have their origins in the processes that influenced the formation of the Earth and the moon.

However, there are alternatives theories that have been proposed to explain the origins of some of the other ultra-low velocity zones.

Some experts believe, for example, that some of the zones are derived from the melting of ocean crust that has sunk back into the mantle.  

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The investigation was undertaken by geologist Michael Thorne of the University of Utah and his colleagues.

'Of all of the features we know about in the deep mantle, ultra-low velocity zones represent what are probably the most extreme,' explained Professor Thorne.

'Indeed, these are some of the most extreme features found anywhere in the planet,' he added.

Geologists had originally speculated that these ultra-low velocity zones might represent areas where the base of the mantle had partially melted, perhaps providing the source of magma for volcanic 'hot spots' like that seen in Iceland.

'But most of the things we call ultra-low velocity zones don’t appear to be located beneath hot spot volcanoes, so that can't be the whole story,' said Professor Thorne.

In their study, the researchers set out to put an alternate theory to the test — that of whether ultra-low velocity zones may be comprised of rock with a different and more archaic composition to that of the rest of the mantle.

According to Thorne, it is possible that ultra-low velocity zones are accumulations of iron oxide. While this compound is most familiar to us as rust, it would behave like a metal at the conditions found in the deep metal.

And, he added, were pockets of such to be found just above the core, they could serve to influence the Earth's magnetic field, which is generated in the outer core.

'The physical properties of ultra-low velocity zones are linked to their origin,' said paper author and seismologist Surya Pachhai of both the Australian National University, and the University of Utah.

Their origins,  he added, in turn provide 'important information about the thermal and chemical status, evolution and dynamics of Earth's lowermost mantle — an essential part of mantle convection that drives plate tectonics.'

Researchers led from the University of Utah studied 'ultra-low velocity zones' on the core–mantle boundary. Pictured: a model of one hemisphere, showing the location of two ultra-low velocity zones (red) and the pacific superplume (transparent red)

Researchers led from the University of Utah studied 'ultra-low velocity zones' on the core–mantle boundary. Pictured: a model of one hemisphere, showing the location of two ultra-low velocity zones (red) and the pacific superplume (transparent red)

The researchers focussed their study on those ultra-low velocity zones which are located beneath the Coral Sea, which lies between Australia and New Zealand. 

This area is ideal for study because it experiences a high level of seismic activity — allowing the team to build up a high-resolution map of the core–mantle boundary that might shine more light on the mysterious ultra-low velocity zones.

Creating a seismic image through nearly 1,800 miles of crust and mantle, however, is no mean feat, especially given how a thick layer of low-velocity material might produce the same reflection pattern as a thinner layer of even lower-velocity rock.

To overcome this challenge, the team adopted a reverse-engineering approach known to scientists as 'Bayesian inversion'.

As Dr Pachhai explained: 'We can create a model of the Earth that includes ultra-low

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