by Ryan Morrison
June 04, 2021
A cut-away of Earth's
shows the solid iron inner core (red)
slowly growing by
freezing of the liquid iron outer core (orange).
travel through the Earth's inner core faster
between the north and
south poles (blue arrows)
than across the equator (green arrow)
studied seismic waves from earthquakes as they travel in
them to detect signs that the core was growing faster in
say it hasn't left the core lopsided as gravity evens
It allowed them to pin down the age of the inner core to
0.5 to 1.5 billion years
Our planet's solid-iron inner core has been growing faster on one
side than the other for more than half a billion years, study finds
The solid-iron inner core of the Earth has been growing faster on
one side than the other for over 500 million years, according to a
It is growing faster under Indonesia's Banda Sea than under Brazil,
but this uneven growth pattern hasn't left the core lopsided, say
seismologists at the University of California, Berkeley, who have
been investigating the phenomenon.
Gravity has acted to evenly distribute the new growth, made up of
iron crystals that form as molten iron begins to cool, maintaining a
spherical inner core.
Even though it doesn't leave the core lopsided, this uneven growth
rate suggests something in the outer core under Indonesia is
removing heat from the inner core at a faster rate than it is under
Brazil on the opposite side of the planet, the team said.
Researchers say this discovery has helped them 'prove rather loose
bounds' for the age of the inner core, to between half a billion and
1.5 billion years.
This boundary for the age of the solid core of the Earth can help
scientists learn more about the magnetic field, which protects us
from harmful solar radiation.
'It can help in the
debate about how the magnetic field was generated prior to the
existence of the solid inner core,' said Barbara Romanowicz,
'We know the magnetic field already existed 3 billion years ago,
so other processes must have driven convection in the outer core
at that time.'
The youngish age of the
inner core may mean that, early in Earth's history, the heat boiling
the fluid core came from light elements separating from iron, not
from crystallization of iron, which we see today.
'Debate about the age
of the inner core has been going on for a long time,' said
Daniel Frost, assistant project scientist.
'The complication is: If the inner core has been able to exist
only for 1.5 billion years, based on what we know about how it
loses heat and how hot it is, then where did the older magnetic
field come from?
'That is where this idea of dissolved light elements that then
freeze out came from.'
Asymmetric growth of the
inner core, that is growing at different rates on each side of the
planet, explains a three-decade-old mystery, Daniel Frost explained.
The mystery is that the crystallized iron in the core seemed to be
more likely to be aligned to the west than the east of Earth's
Map showing the
at which the researchers measured seismic
from earthquakes (circles) to study Earth's inner core
The team say scientists would expect the crystals to be randomly
oriented than have them favor one side of the planet than the
In an attempt to explain the observations, they created a computer
model of crystal growth in the inner core.
Their model incorporated geodynamic growth, how materials on earth
deform and form, and the mineral physics of iron at high pressure
and high temperature.
'The simplest model
seemed unusual - that the inner core is asymmetric,' Frost said.
'The west side looks different from the east side all the way to
the center, not just at the top of the inner core, as some have
suggested. The only way we can explain that is by one side
growing faster than the other.'
The model describes how
asymmetric growth - about 60 per cent higher in the east than the
west - can preferentially orient iron crystals along the rotation
axis, with more alignment in the west than in the east.
'What we're proposing
in this paper is a model of lopsided solid convection in the
inner core that reconciles seismic observations and plausible
geodynamic boundary conditions,' Romanowicz said.
Even though it doesn't leave the core lopsided,
this uneven growth
rate suggests something
in the outer core under Indonesia
removing heat from the inner core
at a faster rate than it is under
on the opposite side of the planet,
the team said
Earth's interior is layered like an onion.
The solid iron-nickel
inner core is 745 miles in radius, or about three-quarters the size
of the moon and is surrounded by a fluid outer core of molten iron
and nickel about 1,500 miles thick.
The outer core is surrounded by a mantle of hot rock 1,800 miles
thick and overlain by a thin, cool, rocky crust at the surface.
Convection occurs both in the outer core, which slowly boils as heat
from crystallizing iron comes out of the inner core, and in the
mantle, as hotter rock moves upward to carry this heat from the
centre of the planet to the surface.
A new model by UC
proposes that Earth's inner core grows faster
on its east side (left) than on its west.
Gravity equalizes the
by pushing iron crystals toward
the north and
south poles (arrows)
The vigorous boiling motion in the outer core produces Earth's
WHAT IS EARTH'S
HOW DOES IT PROTECT US?
field is a layer of electrical charge that surrounds our planet.
The field protects life on our planet because it deflects
charged particles fired from the sun known as 'solar wind'.
Without this protective layer, these particles would likely
the Ozone layer, our only line of
harmful UV radiation.
Scientists believe the Earth's core is responsible for creating
its magnetic field.
As molten iron in the Earth's outer core escapes it creates
These currents generate electric currents which create the
magnetic field in a natural process known as a
According to Frost's
computer model, as iron crystals grow, gravity redistributes the
excess growth in the east toward the west within the inner core.
Movement of crystals in the inner core, close to the melting point
of iron, aligns the crystal lattice with the Earth's rotational axis
- doing so more to the west than the east, they found.
The model correctly predicts the researchers' new observations about
seismic wave travel times through the inner core.
The anisotropy, or difference in travel times parallel and
perpendicular to the rotation axis, increases with depth.
The strongest anisotropy is offset to the west from Earth's rotation
axis by about 250 miles.
The model of inner core growth also provides limits on the
proportion of nickel to iron in the centre of the earth, Frost said.
His model does not accurately reproduce seismic observations unless
nickel makes up between four per cent and eight per cent of the
This is close to the proportion in metallic meteorites that once
were the cores of dwarf planets in our solar system.
The model also tells geologists how viscous, or fluid, the inner
'We suggest that the
viscosity of the inner core is relatively large,' said
This is 'an input parameter of importance to geodynamicists
studying the dynamo processes in the outer core.'
The findings are due to
be presented in the the journal Nature Geoscience.
IRON CORE CREATES THE MAGNETIC FIELD
Our planet's magnetic field is believed to be generated deep
down in the Earth's core.
Nobody has ever journeyed to the centre of the Earth, but by
studying shockwaves from earthquakes, physicists have been able
to work out its likely structure.
At the heart of the Earth is a solid inner core, two thirds of
the size of the moon, made mainly of iron.
At 5,700°C, this iron is as hot as the Sun's surface, but the
crushing pressure caused by gravity prevents it from becoming
Surrounding this is the outer core there is a 1,242 mile (2,000
km) thick layer of iron, nickel, and small quantities of other
The metal here is fluid, because of the lower pressure than the
Differences in temperature, pressure and composition in the
outer core cause convection currents in the molten metal as
cool, dense matter sinks and warm matter rises.
The 'Coriolis' force, caused by the Earth's spin, also causes
This flow of liquid iron generates electric currents, which in
turn create magnetic fields.
Charged metals passing through these fields go on to create
electric currents of their own, and so the cycle continues.
This self-sustaining loop is known as the geodynamo.
The spiraling caused by the Coriolis force means the separate
magnetic fields are roughly aligned in the same direction, their
combined effect adding up to produce one vast magnetic field
engulfing the planet.