The Earth’s solid iron inner core has been growing faster on one side than the other for more than 500 million years, according to a new study.
It grows faster under Indonesia’s Banda Sea than under Brazil, but this uneven growth pattern hasn’t left the core skewed, say seismologists at the University of California, Berkeley, who have studied the phenomenon.
Gravity has acted to evenly distribute the new growth, made up of iron crystals that form when molten iron begins to cool, preserving a spherical inner core.
While it doesn’t skew the core, this uneven growth rate suggests that something in the outer core under Indonesia removes heat from the inner core faster than under Brazil on the other side of the planet, the team said. .
Researchers say this discovery helped them “prove fairly loose boundaries” for the inner core’s age, between half a billion and 1.5 billion years.
A cutout of the Earth’s interior shows the solid inner iron core (red) growing slowly due to freezing of the liquid outer iron core (orange). Seismic waves travel faster through the Earth’s inner core between the north and south poles (blue arrows) than across the equator (green arrow)
FOUR LAYERS OF THE PLANET EARTH
Crust: Up to a depth of 70 km, this is the outermost layer of the earth, covering both the ocean and the land.
Coat: Descending to 2,890 km with the lower mantle, this is the planet’s thickest layer and made of silicate rocks richer in iron and magnesium than the crust above it.
outer core: This region runs from a depth of 2,890 to 5,150 km and is made of liquid iron and nickel with trace elements that are lighter.
inner core: Descending to a depth of 6,370 km in the center of planet Earth, this region is believed to be made of solid iron and nickel.
This limit for the age of Earth’s solid core could help scientists learn more about the magnetic field, which protects us from harmful solar radiation.
“It may help debate how the magnetic field was generated before the solid inner core existed,” said Barbara Romanowicz, co-author of the study.
“We know that the magnetic field already existed 3 billion years ago, so other processes must have been driving convection in the outer core at that time.”
The young age of the inner core may mean that, early in Earth’s history, the heat that cooked the liquid core came from light elements separating from iron, not from iron crystallization, as we see today.
“Debate about the age of the inner core has been going on for a long time,” says assistant project scientist Daniel Frost.
“The complication is, if the inner core can only exist for 1.5 billion years, based on what we know about how it loses heat and how hot it is, where does the older magnetic field come from?
‘That’s where the idea of dissolved light elements that then freeze out came from.’
Asymmetric growth of the inner core, which grows at different rates on each side of the planet, explains a three-decade-old mystery, Frost explained.
The mystery is that the crystallized iron in the core appeared more likely to align with the west than the east of the Earth’s axis of rotation.
Map showing the seismometers (triangles) on which the researchers measured earthquake seismic waves (circles) to study the Earth’s inner core
The team says scientists would expect the crystals to be randomly oriented rather than favor one side of the planet over the other.
In an effort to explain the observations, they created a computer model of crystal growth in the inner core.
Their model included 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 that one side is growing faster than the other.’
The model describes how asymmetric growth — about 60 percent higher in the east than in the west — can preferentially orient iron crystals along the axis of rotation, with more alignment in the west than in the east.
“What we propose in this paper is a model of skewed solid convection in the inner core that reconciles seismic observations and plausible geodynamic boundary conditions,” Romanowicz said.
While it doesn’t skew the core, this uneven growth rate suggests that something in the outer core under Indonesia removes heat from the inner core faster than under Brazil on the other side of the planet, the team said.
The interior of the earth is layered like an onion. The solid iron-nickel inner core has a radius of 745 miles, or about three quarters the size of the moon, and is surrounded by a liquid 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 covered by a thin, cool, rocky crust on the surface.
Convection occurs both in the outer core, which boils slowly 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 center of the planet to the surface.
A new model from seismologists at UC Berkeley proposes that the Earth’s inner core is growing faster on the eastern side (left) than on the western side. Gravity evens out asymmetric growth by pushing iron crystals toward the north and south poles (arrows)
WHAT IS THE EARTH’S MAGNETIC FIELD AND HOW DOES IT PROTECT US?
The Earth’s magnetic field is a layer of electrical charge that surrounds our planet.
The field protects life on our planet because it repels charged particles fired by the sun, known as “solar wind.”
Without this protective layer, these particles would likely take away the ozone layer, our only line of defense against harmful UV radiation.
Scientists believe that the Earth’s core is responsible for creating the magnetic field.
When molten iron escapes into the Earth’s outer core, convection currents are created.
These currents generate electrical currents that create the magnetic field in a natural process known as a geodynamo.
The powerful boiling motion in the outer core produces the Earth’s magnetic field.
According to Frost’s computer model, as iron crystals grow, gravity distributes the excess growth east to west within the inner core.
Movement of crystals in the inner core, close to the melting point of iron, aligns the crystal lattice with Earth’s axis of rotation — they do this more west than east, they found.
The model correctly predicts the researchers’ new observations about the travel times of seismic waves through the inner core.
The anisotropy, or the difference in travel times parallel to and perpendicular to the axis of rotation, increases with depth.
The strongest anisotropy has shifted about 250 miles west of the Earth’s axis of rotation.
The model of inner core growth also puts limits on the ratio of nickel to iron at the center of the Earth, Frost said.
His model does not accurately represent seismic observations unless nickel makes up between 4% and 8% of the inner core.
This is close to the proportion of metallic meteorites that were once the cores of dwarf planets in our solar system.
The model also tells geologists how viscous or liquid the inner core is.
‘We suggest that the viscosity of the inner core is relatively high,’ says Romanowicz.
This is ‘an input parameter of interest to geodynamics studying the dynamo processes in the outer core’.
The findings will be presented in the journal Nature Geoscience.
THE LIQUID IRON CORE OF THE EARTH CREATES THE MAGNETIC FIELD
It is believed that our planet’s magnetic field is generated deep within the Earth’s core.
No one has ever traveled to the center of the Earth, but by studying shock waves from earthquakes, physicists have been able to figure out the likely structure.
At the heart of the Earth is a solid inner core, two-thirds the size of the moon, made primarily 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 liquefying.
Surrounding the outer core is a 1,242-mile (2,000 km) thick layer of iron, nickel, and small amounts of other metals.
The metal is liquid here, because of the lower pressure than the inner core.
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 rotation of the Earth, also causes swirling eddies.
This flow of liquid iron generates electric currents, which in turn create magnetic fields.
Charged metals passing through these fields continue to create their own electrical currents, and so the cycle continues.
This self-perpetuating loop is known as the geodynamo.
The spirals created by the Coriolis force cause the individual magnetic fields to align roughly in the same direction, and their combined effect creates one massive magnetic field engulfing the planet.