According to a new study, the solid iron core of the earth has been growing faster on one side than on the other for over 500 million years.
It’s growing faster under the Indonesian Banda Sea than under Brazil, but that uneven growth pattern hasn’t left the core askew, say University of California, Berkeley seismologists who studied the phenomenon.
Gravity has worked to evenly distribute the new growth, which is made up of iron crystals that form when molten iron begins to cool, and to maintain a spherical inner core.
Even if the core doesn’t stay crooked, this uneven rate of growth suggests that something in the outer core below Indonesia is drawing heat away from the inner core faster than below Brazil on the opposite side of the planet, the team said.
Researchers say this discovery helped them prove “rather loose boundaries” for the age of the inner core, which is between half a billion and 1.5 billion years old.
A section from the interior of the earth shows the solid iron core (red), which slowly grows due to the freezing of the liquid iron core (orange). Seismic waves cross the earth’s inner core faster between the north and south poles (blue arrows) than across the equator (green arrow)
FOUR LAYERS OF PLANET EARTH
crust: To a depth of 70 km, this is the outermost layer of the earth, covering both ocean and land areas.
coat: With the lower mantle at 2,890 km, this is the thickest layer on the planet and consists of silicate rock, which is richer in iron and magnesium than the crust above.
Outer core: This region extends from a depth of 2,890-5,150 km and consists of liquid iron and nickel with light trace elements.
Inner core: This region extends to a depth of 6,370 km in the center of planet Earth and is believed to be made of solid iron and nickel.
This limit on the age of the Earth’s solid core can help scientists learn more about the magnetic field that protects us from harmful solar radiation.
“It can help in the debate about how the magnetic field was created before the solid inner core existed,” said Barbara Romanowicz, co-author of the study.
“We know that the magnetic field existed 3 billion years ago, so other processes must have driven the convection in the outer core back then.”
The younger age of the inner core may mean that early in Earth’s history, the heat that boiled the liquid core came from light elements separating from the iron, rather than the crystallization of iron that we see today.
“The age of the inner core has been debated for a long time,” said Daniel Frost, assistant project scientist.
“The complication is, if the inner core has only been around 1.5 billion years ago based on what we know about how it loses heat and how hot it is, then where did the older magnetic field come from?
“Hence the idea of dissolved light elements that then freeze out.”
The asymmetrical growth of the inner core, which grows at different rates on each side of the planet, explains a three-decade-old mystery, explained Frost.
The riddle is that the crystallized iron in the core appeared to be oriented to the west rather than east of the Earth’s axis of rotation.
Map showing the seismometers (triangles) that researchers used to measure seismic waves from earthquakes (circles) to study the inner core of the earth
The team says scientists would expect the crystals to be oriented randomly rather than preferring one side of the planet over the other.
To explain the observations, they made a computer model of the crystal growth in the inner core.
Their model took into account geodynamic growth, how materials deform and shape on earth, as well as the mineral physics of iron at high pressure and temperature.
“The simplest model seemed unusual – that the inner core is asymmetrical,” said Frost.
“The west side looks different than the east side up to the center, not just at the tip of the inner core as some have suggested. We can only explain that if one side is growing faster than the other. ‘
The model describes how asymmetrical growth – about 60 percent higher in the east than in the west – can align iron crystals preferentially along the axis of rotation, with stronger alignment in the west than in the east.
“What we propose in this paper is a model of unilateral solid-state convection in the inner core that reconciles seismic observations and plausible geodynamic boundary conditions,” said Romanowicz.
Even if the core doesn’t stay crooked, this uneven rate of growth suggests that something in the outer core below Indonesia is drawing heat away from the inner core faster than below Brazil on the opposite side of the planet, the team said
The interior of the earth is layered like an onion. The solid iron-nickel inner core is 745 miles in radius, or about three-fourths the size of the moon, and is surrounded by a molten iron-nickel outer core 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 slowly boils when the heat from the crystallizing iron comes from the inner core, and in the mantle as hotter rock moves up to carry that heat from the center of the planet to the surface.
A new model from seismologists at UC Berkeley suggests that the inner core of the Earth is growing faster on its east side (left) than on its west side. Gravity compensates for the asymmetrical growth by pushing iron crystals towards 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 by deflecting charged particles fired by the sun, known as the “solar wind”.
Without this protective layer, these particles would likely remove the ozone layer, our only line of defense against harmful UV radiation.
Scientists believe that the Earth’s core is responsible for generating its magnetic field.
When molten iron escapes in the outer core of the earth, convection currents are created.
These currents create electrical currents that create the magnetic field in a natural process called geodynamo.
The violent boiling movement in the outer core creates 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.
The movement of crystals in the inner core, near the melting point of iron, aligns the crystal lattice with the Earth’s axis of rotation – more to the west than to the east, they found.
The model correctly predicts the researchers’ new observations about the transit time of seismic waves through the inner core.
The anisotropy or difference in transit times parallel and perpendicular to the axis of rotation increases with depth.
The strongest anisotropy is about 250 miles west of the Earth’s axis of rotation.
The inner core growth model also restricts the ratio of nickel to iron in the center of the earth, Frost said.
His model does not accurately reproduce seismic observations unless nickel makes up between four and eight percent of the inner core.
This roughly corresponds to the proportion of metallic meteorites that were once the nuclei of dwarf planets in our solar system.
The model also tells geologists how viscous or fluid the inner core is.
“We suspect that the viscosity of the inner core is relatively high,” said Romanowicz.
This is “an important input parameter for geodynamicists who study the dynamo processes in the outer core”.
The results are to 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 in the Earth’s core.
No one has ever traveled to the center of the earth, but studying shock waves from earthquakes has enabled physicists to figure out their likely structure.
At the heart of the earth is a massive inner core that is two-thirds the size of the moon and is mainly made of iron.
At 5,700 ° C, this iron is as hot as the surface of the sun, but the pressure caused by gravity prevents it from turning liquid.
Around this 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 due to the lower pressure than the inner core.
Differences in temperature, pressure and composition in the outer core cause convection currents in the molten metal when 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 creates electrical currents, which in turn create magnetic fields.
Charged metals passing through these fields then generate electrical currents themselves, and so the cycle continues.
This self-sustaining loop is known as the geodynamo.
The spiral created by the Coriolis force means that the individual magnetic fields are roughly oriented in the same direction, and their combined effects add up to a huge magnetic field that envelops the planet.