As seismic waves race through the body of our planet, they appear to move 3 percent faster when moving vertically from pole to pole than horizontally from east to west.
New models suggest that the solid core of the earth on the one hand, deep under the Indonesian Banda Sea, is growing faster and on the other hand, under Brazil, is growing more slowly.
There was a time when our planet did not have a solid core. The deepest interior of our planet contained a mass of molten material likely billions of years before liquid iron began to cool and solidify in the center.
This means that the center of the earth could be a huge, growing cluster of crystallized iron, and if these crystals are oriented in a certain way, seismic waves can likely travel faster in some directions.
While creating models of how this particular alignment might have come about, researchers came across an unexpected explanation: the inner core of the earth is growing crookedly.
“The simplest model seemed a bit unusual – that the inner core is asymmetrical,” says global seismologist Daniel Frost of the University of California Berkeley.
“The west side looks different from the east side up to the middle, 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.”
It is impossible to drill into the inner core of the earth to check what is going on, so this is an area of research that is up for debate. Seismic wave propagation and computer simulations are some of the only ways we can test possible explanations for why our planet is shaped the way it is.
Using various computer models that take into account the geodynamics of the earth and the physics of iron minerals under high pressure and high temperature, researchers have now tried to find out why the inner core of our planet is oriented in this way.
The simplest explanation they found was that the crystal core of our world grows fastest on the equator and especially on the east side.
“This corresponds to a 40 percent lower growth rate at the poles and a 130 percent higher growth rate at the equator compared to the global average,” the authors conclude.
“The rate of growth at the equator varies between the eastern and western hemisphere by 100 percent and 160 percent of the global average, respectively.”
This asymmetrical growth rate suggests that some parts of the Earth’s inner core are warmer while other parts are cooler, which allows iron crystals to form faster. Gravity then distributes this excess growth evenly in the soft but firm core, maintaining the overall spherical shape and propelling the crystals towards the north and south poles.
Ultimately, researchers explain, it is this movement caused by gravity that aligns the crystal lattice of the Earth’s inner core along the axis of rotation of our planet.
And so it was from the beginning. The model shows that this type of asymmetrical growth has occurred since the planet’s interior first cooled and solidified, with the radius growing an average of one millimeter per year.
If the model is correct, and this is the true rate of growth, it means that the Earth’s solid inner core is a relatively new phenomenon that only showed up half a billion and 1.5 billion years ago, but likely on the younger side.
This is confusing because the Earth’s magnetic field is at least 3 billion years old, and it is believed that this field arises when heat from the crystallization of iron in the inner core boils up molten material in the outer core.
If the Earth’s core is really that young, it could mean that our planet’s magnetic field was not always generated in the same way.
For example, some scientists have suggested that the original magnetic field was much weaker than it is now and was created by dissolved light elements that accumulated on the outer edge of the inner core of our planet.
It wasn’t until these elements began to crystallize, researchers argue, that the magnetic field got stronger. Seismic waves that propagated throughout the crystal core then induced the electromagnetic field we know today.
Even from the movements of tiny crystals deep in the core of our planet, great forces can grow.
The study was published in Natural geosciences.