New paleomagnetic research suggests that Earth’s solid inner core formed 550 million years ago, restoring our planet’s magnetic field.
About 1,800 miles below our feet, swirling liquid iron in Earth’s outer core creates our planet’s protective magnetic field. This magnetic field is invisible but vital to life on Earth’s surface as it shields the planet from solar winds – radiant currents from the Sun.
However, about 565 million years ago, the strength of the magnetic field decreased to 10 percent of its current strength. Then the field mysteriously revived, regaining strength just before the Cambrian explosion of multicellular life on Earth.
What caused the magnetic field to bounce back?
According to new research from University of Rochester scientists, this rejuvenation happened within a few tens of millions of years — rapidly on geologic timescales — and coincided with the formation of Earth’s solid inner core, suggesting the core is likely a direct cause.
“The inner core is hugely important,” says John Tarduno, William R. Kenan, Jr., professor of geophysics in the Department of Earth and Environmental Sciences and research dean for Arts, Sciences & Engineering in Rochester. “Just before the inner core started growing, the magnetic field was about to collapse, but as soon as the inner core started growing, the field was regenerated.”
In the article published in Nnature communication, the researchers determined several key dates in the inner core’s history, including a more accurate estimate of its age. The research provides clues to the history and future evolution of Earth and how it became a habitable planet, as well as the evolution of other planets in the solar system.
Unlock information in ancient rocks
The earth is made up of layers: the crust, where life resides; the mantle, the thickest layer on earth; the molten outer core; and the solid inner core, which in turn is composed of an outermost inner core and an innermost inner core.
The Earth’s magnetic field is created in its outer core, where swirling liquid iron causes electric currents and powers a phenomenon called geodynamo, which creates the magnetic field.
Because of the relationship of the magnetic field to the Earth’s core, scientists have been trying for decades to determine how the Earth’s magnetic field and core have changed throughout our planet’s history. You cannot directly measure the magnetic field due to the location and extreme temperatures of the materials in the core. Fortunately, minerals that rise to the Earth’s surface contain tiny magnetic particles that set the direction and intensity of the magnetic field as the minerals cool from their molten state.
To better constrain the age and growth of the inner core, Tarduno and his team used a CO2 laser and the lab’s superconducting quantum interference device (SQUID) magnetometer to analyze feldspar crystals from the rock anorthosite. These crystals contain tiny magnetic needles that are “perfect magnetic recording devices,” says Tarduno.
By examining the magnetism trapped in ancient crystals — a field known as paleomagnetism — the researchers determined two new important dates in the history of the inner core:
- 550 million years ago: the time when the magnetic field began to be rapidly renewed after a near collapse 15 million years earlier. The researchers attribute the rapid renewal of the magnetic field to the formation of a solid inner core, which recharges the molten outer core and restores the strength of the magnetic field.
- 450 million years ago: The point in time when the structure of the growing inner core changes, marking the boundary between the innermost and outermost inner cores. These changes in the inner core coincide with changes in the structure of the overlying mantle at about the same time, due to plate tectonics at the surface.
“Because we were able to narrow down the age of the inner core, we were able to examine the fact that today’s inner core actually consists of two parts,” says Tarduno. “Plate tectonic movements on the Earth’s surface indirectly affected the inner core, and the history of these movements is imprinted deep within the Earth in the structure of the inner core.”
Avoiding a Mars-like fate
A better understanding of the dynamics and growth of the inner core and magnetic field has important implications, not only in uncovering Earth’s past and predicting its future, but also in unraveling how other planets form magnetic shields and able to maintain the conditions necessary for life.
Researchers believe that Mars, for example, once had a magnetic field, but the field dissipated, leaving the planet vulnerable to solar wind and the oceanic surface. While it’s unclear whether the absence of a magnetic field would have inflicted the same fate on Earth, “certainly Earth would have lost a lot more water if Earth’s magnetic field hadn’t been regenerated,” says Tarduno. “The planet would be much drier and very different than the planet today.”
Regarding planetary evolution, research emphasizes the importance of a magnetic shield and a mechanism to maintain it, he says.
“This research really underscores the need to have something like a growing inner core that sustains a magnetic field throughout the lifespan of a planet — many billions of years.”