Earth's Magnetic Field Almost Completely Collapsed 550 Million Years Ago
More than half a billion years ago, Earth experienced a near-complete collapse of its magnetic field. It begins in the early Cambrian period. Then, after a period of about 15 million years, the field began to grow again. The cause of the collapse and the reflection of the field is a mystery. Then, a group of geologists studied the rocks from Oklahoma that formed during that time. Magnetic markers in rock minerals point to events that began about 550 million years ago. That was before multicellular life was introduced on our planet.
Look Deep Into The Core
To understand what’s going on, look at the structure of our planet. Most of us learned in school that the Earth is made up of layers. There’s a crust, where you’re sitting reading this now. Beneath it is the mantle, the thickest layer of the earth. It lies above the molten outer core, which surrounds the solid inner core. The inner core has two parts—the outermost inner core and the innermost core. The core region lies about 2,900 kilometers below the surface. The swirling action of molten iron in the outer core is what generates our magnetic field. If it weren’t for that activity, we wouldn’t have a protective shield against the solar wind. In fact, without it, our planet might be more like Mars today.
So, what happens at the core? Why does our magnetic field fade to almost 10 percent of its strength and then regenerate again? According to John Tarduno, a professor of geophysics at the University of Rochester in New York, the cause is the formation of the Earth’s dense core.
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“The inner core is very important,” he said. “Just before the inner core begins to grow, the magnetic field is at the point of collapsing, but as soon as the inner core begins to grow, the field is regenerated.”
Paleomagnetism Reveals Changes in Our Magnetic Field
In a recent paper, Tarduno and his research team cite key dates in the history of the inner core. They also provide precise age estimates for collapse and regeneration. Since they couldn’t reach into the core and observe it directly, how would they know when this event occurred? The team turned to paleomagnetism to find answers. That’s the study of magnetic markers on rocks that were created when the rocks were formed. Geologists often use this to track records of other changes in the Earth’s magnetic field, such as polar reversals.
Earth’s magnetic field stretches from the core through the mantle and crust and out into space. It is not possible to directly measure the magnetic field inside the Earth. That’s because of the location and extreme temperature of the material in the core. So geologists thought of a better way. They looked for paleomagnetic markers in rocks and minerals that came to the surface. The markers are like tiny needles that lock onto the direction and intensity of the magnetic field that exists as the mineral cools after it forms.
Tarduno and his team wanted to determine the age and growth of the Earth’s core using paleomagnetism to measure the particles. So, they used a CO2 laser and a superconducting quantum interference device magnetometer (SQUID) to analyze feldspar crystals from rock anorthosites and study their perfect magnetic markers.
Rock Dating Use Magnetism To Win
By studying the magnetism locked in the ancient crystal, the researchers determined two important new dates. The first was when the magnetic field began to strengthen after nearly collapsing 15 million years earlier. The rapid regrowth is due to the formation of a solid inner core. It actually recharges the melted outer core and restores the strength of the magnetic field.
Another interesting thing happened about 450 million years ago. That’s when the structure of the growing inner core changes. The result is a boundary between the inner and outer core. Far above the core, mantle changes occur due to plate tectonics at the surface.
Paleomagnetism is enabling new understanding of the Earth’s core, according to Tarduno. “Because we demarcated the age of the inner core more accurately, we were able to explore the fact that the current inner core actually consists of two parts,” he said. “The tectonic movements of plates at the Earth’s surface indirectly affect the inner core, and the history of this movement is imprinted deep within the Earth in the structure of the inner core.”
What About Magnetic Fields Elsewhere?
The team’s research into paleomagnetic clues to the evolution of Earth’s interior provides clues to the history and evolution of our planet. It also offers insight into how it became livable. Finally, their work has implications for understanding the evolution of other planets in the solar system. Things could be very different if they didn’t have a magnetic field. For example, Mars once had a magnetic field, but it disappeared more than 4 billion years ago. That makes the planet vulnerable to the solar wind and likely played a role in the disappearance of the Martian oceans.
It is unclear whether Earth would have suffered the same fate had its magnetic field not regenerated. Tarduno said that our planet would lose a lot of water if the magnetic field did not return. “This planet will be much drier and very different from today’s planets,” he said. “This research really highlights the need to have something like a growing inner core that sustains the magnetic field throughout the lifetime—billions of years—of a planet.”
For more information
How can Earth avoid a fate like Mars? Ancient rocks hold clues
Early Cambrian renewal of the geodynamo and origins of deep core structures
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