More than a half A billion years ago, the Earth experienced an almost complete collapse of its magnetic field. It began at the beginning of the Cambrian period. Then, after a period of about 15 million years, the field started to grow again. The cause of that collapse and rebound of the field was a mystery. Then a group of geologists studied Oklahoma rocks that were created during that time. Magnetic markers in the minerals in the rocks pointed to an event that began about 550 million years ago. That was before the introduction of multicellular life on our planet.
Look deep into the core
To understand what happened, look at the structure of our planet. Most of us learn in school that the Earth is made up of layers. There’s the crust where you’re sitting reading this right now. Beneath it is the mantle, the thickest layer of the Earth. It sits on top of the molten outer core, which surrounds the solid inner core. That inner core has two parts: an outermost inner core and an innermost inner core. The central region lies about 2,900 kilometers below the surface. The swirling action of liquid 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 look more like Mars today.
So what happened in the core? Why did our magnetic field fade to almost 10 percent of its strength and then regenerate again? According to John Tarduno, professor of geophysics at the University of Rochester in New York, the cause was the formation of the solid inner core of the Earth.
“The inner core is tremendously important,” he said. “Just before the inner core started to grow, the magnetic field was about to collapse, but as soon as the inner core started to grow, the field regenerated.”
Paleomagnetism reveals changes in our magnetic field
In a recent paper, Tarduno and a team of researchers cited crucial dates in the history of the inner core. They also gave a precise estimate of the age of collapse and regeneration. Since they can’t reach the core and observe it directly, how did they find out when these events occurred? The team turned to paleomagnetism to find an answer. That is the study of magnetic markers in rocks that were created when rocks were formed. Geologists often use this to track records of other changes in the Earth’s magnetic field, such as the pole shift.
Earth’s magnetic field extends from the core through the mantle and crust to space. It is impossible to measure the magnetic field inside the Earth directly. That’s due to the location and extreme temperatures of the materials in the core. So the geologists thought of a better way. They looked for paleomagnetic markers in rocks and minerals that rose to the surface. Those markers are like tiny needles that block the direction and strength of the magnetic field that existed when the minerals cooled after they formed.
Tarduno and his team wanted to identify the age and growth of Earth’s inner core by using paleomagnetism to measure those particles. Therefore, they used a CO2 laser and a superconducting quantum interference device (SQUID) magnetometer to analyze feldspar crystals from the rock anorthosite and study their perfect magnetic markers.
Dating Rocks Using Magnetism to Win
By studying the magnetism locked up in those ancient crystals, the researchers determined two important new dates. The first was when the magnetic field began to strengthen after nearly collapsing 15 million years earlier. That rapid regrowth was due to the formation of a solid inner core. In fact, it recharged the molten outer core and restored the strength of the magnetic field.
Another interesting thing happened about 450 million years ago. It was then that the structure of the growing inner core changed. The result was a boundary between the innermost and the outermost inner core. High above the core, changes in the mantle occurred due to plate tectonics at the surface.
Paleomagnetism made this new understanding of the Earth’s core possible, according to Tarduno. “Because we constrained the age of the inner core more precisely, we were able to explore the fact that the current inner core is actually composed of two parts,” he said. “Tectonic plate 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.”
What about magnetic fields elsewhere?
The team’s research on paleomagnetic clues to Earth’s interior evolution provides clues to the history and evolution of our planet. It also offers insight into how it became habitable. Ultimately, their work has implications for understanding the evolution of other planets in the solar system. Things could well be very different if they didn’t have magnetic fields. For example, Mars once had a magnetic field, but it dissipated more than 4 billion years ago. That left the planet vulnerable to solar wind and likely played a role in the loss of the Martian oceans.
It is not clear whether the Earth would have suffered the same fate if its magnetic field had not regenerated. Tarduno said that our planet would have lost a lot of water if the magnetic field had not returned. “The planet would be much drier and very different from today’s planet,” he noted. “This research really highlights the need to have something like a growing inner core that sustains a magnetic field throughout the life, many billions of years, of a planet.”
This article was originally published on universe today by Carolyn Collins Petersen. Read the original article here.