Annick van der Bonn
Center for Earth Development and Dynamics (CEED), University of Oslo
Let me tell you about the shield that is invisible from within the Earth.
This is history. Opinions expressed in the text are at the author’s expense.
Although you might not notice it’s here, Earth’s magnetic field plays an important role in our daily lives. It protects us from solar storms, which are made up of charged particles and can be harmful to satellites, and our phones use computer models of the magnetic field for navigation.
The magnetic field also protects our atmosphere. In fact, we think that the Martian atmosphere was blown away by the solar wind after the magnetic field was turned off about 3.7 billion years ago. The interaction between the Earth’s magnetic field and solar storms also creates a beautiful phenomenon that many Norwegians are familiar with: the northern lights.
We know surprisingly little about why the magnetic field behaves the way it does, despite the fact that Earth’s magnetic field plays an important role in our lives. To understand this, we must study how the Earth’s magnetic field behaved in the past, over hundreds of millions of years.
The Earth’s magnetic field is formed in the depths of our planet, in the outer core where liquid iron, as hot as water, flows. It is this movement of iron that creates our magnetic field, and this is called a “geodynamo”.
As early as the 19th century, Norwegian scientist Christopher Hanstein realized that something deep in our planet was responsible for moving his compass needle. He said poetically, “The Earth speaks of the motions within it using the stupid language of magnetic needle, and if we correctly understand how to interpret the flame writing of the northern lights, it will be no less informative for us.”
Imagine the Earth’s magnetic field as bar magnet Inside the Earth, roughly in line with the Earth’s axis of rotation. A magnet has two poles: a north pole and a south pole. This “dipole” is responsible for about 95 percent of the Earth’s magnetic field. The remaining 5 percent are more complex magnetic structures.
The magnetic field varies on time scales from seconds to hundreds of millions of years. This moves the position of the magnetic poles. Sometimes the North and South Pole changes completely. When they turn, our compasses will no longer point north, but south instead.
Throughout human history, the Earth’s magnetic field has been strong, but we are now beginning to understand that this was not always the case.
Today we use satellites to constantly measure the magnetic field. But how can we study something we couldn’t even see or touch, millions of years in the past?
The rocks can “pick up” the magnetic field when they form. The magnetic minerals inside the rocks orient themselves in the direction of the magnetic field, and these “magnets” are trapped in the rocks. In this way, the rocks can “remember” the magnetic field of the Earth at the time when they formed.
This is how scientists can study rocks that are millions of years old to characterize the Earth’s magnetic field at that time. This is called “ancient magnetism”.
Scientists have been collecting ancient magnetic data for more than 60 years. The understanding of the long-term behavior of the Earth’s magnetic field has increased sharply.
We now have a pretty good understanding of how the magnetic field has behaved over the past 500 million years, except for a huge gap that has plagued scientists for decades: Devon, the time period from 420 to 360 million years ago.
The rocks from Devon had a poor memory of the magnetic field at that time. It turns out that they instead “remember” a magnetic field from a smaller period.
There have been many more setbacks than mass extinctions throughout Earth’s history. interval with one polarity It’s called a “crown”. “Super crowns” have one polarity for a very long time. The last time the Earth’s magnetic poles shifted was 780,000 years ago.
New research on various ancient Devonian data shows that the magnetic field at the time was so weak and variable that it prevented rocks from being properly magnetized. The magnetic minerals in the Devonian rocks turned out to have snapped off toward the inverted “super crown” at Caiaman, where the field was much stronger.
We now believe that Earth’s magnetic field in Devon may have been so weak that it did not adequately protect life on Earth, and may have affected plants and animals by exposing them to increased ultraviolet radiation. Fortunately, the Devonian fish did not need satellites to navigate …
Looking back in time, our research showed that Earth’s magnetic field behaved very differently than it does today. This knowledge can help us understand when and how Earth’s inner and outer cores formed, which can help us improve computer models of the Earth’s magnetic field.
Thanks to Peter Silkoset, Mat Domeier, and Nina Buenaventura for the edits (in Norwegian).
Hanstein, Christopher. Studies on Earth’s magnetism. vol. 1. Lehmann und Gröndahl, 1819.
A. van der Boon, A. J. Biggin, D. Thallner, M. W. Hounslow, R. Bono, K. Wójcik, M. Paszkowski, P. Königshof, T. de Backer, P. Kabanov, R. Vandenberg, A Persistent Non-uniformitarian The ancient magnetic field in the Devonian period? Prepress EarthArXiv (2021), doi: 10.31223/X53W56. In Review of Earth Science Reviews.
a. van der boon, Ten things you may not know about the Earth’s magnetic field, 2021.
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