Written by: on January 4, 2021 @ 8:00 am

Image Source: Pixabay.com

Is the earth round? I think we can all agree that it is. What we are going to tell you next, spoiler alert, will remind you of your high school geometry class.

A great circle is the largest possible circle that can be drawn around a sphere. All spheres have great circles. If you cut a sphere at one of its great circles, you’d cut it exactly in half. The Earth is not a perfect sphere, but it maintains the general shape. All the meridians on Earth are great circles. Meridians, including the prime meridian, are the north-south lines we use to help describe exactly where we are on the Earth. All these lines of longitude meet at the poles, cutting the Earth neatly in half. These halves are called the Northern & Southern hemispheres.

Image Source: Pixabay.com

Let’s talk about the hemispheres of the earth. The earth is divided into 2 by the equator. Can you go to the equator? Actually, you can! Many countries have now created tourist attractions at the equator! However, you cannot “see” the equator as it is an imaginary line splitting the earth in half. Creating the Northern hemisphere and the Southern hemisphere.

Image Source: Pixabay.com

Have you ever wondered when man first thought about the equator and the hemispheres? The earliest documented mention of the round earth concept dates from around the 5th century BC, when it was mentioned by ancient Greek philosophers. In the 3rd century BC, Hellenistic astronomy established the roughly spherical shape of the earth, as a physical fact and calculated the Earth’s circumference. This knowledge was gradually adopted throughout the “old world” during the 3rd-8th centuries and ongoing during the “Middle Ages.”  A practical demonstration of Earth’s sphericity was achieved by Ferdinand Magellan and Juan Sebastian’s circumnavigation from 1519-1522. The argument had officially been settled…the earth is round!

Back to the equator, or the line of 0 degrees latitude, divides the earth into the Northern and Southern hemispheres. There are differences in the climates of the Northern and Southern hemispheres because of the Earth’s seasonal tilt toward and away from the sun.

Image Source: Wikimedia Commons

This occurs because deep inside the earth, liquid iron is flowing and generating the earth’s magnetic field, which protects our atmosphere against harmful radiation from the sun. This field changes over time and behaves differently in different parts of the world. The field can even change polarity completely! The magnetic north and south poles can actually switch places. This is called a reversal and last happened 780,000 years ago. Quite some time ago!

Image Source: Pixabay.com
Magnetic Field Lines

Between South America and Southern Africa, there is an enigmatic or difficult to determine magnetic region called the South Atlantic Anomaly, where the field is a lot weaker than one would expect. Weak and unstable magnetic fields are thought to precede magnetic reversals, so some have argued this feature may be evidence that we are facing one.

A new study published June 12, 2020, in the Proceedings of the National Academy of Sciences,  has uncovered how long the field in the South Atlantic has been acting up – and sheds light on whether it is something to worry about.

Weak magnetic fields make us more prone to magnetic storms that have the potential to knock out electronic infrastructure, including power grids. The magnetic field of the South Atlantic Anomaly is already so weak that it can adversely affect satellites and their technology when they fly past it. The strange region is thought to be related to a patch of magnetic field that is pointing a different direction to the rest at the top of the planet’s liquid outer core at a depth of 1,795 miles (2,889 km) within the Earth.

This “reverse flux patch” itself has grown over the last 250 years. But we don’t know whether it is simply a one-off product of the chaotic motions of the outer core fluid or rather the latest in a series of anomalies within this particular region over long time frames.

If it is a non-recurring feature, then its current location is not significant – it could happen anywhere, perhaps randomly. But if this is the case, the question of whether its increasing size and depth could mark the start of a new reversal remains.

If it is the latest in a string of features reoccurring over millions of years, however, then this would make a reversal less likely. But it would require a specific explanation for what was causing the magnetic field to act strangely in this particular place.

Image Source: Wikimedia Commons
Saint Helena Island

To find out, scientists travelled to Saint Helena – an island in the middle of the South Atlantic Ocean. This island, where Napoleon was exiled to and eventually died in 1821, is made of volcanic rocks. These originate from two separate volcanoes and were erupted from between eight million and 11.5 million years ago.

Scientists discovered when volcanic rocks cool down, small grains of iron-oxide in them get magnetized and therefore save the direction and strength of the Earth’s magnetic field at that time and place. A group of scientists collected some of those rocks and brought them back to their lab in Liverpool, where they carried out experiments to find out what the magnetic field was like at the time of eruption, potentially 11 million years ago.

The results showed us that the field at Saint Helena had vastly different directions throughout the time of eruption, showing us that the magnetic field in this region was much less stable than in other places. It therefore challenges the idea that the abnormality has only been around for only a few centuries. Instead, the whole region has likely been unstable on a timescale of millions of years. This implies the current situation is not as rare as some scientists had assumed, making it less likely that it represents the start of a reversal. Please feel free to re-read the last 2 paragraphs!

What could explain the odd magnetic region? The liquid outer core is generating moves (by convection) at such high speeds that changes can occur on truly short, human timescales. The outer core interacts with a layer called the mantle on top of it, which moves far slower. That means the mantle is unlikely to have changed very much in the last ten million years.

From seismic waves passing through the Earth, we have some insight into the structure of the mantle. Underneath Africa there is a large feature in the lowermost mantle where the waves move extra slow through the Earth – meaning there is most likely an unusually warm region of the lowermost mantle. This possibly causes a different interaction with the outer core at that specific location, which could  explain the strange behavior of the magnetic field in the South Atlantic.

Another aspect of the inside of the Earth is the inner core, which is a solid ball the size of Pluto beneath the outer core. This solid feature is slowly growing, but not at the same rate everywhere. There is a possibility that it is growing faster on one side, causing a flow inside the outer core that is reaching the outer boundary with the rocky mantle just under the Atlantic ocean. This may be causing irregular behavior of the magnetic field on the long timescales found on Saint Helena.

Although there are still many questions about the exact cause of the irregular behavior in the South Atlantic, this study shows us that it has been around for millions of years and is most likely a result of geophysical interactions in the Earth’s mysterious interior.

Interested in learning more about the hemispheres and magnetism? Try our at-home experiment and make your very own compass! Grab your materials and follow the instructions here: https://sciencemadefun.net/downloads/compass.pdf

Yael Annemiek Engbers, Ph.D. candidate, University of Liverpool 
Andrew Biggin, Professor of Palaeomagnetism, University of Liverpool

Tags: , , , , ,

Catogories: Experiments: Science Made Fun, Science, Uncategorized

Leave a Reply