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