IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
Could the north and south poles really switch?
The magnetic poles move around the surface of the Earth. Could they ever completely
switch, so that north became south and south became north?
The magnetic poles do switch, and if you’re talking geological timescales, they switch fairly regularly. Figur-
ing out when the next switch will occur, though, might be impossible ….
The Earth’s magnetic ﬁeld randomly changes
direction over timescales of a few tens of thousands
to millions of years. Each period is called a “chron,”
with each ﬂip of the direction of the magnetic ﬁeld
being the start of a new chron.
Our magnetic ﬁeld, which protects us from
harmful radiation coming from the Sun and other
objects in space, has two distinct poles: north and
Lines of magnetic force ﬂow between the
poles, and as a result any magnetic material on the
surface tends to align itself with the magnetic ﬁeld.
This is why our compasses work: the magnet inside
turns to point along the lines of force (see previous
pages for more).
All magnets have a north and south pole, and
if two magnets are close together, their opposite
poles will attract and—if the magnets are strong
enough—stick together. So one magnet’s north
pole will attach to another magnet’s south pole.
If the polarity of one magnet is reversed
(most easily by just turning it around!) so that
the south poles are facing each other, the mag-
nets will move apart.
When one magnet is much bigger than the
other (as in the case with a tiny compass needle
and the entirety of our planet), then the smaller
compass just turns to align itself in the direc-
tion of the bigger magnet’s opposite pole.
When the Earth’s magnetic ﬁeld reverses,
compass needles will swing around and point
south. This won’t be a big problem, as people
will simply adjust the labeling on their com-
passes and continue as normal. The compass
is still pointing reliably in a single direction,
which is what enables navigation (see the next
page for more on this).
Unfortunately, when the poles do reverse, they won’t
necessarily do it instantly. There can be periods when the
magnetic ﬁeld more or less shuts down, to as little as 5
percent of its maximum strength.
Instead of two distinct poles, the magnetic ﬁeld could
have several poles that move around chaotically until a
stable ﬁeld returns.
Computer modeling of the magnetic ﬁeld shows that a
normal north-south (or south-north) ﬁeld is the most sta-
ble, so the Earth’s giant magnet usually ends up like this.
What causes these reversals? Because the Earth is not
a perfectly geometric structure and has many odd lumps
and bumps and dierent densities and irregularities,
there’s inherent instability in the way the core generates
our magnetic ﬁeld.
Again, extremely detailed computer modeling of the
internal structure of the Earth and our magnetic ﬁeld
actually shows that magnetic ﬁeld reversals—poles
swapping—occur over long enough time periods. The
period between reversals is quite random—sometimes
every 10 thousand years, sometimes every 10 million.
This is backed up by evidence from rocks on the sea-
bed, which show lines of magnetic alignment from when
they were formed at mid-ocean ridges. When molten rock
comes up from the mantle, its various magnetic elements
are locked into a particular conﬁguration—based on the
direction of the magnetic ﬁeld—as the rock cools.
Rocks from dierent time periods show dierent
magnetic alignments. Pole swaps are just a normal part of
life on Earth.