logs take Leeds researcher on journey to the centre of
through a mass of historical documents is not the usual
way that geophysicist Dr Andrew Jackson conducts his research.
But his recent work looking at changes in the earth's
magnetic field sent him and his colleagues all over Europe,
examining the maritime records of trading vessels, naval
ships and scientific expeditions as far back as 1590.
to the earth's core - the logs of ships such as Darwin's
Beagle (above), the King George (below
and the Betsy (below
have helped Dr Andrew Jackson (below)
probe the secrets of the earth's core.
the late 16th century to the early 19th century, seafarers
were crossing the oceans, steering by compass, measuring
the declination between true north and magnetic north
and recording it in their ships logs. Declination
is not a fixed value: it differs across the earths
surface and over time. On the well-travelled trade routes,
declination measurements have been taken across the centuries,
and the logs now offer unrivalled data on changes in the
earths magnetic field.
Jackson started to study this little understood phenomenon
fifteen years ago. He was able to easily access data on
declination for the last hundred years, but as changes
in the field happen so slowly he soon realised he needed
a much longer observational record.
Dr Andrew Jackson
earths magnetic field is as variable as the weather
and even harder to predict. Five days in meteorological
terms is equivalent to about 300 years in the magnetic
field, in terms of how changes take place. We used land
surveys to gain detailed information for the 20th century,
but I needed to go back much further. The ships
logs provided the answer.
Jackson of earth sciences teamed up with science historian
Art Jonkers from the University of Amsterdam to comb the
maritime records. They looked at logs from scientific
expeditions, naval ships and from merchant companies such
as the British and Dutch East India Companies, the French
Compagnie des Indes and the Hudsons Bay Company.
In Britain, the records were held mostly in the British
Library, the public records office at Kew, and the national
maritime museum, but the researchers also looked at the
other seafaring nations of Europe, such as France, Denmark
and Holland. In Spain, where the logs should have gone
back the furthest, the researchers were unfortunately
thwarted by unreliable archiving and the disappearance
of many records.
of the companies followed standard routes, down the Atlantic,
round the Cape of Good Hope and then either west or east
of Madagascar to India or China, said Dr Jackson.
On the return journey the ships would stop at St
Helena to pick up provisions, and crews would plant seeds
to harvest on their next trip.
logs are stories in themselves, charting not only declination,
but important events on board like woodworm infestations
in the rudder, comet sightings and interesting ocean wildlife.
Although fascinating, the researchers didnt have
much time for browsing the more colourful sections of
the logs. The work has taken five years, and around 200,000
observations have been fed into the researchers
used everything we could get our hands on, says
Dr Jackson. There are a few gaps, particularly across
the Pacific in the early 17th century. These were routes
used by the Spanish, where the data has been lost. The
Atlantic and Indian oceans were very well travelled, and
many of the expedition logs were especially useful as,
by their nature, these journeys were off the beaten track.
researchers even used the logs of well-known ships such
as the Bounty, Darwins Beagle and the 1882 Challenger
information has enabled Dr Jackson to map the changes
which have taken place over the last 400 years, search
for patterns, and look at the possibility of predicting
The earths magnetic field is created by the movement
of the molten iron which makes up its core and is asymmetrical
and unstable because of the erratic movement of the liquid.
While the spinning of the earth on its axis ensures the
field roughly relates to the geographical poles, Dr Jacksons
research has shown that other factors come into play.
to the longer time-frame which the ships logs have given
me, Ive already managed to identify certain features
which are long-lived, explains Dr Jackson. They
seem to show that the magnetic field is also influenced
by the earths solid mantle, as well as its liquid
core. As the mantle cools, it moves the tectonic plates,
and the temperature changes affect the flow of liquid
in the core, so affecting the magnetic field.
changes have been fairly minor, but in the last few million
years there have been total reversals of the magnetic
field, making the south pole magnetic north. The clue
to this is found in lava, preserved on the sea floor.
While fluid, the magma particles are fixed in a certain
way which reflects the polarity of the magnetic field
at the time, and this is trapped into the lava as it cools.
The rock preserves records of magnetic field reversals
going back 180 million years. There have been several
reversals in the last few million years, and more are
likely to take place.
magnetic field has an important role in protecting us
from cosmic rays which can be harmful to biological tissue,
but Dr Jackson doesnt feel a reversal would be dangerous.
He explains: There is no evidence that previous
reversals caused major extinctions. Reversals do seem
to reduce the magnetic field by about a factor of ten,
but they happen very slowly, taking up to ten thousand
of the likely effects would be more displays of aurora
and problems using a compass. It could make things difficult
for animals who use magnetic fields to navigate, like
turtles or homing pigeons, but the slow speed of the change
would give animals time to adapt.
Jackson is continuing to work on the magnetic field, but
has left the historical archives behind him. He nows receives
highly accurate data from Danish, German and Argentinian
satellites, which are measuring the earths magnetic
field from space. .