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Issue 470, 8 October 2001

Ships logs take Leeds researcher on journey to the centre of the earth

Trawling through a mass of historical documents is not the usual way that geophysicist Dr Andrew Jackson conducts his research.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.


Journey to the earth's core - the logs of ships such as Darwin's Beagle (above), the King George (below left) and the Betsy (below right) have helped Dr Andrew Jackson (below) probe the secrets of the earth's core.

From 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 earth’s 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 earth’s magnetic field.

Dr 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

“The earth’s 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.”

Dr 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 Hudson’s 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.

“Many 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.”

The 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 didn’t 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 researcher’s database.

“We 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.”

The researchers even used the logs of well-known ships such as the Bounty, Darwin’s Beagle and the 1882 Challenger expedition.

The 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 future fluctuations.
The earth’s 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 Jackson’s research has shown that other factors come into play.

“Thanks to the longer time-frame which the ships logs have given me, I’ve 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 earth’s 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.”

Most 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.

The magnetic field has an important role in protecting us from cosmic rays which can be harmful to biological tissue, but Dr Jackson doesn’t 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 years.

“Some 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.”

Dr 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 earth’s magnetic field from space. .



 
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