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Putting the Pieces Together

Lava expelled from volcanoes also holds valuable clues into the magnetic field's ancient history. In an effort to look at the ancient field from a global viewpoint, Scripps researchers including Constable, Tauxe, Hubert Staudigel and Catherine Johnson led an ambitious multi-institutional sampling effort that ended last year, which Constable refers to as "The Million Year Study."

"The goal was to get as many samples of lava flows of a broad time interval over as much of the globe as possible to map the magnetic field over the last few million years," said Constable.

The field campaign took Scripps researchers from pole to pole, from McMurdo Sound in Antarctica to Costa Rica, from the Azores to Spitsbergen, collecting lava sources that date back several million years.

"It turns out to be a very challenging project because even though the dates we get for million-year-old lavas seem very accurate, they usually don't allow us to put flows from different places in the right relative time order," said Constable.

Besides trying to estimate the strength and direction of the magnetic field over the last million years, the study could help researchers identify what drives the magnetic field.

Scientists believe that motions in the earth's liquid outer core are the source of the magnetic field. The main energy source for the geodynamo, as most earth scientists refer to it, arises from heat released when iron solidifies to make the solid inner core and from buoyancy effects because some lighter material in the liquid outer core doesn't freeze along with the iron that makes up the inner core.

"It's a generator that is not made of mechanical parts but of liquid moving around," explains Constable.

Tectonic plates that make up the earth's crust are subducted back into the earth and some at least are thought to reach the core-mantle boundary. The subducted plates are cooler than the surrounding rock at that depth in the mantle at the boundary. Some scientists believe that these temperature variations influence the way material moves around in the hot iron core and plays a major role in the magnetic field changes fluctuations we see at Earth's surface.

Constable used the information from global field studies by Tauxe herself and others to create the latest generation of models to explain the field. Current models only calculate the field's strength and do not account for changes in direction, which according to Constable doesn't tell the whole story.

Earth's magnetic north and the geographic North Pole are only rarely lined up perfectly. This deviation, which results in a slight misdirection for the present field if compass navigators forget to take this into account, would be a major problem during times when the field strength is low as is expected during a reversal.

When Constable's models are lined up next to the current field model, the downward trend appears much less significant, suggesting the present-day field is still above average compared to the last several million years. Constable developed the latest set of models to provide a better understanding of the past field, but some researchers are already interested in more accurate models that they believe may one day be used to forecast future fluctuations and reversals.

There are major challenges to forecasting reversals, but the model has already contributed to one important historical debate. A historian has used it to track Christopher Columbus' voyages to the New World and back via the Azores, and believes it can help determine the exact island where he first made landfall in his search for the New World. Scholars plugged his compass position into the magnetic field model and were able to show that most evidence points to Plana Cays in the southern Bahamas as his landing point on his first voyage.

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