It is, essentially, the top of a volcano sticking out of the Atlantic Ocean, about halfway between South America and Africa, a hop and a skip south of the equator. But Ascension Island is where Puget Sound professor Mike Valentine and student Andy Caruthers will spend a chunk of their summer vacation. It is a hunt for evidence of variations in the earth s magnetic field. Valentine also will be looking for evidence of a complete reversal of the polarity of the field, though he said those more dramatic changes are not everyday occurrences. The last big one occurred 780,000 years ago, he explained.
Brief flips occur more frequently than that.
And they take ten thousand years to happen, so it s not something where you wake up the next day and see the field is the other way. Permanent records of the magnetic field are left in volcanic rocks. Valentine says it has to do with minerals in lava flows. The minerals become magnetized parallel to the earth s field at the time that the lava cools, said Valentine. By taking rock samples and determining the age of the rocks and the direction of the magnetic field within the samples, geologists get a snapshot of the earth s magnetic field at that place and that time in the distant past.