Scientists now know that Earth's magnetic field also changes with time, so that the entire thing resembles a jigsaw puzzle in which the pieces are constantly changing shape.
These fluctuations in Earth's magnetic field are known as secular variation.
Secular variation happens over decades to centuries, and some geophysicists think it may play a role in the mysterious reversals of Earth's magnetic field that take place every half million years or so.
Supporters of the dynamo theory think secular variation occurs because of circulation patterns within Earth's liquid core.
As the solid core heats the liquid core, the molten iron becomes less dense and floats upward, which in turn causes cooler material to sink.
Variations in the rate that the liquid iron rises and falls show up on Earth's surface as differences in the magnetic field.
The two phenomena are thought to be so tightly linked that scientists use secular variation to infer the pattern of molten iron flow in Earth's core.
In fact, since drilling to Earth's core is a technical impossibility, secular variation is the only evidence scientists currently have that Earth even has an active core.
(Related: "The Core: Hollywood Fiction or Science?")
In the new paper, published in this month 's issue of the New Journal of Physics, Northwestern University's Ryskin argues that secular variation may instead be due to circulating seawater.
It's well established that ocean currents, such as the Gulf Stream in the Atlantic, form the circulatory system of the seas.
These currents bring up nutrient-rich cold water from the ocean depths and carry it to different parts of the Earth.
It's also known that dissolved salts in seawater conduct electricity, and as ocean currents move within Earth's main magnetic field, they generate their own secondary magnetic field.
Scientists had previously made spot measurements of this "oceanic" magnetic field and concluded that it is too weak to explain secular variation.
But based on his new mathematical model, Ryskin says, the total field generated by all the ocean currents in the world is roughly equal to measurements of secular variation.
Ryskin stresses that his hypothesis does not challenge the theory that Earth has a dense iron core. But he does question whether that core is active.
"The fact that the outer core is fluid, made of heavy minerals like iron, and is extremely hot—all these facts are probably true," Ryskin said.
"However, if secular variation is not produced in the core, then we have no evidence whatsoever that there is flow there."
Ryskin says he has theories about what, if not the dynamo, might be creating the main magnetic field. He has a paper on the subject coming out soon, he said, and is not yet ready to discuss his findings.
Meanwhile, for many geophysicists, the idea of an inert core raises more troubling questions than it answers.
"How is the planet's [main] magnetic field generated if not by motion in the core?" said Michael Purucker, a geophysicist at NASA's Goddard Space Flight Center in Maryland.
The ocean's magnetic field, he added, "may mimic secular variation, but it's so much weaker that it's not even in the ballpark."
Geophysicist Jeffrey Love of the U.S. Geological Survey called Ryskin's ideas "highly unconventional."
"It's true that if secular variation were caused by the ocean, it could be argued that we know nothing about what's going on in the Earth's core," Love said.
"But our understanding of what the ocean can do to the Earth's magnetic field says that [the new study's suggestion] is simply not true."
Some experts also criticize Ryskin's mathematical approach as unnecessarily complex, even for experts in the field.
"His method is so involved," said David Loper, an emeritus professor of geology at Florida State University, "that its pitfalls are obscured from all but the most dedicated reader."
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