The fact that our planet is surrounded by a magnetic field — which acts as a protective blanket against harmful solar radiation — is what makes life on Earth possible. Without it, Earth’s surface would have been continuously bombarded by cosmic radiation through its 4.5 billion-year history, instantly frying all but the hardiest of microbes.
This magnetic field, as it exists today, has two poles — north and south. But, as a new study published in the Geophysical Research Letters explains, this was not always the case. There were periods in Earth’s history, between 500 million and 1 billion years ago, when the planet may have been enveloped in a multipolar magnetic field.
Our planet’s magnetic field is generated by molten iron swirling around in the planet’s outer core around a smaller, solid core — a process that creates a self-sustaining geomagnetic dynamo. The motion of liquid iron in the outer core is itself driven by the continuous loss of heat from the inner core.
However, Earth’s inner core was not always solid. According to the study, which is based on three-dimensional simulations of geodynamos created by turbulent liquids and models of Earth’s thermal history, the inner core is believed to have begun solidifying roughly 650 million years ago.
“What I found was a surprising amount of variability,” Peter Driscoll, a scientist at the department of terrestrial magnetism at the Carnegie Institution of Washington, said in a statement. “These new models do not support the assumption of a stable dipole field at all times, contrary to what we’d previously believed.”
For instance, the models suggest that around 1 billion years ago, Earth may have transitioned from a strong dipolar field to a weak magnetic field that fluctuated wildly in terms of intensity and direction and originated from several poles. Then, shortly after the predicted timing of the core solidification event nearly 650 million years ago, the geodynamo simulations predict that Earth’s magnetic field transitioned back to a two-pole system.
“These findings could offer an explanation for the bizarre fluctuations in magnetic field direction seen in the geologic record around 600 to 700 million years ago,” Driscoll said. “And there are widespread implications for such dramatic field changes.”
For one, the findings could have an impact on how magnetic measurements are used to reconstruct the movement of Earth’s continental plates and the planet’s ancient climate.
Scientists can visualize how Earth’s magnetic field looked like in the past by analyzing rocks and crystals that have remained untouched since their formation. Magnetite — magnetic variant of rust, or iron oxide — is one such mineral. Microscopic grains inside magnetite are capable of storing information about the planet’s magnetic field, including the direction and intensity, from the time the minerals cooled and solidified. With proper instruments, these magnetite grains can be read like a tape recorder.
The predictions made by Driscoll’s models still need to be compared with data gathered from magnetized rocks. Once that is done, it may yield a more accurate and comprehensive record of our planet’s magnetic field.