| Powering the Planet | |
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It was just before the 20th century began when physicists discovered radioactivity. This was a tantalizing clue about how atoms are put together, so radioactivity drew a lot of attention from the start. One thing learned early on was that radioactive atoms released energy without undergoing any chemical reaction—heat without burning!
This discovery was important for physics, but it meant just as much in a different way for geology. If Earth's radioactive elements created heat, then that changed the rules.
Throughout the 1800s, geologists established that Earth was essentially eternal. As James Hutton had memorably put it in 1788, the record of the rocks showed "no vestige of a beginning—no prospect of an end." It appeared that the accumulation of rocks and the evolution of living things must have taken many hundreds of millions of years. Yet the best physicists of the day could not account for the age of the Earth. Their argument was based on the flow of heat from within the planet, as estimated from the high temperatures of deep mines. If heat flowed from the Earth, then it must be cooling.
The physicist Lord Kelvin argued that even if the Earth had started out white-hot, it would take about 400 million years, tops, to cool to its present state—but that didn't matter because, he said, the Sun could be no older than 50 million years for the same reason. Geologists grumbled, because they could see that too much had happened during the course of Earth's history to fit in that amount of time. How could Kelvin's logic outweigh the evidence of the rocks?
It turned out that Kelvin's premise was wrong. Earth was not doomed to go from white heat to a Big Chill; natural radioactivity keeps it constantly warm. The planet has an almost eternal power source and almost infinite life span. The rest of the 20th century involved rethinking the Earth's history and its dynamics.
The vision of Alfred Wegener, published in 1912, fit this new setting. His "continental drift" theory was the first to treat the planet as active and mobile, not passive and fixed. He could be called the father of geologic mobilism.
As the century wore on, mobilist thinking continued to emerge in new theories. Whether in Warren Carey's expanding Earth, in Harry Hess's convection-based seafloor spreading model that gave rise to plate tectonics, or in the new radical top-down tectonic model of Don Anderson, Earth in the eyes of geologists would never again be a quiet place.
