Continental Hearts: Ancient Expanses Called Cratons Pose a Geological Puzzle

Article excerpt

When Viennese geologist Leopold Kober gave geology a new word--kratogen, soon shortened to craton--for the flat, stony interiors of continents, he thought such places to be among the duller places for geological study. For him, the more flexible expanses of crust he called orogens, full of rising mountains and earthquake faults, were where the action was.

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Kober could not know that today, 90 years later, cratons would be objects of intense study. A man of his time, he got little right. He was a fixist and a contractionist. A skeptic of the theory of continental drift first espoused by Alfred Wegener in 1912, Kober believed that the cooling Earth had shrunk just enough over time to explain the folds, mountains and valleys that rumple its crust. He thought ocean floors to be drowned cratons, unyielding and driftproof, and he laid out his scheme in his influential 1921 book Der Bau der Erde, or "the construction of the Earth."

The discovery in the 1960s that ocean basins are made of young, spreading crust confirmed Wegener's basic ideas. Growing continuously from volcanic rifts, the seafloor dives back down when it cools, shrinks and gets dense--pushing continents and the tectonic plates they rest on about like rock scum. Ancient, stable cratons play no part in this underwater action.

But, far from boring, the unyielding hearts of continents to which Kober applied his term hold deep mysteries. Serene and nearly unchanging now, in their youth cratons were places of immense violence and dynamic geological behavior largely impossible on today's cooler Earth. How exactly cratons formed remains an open question, and geologists continue chipping away at the dogged rocks to try to uncover vital clues to an earlier age.

"Kober was a great field geologist and made great contributions to our understanding of the eastern Alps," says Turkish geologist Celal Sengor, a top historian of the field who has at least 30,000 early geology books in his home overlooking the Bosporus. But as time went on, says Sengor, Kober got only weirder. "He developed the concept of a mysterious 'Atomgas' that welled up from the interior of the Earth and caused magmatism ... he was not entirely rational," Sengor says.

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Kober got one thing right: craton. It is an almost perfect word, short and solid as a stone. It derives from kratos, meaning strong and unyielding. Greek myths personified the concept as the minor god Kratos (and the term also inspired the ends of the words democracy and meritocracy, not to mention the relentless warrior antihero of the popular video game God of War).

Cratons started gaining appreciation in the 1970s, when geologists first recognized that the rocks made no outward sense. As solid and cool as they are, cratons should have sunk below the Earth's surface. A good peg on which to fix the birth of modern craton theory and discovery is the early investigations of one man, Thomas Jordan.

"It became clear to me that cratons were just about the most puzzling things," says Jordan, of the University of Southern California in Los Angeles. Now 62, he holds an endowed chair in geological sciences at the university, is director of the Southern California Earthquake Center, a member of the National Academy of Sciences, former department chair at MIT, coauthor of a popular geology textbook and winner of many honors.

His role in the field is appropriate. In an odd way, his life history resembles that of a craton: wildly tumultuous youth full of disruption and crisis, followed by a sudden maturity, early adulthood and extended stability.

Jordan was 16 when he graduated from high school in Panama--where his Army-officer father was stationed. But he'd attended a dozen schools and, the eldest of seven kids, had lived in more than 20 houses. With ace scores in mathematics, Jordan went straight to Caltech. …