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The Mountain Problem

The Century in Review

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tilted rocks

Whole mountain ranges are made of tilted strata. How does it happen?

Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Nothing shows how science changes like old textbooks. Here's a hobby—next time you're at a flea market or rummage sale, find Dad's or Grandma's old geology textbook. It's not hard to find items like Thomas Chrowder Chamberlin's 1914 classic Introductory Geology, or Harold Jeffreys's 1929 volume The Earth. And the first true text of geology, Sir Charles Lyell's Principles of Geology of 1830, is available in an inexpensive reprint or online.

In 1900, the texts had a great deal to teach about mountains (and beautiful line drawings of their structure). Mountains are exceptionally important for geology, and not just because that's where the rocks are. What keeps them high?

Almost everything on the Earth's surface is eroded, worn down by time and weather, turned to soil and sand. The rocks of mountains, too, are softened and crumbled into sediment, carried downhill by streams, slides, and glaciers, sifted into the oceans. Yet after billions of years, the mountains remain. What keeps building mountains is not obvious.

Arm-Wavers of Previous Centuries

Oh sure, volcanoes are mountains that build themselves. But most mountains are pushed upward, their rock beds wrinkled like bedsheets or tilted like stacks of newspapers. A hundred years ago, geology textbooks described mountains in detail, but couldn't explain them. In 1914 when Chamberlin got to that part of his textbook, what he had to say boiled down to some arm-waving about geosynclines, a 19th-century concept involving great piles of offshore sediments gradually accumulating, sinking under their own weight until something pushed them back up into mountains.

This was a slight improvement over James Hutton's first speculations in 1795, in Theory of the Earth with Proofs and Illustrations, that oceans and continents must slowly rise and fall to exchange positions, spilling sediments back and forth between them like a Slinky in your two hands. Between Hutton and Chamberlin, James Dwight Dana had shown that continents and ocean floors are made of very different rocks, which could never exchange places. (Dana also gave geosynclines their name.)

But mountains also expose evidence that large piles of solid rocks are shoved sideways along thrust faults, like scraps pushed by a janitor's broom, for dozens of kilometers. Chamberlin didn't have a good explanation for that either. It used to be thought that the Earth had shrunk as it cooled over geologic time, forcing the crust to buckle like an apple's skin as it dries. But by 1914 it was known that the crust made its own heat through radioactivity, and the Earth did not cool. So that theory was out the window.

From Arm-Waving to Hand-Wringing

By 1932, when Chester Longwell, Adolph Knopf and Richard Flint first published their Textbook of Geology, the problem of mountains was the subject of a whole chapter. "Why have sea floors of remote periods become the lofty highlands of today? What generates the enormous forces that bend, break, and mash the rocks in mountain zones? These questions still await satisfactory answers," they wrote.

The evidence was more and more puzzling.

  • First of all, seismology showed that Earth is solid rock at least 1,500 kilometers down, which seemed to rule out large movements of the continents.
  • Yet, second, in places like the eastern ranges of America, petrology made it clear that the rocks consist of sediments from still older mountains to the east—where nothing but the Atlantic Ocean stands today.
  • Third, paleontology found that more and more fossils in widely separated continents are closely similar, and the theory that land bridges had risen for the ancient creatures to migrate on was looking clunkier as the evidence grew.

Continental Drift Prevails

Finally—and this was not entirely a scientific development—German meteorologist Alfred Wegener had popularized a bold theory in which the continents are moving around the Earth's surface like ice cubes in a punchbowl. It was the first true theory of Earth tectonics, a word meaning "large-scale architecture" that geologists borrowed (along with many other words from architects). Certainly Wegener had real evidence, but his books had so much speculation, too, that few people working in geology cared much for them, especially in America.

Thus Longwell, Knopf and Flint mentioned his theory of "continental drift" briefly, using quotation marks. And while they cited Wegener's book The Origin of Continents and Oceans in the reading list at chapter's end, they added a comment: "This book outlines the theory of continental drift and cites supporting evidence of several kinds. It is an entertaining volume." They were actually being kinder than most American geologists, perhaps because Wegener had recently died.

But Wegener had the right idea after all, if not the right theory. He saw the continents as chunky, iceberg-like objects that shouldered their way around, deep roots and all, like the blocks on a Rubik's Cube. The real Earth is too stiff and solid to work that way. Instead we have the 1960s theory of plate tectonics, in which the continents and sea floor alike cover the world in thin rocky plates that move fairly easily over a shallow layer where the rocks are relatively soft. The plates bump each other's edges in a slow-motion Demolition Derby, and that's where mountain ranges get their start. (See Plate Tectonics in a Nutshell for more.)

The textbooks quickly changed, and by the time I was in college, in the early 1970s, everyone was on the same page they're on today. Instead of the arm-waving and hand-wringing of the older texts, we have the straightforward explanation, "Mountain belts are typically formed by plate tectonic activity, specifically continental collision." It's been a great century.

PS: Plate tectonics does a good job of explaining the Earth's crust. Today, the arm-waving is about the deep Earth—the mantle and core. Here's a seven-part introduction to the mantle and a closer look at the core.

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