Clocks in the Rocks and Terra Mobile:
How Geologists Discovered the Age and Motion of the Earth

Chronological (or Absolute) Dating

As of 1900: Geologists had done a good job os assessing the relative ages of rocks, but attempts at hanging numerical ages on them had been frustrated.

Early attempts: Initially, three lines of evidence were pursued:

The discovery of radioactivity: Ironically, radioactive decay, which frustrated Kelvin's purpose, ended up providing the true key to the absolute dating of rocks.

Radiometric dating:

Although only igneous rocks can be radiometrically dated, ages of other rock types can be constrained by the ages of igneous rocks with which they are interbedded.

Finally, by the early 20th century, rocks could be chronologically dated. Often, however, knowledge of their age raised bigger enigmas. For example: If erosion has been constantly wearing down the land's surface for 4.6 billion years, why are there mountains?

A possible answer: Upwellings of molten rock form them. That's doubtless true in the case of volcanic mountains like the Andes or Galápagos.

But what about the many mountain ranges that aren't made up of volcanos, like the Himalayas or the Alaska Range (below)?

For that matter, why do any continents stand above sea level?

Here's the answer:

Plate Tectonics

In Darwin and Lyell's time, people expected that the topography and composition of the ocean's floor should resemble that on land. By Holmes' time, the exploration of the deep oceans was underway, and had revealed some strange things:

Clearly the geology of the oceans was unlike that of the continents. Geologists soon regarded continental and oceanic crust as very different beasts. WTF?

The beginnings of an answer came from an improbably source. The German meteorologist Alfred Wegener (1880-1930) performed field work in Greenland, covered by a continental ice sheet.

There, he had ample opportunity to observe the behavior of glaciers. He observed that ice, when greatly compressed, flowed plasticly, allowing the ice sheet to glide slowly across the underlying rock, and apparently began wondering if rock did the same thing on a larger scale.

He noticed the following patterns:

Continental Drift: To explain this, Wegener proposed the hypothesis of continental drift: i.e. that the location of continents was not fixed, and that they had "drifted" across the globe. Note: Wegener thought that the continental crust slid over the oceanic crust like glacial ice sliding over bedrock. Problem: While Wegener was a genius at making observations and recognizing patterns, he was not able to provide a theory to explain and predict the movements of continents, i.e. to say how it happened. Of course, no one else could, either, but that didn't really matter. The Geological profession didn't like amateurs claiming to solve puzzles that had defied them for 40 years, so....

From 1929 until 1960, no US textbook mentioned continental drift. In 1930 Wegener died in a freak storm while doing field work in Greenland. (Note: Wegener never really fell out of favor in South America, where Geologists routinely walked over the rocks that formed the basis of his argument.)

New evidence and reconsideration

Paleomagnetism After WWII, several lines of evidence from the study of the intrinsic magnetic fields of igneous rocks began to come together.