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5.12 The Evolving Earth

When Charles Lyell was a young man early in the 19th century, he returned to a coast in England that he had visited as a child. His keen scientist’s eye noted that the shape of the shoreline had subtly changed. Erosion by wind and sea was steadily eating away at the landscape. Lyell saw this as one instance of the planet’s ancient, but steadily and constantly changing surface. Lyell played a major role in arguing that the Earth had a long geological history. He believed that gradual change, exerted over a very long time, could dramatically alter the Earth’s surface. The name of this theory, uniformitarianism, refers to the long timescales over which the surface of the Earth transforms. This idea was opposed by those scholars who subscribed to “catastrophism:” a relatively short geologic history, characterized by sudden cataclysmic events. 

Sir Charles Lyell (1797-1875) was a British geologist. Click here for original source URL

Lyell was not just a theorist, alone with his equations in a book-lined study. He was an active naturalist and explorer. Until his seventies, he roamed up volcanoes and scrambled into canyons all around the world. In Chile, he decided that if one earthquake could elevate a terrain by a few feet, then thousands of earthquakes spread over millions of years could raise a chain of mountains. At Mount Etna in Sicily, he observed the freshly hardened lava, and concluded that the great volcano had been built from many lava flows. Back in England, engineers were cutting through hillsides to lay tracks for the newly invented railways. Lyell saw the many layers of corrugated sediments that were revealed. He knew that he was looking at the record of a constantly and slowly changing Earth.

We know now that this constant geologic activity extends deep within the interior of the Earth. Thousands of small earthquakes shake the planet every day. Churning convection cells in the mantle drive plate tectonics, constantly creating new surface rock and destroying old rock at subduction zones. Even the continental crust, which is not subducted, is constantly subject to metamorphism, tectonic movements, and erosion by wind and water. All this activity leaves so few ancient rocks on the Earth that we had to go to the Moon to find rocks old enough to date the formation of the two bodies.

Lyell’s idea of continuing change can be extended to living creatures. A fossil is the remains of a living organism, after minerals have seeped in and hardened. Bone and shell turn to stone. Fossils were trapped in layers of rock below the Earth’s surface. Lyell’s contemporaries noticed that they often seemed to correspond to creatures that were not alive today. Perhaps these creatures had migrated to distant lands? Thomas Jefferson told pioneers headed west to search for wooly mammoths, whose bones had been found on the East Coast of the United States. Georges Cuvier, the French founder of the field of paleontology in the early 19th century, cataloged dozens of extinct species. Lyell also studied fossils, and speculated that climate change had caused the extinction of many species. Like rock, life too was subject to the ever-changing Earth.

Today, a combination of uniformitarianism and catastrophism has evolved as the favored theory of the planet’s evolution. Earth’s history is definitely a long one, and the surface is slowly shaped by prolonged, perpetual geological processes. But catastrophes occur, too, and they can have profound effects on the geological and biological history of the planet.