British naturalist James Hutton was one of the first to recognize that Earth was much older than anyone had imagined. In 1795, he proposed that understanding the processes of the present was a key to understanding Earth’s history. Hutton noticed that the old Roman roads crossing his Scottish homeland were in remarkable condition, considering their age. If the forces of erosion had had little effect on roads more than 1,500 years old, much more time must have been needed to form the massive sedimentary rocks he detected throughout Great Britain.
He reasoned that they must have been formed by a sequence of processes that led from the eroding of older rocks to the depositing of their debris as sediments subsequently transformed into the younger rocks.
In that same decade, a British surveyor named William “Strata” Smith made a major contribution toward visualizing Earth’s history. He became quite good at recognizing similar rock formations or strata by the fossils he found in them. In the deeply buried older rocks he often found fossils with no living equivalents. He also learned that younger formations near the surface were much more likely to contain fossils resembling presently living species.
At the time, most Europeans accepted Archbishop Usher’s declaration, made in 1658, that Earth was created on Sunday, October 23, 4004 b.c. at precisely 6:00 a.m.–a date extracted from the Bible by back-calculating begats and exercising a bit of imagination. British geologist Charles Lyell challenged this orthodox worldview when he published the first volume of his Principles of Geology in 1830. Based on a plethora of evidence collected primarily from his work in Italy, he was able to show that a mere 6,000 years could not possibly have allowed enough time for the great geologic processes displayed in the rock record.
In Italy, British geologist Charles Lyell found strong evidence that Earth’s history must be extended back into time. Prime sources were mud flats surrounding the ancient Roman port of Classis and two volcanoes, one in the Bay of Naples and the second, Mt. Etna, on Sicily. Each red dot represents one of Italy’s volcanoes. of erosion had had little effect on roads more than 1,500 years old, much more time must have been needed to form the massive sedimentary rocks he detected throughout Great Britain. He reasoned that they must have been formed by a sequence of processes that led from the eroding of older rocks to the depositing of their debris as sediments subsequently transformed into the younger rocks.
One critical component of the rock record was sedimentary rocks. By Lyell’s time, the concept of sedimentary rocks was already well established: sediments laid down by erosional processes somehow, over time, solidified into rocks. Time was one of the unknowns of the process, but in Italy Lyell found a clear time reference. He noted that the ruins of the old Roman port of Classis were separated from the coast by five miles of mudflats, sediments laid down over 1,800 years. But the sediment remained loose, unlike the large formations of solidified sedimentary rock common everywhere. How long did it take for sediments to be turned into rock? Much longer, certainly, than the time since the Roman Empire.
Thinking of time and geological processes, Lyell noticed other disturbing evidence. Sedimentary rocks lying hundreds of feet above sea level on the sides of a volcano in the Bay of Naples particularly caught his attention. These rocks contained fossils that resembled living species. To Lyell, it could only mean that the large volcano formed recently in geologic time if the sedimentary rocks resting on its side had no fossils of extinct species.
Later, on the side of Mount Etna, a volcano on the island of Sicily, he discovered small cones formed during ash eruptions. These cones had existed as long as anyone in the region could remember. If no one had witnessed the formation of the small cones, then how long ago had Etna formed? He found the answer in layer after layer of lava flows, ash deposits, and buried cones in valleys exposed by water erosion along the sides of the mountain. He realized that the 10,000-foot volcano had grown from hundreds or perhaps thousands of small eruptions that must have occurred over millennia.
But what really astounded Lyell was the limestone layer that clearly ran underneath Etna and across the plains surrounding the volcano. The limestone contained fossils of animals that were exactly the same as those living in the Mediterranean of his day. The limestone, in turn, was deposited on deeper sedimentary formations, layers that must have existed before the limestone was deposited on top. These older layers contained fossils of extinct animals. Mount Etna, ancient and enduring though it is in human terms, must be a recent geologic event, built on top of young limestone.
These were the earliest inklings of evidence that geological processes take great expanses of time to run their course. Today we know that Mount Etna formed during the past 500,000 years through periodic and often violent eruptions. If we average out its growth to a towering 10,875 feet above the foundation limestone, Etna has grown at a rate of about 2.5 inches per 1,000 years. After slowly precipitating from seawater along the ocean floor over thousands of years, the underlying limestone was eventually pushed above sea level by tectonic forces.