HOW OLD IS THE EARTH 🌎

For many years, even as late as the 1700s, natural historians still thought that the earth was only a few thousand years old, perfect and unchanging, except for erosion and decay brought about by sin. According to church dogma, the world otherwise would have been perfect and unchanging. Ideas about the age of the earth were dictated by dogma, which is a set of ideals that cannot be questioned. For example, in 1654, James Usher, the Archbishop of Ireland, used the ages of people in the Bible to calculate that creation happened on October 23, 4004 BCE. Another scholar, John Lightfoot, placed the time at 9am. Of course, the Bible doesn't give consistent accounts of how much time lapsed between events, so a lot of guesswork had to be involved. Nevertheless, Usher's estimates was the pinnacle of scholarship for its time, incorporating what was known of the history of Hebrews, Babylonians, Persians, Greeks, and Romans. So we must respect this estimate for the honest attempt that it was, even though we now know that it was, well, a little bit short. For more than a century, the power of the church over European scholarship meant that this estimate was not challenged. However, during the Enlightenment, the hold of religious dogma over scholars and scientists began to weaken. In the second half of the 18th century, scholars and natural historians began to question both the power of the church and the aristocrats. They used rationality, evidence, and critical thinking to challenge the powers that be and the way things were. They focused on examining the sources of human knowledge, the justification for the power of governments and religious leaders, and the unquestioned assumption of past centuries. The Enlightenment in England was first inspired by Isaac Newton's transformation of physics and our understanding of the universe. It was also led by John Locke, whose ideas about government and religion were an inspiration to the Americans such as Thomas Jefferson, Benjamin Franklin, and other founding fathers. To the surprise of many people, Edinburgh was a major intellectual center and hub, along with Glasgow, of the Scottish Enlightenment. Nicknamed the Athens of the North, Edinburgh featured many neoclassical buildings and had a reputation for learning like that of its ancient namesake. Why was such a small provincial city like Edinburgh, one of the intellectual centers of the world, surpassing even great cities like London and Paris? As Arthur Herman pointed out in his book How the Scots Invented the Modern World, a number of factors contributed to the ideal environment for freethought and intellectualism. The first was the political stability and economic boom that came after the union with England in 1707. Scottish traders had become rich with their transatlantic trade and their wealth endowed many institutions, especially universities. Edinburgh experienced political stability and peace for most of the 1700s. A second important influence was the religious climate of the city and the absence of religious persecution. After a young Scotsman, Thomas Aikenhead, was hanged in 1697 for blasphemy, the power of religious leaders in Edinburgh rapidly began to wane. Part of the reason was that Scotland was split between Catholics and the mostly Presbyterian followers of John Knox. The Presbyterians were great believers in every man reading the Bible for himself, and they instituted public schools throughout Scotland so that by the late 1700s, Scotland had the highest literacy rate in the world. By that time, Scotland had five major universities, while England had only two. There were many newspapers and book publishers. In Scottish intellectual life, culture was oriented toward books. Among the geniuses of the Scottish Enlightenment was the legendary sceptical philosopher David Hume, the economist Adam Smith, whose work The Wealth of Nations first described capitalism, James Watt, the co-inventor of the modern steam engine that powered the Industrial Revolution, and a quiet young gentleman named James Hutton. Hutton was born in Edinburgh in 1726. James managed to get an education at a local grammar school at the University of Edinburgh, and he turned to the study of medicine, as it was the only way at the time to study chemistry or any other kind of natural science. But the actual practice of medicine held no appeal for Hutton. He used his training to experiment with the latest techniques in agriculture with great success, clearing the landscape, cutting the ditches, draining his farm, creating many fresh cuts in the local bedrock, which fascinated him. In 1753, he wrote that he had become very fond of studying the surface of the earth, and was looking with anxious curiosity in every pit or ditch or bed of a river that fell his way. As a gentleman farmer and landowner, maintaining and improving his family's farms in southeast Scotland, Hutton had studied how soils form, how sediments erode, and how layers of sediment are deposited as they flow down rivers into the sea. From this, Hutton gained insights about rock weathering, and how slowly sediments are formed and deposited. He visited the ancient Roman fort near the Scottish-English border, known as Hadrian's Wall, and noticed that it had not weathered or broken down much in more than 1600 years, since it was built in 122. From this, Hutton realized that the process of weathering down whole mountain ranges would take much, much longer. Hutton devoted his time to extensive scientific reading, and traveled widely to inspect rocks and observe the action of natural processes. Using the basic principles of natural law that all Enlightenment scholars followed, Hutton applied the principle of naturalism to earth as well. In his mind, supernatural catastrophes, known as catastrophism, were useless scientific explanations because they could not be subjected to tests or examination by natural principles or evidence. Instead, Hutton argued that natural laws and processes that operate today must have operated the same way in the past. This is often known as the principle of uniformitarianism, or in other words, the present is the key to the past. Hutton's ideas, including the uniformitarian principle, were first formally presented to the Royal Society in 1785. Finally, in 1795, he published a book called The Theory of Earth. Hutton's ideas were astonishing and remarkably advanced for their time. By the late 1700s, scholars knew a lot about rocks, strata, and fossils, but there was no general theory of geology. One of the impediments was the still widely held belief that Earth was only created a few thousand years earlier, according to the Usher-Lightfoot interpretation of the Bible. Some geologists thought that sedimentary rocks were formed when immense quantities of minerals precipitated out of the waters. Many scholars realized that the destructive processes of erosion were important, but there was no equivalent explanation for the uplift and creation of landscapes. Hutton gained his greatest insight on how long this process must have taken when he found outcrops of what is known as an angular unconformity. To Hutton, an angular unconformity was proof of the immense age of Earth. He saw the tilted layers at the bottom as having once been deposited horizontally, on a bed of a stream or the floor of an ocean, and then hearted into sandstone and shale, and then tilted vertically by immensely strong forces. The sharp erosional surface cutting through the lower tilted beds must represent their uplift into mountain ranges, as subsequent erosion wearing them down again over millions of years. Finally, the horizontal bed on top represent the accumulation of yet another long piling up of sediment on the bed of a river or the ocean, which takes millions of years if you assume the modern rates of sedimentation. Altogether, any singular angular unconformity must represent millions of years of time at the minimum, not the thousands that were supposedly indicated. Hutton continued to explore outcrops around Scotland, for evidence to support his views, which were later published in his book. He found additional examples of angular unconformities all around Scotland. But on his last field trip in 1788, he took a small boat along the Berwickshire coast with his friends and followers James Hall and John Playfair. He knew that coming down the coast of southeast from Edinburgh, the outcrops were vertically tilted sandstones. If you come up from the south, the outcrops were mainly horizontal beds of old red sandstones. Like any good detective, Hutton knew there must be someplace along the coast where the two must meet. And finally, he found it in a place called Sicker Point. As Playfair wrote about this momentous day, Hutton's ideas were a huge departure from contemporary geological thinking. He asserted that sedimentary rocks were one of the sands and muds that had been washed off the land from rivers into the oceans, accumulated in beds there, and then solidified into rocks. But he argued that the hardening into rocks was not due to simple precipitation of sands and muds out of a watery solution, but rather due to the effects of heat and pressure, which modern geology has confirmed. Hutton claimed that the totality of these geological processes could fully explain the current landforms all over the world, and no other explanation was necessary. Finally, he argued that the processes of erosion, deposition, sedimentation, and uplift of mountain ranges were cyclical, and must be repeated many times in Earth's history. Given the enormous spans of time taken by such cycles, Hutton asserted that the age of the Earth must be inconceivably great. As Hutton himself wrote, geological time is immense and virtually endless, with no vestige of a beginning and no prospect of an end. Hutton burst the boundaries of time, therefore establishing geology's most distinctive and transforming contribution to human thought. Deep time. Thanks for watching. If you like geology, here's a video about the natural history of New York City. And you can write in the comments below a topic you think I should cover next.