James Hutton (1726-1797) was an important contributor to the Scottish Age of Enlightenment especially in the science of Geology. For centuries scholars had believed that the age of the earth could be calculated by counting back the number of generations listed in the Bible until the creation of the earth and the Garden of Eden . In fact in the early 17th Century Archbishop Ussher had stated that the earth was created precisely at 6pm on the 22nd October 40004 BC ! In the early 18th Century scientists began to seriously doubt this figure and Hutton using observation and reasoning was the first to convincingly demonstrate an alternative chronology. Visiting the coast of Berwickshire with his friends in 1788 he noted that two types of rocks were exposed. The lower sequence were tilted and twisted, the upper made of almost horizontal layers. Between the two was a flat surface which he called an ‘Unconformity’. He reasoned that the lower twisted rocks had in the remote past been convulsed and lifted upwards forming mountains. These were then eroded to a flat plane and on this surface the upper rocks were eventually deposited. This all took a lot of time much longer than Ussher’s age of the earth. In fact one of his friends noted that ‘Hutton’s mind seemed to grow giddy by looking far into the abyss of time’
Fast forward into the 19th Century and to the life of Charles Lyell (1797-1875). Lyell took up Hutton’s mantle. Visiting Mount Etna he noted as had many visitors before him that the volcano had been almost continually active as far back at least as the Roman Empire. Using his observations and those of others he found that over the last few hundred years Etna had erupted five times a century on average and each lava flow was roughly 0.02 cu km in volume so that 5x 0.02 = 0.1cu km of lava was erupted each century. Now using recent calculations of the total size of Mt Etna as 2000 cu km he calculated the approximate age of Mt Etna (see if you can do the Maths!). The answer is 20,000 centuries or two million years! This was an incredible age to come up with at this time especially as Lyell noted that the volcano lay on much older rocks.
The pioneering work of Hutton, Lyell and many other (mainly British ) geologists established a detailed chronology of the last 600 million years of the earth’s history during the later Victorian period. Modern methods, using radioactive minerals, have extended our knowledge so that today we believe that the age of the earth is much older – approximately 4,500,ooo,ooo years.