The Age of the Earth (The Search Begins)

FAS Astronomers Blog, Volume 33, Number 4.

The Earth was formed … some time ago … but when?

To answer this question, I decided to focus on three things.

1. How old is the Earth?
2. How do we know it is that old?
3. Who figured it out?

This is part 1 of the quest, describing how geologist unraveled the history of the Earth. 

Early attempts

Early attempts to determine the Earth’s age were based mostly on theology and the religious knowledge and traditions of the day. The Bible was the main source of information with John Lightfoot and James Ussher producing the two most well-known estimates. 

John Lightfoot was a religious scholar who became the vice-chancellor of Cambridge University in 1654. His extensive writings have been captured in a 13-volume work published in 1825. Two of his works, initially published in 1642 and 1644, are often cited as the source of his estimate for the Earth’s age. As best as I can determine, Lightfoot placed the Earth’s origin on September 12, 3928 BCE followed by the beginning of man on September 17, 3928 BCE, the latter at 9 am. 

James Ussher was an Irishman who eventually became the head of the Church of Ireland. He performed an extensive analysis of the Bible and other works in the mid 17^th century. In his lengthy 1650 publication, The Annals of the World, he concluded that the Earth began at sunset on October 22, 4004 BCE, which puts the Earth’s age at around 6,000 years. 

Ussher’s timeline and the 6,000-year age for the Earth is still referenced in some religious circles today. It is likely that it has stayed with us through the initial printings of the King James Bible. Ussher’s timeline and dates were included in many of these copies and, as such, probably spread through the English-speaking world in the early 1700s. 

Although, most scientists now discredit Ussher’s conclusions, Paleontologist Stephen Jay Gould, in a 1991 article “Fall in the House of Ussher”, defended Ussher as a serious scholar who did the best he could based on the beliefs and information available at the time. 

Geology takes over

Today, our best guess is that the Earth is probably older than 6,000 years – quite a bit older. The first scientific estimates of the Earth’s age came with the advent of geology in the late 18^th and 19^th centuries. 

George Cuvier was a French scientist who first identified gaps in fossil records. He noticed that species would exist and then suddenly disappear and theorized that natural disasters (catastrophes) led to the extinction of these species. He began to look at the Earth in the same light. His ideas eventually became known as Catastrophism, which postulated that the Earth’s history was driven by periodic catastrophic events. 

Scottish farmer James Hutton took a different view. He developed a theory, later called Uniformitarianism, whichsaid the Earth underwent long slow “uniform” changes rather than the abrupt changes Cuvier favored. Hutton published his ideas in a series of articles, ”Theory of the Earth”, during the late 1780s. He later expanded his views into a book with the same title in 1795. However, his theory didn’t garner much acceptance at the time.

Bill Bryson in his book about everything (A Short History of Nearly Everything) describes Hutton’s prose as nearly incomprehensible. John Playfair published a more readable summary of Hutton’s ideas in his 1802 Illustrations of the Huttonian Theory of the Earth and geologists of the time started to take notice.

It was Scottish Charles Lyell who eventually popularized Hutton’s ideas. Lyell was trained as a lawyer, but instead of pursuing the law, he travelled Europe looking for evidence of the slow geological process proposed by Hutton. From 1830 to 1833, he published his three volume Principles of Geology, which brought Hutton’s Uniformitarianism to a wide audience. Michael Benton (Benton. When Life Nearly Died. 2005) points out the Lyell might have gone too far. Today, we recognized that mass extinctions do occur, and in some respects, Cuvier might have been correct, particularly as it relates to the evolution of life. However, with the work of Lyell, it started to become evident that the Earth was ancient, although no one knew just how old it was.

William Thomson (Lord Kelvin) didn’t quite buy in to the uniformitarianism of Hutton and Lyell. Beginning in the 1860s, he applied a mathematical model of the heat transfer from an initial molten Earth and from the Sun using thermodynamics. Based on this, he concluded that the Earth was somewhere between 20 and 100 million years old. 

Stratification

If you’ve ever been to the Grand Canyon, you can see the history of the Earth displayed in the layers of rock that have been cut away by the Colorado river. This stratification provides more clues as to the age of the Earth.

Nicholas Steno introduced the concept, now known as the Law of Superposition, back in the 17^th century by noting that sedimentary rocks form in layers with the younger rocks at the top and older rock underneath. He was followed by Giovanni Arduino, who, based on the study of rock stratification in northern Italy, classified strata into four layers from the oldest to newest as Primary, Secondary, Tertiary, and Quaternary. 

It was William “Strata” Smith who brought the idea of stratification to the forefront of geology. During the late 1700s and early 1800s, Smith travelled the countryside of England performing surveys for the building of canals. He recognized that rock (and the corresponding fossils) are layered one on top of each other in a chronological order. He realized that this stratification (and fossilization) repeated the same pattern throughout the country. This is called the Principle of Faunal Succession. He also identified outcroppings that corresponded to each of the various levels of stratification. He then created detailed color-coded surface maps identifying where various strata were found, and in the process, he defined the geology of each region. He ended up with maps showing the stratification of the Earth but spread horizontally across the country. 

In 1801, he published his initial findings with a map of the geological stratification in the Bath area. In 1815, he followed with his A Delineation of the Strata of England and Wales with a part of Scotland, a map covering the entire country of England. 

Unfortunately, Smith’s common origins were not looked upon well by the members of the English’s Geological Society and Smith received little support and only sold a few hundred copies of his book. He eventually fell on hard times and spent several weeks in a debtor’s prison in 1819. However, he recovered, and in 1831, the Geological Society awarded Smith the first Wollaston Medal, their highest honor. The next year he received a pension from King William IV. Smith, who passed away in 1839, is known today as the father of English geology.

Stratification would eventually become a method of identifying and defining the history of the Earth. This Geologic Time is broken into four eons (Hadean, Archean, Proterozoic, and Phanerozoic) and then several eras, periods, epochs, and so on. 

• The first three eons are often grouped together into the Precambrian supereon.
• The last, the Phanerozoic eon, is divided into three eras (Paleozoic, Mesozoic, and Cenozoic).
  • The Paleozoic era was when life began to flourish. Modern plants, invertebrates and vertebrates appeared and spread across the planet.
  • The Mesozoic era was the time of reptiles, including the dinosaurs, and is segmented into three periods (Triassic, Jurassic, and Cretaceous). 
  • The Cenozoic era is the age of mammals and covers the time from the demise of the dinosaurs up through the Holocene epoch, which is the time of humans on the Earth. 

Stratification does not provide a perfect map of the Earth’s history. There is something called the Great Unconformity, first described by John Wesley Powell during his exploration of the Grand Canyon in 1869 and possibly by James Hutton a hundred years earlier. The Unconformity refers to a gap of up to a billion years in the rock stratification found throughout the world. 

Today, the International Commission on Stratigraphy determines “Global Boundary Stratotype Section and Points (GSSP)” that explicitly delineate the various strata associated with the boundaries of Geologic Time.   

More from Geology

Geology offers some other methods of dating the Earth [see Hazen. How Old is Earth, and How Do We Know? 2010]. Annual deposits of ice and annual layers of sedimentary rock are thousands of years old. The formation and erosion of mountains occur over millions of years. The gradual drift of the continents from Pangaea to what we see today is thought to have taken some 200 million years. 

Conclusion

Geology showed that the Earth was many millions of years old. However, the determination of the exact age would take several modern discoveries from the world of physics. This story follows in part 2.