If astronomy revealed us the vastness of space, geology introduced us to the vastness of time. How did the geologic time scale developed? How is the age of the Earth determined?
The infinite age of the Earth in the Ancient World and Middle Ages
The inexorable evolution of the Earth's surface produced by sudden and violent phenomena (Earthquakes, volcanic explosions) but also by slow and progressive phenomena (erosive action of water, transport of alluvium by rivers) begs a question: did this evolution have a beginning, will it have an end? The answers diverge.
For Aristotle (384-322 BC), the Earth is eternal and perpetually keeps its physical identity because the changes in the surface offset each other on average. For the Stoics, on the contrary, the Earth is engaged in a succession of creations and destructions where each time the same world is created with the same beings and the same events.
The interlude of short chronologies
With the Copernican revolution, Aristotle's theses were discarded in favor of Christian theses. The ideas of creation and history were implanted in the common conscience, becoming the references. Biblical and historical chronologies were then completely renewed, bringing a transition from long chronologies to short chronologies. These will prevail for two centuries. During the 16th and 18th centuries, there were two hundred different calculations: the shortest gives 3483 years from the birth to Christ, the longest 6984 years.
The most famous is due to James Ussher (1581-1656), who affirmed that Creation took place at nightfall on 22 October 4004 BC. Christopher Columbus (1450-1506), Gerardus Mercator (1512-1594), Johannes Kepler (1571-1630), Isaac Newton (1642-1727) each made their proposal.
The whole history of the Earth (the formation of mountains, the piling up of geological layers, the carving of valleys) was concentrated in very short periods without this being a problem. The Earth was formed and structured during brief and violent events, of which the Flood was the main one.
The first geological chronometers
Does nature have an internal chronometer allowing to estimate the age of all things? In 1715, Edmond Halley (1656-1743) thought to find it in the saltiness of the ocean which must increase regularly over time according to the continual salt intake by the rivers. He claimed that we may find out that the world is much older than we think, but he did not provide any figures.
In 1721, Henri Gautier (1660-1737) had the idea of dating the age of the Earth by estimating the time of erosion of continental reliefs by measuring the turbidity of rivers. He claimed that in 35,000 years a continent would be completely leveled. The law his measures invoked, suggested that the Earth exist much longer than previously thought. Gautier did not even dare to record the immense duration of time he obtained.
The return of long chronologies
From the years 1760-1770, long chronologies became relevant again. At that time, geology took off and meticulous observations on the ground multiplied. Geology ceased to be a speculative science and became an observational science. The time was finally ripe for both freeing humanity from biblical constraints and fully recognizing the durations of geological phenomena.
By estimating the rate of sedimentation or the rate of erosion of mountains and assuming them constant over time, geologists, such as John Turberville Needham (1713-1781), Pierre-Bernard Palassou (1745-1830) and Jean-Louis Giraud Soulavie (1752-1813), arrived at ages of several million years for the formation of sedimentary layers or the abrasion of reliefs. It was the discovery of the concept of geologic time (deep time) that was spreading like wildfire in 17th century Europe.
Physical timers also revealed the immensity of time. In 1676, Ole Roemer (1644-1710) had shown that light travels with a finite speed. Leonhard Euler (1707-1783) then William Herschel (1738-1822) deduced that the light of the stars travels for several million years before reaching Earth.
Buffon (1707-1788), had the idea of using the cooling rate of the globe as an indicator, in 1778.
He claimed that the Earth was entirely in fusion at the beginning of its history, then it consolidated and slowly cooled. By extrapolating a series of experiments on globes of metals, glass and stone, he found that it took 2,905 years for consolidation to reach the center of the planet, 33,911 years for it to be possible to touch it and 74,047 years to acquire its current temperature. These ages still seem insufficient to him when he sought to reconcile them with geological phenomena.
Lord Kelvin and the controversy over the age of the Earth
At the beginning of the 19th century, deep time geology is accepted. But what is the true age of the Earth? From one extreme, humanity fell to the other: the partisans of uniformitarianism - of which Charles Lyell (1797-1875) was a leading figure - affirmed that the globe has not undergone any particular evolution over time and that its age stretches endlessly. They also believed that the agents that shaped the surface of the Earth have always been of the same nature and the same intensity as those seen at their time.
A valuable insight to the age of the Earth was given by Charles Darwin (1809-1882) in 1859: he estimated that it took 300 million years for the sea to carve the Weald Valley in the south of England (he extrapolated then current erosion rates). The number he calculated seemed compatible with biological evolution and the slow processes of natural selection.
In 1862, Lord Kelvin (1824-1907) attacked the foundations of uniformitarianism : if the Earth cools, its thermal energy has necessarily decreased over time bringing a drop in the intensity of geological phenomena and a modification of its general appearance.
Kelvin calculated the age of the Earth by using thermal gradients; during the cooling of the planet, the surface geothermal gradient decreases. Assuming that the initial temperature of the globe was homogeneous and equal to the melting temperature of the rocks, and by applying Fourier's law of heat conduction, Kelvin suggested that the Earth was between 20 and 100 million years old.
Lord Kelvin's calculation started a famous controversy because geologists, who suggested lengthy geological durations without being able to prove it, refused to be constrained by physics. Even if they no longer assumed infinite times, they did not want to conceive of figures below 300-400 millions of years.
The controversy escalated again at the end of the century when Lord Kelvin and other physicists agreed on the low estimate of 24 million years. They determined that if the Sun gets its energy from its gravitational contraction, it cannot have an age greater than 20-25 millions years, thereby limiting the age of the Earth.
Geological estimates of Earth's age, based on the speed of sediment deposition or on the increase in salinity of the oceans (the idea of Edmond Halley was taken up in 1900 by John Joly), continued to be 90-100 million years old. The misunderstanding between geologists and physicists therefore remained unresolved.
The discovery of radioactivity
The intensive research on radioactivity by Henri Becquerel (1852-1908) in 1896 resolved the controversy. In 1898 physicists Marie Skłodowska-Curie (1867-1934) and her husband Pierre Curie (1859-1906), discovered two radioactive elements; polonium and radium. Marie Curie actually coined the term radioactivity, to describe the recently discovered phenomenon - the emission of elementary particles by certain atoms when their unstable nuclei disintegrate.
The production of heat by radioactive decay invalidated Lord Kelvin's thermal model and made his estimate of the age of the Earth obsolete. Geology was no longer limited by physics. Lord Kelvin had disregarded convection, which completely modifies the mode of cooling of the planet by continuously bringing to the surface heat from the depths.
It was the lack of knowledge of internal and continuous production of heat that made it impossible at the time to develop a correct thermal model for calculating the age of the Earth. Radioactivity provides a new way of determining the age of Earth. The ratio of radioactive elements to radiogenic elements (products of decay) depends only on time and constitutes a clock, if we know the initial proportion of radiogenic elements.
The first such attempt to calculate the age of Earth was based on the measurement of the helium produced by the decay of radium. In 1904, Ernest Rutherford (1871-1937) estimated a sample age of 40 million years, which he revised in 1905 to 140 million years, and then revised it again in 1906 to 500 million years.
Lord Rayleigh (1875-1947), by the same method, was the first in 1905 to exceed a billion years. The measurements of helium are however biased because on the one hand the helium does not remain confined in the rock but can escape and on the other hand it can come from the disintegration of other elements.
Bertram Boltwood (1870-1927) developed in 1905 a more reliable dating method based on the measurement of the uranium / lead ratio. Thanks to this method, the age of the oldest rocks was fixed in 1915 to be around 1600 million years. This sudden lengthening of geological times considerably disturbed geologists because to accept this fact is to recognize that their vision of Earth's history was only very partial.
In 1935, Alfred O. C. Nier (1911-1994) advanced radio chronological methods using the concept of isotopes. Three radioactive chains (uranium-238 / lead-206, uranium-235 / lead-207 and thorium-232 / lead-280) allow dating, while the lead-206 / lead-207 ratio serves as a function of time. Nier estimated the oldest rock samples to be 2,570 million years old.
In 1946, Arthur Holmes (1890-1965) and Friedrich Houtermans (1903-1966) showed that Nier's method gives a precise estimate of the age of the Earth, which they fixed between 3 and 3.4 billion years old.
This age, accepted by geologists, now posed a serious problem for astronomers because they determined that the age of the universe was between 1.8 and 2 billion years. To arrive at this estimate, they used the Hubble distance-velocity relationship which provides the age of the universe if a constant rate of expansion is assumed. The conflict was resolved in the 1950's when astronomers revised their intergalactic distance scale.
In 1953, further progress in estimating the age of the globe came from the analysis of the isotopic composition of meteorites. Clair Patterson (1922-1995) showed, using the uranium / lead method, that the Earth and meteorites were formed at the same time from an identical reservoir, 4.55 billion years ago. This age was confirmed by other radio chronological methods (potassium / argon and rubidium / strontium) and definitively accepted in the 1970s by dating the lunar rocks.