Geoscience Reference
In-Depth Information
called neptunists, was precipitated from an ancient ocean. The idea of extreme acts of God
such as the flood helped people to imagine that the Earth had been shaped by catastrophes,
and this was the generally accepted theory until the end of the 18th century.
In 1795 the Scottish geologist James Hutton published his now famous
Theory of the Earth
.
The much quoted though paraphrased summary of its message is that 'the present is the key
to the past'. This is the theory of gradualism or uniformitarianism, which says that if you
want to understand geological processes you must look at the almost imperceptibly slow
changes occurring today and then simply trace them out through history. It was a theory
developed and championed by Charles Lyell, who was born in 1797, the year Hutton died.
Both Hutton and Lyell tried to put religious beliefs in events such as the creation and the
flood to one side and proposed that the gradual processes at work on the Earth were without
beginning or end.
Dating creation
Attempts to calculate the age of the Earth came originally out of theology. It is only com-
paratively recently that so-called creationists have interpreted the Bible literally and there-
fore believe that Creation took just seven 24-hour days. St Augustine had argued in his
commentary on Genesis that God's vision is outside time and therefore that each of the
days of Creation referred to in the Bible could have lasted a lot longer than 24 hours. Even
the much quoted estimate in the 17th century by Irish Archbishop Ussher that the Earth
was created in 4004
BC
was only intended as a minimum age and was based on carefully
researched historical records, notably of the generations of patriarchs and prophets referred
to in the Bible.
The first serious attempt to estimate the age of the Earth on geological grounds was made
in 1860 by John Phillips. He estimated current rates of sedimentation and the cumulative
thickness of all known strata and came up with an age of nearly 96 million years. William
Thompson, later Lord Kelvin, followed this with an estimate based on the time it would
have taken the Earth to cool from an originally hot molten sphere. Remarkably, the first
age he came up with was also very similar at 98 million years, though he later refined it
downwards to 40. But such dates were considered too recent by uniformitarianists and by
Charles Darwin, whose theory of evolution by natural selection required more time for the
origin of species.
By the dawn of the 20th century, it had been realized that additional heat might come from
radioactivity inside the Earth and so geological history, based on Kelvin's idea, could be
extended. In the end, however, it was an understanding of radioactivity that led to the in-
creasingly accurate estimates of the age of the Earth that we have today. Many elements
exist in different forms, or isotopes, some of which are radioactive. Each radioactive iso-
