Sabancı Üniversitesi Proficiency Sınavı Hazırlık 4
Sabancı Üniversitesi Proficiency Sınavı Hazırlık
1 – Sabancı Üniversitesi Proficiency Sınavı Hazırlık (Sabancı Üniversitesi Proficiency Sınavı Hazırlık için Bireysel – Özel Ders )
2 – Sabancı Üniversitesi Proficiency Sınavı Hazırlık ( Sabancı Üniversitesi Proficiency Sınavı Hazırlık için 4 kişilik Gruplarla Ders )
Birth of Geology
5. Humans have noticed fossil remains since prehistoric times. There are ancient stone tools that appear to have been chipped so as to show off a fossil shell. The fossilized stem of a giant cycad was placed in an ancient Etruscan burial chamber. But attempts to understand the nature of fossils are comparatively recent. The science of geology arose primarily in Christian Europe where beliefs based on biblical stories made it unsurprising to discover the shells and bones of extinct creatures high up in mountainous regions:
they were the remains of animals that perished in the biblical flood. Even granite, it was suggested by socalled 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 l8th century.
6. 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 tracethem 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.
7. Attempts to calculate the age of the Earth came originally out of Christian theology. It is only comparatively recently that so-called creationists have interpreted the Bible literally and therefore 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 bit longer than 24 hours. Even the much quoted estimate in the l7th 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.
8. 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.
9. 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 increasingly 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 isotope has a characteristic half-life, a time over which half of any given sample of the isotope will have decayed. By itself, that is not much use unless you know the precise number of atoms you start with. But, by measuring the ratios of different isotopes and their products it is
possible to get surprisingly accurate dates. Early in the 20th century, Ernest Rutherford caused a sensation by announcing that a particular sample of a radioactive mineral called pitchblende was 700 million years old, far older than many people thought the Earth to be at that time. Later, Cambridge physicist R. J.Strutt showed, from the accumulation of helium gas from the decay of thorium, that a mineral sample from Ceylon (now Sri Lanka) was more than 2,400 million years old.
10. Uranium is a useful element for radio dating. It occurs naturally as two isotopes — forms of the same element that differ only in their number of neutrons and hence atomic weight. Uranium-238 decays via various intermediaries into lead-206 with a half-life of 4,510 million years, whilst uranium-235 decays to lead-207 with a 713-million-year lifetime. Analysis of the ratios of all four in rocks, together with the accumulation of helium from the decay process, can give quite accurate ages and was used in 1913 by
Arthur Holmes to produce the first good estimate of the ages of the geological periods of the past 600million years.