Thursday, 7 January 2016

Radio-Isotope Dating: Age of the Earth and Plate Tectonics - Introduction

'The world was created on Sunday, 23rd October 4004 BC"     …James Ussher, Irish Archbishop of Armagh
'..., this world has neither a beginning nor an end".     …James Hutton (1726 – 1797)
For the past few years, I have been giving an annual seminar to the Glasgow University Nuclear Physics Group on a topic that has something to do with nuclear physics but definitely it would have no overlap with the current research programme of the group.  I am happy to say that this year I chose to talk about radioisotope dating as applied to the determination of the age of the Earth and its contribution in establishing plate tectonics on a firm scientific basis.  With no real background in Geology, it was hard work but rather exciting to get to know better the planet we live on. 
In fact, the impact of radioisotope dating on geology and the way we now see the historic evolution of the Earth can only be described as dramatic.  In 1963, geophysicist John Tuzo-Wilson summarized:  "It will be difficult for most of us to accept that large amounts of what we have written and taught has been erroneous".
To an extent, it is not surprising that geology was in such a mess until about 1950.  There was really no absolute time scale - it was a qualitative subject - not really a proper science. This has a feeling of deja vu.  Physics suffered similar fate for almost 2000 years, biology and medicine are definitely showing signs of new life and are developing real understanding about the basis of diseases and how to cure them.
Geology is the science that deals with the history of the Earth as recorded in the rocks.  The problem has been that there was no clock, and rocks have the habit of moving about, weathering and generally changing in every way possible - typically over geological time scales - over millions of years.  Until radioisotope dating arrived properly, around 1950, only thing we could do was to visually examine rocks and chemically find out the minerals it contained.  A totally unsatisfactory situation which lends to a lot of speculative wacky  hypotheses; and the Church got involved too!
Radioactivity was discovered in 1896 and the first ideas that it can be used to date minerals in rocks came within a decade.  The first results were that many of the rocks were hundreds of millions of years old - imagine the reaction of established geologists who believed that the Earth was no more than 20 million years old.  The Church wanted us to believe that it was a mere 6000 years old.  The literature is still full of young-earth 'scientists' who are spending lot of energy in trying to prove that radioisotope dating is a flawed science and its results are misleading.  These are things that make life unique.
It took 50 years to sort out all the details in radioisotope dating method and it is now possible to say with good certainty that the Earth was created at the same time when the Solar System formed from a nebula 4.67 billion years ago.  The initial Earth was in a molten form and the crust took a few hundred million years to form.  Strictly, the Earth is slightly younger than the Solar System.  Meteorites, remnants of the early structures formed in the Solar Nebula, have proved invaluable to fix the date when the Solar System formed. 
While we are talking about the formation of the Solar System, it might be good to reflect on the age of the Milky Way - our galaxy.  It is considered that stars and galaxies started to form within about 100 to 200 million years after the big bang that created the Universe 13.7 billion years ago.  Independent determination of Be-9 abundance on the oldest stars in globular clusters at the periphery of the Milky Way puts their age at 13.6 +- 0.8 billion years. Therefore, the Solar System is much younger than the Milky Way.
Radioisotope dating provided an absolute clock and it was then possible to date past events as they happened on the Earth.  One of the most remarkable discovery was the confirmation of sea floor spreading and thus establishing the theory of continental drift or plate tectonics on a firm scientific footing.  Over the next few weeks, I shall be discussing some of these subjects from a physicist view point. 

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