Estimates on the Age of the Earth James Ussher was the Anglican Archbishop of Ireland. In 1650 he published a book titled; “Annals of the Old Testament, deduced from the first origins of the world”. For many years, Ussher relied on Middle-Eastern and Mediterranean histories to help determine the Earth’s age. In addition, he used at least 3 versions of the Bible, the Hebrew text being the main foundation of his studies.

Ussher’s method was similar to others before him, including Isaac Newton. After years of exhaustive research, Ussher determined the earth was created on Sunday, October 23rd 4004 BC. For the next 50 years, Ussher’s date was widely accepted, but that would start to change. James Hutton, a Scottish physician, reasoned that the processes at work on the earth were generally slow to occur, and therefore the earth must be much older than Ussher’s 6,000 years.

Hutton and his colleagues recognized the tremendously long amounts of time required for many of the earth’s formations to form.

Hutton also recognized the relationship between rock layers and their relative age. He was familiar with an idea that was first proposed in the late 1600’s by a guy named Nicholas Steno. Steno’s idea has become known as the Law of Superposition, and can be used as a relative dating method for rock.

Relative Dating

In 1896 Henry Becquerel discovered that uranium will naturally decay (break down) because of its radioactive properties. In 1905, the British physicist Lord Rutherford made the first suggestion for using radioactivity as a tool for measuring geologic time. In 1907, Professor B. B. Boltwood, radiochemist of Yale University, used Rutherford’s idea to date igneous rock layers He published a list of geologic ages based on radioactivity.

Many of Boltwood's ages had to be revised, because of our increased knowledge of the atom. The significance of Boltwood’s work was, it showed that geologic time should be measured in terms of millions or billions of years, instead of thousands of years. By 1950, methods of dating the earth had become much more precise.

Radiometric dating is the process of determining the age of rocks from the decay of radioactive elements. As an element decays, the amounts of parent element and daughter product are measured. When half of the amount of original material is turned into daughter product, a half life has occurred. Each radioactive element decays at a different specific rate. Knowing the rate of decay allows us to accurately determine the age of an item.

One of the benefits of radiometric dating is that it will allow the dating of very young material, or very old material. Parent Isotope Stable Daughter Product Currently Accepted Half-Life Values Uranium-238 Lead-206 4.5 billion years Uranium-235 Lead-207 704 million years Thorium-232 Lead-208 14.0 billion years Rubidium-87 Strontium-87 48.8 billion years Potassium-40 Argon-40 1.25 billion years Samarium-147 Neodymium-143 106 billion years

Absolute Dating- Radiometric Dating Half life Parent material C14 Starts at 100% → Daughter product N14 Starts at 0% Number of Years elapsed 1 2 3 4 5,730 50% 50% 75% 11,460 25% 17,190 12.5% 87.5% 6.25% 22,920 93.75%