How Old Is The Earth…

Posted on by SFScienceCategories:Scientific Method, Chemistry, Physics

4.54 billion years old (4.54 × 109 years old)

Well, I am glad that is settled.

I heard a fellow on YouTube deriding science by saying…

“First they told us that the Earth was less than 10,000 years old, then it was hundreds of thousands of years old, then millions, now it is billions of years old. They can’t even agree on something as fundamental as the age of the Earth.”

The age of the Earth has not changed, but our ability to correctly measure it has. Each of the stages listed above is a refinement towards a more accurate answer. Of course, what the chap was criticising is really science’s great strength; the principle of trying to explain what the evidence suggests, of going where the evidence leads.

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(It may help to read this post on atoms and molecules before reading on.)

The chief tool used to determine the age of rocks and meteorites is radiometric dating – chiefly a technique called potassium argon dating. Potassium is the 19th element on the periodic table, in Group I with the other alkali metals. It has 19 protons in its nucleus and a cloud of 19 electrons in orbit around its nucleus. Roughly 93% of potassium atoms also have 20 neutrons in their nucleus. Neutrons have the same mass as protons but have no electrical charge. These “normal” potassium atoms are known as potassium 39 atoms (sometimes written as 39K). Almost 7% of potassium atoms contain 22 neutrons (41K). Potassium 41 is a stable isotope of potassium. This means that it is not radioactive. Finally, there is a tiny fraction of potassium atoms that have 21 neutrons (40K). This is a radioactive (unstable) isotope of potassium. The unstable atom of potassium 40 can decay into stable calcium 40 or stable argon 40 atoms. 89.1% of the time it decays into calcium and 10.1% of the time into argon. The half-life of this decay is 1.25 × 109 years. The half-life is the time it takes for half of its atoms to decay into either calcium or argon.

If a rock formed from lava after a volcanic eruption (igneous rock), then we can make some assumptions about it. Firstly, it will contain the same proportion of 40K (potassium 40) as any other naturally occurring material. Secondly, any argon (a noble gas) in the liquid rock will evaporate out. It cannot react with the elements in the rock so it is free to leave and will evaporate just as dissolved oxygen or carbon dioxide leave water when it is heated. Once the lava solidifies, any movement of atoms in and out of the rock ceases. A time capsule has been formed locking the atoms in place. Over time, 40K in the solid rock will slowly decay and some argon atoms will form within the rock. Because the rock is solid, the atoms will not be able to escape. When a scientist comes along and digs up the rock, millions of years later, he can heat the rock up and release any argon that has accumulated. By comparing the ratio of argon and 40K within the rock he can work out how long it has been since the rock solidified.

There are a number of other radiometric dating systems (e.g. uranium lead or argon argon) and they can sometimes be used to compare results. The different dating methods agree with one another which lends weight to the accuracy of the measurements made. The results are repeatable, meaning that the same result is produced whoever does the test. Repeatability is one of the cornerstones of the scientific method.

Questions…

  1. Name two other Group I metals.
  2. Name two Group II metals.
  3. Igneous rock is a form of rock produced by cooling magma. Can you name another type of rock and describe it?
  4. How many electrons does argon have in its outer shell?
  5. Name another noble gas.
  6. What force holds an atom together?
  7. Protons and neutrons each have a relative mass of 1. Why is the relative atomic mass (RAM) of argon (39.95) not a whole number?

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