Page 18 - Curriculum Visions Dynamic Book

Radioactive clocks
Every radioactive element decays by emitting radiation and particles until it changes into a stable element. Sometimes it passes through a series of transformations into other radioactive elements before finally becoming stable. Each radioactive element also has its own steady rate of decay.
To see how this happens, consider the following example. Suppose a piece of an element loses half its surplus particles and radiation in the first 10 years after it has formed. This is taken as the “tape measure” for radioactive decay, or half-life (see page 16). Half of what remains would be lost over the next ten years. At the end of 20 years there would be half of a half, or one-quarter of the starting amount. Ten years later there would be half of one-quarter (one- eighth) of the starting amount and so on. Because of this constant and predictable rate of change, it is possible to use it as an extremely accurate clock.
Several radioactive elements have very long half- lives (they decay very slowly) and they can
be used for dating rocks. For example, radioactive potassium occurs in minerals such as hornblende and mica, and these minerals are found very widely in igneous rocks.
Rubidium is another radioactive element
found in ancient volcanic rocks. The decay of rubidium to the stable element strontium has
been used to date the world’s oldest surviving rocks (at 3.8 billion years).
The Moon’s rocks have also been dated by
this method, providing us with a date for the origin of the Solar System of about 4.6 billion years ago.
 The minerals from igneous rocks such as granite and basalt contain radioisotopes. Their decay can be accurately measured to estimate
a date for the rock formation. This sample is granite, which is widely found on continents. Basalt is the most common rock of the ocean floors. By using such samples, both land and sea rocks can be dated to help build a clearer picture of geological change.
 These archaeologically significant human remains were trapped in the peaty fibres of a marsh. By the use of carbon dating it was possible for scientists to ascertain the exact age of “Pete Bog”.
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