Igneous Rock: Granite

is a coarse, textured, igneous rock

it is normally acidic in nature (pH<7) It often contains visible quartz, feldspar and coloured minerals (usually mica.)

It is formed when molten rock material cools slowly at considerable depth beneath the earth's crust.

It tends to contain higher concentrations of radioactive materials than other rocks.

Sedimentary rock: limestone or sandstone

is formed when layers of pre-existing rock particles that have been eroded from other rock sources are laid down as sediment.

Some of these sediment particles may have been eroded from rocks containing radioisotopes (e.g. tiny particles of granite may contain uranium deposits).

Another way in which a sedimentary rock can become radioactive is if uranium-bearing fluids have migrated though them at some time in the past.

Metamorphic rock

is formed when a pre-existing rock is subjected to high temperature and/or pressure.

It therefore will be radioactive if the rock it is formed from was radioactive before the metamorphosis.

If a rock contains a vein of uranium ore the concentration of uranium can be as great as 5000 parts per million but in general the levels are much lower.

Radioisotopes decay to give other radioisotopes in a radioactive decay series. Part of the decay series for uranium is the gas radon. This means that rocks with uranium content are not only potentially hazardous when tiny grains are transported by erosion but also from the radon gas they emit. (See Radon Gas)

Age of rocks

The most common and accepted method of establishing the age of rocks is based on the natural radioactivity of certain minerals found in rocks. Once the rate of radioactive decay of any particular isotope has been found, the age of a specimen can be worked out from the ratio of the remaining isotope to its decay product. The older it is the more decay product will be present.

In recent geology carbon-dating can be used to date specimens as old as 35,000 years.