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Radioactive
rocks contain atoms of radioisotopes
such as thorium or uranium. Uranium has a very long half-life
so deposits of it are still quite abundant around the world. It is
an alpha particle emitter so it is not dangerous to living things
when it is outside the body. However, tiny particles of rock are dissolved
in water or broken off by the wind during the process of erosion.
These tiny alpha emitters can be ingested (drank or eaten) or inhaled
(breathed into the lungs). Alpha emitters inside the body are very
dangerous.
Igneous Rock: Granite
- is a coarse,
textured, igneous rock
- it is normally
acid 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)
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.
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