The Principle of Superposition- younger rock layers form on top of older
    layers.  This is relative age dating.

Absolute Age Dating -
    Radioactive elements decay and form a new product.

    Over a certain length of time a specific amount of matter will decay
    form a new product.

    Scientist measure how long it takes 1/2 of the atoms to decay -- they
    call this the element's half-life.

    They compare the amount of the original element to the amount of the
    decay product and can calculate the length of time since the rock formed.

    For example:  Carbon-14 takes 5,730 years for 1/2 of its atoms to decay.
    Another half will decay in another 5,730 years.