The principle of superposition builds on the principle of original horizontality.
This is the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally (Figure 2).
A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.
Half Lives for Radioactive Elements Radioactive Parent Stable Daughter Half life Potassium 40 Argon billion yrs Rubidium 87 Strontium billion yrs Thorium 232 Lead billion years Uranium 235 Lead million years Uranium 238 Lead billion years Carbon 14 Nitrogen years The proportion of parent to daughter tells us the number of halflives, which we can use to find the age in years.
There are three general approaches that allow scientists to date geological materials and answer the question: "How old is this fossil?