This table lists the unstable “parent” elements and the more stable “daughter” elements that they become. Half-lives refer to the length of time it takes for half the parent element in a rock or other object to decay. Because different elements have different half-lives
, their useful ranges vary.
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Leonardo Da Vinci’s Findings and the Ages of Fossils
Leonardo Da Vinci
, 15th century
Italian artist, scientist and inventor, was one of the first to document the
types of fossils he found and noticed that different fossils were found in different
rocks. He was one of the first people to recognize that fossils were the remains
of living things and that the Earth must be much older than most of his contemporaries believed it to be. In the following centuries, scientists built on
his ideas. 19th century scientists studied sedimentary rocks and fossils from
all over the world developing an understanding of the relative ages of rock
layers and fossils. They could tell that some layers were older than others,
but they didn’t know how old rocks and fossils within them were. More
recently, in the 20th century, scientists have developed technology to allow
the numerical ages of rocks and fossils to be determined. See below for further
explanation of relative and numerical ages.
- Relative age dating of sedimentary rocks layers
and the fossils they contain means establishing which rock layers are older
than others. This method of dating does not give ages in years. Instead it
establishes which rocks and fossils are older and which are younger. Relative
age dating relies on several methods. One of the most important involves the
principle that younger rock layers are formed on top of older rock layers.
- Numerical age dating means establishing the approximate
age in years of a particular layer of rock by analyzing the decay of radioactive
elements. Radioactive decay occurs when an unstable form of a chemical element
naturally converts to a stable form of another chemical element. Volcanic
ash layers, for example, contain radioactive elements. When the ash layer
forms, the radioactive elements begins to decay at a constant rate. Because
we know the rates of decay of different radioactive elements, we can understand
the age of the ash layer by examining the amount of unstable radioactive element
and stable product element within the layer.
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