Absolute dating volcanic ash layers
The thin, dark part of each ring represents slow autumn and winter growth.Several other processes result in the accumulation of distinct yearly layers that can be used for dating.Probably the most reliable of these estimates was produced by the British geologist Charles Lyell, who estimated that 240 million years have passed since the appearance of the first animals with shells.Today scientists know his estimate was too young; we know that this occurred about 530 million years ago.Droughts and other variations in the climate make the tree grow slower or faster than normal, which shows up in the widths of the tree rings.These tree ring variations will appear in all trees growing in a certain region, so scientists can match up the growth rings of living and dead trees.
During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques.
The discovery of radioactive materials did more than disprove Thomson's estimate of Earth's age.
It provided a way to find the absolute age of a rock.
Radioactive materials in Earth's interior provide a steady source of heat.
Calculations of Earth's age using radioactive decay showed that Earth is actually much older than Thomson calculated.