Flowing lava erases nearly everything in its path. An entire forest can be wiped out by streams of molten rock. In some circumstances however, a flow may encase a small amount of plant material before it is entirely incinerated. Starved of the oxygen required for combustion, these bits of organic material are preserved inside the cooling lava. When geologists find these artifacts, they can use them to date the lava flows that contain them.
Geologists use two main techniques to determine the age of objects they find. Relative- age dating compares past geologic events based on where objects appear relative to one another in layers, or strata, of rock. In contrast, absolute-age dating provides an accurate estimate of an object's actual age. This technique, also called radiometric dating, measures the stage of decay of specific radioactive isotopes contained in the object. Depending on which isotope is analyzed, objects from several billion years old to just a few thousand years old can be accurately dated.
Scientists use radiocarbon dating, a radiometric dating analysis of carbon isotopes, to date objects that are 60,000 years old or less. This technique provides very accurate age estimates of relatively young objects. However, it can only be used to date objects that were once alive. It cannot be used to date rocks, unless those rocks happen to contain organic material.
Radiocarbon dating relies on an understanding that some isotopes of carbon are radioactive and decay at constant rates over time. These unstable isotopes lose particles from their nuclei, thus becoming different elements. Living plants exchange their carbon with the carbon in the air. The air contains mostly carbon-12, the most common carbon isotope. However, it also contains small amounts of other isotopes, including carbon-14, a radioactive isotope produced in the atmosphere when cosmic rays bombard nitrogen atoms. All living plants -- and all organisms that depend on plants -- contain these two isotopes in a known ratio. After an organism's death, its carbon-12 content remains constant. However, its carbon-14 content decreases because carbon-14 is radioactive and slowly decays into nitrogen. Hence, the ratio of the two isotopes changes over time.
The half-life of carbon-14 is 5,730 years, which means that in 5,730 years, half of the original carbon-14 atoms will have changed to nitrogen. Scientists rely on other data sources, such as tree rings, to estimate variations in carbon-14 concentrations over time. They use these estimates to interpret the ratio of carbon-12 and carbon-14 and ultimately to produce an accurate date of death for an organic object.