Absolute Dating and Radioactive Decay
From Relative to Absolute Age
Relative dating tells us which rock came first, but not how long ago. To attach a number of years to a rock, geologists use absolute dating (also called numerical dating). The most reliable methods are based on radioactive decay — a process that happens at a fixed, predictable rate regardless of temperature, pressure or chemical environment, making it an ideal geological clock.
Every atom of a radioactive element is unstable. Over time, each unstable parent isotope spontaneously transforms into a more stable daughter isotope. The key insight is that this decay happens at a rate that is constant for each isotope pair — it does not slow down as the rock gets older, and it is not affected by what happens around the mineral.
Half-Life: The Geological Clock
The rate of radioactive decay is expressed as a half-life: the time it takes for exactly half of the parent isotope atoms in a sample to decay into the daughter. After one half-life, the ratio of parent to daughter is 1:1. After two half-lives, the ratio is 1:3 (one quarter parent, three quarters daughter). After three half-lives it is 1:7, and so on.
This halving pattern means the age of a rock can be calculated if two things are known: the half-life of the isotope pair being used, and the current ratio of parent to daughter atoms in the mineral. By measuring these in the laboratory, geologists calculate how many half-lives have passed since the mineral crystallised, and from that, the age of the rock.
Three isotope pairs are used in radiometric dating: uranium-238 decays to lead-206 (half-life approximately 4.47 billion years — used for very old igneous and metamorphic rocks); potassium-40 decays to argon-40 (half-life approximately 1.25 billion years — used for volcanic rocks and minerals); carbon-14 decays to nitrogen-14 (half-life 5,730 years — used only for organic material up to about 50,000 years old).
Calculating Age from the Parent-Daughter Ratio
The calculation is straightforward once you know how many half-lives have passed. Start by working out the ratio of parent to total (parent + daughter) atoms. Then count how many halvings are needed to get from 100% parent (at the moment of crystallisation) to the measured proportion.
For example: a mineral contains equal numbers of uranium-238 and lead-206 atoms, so the ratio of parent to daughter is 1:1. This means half the original uranium has decayed — one half-life has passed. Since the half-life of uranium-238 is approximately 4.47 billion years, the mineral is approximately 4.47 billion years old. If instead the ratio were 1:3 (one parent for every three daughters), two half-lives have passed, giving an age of approximately 8.94 billion years — which would be older than the Earth itself and therefore impossible, so always check your answer is geologically reasonable.
The choice of isotope pair depends on the age of the material and what minerals are present. Carbon-14 is only useful for organic material (wood, bone, shell) less than about 50,000 years old — older than that, the amount of carbon-14 remaining is too small to measure reliably. It cannot date rocks at all because rocks do not contain significant amounts of organic carbon.
Uranium-lead dating is ideal for ancient igneous and metamorphic rocks because the 4.47-billion-year half-life means measurable amounts of both parent and daughter are present even in rocks billions of years old. Potassium-argon dating works well for volcanic minerals such as feldspar and mica. However, argon is a gas and can escape if the rock is reheated, so this method gives the date of the last cooling event, not necessarily the original crystallisation. Understanding which method suits which rock is an important part of interpreting radiometric dates correctly.
The half-life of carbon-14 is 5,730 years. This makes it invaluable for dating archaeological material and very recent geological deposits, but it is completely unsuitable for dating ancient rocks. After about 10 half-lives (roughly 57,000 years), so little carbon-14 remains that measurements are unreliable.
What Radiometric Dating Actually Dates
Radiometric dating measures the time since a mineral crystallised from magma (for igneous rocks) or recrystallised during metamorphism. It cannot directly date sedimentary rocks, because the minerals in a sedimentary rock were eroded from older rocks and their radiometric clock was set when the original igneous or metamorphic rock formed — not when the sediment was deposited. To date a sedimentary succession, geologists look for interbedded volcanic ash layers (tephras) or lava flows and date those instead, bracketing the sedimentary beds between dated igneous events.
Step 1: Identify the ratio of parent to (parent + daughter). If you are told that a sample has 25% parent and 75% daughter, the proportion of original parent remaining is one quarter — that is two half-lives.
Step 2: Multiply the number of half-lives by the length of one half-life. If the isotope pair is potassium-40 to argon-40 (half-life 1.25 billion years) and two half-lives have passed, the age is 2 × 1.25 billion years = 2.5 billion years. Always state your working and include the correct units. A number without Ma or Ga is an incomplete answer.