wunder beta⏳ Radiometric Dating
How do scientists know a rock is a billion years old? Inside every rock sits a hidden clock made of decaying atoms. Learn how half-lives let geologists read the true age of rocks, fossils, and the Earth itself.
What you’ll learn
- Clocks Inside RocksUnderstand radioactive decay and define the half-life as the time for half the parent atoms to decay.Unstable parent atoms decay into steady daughter atoms at a fixed rate. A half-life is the time for half the parent to decay. Because the rate never changes, the shrinking amount of parent forms a reliable clock, dropping by half with each half-life.
- Reading the ClockExplain how the parent-to-daughter ratio gives an age, and match isotopes to the objects they date.The ratio of parent to daughter atoms reveals how many half-lives have passed, which converts to years. Carbon-14, with a short half-life, dates once-living things, while potassium-40 and uranium-238, with long half-lives, date ancient rocks.
- Dating the Deep PastApply dating to fossils by bracketing, and describe how it reveals the age of the Earth.Fossils in sedimentary rock are dated by the volcanic layers above and below them. Using uranium and lead on the oldest rocks, meteorites, and zircon crystals, scientists date the Earth at about 4.54 billion years. Cross-checking isotopes makes the results reliable.
Questions this course answers
What is a half-life?
A half-life is the time it takes for half of the parent atoms in a sample to decay into daughter atoms.
After two half-lives, how much of the original parent is left?
One half-life leaves 50 percent, and a second half-life halves that again to 25 percent, or one quarter.
How do scientists work out a rock's age from its atoms?
The ratio of parent to daughter atoms tells how many half-lives have passed, which gives the age.
Why is carbon-14 a poor choice for dating a billion-year-old rock?
Carbon-14's half-life is about 5,730 years, so after tens of thousands of years almost none is left to measure.
How do geologists date a fossil in sedimentary rock?
Sedimentary rock usually cannot be dated directly, so the volcanic layers bracketing the fossil are dated instead.
About how old is the Earth, according to radiometric dating?
Dating of the oldest rocks and meteorites gives an age of about 4.54 billion years for the Earth.
Grounded in trusted sources
- United States Geological Survey (USGS)
- Britannica
- Smithsonian National Museum of Natural History
- National Geographic
Every Wunder lesson is built from real, reputable sources — never invented.
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