What is the difference between relative and absolute dating?

Relative dating establishes the order of events (which rock or event is older or younger) without numbers, using principles like superposition, cross-cutting relationships and faunal succession. Absolute (radiometric) dating gives an age in years by measuring the decay of radioactive isotopes. The geological timescale combines both.

How old is the Earth and how do we know?

About 4.54 billion years. The figure comes from radiometric dating — measuring the ratio of radioactive parent isotopes to their decay products — in the oldest meteorites and minerals, which share the age of the early Solar System.

What is the law of superposition?

In an undisturbed sequence of sedimentary layers, each layer is younger than the one below it and older than the one above. So the bottom layer is the oldest. It was set out by Nicolas Steno in 1669 and is the basis of relative dating.

How does radiometric dating work?

A radioactive isotope decays into a daughter atom at a constant rate, described by its half-life — the time for half of it to decay. Because the rate never changes, the measured ratio of parent to daughter atoms gives the elapsed time in years. Carbon-14 (half-life ~5,730 years) suits recent material; uranium-lead suits rocks billions of years old.

The geological timescale

How does anyone know a rock is 300 million years old? The timescale wasn't handed down — it was built in two stages: first ordering events straight from the rocks with no dates at all, then, much later, pinning real years onto that order using radioactivity. The powerbook below walks that whole story, from a stack of layers to the dated chart of Earth's history.

4.54 Ga

age of the Earth

~88%

of time is Precambrian

ways to date rock

The Geological TimescaleSection 1 of 6

0 of 6 locked in

This book has 6 short sections. Read each one, answer the check, then use Next to move on — the bar above tracks how far you've come.

The problem of deep time

The Earth is about 4.54 billion years old. That number is easy to write and impossible to feel — a billion is already beyond us, and this is more than four of them.

Try a trick. Squeeze the whole 4.54 billion years into a single calendar year, January to December. On that scale the oldest known rocks form in late February; the first complex animals don't appear until mid-November; the dinosaurs are wiped out around December 26; and the entire history of our own species — every cave painting, pyramid and city — fits into the last half-hour of December 31. All of recorded human history is the final thirty seconds.

So the first problem of geology is not what happened, but how anyone could measure time on that scale. You can't watch it, and nobody was there. The rest of this book is the answer, and it arrives in two stages: first putting events in order, then — much later — putting numbers on them.

New words

deep time

the vast millions-to-billions-of-years scale of Earth history, far beyond human experience

🔒 Lock it in

Why can't we simply measure the Earth's age by direct observation?

Answer the check to continue

Quick reference & sources

The interactive powerbook above is the lesson — read it section by section with the Next button. What follows is a plain recap and the sources.

Relative vs absolute dating — the two toolkits

Every age on the timescale comes from one of two kinds of evidence.

Relative — the order

Superposition, original horizontality, cross-cutting relationships, inclusions and faunal succession. Gives older/younger, no numbers. Came first (from 1669).

Absolute — the years

Radiometric dating via isotope half-lives (carbon-14, uranium–lead). Gives an age in years. Arrived in the 20th century and dated the Earth at ~4.54 Ga.

How a rock actually gets an age

In practice the two toolkits work together. First you place a rock in the local sequence by superposition and cross-cutting. Then you correlate it to other regions using index fossils, so the same fossil means the same age worldwide. Finally you find something datable nearby — often a volcanic ash bed or an igneous intrusion — and radiometrically date it, transferring real years onto the whole correlated sequence. Order first, then numbers.

For HSC Earth & Environmental Science students

This powerbook covers the “geological timescale” thread of Module 1: Earth's Resources: relative dating principles, absolute (radiometric) dating and half-life, and how they build the timescale of eons, eras and periods. Superposition links straight back to Rocks & the rock cycle, and the deep-time framework underpins later modules on Earth's processes and the fossil record.

See the whole course on the Earth & Environmental Science hub.

Sources & notes

Law of superposition and original horizontality: Nicolas Steno, 1669.
Faunal succession and the first detailed geological mapping: William Smith, early 1800s (his geological map of England, 1815).
Age of the Earth ~4.54 billion years, from radiometric dating of meteorites and ancient minerals (standard geochronology).
Half-lives: carbon-14 ≈ 5,730 years; uranium-238 ≈ 4.47 billion years (standard nuclear data).
Eon/era/period divisions and ages (Phanerozoic from ~539 Ma; K–Pg extinction 66 Ma) follow the International Commission on Stratigraphy chart; the Precambrian spans ~88% of Earth history.
Course framing follows the NSW NESA Stage 6 Earth and Environmental Science Syllabus (2017). Explanations here are original and not a reproduction of the syllabus; this site is not affiliated with NESA.

Keep exploring

For 150 years geology knew the whole story of the Earth in order, with not a single date on it. Radioactivity finally printed the years — and turned a sequence into a calendar.