How old rocks are
Tiny zircon crystals from Western Australia are about 4.4 billion years old, older than any whole rock found on Earth.
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Geologists answer two different questions about age. Which of these rocks is older? That is relative dating, and it uses the order of layers and the fossils in them. How many years old is this rock? That is absolute dating, and it uses the steady decay of radioactive atoms. The two methods grew up separately and now check each other.
Older below, younger above
Sedimentary rocks form particle by particle and bed by bed, each layer laid on top of the last. So in any undisturbed sequence of layers, a bed must be older than any bed above it. The USGS calls this the Law of Superposition. Fossils add a second clue. In the late 1700s and early 1800s the English canal engineer William Smith and the French scientists Georges Cuvier and Alexandre Brongniart found that rocks of the same age contain the same kinds of fossils, even far apart. Smith used this to predict which rocks lay under the hills his canals had to cross, and geologists used it to recognize rocks of the same age on opposite sides of the English Channel. The idea behind it is now called the Law of Fossil Succession.
Clocks inside minerals
Superposition says which layer is older but not by how much. For numbers in years, geologists use radiometric dating. Some elements in rocks and minerals have radioactive isotopes that decay into stable isotopes of other elements at a fixed rate. The ones used for the oldest rocks have half-lives from about 700 million to more than 100 billion years (USGS). Measure how much of the original isotope is left and how much of its product has built up, and the time since the mineral formed follows. Strictly, the method measures the last time the rock was melted or disturbed enough to reset its radioactive elements.
A half-life is the time for half of the radioactive atoms to decay. After one half-life, half are left; after two, a quarter; after three, an eighth. Because the starting amount does not matter, only the ratio of what remains to what has formed, the clock works on a crystal far too small to weigh by hand.
The oldest things on Earth
- 4.54 Ga: Age of the Earth and the Solar System, from lead isotopes in meteorites and the oldest lead ores (uncertainty under 1%)
- 4.53 to 4.58 Ga: More than 70 meteorites dated
- 4.4 to 4.5 Ga: Oldest dated Moon rocks
- 4.4 Ga: Zircon grains in sedimentary rocks of west-central Australia
- 4.03 Ga: Acasta gneisses near Great Slave Lake, Canada: among Earth's oldest dated rocks
- 3.7 to 3.8 Ga: Isua supracrustal rocks, West Greenland
The figures come from the USGS. Ga stands for billions of years ago. Ancient rocks over 3.5 billion years old are found on every continent, but none of them is the planet's first crust. They are lava flows and sediments laid down in shallow water, which means Earth already had a history before they formed. To get the age of Earth itself, scientists treat Earth and meteorites as parts of one system and follow how the ratio of lead-207 to lead-206 has changed as uranium decayed. That gives 4.54 billion years.
- In undisturbed layers of sedimentary rock, the oldest layer is…
- at the bottom
- at the top
- in the middle
- Who used fossils to predict the rocks under hills his canals had to cross?
- William Smith
- Friedrich Mohs
- Robert Boyle
- What fraction of a radioactive isotope is left after three half-lives? Give a decimal.
- A mineral has 25% of its original radioactive isotope left. How many half-lives have passed?
- Earth is 4.54 billion years old and the Acasta gneisses 4.03 billion. How many million years older is Earth?
- What is the oldest material dated on Earth so far, according to the USGS?
- Zircon grains from Western Australia, about 4.4 Ga
- The Acasta gneisses, 4.03 Ga
- Isua rocks, 3.7 to 3.8 Ga
- How was the 4.54 billion year age of Earth found?
- From lead isotopes in meteorites and the oldest lead ores
- From the oldest rock on Earth
- By counting sediment layers
- Radiometric dating measures the time since the rock was last…
- melted or disturbed enough to reset its radioactive elements
- touched by water
- exposed to sunlight
- More than 70 meteorites give ages from 4.53 to 4.58 billion years. What is the width of that range in million years?
- Order from oldest to youngest.
- Relative dating: superposition (older below) and fossil succession.
- Absolute dating: radioactive decay; half-lives of 700 million to over 100 billion years.
- Ancient rocks: Acasta gneisses, 4.03 Ga; oldest crystals: zircons, about 4.4 Ga.
- Earth is 4.54 billion years old, from lead isotopes in meteorites.
Sources for this course (18)
- USGS FAQs (usgs.gov/faqs): What is the difference between a rock and a mineral? (mineral definition; common minerals quartz, feldspar, mica, amphibole, olivine, calcite; rock = aggregate of one or more minerals)
- What are igneous rocks? (Latin for fire; intrusive/plutonic slow cooling over thousands or millions of years, coarse grained, granite, diorite, gabbro, peridotite; extrusive/volcanic fine grained or glassy, vesicular, rhyolite, andesite, basalt, obsidian)
- What are sedimentary rocks? (layering; siltstone, sandstone, conglomerate, limestone, shale; clay, silt, sand, pebbles larger than 2 mm; chert; limestone precipitated from water)
- What are metamorphic rocks? (heat, pressure, hot mineral-rich fluids; not melted; denser; phyllite, schist, gneiss, quartzite, marble; foliation; contact metamorphism)
- USGS Find-A-Feature: Fossil (fossils almost exclusively in sedimentary rocks)
- USGS Fossils, Rocks, and Time (pubs.usgs.gov/gip/fossils: most rocks exposed at the surface are sedimentary; gravel to conglomerate, sand to sandstone, mud to mudstone or shale; Nicholas Steno mid-1600s; Law of Superposition; William Smith; Law of Fossil Succession)
- USGS Geologic Time: Age of the Earth (pubs.usgs.gov/gip/geotime/age.html: radiometric dating, half-lives 700 million to more than 100 billion years, Acasta Gneisses 4.03 Ga, Isua 3.7 to 3.8 Ga, zircons 4.3 Ga and 4.4 Ga in Western Australia, Moon rocks 4.4 to 4.5 Ga, meteorites 4.53 to 4.58 Ga, Earth 4.54 Ga with uncertainty under 1%, rocks over 3.5 billion years on all continents)
- USGS Mohs Hardness Scale image page and USGS Bulletin 1042-G Gem Stones of the United States (ten minerals, relative scale, diamond many times harder than corundum, much dust is silica, quartz 7)
- NPS Mohs Hardness Scale (Theophrastus, On Stones, about 300 BC)
- Physical Geology H5P Edition 5.6 Mineral Properties (opentextbc.ca, reference materials: natural fingernail 2 to 2.5, copper 3, steel nail 4, glass and knife 5 to 5.5, streak plate 6.5 to 7)
- Physical Geology 2nd ed. 7.2 Classification of Metamorphic Rocks (Steven Earle, opentextbc.ca: slate, phyllite, schist, gneiss by grade; grade table 150 to 300, 300 to 450, 450 to 550, above 550 °C; marble, quartzite, hornfels; migmatite)
- British Geological Survey, Discovering Geology: Rocks and minerals (rock cycle, Peterhead granite feldspar, quartz and biotite, limestone to marble, shale to slate, contact metamorphism, mineral definition)
- Weathering (biological, chemical, physical, freeze-thaw)
- Geological processes (erosion, deposition)
- BGS Geology dataset page (schist and gneiss recrystallized in the solid state)
- Geological Society of London, Compaction and Cementation (calcite, silica, iron oxides, clay minerals as cements)
- OpenStax Chemistry 2e 1.2 (oxygen 49.20% and silicon 25.67% of the mass of Earth's crust and atmosphere)
- USGS Water Density (ice about 9% less dense than water).
Checked: 2026-10-17