50 Fun Facts About Geologic Time Scale

Which dinosaur died out so long before Tyrannosaurus rex that T. rex lived closer in time to humans?

Stegosaurus vanished about 145 million years ago, some 76 million years before T. rex appeared. Only 66 million years separate the last T. rex from us.

Which geologic period takes its name from a mountain range on the border of France and Switzerland?

Jurassic rocks were first picked out in the limestone of the Jura Mountains, just north of the Alps. Alexander von Humboldt studied them on a tour of the region in 1795.

Which geologic period, the last of the dinosaur age, takes its name from the Latin word for chalk?

Cretaceous comes from creta, Latin for chalk, which built up on sea floors from the shells of tiny plankton. The White Cliffs of Dover are made of it.

Which geologic period ended with the 'Great Dying', the worst mass extinction in Earth's history?

The Permian closed with about 81% of marine species wiped out. It was also the greatest known mass extinction of insects.

Which crater, buried beneath Mexico's Yucatán Peninsula, marks the asteroid strike that wiped out the dinosaurs?

Chicxulub was found in the late 1970s by geophysicists who were hunting for oil, not dinosaurs. The crater is about 200 km wide and lies under a kilometre of younger rock.

Which group of fish first appears in the fossil record about 450 million years ago, before trees even existed?

Sharks left their oldest known traces, a few fossil scales, some 450 million years ago. They have since come through five mass extinctions.

Which proposed epoch of geologic time did geologists vote down in 2024?

The Anthropocene was voted down after 15 years of debate, largely because its proposed start was so recent. Mud in Canada's Crawford Lake had been picked as its marker.

Which sail-backed predator, often mistaken for a dinosaur, died out 40 million years before dinosaurs appeared?

Dimetrodon is more closely related to mammals than to any dinosaur. Its famous sail was held up by long spines growing out of its backbone.

About how old is Earth, an age worked out from meteorites that formed alongside the planets?

At about 4.5 billion years, Earth is roughly a third as old as the universe. The figure comes from meteorites, leftovers from the birth of the Solar System.

Which supercontinent, formed about 335 million years ago, began to break apart while dinosaurs roamed it?

Pangaea joins the Greek words for 'all' and 'Earth'. Alfred Wegener used the name in 1920 while arguing that the continents had once been a single landmass.

Which is the largest unit of the geologic time scale, with just four covering all of Earth's history?

An eon is the top rung of the ladder: eons split into eras, eras into periods, and periods into epochs. The longest of the four lasted almost two billion years.

Which era, nicknamed the Age of Reptiles, covers the whole reign of the non-avian dinosaurs?

Mesozoic means 'middle life', a name proposed in 1840 by the geologist John Phillips. The 'Age of Reptiles' tag came from the paleontologist Gideon Mantell.

Which span of geologic time, nicknamed the Age of Mammals, covers the 66 million years since the dinosaurs' extinction?

The Cenozoic saw mammals take over the land once the dinosaurs were gone, growing from mostly small animals into the largest on land. Its name means 'new life'.

Which epoch, popularly called the Ice Age, ended about 11,700 years ago as the great ice sheets retreated?

The Pleistocene was not one long freeze: its ice sheets advanced and melted back again and again, at intervals of roughly 40,000 to 100,000 years.

Which epoch began as the last glacial period ended and takes in all of written human history?

The Holocene takes its name from Greek words meaning 'wholly new'. It is a warm spell between advances of the ice rather than the end of the ice ages.

Which geologic period began about 2.6 million years ago and runs through the ice ages to the present day?

The Quaternary covers roughly the whole time that recognisable humans have existed. Its name was coined in 1829 for young sediments in France's Seine Basin.

Which period, named after a Latin name for Wales, saw most major animal groups appear in a sudden burst?

The Cambrian explosion lasted between 13 and 25 million years, a blink in geologic terms. Almost all modern animal phyla first show up in its rocks.

Which period is nicknamed the Age of Fishes, when armoured giants such as Dunkleosteus hunted the seas?

The Devonian is called the Age of Fishes, but it changed the land too: the first forests took root late in the period. Dunkleosteus was one of its armoured placoderms.

Which period of swamp forests was home to Meganeura, a dragonfly relative with a wingspan of about 70 cm?

The Carboniferous means 'coal-bearing': its swamp forests became coal beds found around the world. Meganeura fossils first turned up in a French coal field in 1880.

Which period saw the first dinosaurs and the first mammals appear as life recovered from the 'Great Dying'?

The Triassic is named after a stack of three distinct rock layers that crops up across southern Germany. Friedrich Alberti coined the name in 1834.

Which principle, first proposed in 1669, states that in undisturbed rock layers the oldest lie at the bottom?

Superposition gives only the order in which layers formed, never an age in years. It also fails where folding has tipped a stack of rock past the vertical.

Which Danish anatomist, later a Catholic bishop, argued that 'tongue stones' found in rocks were really ancient teeth?

Nicolas Steno is counted among the founders of modern geology, yet in 1675 he became a priest. Pope John Paul II beatified him in 1988.

Which Scottish 'father of modern geology' wrote that Earth showed 'no vestige of a beginning, no prospect of an end'?

James Hutton took the tilted rocks of Siccar Point as proof that land is worn down and rebuilt in endless cycles. He also inherited two family farms and set about improving them.

Which principle, summed up as 'the present is the key to the past', says ancient rocks formed by processes seen today?

Uniformitarianism got its name from William Whewell, the scholar who also coined the word 'scientist'. Its rival, catastrophism, put the past down to sudden upheavals.

Which geologist wrote Principles of Geology, which a young Darwin took with him aboard HMS Beagle?

Charles Lyell argued that small changes pile up over enormous spans of time. Darwin was handed the first volume by the Beagle's captain, Robert FitzRoy, just before sailing.

Which English surveyor, nicknamed 'Strata', published the first detailed geological map of an entire country in 1815?

William Smith saw in coal pits and canal excavations that rock layers always sit in the same order. Later ruined financially, he spent time in a debtors' prison.

Which technique, pioneered by Rutherford and Boltwood, dates rocks in years from the decay of unstable isotopes?

Radiometric dating works like an hourglass: unstable atoms turn into stable ones at a fixed rate, so the ratio of the two shows how much time has passed.

Which property of a radioactive isotope is about 5,700 years for carbon-14 and 700 million years for uranium-235?

Half-life began as 'half-life period', a term that dates to Ernest Rutherford's work in 1907. It is the time it takes for half of a radioactive sample to decay.

Which Victorian physicist calculated from Earth's rate of cooling that it was only 20 to 400 million years old?

Lord Kelvin later cut his estimate to between 20 and 40 million years. Darwin, who died before that final cut, had already said Kelvin's figures left too little time for evolution.

Which Irish archbishop worked out from the Bible that the world was created in 4004 BC?

James Ussher even named the day: 23 October. His date was no outlier, since the astronomer Johannes Kepler had put creation at 3992 BC.

Which first chapter of Earth's history is named after the Greek god of the underworld for its hellish conditions?

The Hadean got its name from geologist Preston Cloud, who meant it to cover the time before the oldest known rocks. Earth's surface is thought to have been molten.

Which mineral, found as tiny grains in Australia's Jack Hills, is the oldest known material formed on Earth?

Zircon is tough enough to outlast its parent rock: the Jack Hills grains turned up in a pebbly rock that formed more than a billion years after they did.

Which microbes triggered the Great Oxidation Event by releasing oxygen as a waste product of photosynthesis?

Cyanobacteria began raising oxygen levels about 2.4 billion years ago. The gas was toxic to most life of the time, hence the event's other name, the Oxygen Catastrophe.

Which name is given to the idea that the whole globe once froze over, most famously during the Cryogenian Period?

Snowball Earth was coined by Joseph Kirschvink in 1992. A rival 'slushball' version of the idea leaves a thin band of open water around the equator.

Which armoured sea creatures, common fossils for almost 270 million years, finally died out in the 'Great Dying'?

Trilobites were among the most successful early animals, with more than 22,000 species described. By the end, only a handful of species were left to lose.

What do geologists call the remains of species typical of one span of time, which they use to date rocks?

Index fossils matter because rocks of the same age can look completely different from place to place. Graptolites, tiny colonial sea animals, are classic examples.

Which coiled-shell sea creatures, extinct since the dinosaurs' day, are named after the ram's horns of an Egyptian god?

Ammonites owe their name to Pliny the Elder, who called the fossils 'horns of Ammon'. They are closer kin to squid and octopuses than to the nautilus they resemble.

Which layered mounds, built up by mats of microbes, include fossils some 3,400 million years old from Western Australia?

Stromatolites are rare in modern seas but fill the early fossil record. They peaked about 1.25 billion years ago and then went into a long decline.

Which 375-million-year-old 'fishapod', found in Arctic Canada in 2004, blends features of fish and four-legged animals?

Tiktaalik had the scales and gills of a fish but a flattened, triangular head and perhaps primitive lungs. Inuit elders of Nunavut suggested its name.

Which Arizona landmark cuts down through rock layers from the Kaibab Limestone to the ancient Vishnu Schist?

The Grand Canyon is far younger than its rocks, which span nearly two billion years. Several studies suggest the Colorado River only set its course there five to six million years ago.

Which period did Charles Lapworth define in 1879 to end a feud between geologists who both claimed the same Welsh rocks?

The Ordovician is named after the Ordovices, an ancient tribe of north Wales. Its fossils matched neither rival system, so Lapworth gave the disputed rocks one of their own.

Which period, named after hills in South Australia, became in 2004 the first new period added in 120 years?

The Ediacaran takes its name from the Ediacara Hills, where geologist Reg Sprigg found fossil traces of previously unknown lifeforms in 1946.

Which obsolete period name, dropped from official charts in 1989, supplies the T in the 'K-T boundary'?

Tertiary survives from an 18th-century scheme by Giovanni Arduino that sorted rocks into primary, secondary and tertiary. Its span was later split into the Paleogene and Neogene.

Which age of the time scale, ratified in 2018, opens with a 200-year drought that struck several early civilisations?

The Meghalayan is named after the Indian state of Meghalaya, home of the stalagmite used to mark its start. The drought began about 4,200 years ago.

By what name are GSSPs, the official reference points fixing boundaries on the geologic time scale, better known?

Golden spikes need not be gold: the one in South Australia's Flinders Ranges is a bronze disc. Each fixes the base of a stage at one agreed point in a rock face.

Which American geochemist first accurately dated Earth using lead from a meteorite, then fought to ban leaded gasoline?

Clair Patterson kept finding stray lead in his samples and traced it to industry. His campaigning proved seminal in getting lead out of gasoline and food cans.

Which British geologist dated rocks by uranium decay as a student and later proposed convection currents in the mantle?

Arthur Holmes gave a Norwegian rock an age of 370 million years while still an undergraduate. His idea of a churning mantle later helped plate tectonics win acceptance.

Which vast volcanic province, erupting for roughly two million years, is blamed for the 'Great Dying' extinction?

The Siberian Traps rank among the largest volcanic events of the last 500 million years. Their eruptions are believed to be the main cause of the extinction.

What do geologists call a buried erosion surface marking a gap in the rock record, as at Scotland's Siccar Point?

An unconformity stands for time that left no rock behind. Seeing the one at Siccar Point in 1788, John Playfair wrote that his mind 'seemed to grow giddy'.

Which rare metal, common in asteroids, did the Alvarez team find in the clay layer marking the dinosaurs' extinction?

Iridium turned up at hundreds of times its normal level in boundary layers all over the world. Luis Alvarez, a Nobel-winning physicist, and his geologist son Walter announced the find in 1980.