Geological
Time
The drive up Chapman's Peak cuts through rock 510 million years old — Earth's history written in layers. Earth itself is 4.6 billion years old. The oldest written human record is about 5 000 years. So how can a Grade 10 student honestly say what happened a billion years ago? With two clever tools: stratigraphy (read the layers) and radiometric dating (the clock hidden inside every radioactive rock).
If you stretched all of Earth's 4.6 billion years of history out along your arm — from your shoulder to your fingertip — then all of recorded human history (the pyramids, the Greeks, every empire, every textbook, your great-grandparents) would fit inside the thickness of a single fingernail clipping at the very end. Squash the same 4.6 billion years into one 24-hour day and it comes out the same way: humans appear about one minute before midnight. So how did scientists figure out what happened in the rest of that arm? They didn't watch it. They learned to read it.
Two big tools. First, stratigraphy — the study of rock layers. Just like a slice of cake, the lowest layer was laid down first and the top is most recent. By reading the order of strata in a cliff or a borehole, scientists can tell what came before what — even without knowing exact ages. The Cape Town Cycle Tour route at Chapman's Peak Drive passes layer after layer of 510-million-year-old Table Mountain sandstone.
Second, radiometric dating — the radioactive clock inside the rock. Some elements (uranium, potassium, samarium, carbon) decay at a constant, predictable rate called their half-life. By measuring how much of the radioactive parent is left versus its stable daughter, scientists can calculate exactly how long ago the rock formed. Combine the two and you get the Geologic Time Scale — Earth's autobiography, accurate to a few million years, stretching back 4.6 billion.
Earth's Autobiography, Written in Stone
The Geologic Time Scale is a system that divides Earth's 4.6-billion-year history into named units — eons, eras, periods, epochs and ages — based on the order of rock layers and the radioactive clocks inside them.
For most of human history, no one knew Earth was billions of years old. Scientists tried to estimate it from how long it would take oceans to become salty, or how long Earth's rocks would take to cool — none of it worked. Radiometric dating changed everything: it gave us a precise clock. Combined with the visible strata in cliffs, road cuttings and boreholes, scientists built a single agreed-upon calendar of Earth's history.
For marine sciences this calendar is everything. It tells us when life first appeared in the oceans (early Archean), when the great Cambrian explosion of marine diversity happened (~541 mya), when the supercontinent Pangaea broke up (~200 mya — Topic 11), when dinosaurs vanished (~65 mya), and when the modern oceans we sail and dive in took shape. Every fossil, every rock, every coastline has a date attached — and the Geologic Time Scale is how we know.
Tool 1: Reading the Layers
The simplest tool in geology is almost embarrassingly old-fashioned: stack of pancakes, oldest at the bottom. This is called stratigraphy, and it's the founding principle of the geologic time scale.
The Law of Superposition
Sediment falls from above and settles in layers. Once a layer is buried, it can't be re-arranged. So in any normal sequence of strata, the oldest layer is at the bottom and the youngest is at the top. Each layer records the conditions when it was laid down — what was alive, what the climate was like, whether the ocean covered the area.
Look at Chapman's Peak Drive on the way out of Cape Town. The orange and grey horizontal bands in the cliffs are 510-million-year-old Table Mountain sandstone, laid down when this part of Africa was still in the supercontinent Gondwana. Every layer is a page.
Tool 2: The Clock Inside the Rock
Stratigraphy gives you relative dating — the order of events, what came before what, with no numbers attached. For the absolute age, an actual count of years, scientists use the fact that some elements decay at a steady, predictable rate. That method is radiometric dating, and the property it runs on is called half-life. Learn the pair together: a cliff face tells you the order, a radioactive clock tells you the date.
First, the problem: Earth has destroyed its own birth certificate
Earth is an active planet — and that one fact is the reason this whole topic is difficult. Because Earth is active, none of its original rocks still exist. There is no surviving piece of the surface Earth had when it formed, so there is nothing on this planet you can put in a machine and measure to get the planet's own age directly. Everything below is a way around that problem.
Three processes have been destroying and remaking the surface for four and a half billion years — and you have already met all three:
🌋 Subduction
- Ocean floor is pulled back down into the mantle at trenches (Topic 11).
- It is being destroyed at roughly the rate it is made at the ridges.
- The oldest sea floor anywhere is only about 200 million years old — younger than the dinosaurs.
🌧️ Erosion and burial
- Wind, rain, rivers and waves break rock into sediment.
- The sediment is carried to the sea, buried and compressed into new sedimentary rock.
- The atoms survive. The original rock does not — it has become a different rock.
🔥 Melting and reheating
- Volcanism and mountain-building melt and cook rock back into new rock.
- Melting resets the radioactive clock to zero — see the warning below.
- So even ancient material dates from its last melt, not from Earth's formation.
Why "resets the clock" is the part that catches people out
Radiometric dating does not tell you when the atoms were made. It tells you when the rock last solidified — because that is the moment the daughter isotope started being trapped inside the crystal instead of escaping. Melt the rock again and the trapped daughter is released; the clock starts over from zero.
So on an active planet, every rock reads younger than the planet. The oldest crust we have found — Hudson Bay, 4.3 billion years — is not Earth's age. It is a minimum: Earth is at least that old. To find the real figure we have to go somewhere that stopped being geologically active almost immediately. That is what the calibration points below are for.
Worked example — Cobalt-60
~ half-life: 5.26 years · decays into stable Nickel-60
Start · 0 years
8 g ⁶⁰Co
100% original
After 1 half-life · 5.26 yr
4 g + 4 g Ni
50% original / 50% daughter
After 2 half-lives · 10.52 yr
2 g + 6 g Ni
25% original / 75% daughter
The principle. Half-life is the time it takes for half the atoms of a radioactive isotope to decay into the daughter element. After 1 half-life, half is gone; after 2 half-lives, three-quarters; after 3 half-lives, seven-eighths. The decay rate is constant — temperature, pressure and chemistry can't change it. By measuring the ratio of parent to daughter in a sample, scientists can count how many half-lives have passed, and calculate the rock's exact age.
Different isotopes are useful for different age ranges:
| Isotope | Half-life | Best used for… |
|---|---|---|
| Carbon-14 | 5 730 years | recent organic remains (younger than ~50 000 years) |
| Samarium-147 | 106 billion years† | the oldest rocks and meteorites — see the note below |
| Potassium-40 | 1.25 billion years | very old rocks & volcanic ash |
| Uranium-238 | 4.5 billion years | the oldest rocks on Earth, Moon and meteorites |
† Samarium-147 — why your exam paper says something different
Look carefully at Question 2.1 of your Term 2 Paper 1. It says "the half-life of Samarium-147 is 106 million years" and gives you a decay table running 0 → 106 → 212 → 318 → 424 → 530 million years. The real half-life of ¹⁴⁷Sm is 106 billion years — a thousand times longer, and about eight times the age of the universe.
That is not a trick, and you are not being taught something false. The paper scales the number down deliberately so the question works. With the true half-life, a 160-million-year-old fossil would still hold 99.9% of its original ¹⁴⁷Sm — the graph would be a flat line and there would be nothing to read off it. Shrinking the half-life by 1 000× turns it into a curve you can plot in seven marks.
What to do in an exam: always use the half-life the question gives you. The method — count the half-lives, read the graph — is identical whichever number is on the page, and the method is what earns the marks. Just don't carry "106 million" out of that question and into a general-knowledge answer.
Real ¹⁴⁷Sm dating is used on meteorites and the oldest rocks on Earth, where billions of years actually have passed. → Open Term 2 Paper 1
Both your Topic 12 booklet and the Mapping Deep Time slides give the half-life of Potassium-40 as 4.468 billion years. That is not Potassium-40's half-life — it is Uranium-238's, quoted correctly in the same sentence one line earlier. The two got swapped.
The correct value for ⁴⁰K is 1.25 billion years, which is what the table above uses. You can see the mistake without looking anything up: if the two isotopes had the same half-life there would be no reason to ever choose between them, and the whole point of having several isotopes is that they cover different age ranges.
What to do in an exam: the same rule as the Samarium note above — use the number the question gives you, because the method is what earns the marks. But do not carry 4.468 billion out of the booklet as Potassium-40's half-life in a general answer. Knowing that your own textbook can contain an error, and being able to say why it is an error, is worth more than the number itself.
The fossil-dating paradox
There is one annoying problem. Fossils form in sedimentary rock — but sedimentary rock cannot be radiometrically dated for deep time. Igneous rock (cooled lava) can be dated, but it's so hot when it forms that it destroys every organic trace. How do scientists get around it?
🌋 Igneous rock
- Datable — contains uranium & potassium with very long half-lives.
- No fossils — molten origin destroys all organic matter.
- Best examples: volcanic ash, basalt, granite.
🐚 Sedimentary rock
- Holds the fossils — cool, gentle conditions preserve organic remains.
- Hard to date directly for deep time (Carbon-14 only reaches back ~50 000 years).
- Best examples: sandstone, shale, limestone.
The solution — "bracketing"
Scientists look for a layer of igneous rock or volcanic ash directly above the fossil and another directly below. Radiometric dating gives an exact age for each of those layers. The fossil must be older than the layer above and younger than the layer below — so its age is "bracketed" between two precise numbers. By stacking enough brackets together, scientists have built a complete chronological map of life on Earth.
Calibration points — where does our timeline start?
Three pieces of rock have been radiometrically dated to give us the outer edge of Earth's history:
Earth's oldest crust
billion years
Hudson Bay, North America
Oldest Moon rock
billion years
Apollo lunar samples
Oldest meteorites
billion years
found on Earth's surface
Read those three numbers in order — they answer the "active planet" problem
4.3 billion — Earth's oldest crust. Active planet, so this rock has been remelted, buried or recycled at some point since Earth formed. Its clock was reset. This is a minimum, not an answer.
4.4 billion — the oldest Moon rock. The Moon is much smaller, so it lost its internal heat quickly and its geology effectively stopped. Almost nothing has reset its clocks. It gets us closer — but the Moon formed after Earth did.
4.5 to 4.6 billion — meteorites. These are leftover rubble from the cloud that built the solar system. They were never part of a large active body, so their clocks have never been reset at all. They date the formation of the solar system itself — and Earth formed from the same material at the same time.
That is why the answer to "how old is Earth?" is 4.6 billion years, and why it comes from a rock that fell out of the sky rather than from the ground under your feet. It is a genuinely strange piece of reasoning, and it follows directly from Earth being an active planet.
The page says the oldest Moon rock is 4.4 billion years. So does your Topic 12 booklet, so does the slide deck, and so does the real Apollo sample data. The CAPS document says 4.2 billion.
This matters because "give the ages of the oldest samples from Earth, the Moon and a meteorite" is a standard one-mark list question. If the question comes from CAPS, answer 4.2; if it comes from your booklet, answer 4.4. Either way the reasoning above is unaffected — the Moon sits between Earth's crust and the meteorites on both sets of numbers, and that ordering is the actual point.
The Geologic Time Scale
Earth's history is divided, top-down, into eons → eras → periods → epochs → ages. The boundaries are big geological events: the appearance of oxygen, mass extinctions, the rise of new life. Tap any eon to explore — then tap the Phanerozoic eras to see the part where fossils get really interesting.
Where the boundaries came from — and why they moved
The divisions were not designed. They were discovered, and in the wrong order from the one you would expect. Nineteenth-century geologists had no way to date anything, so they drew the lines where fossils suddenly disappear from the rock record — a whole community of species present in one layer and gone in the next. Those disappearances are mass extinctions, and they are the reason the boundaries fall where they do.
Only much later did radiometric dating refine the periods and epochs and pin an actual number of years on each line. So this is the join between the two halves of this topic: the boundaries were found by stratigraphy and fossils, then dated by radioactive decay. Relative first, absolute second — the same order in which you learned the two tools.
~ inside the Phanerozoic — the three eras
Palaeozoic
541 → 251 mya
Era of Ancient Life · trilobites
Mesozoic
251 → 65 mya
Era of Intermediate Life · dinosaurs
Cenozoic
65 mya → today
Era of Recent Life · mammals
The twelve periods, with their dates
The eras are the headline; the periods are what a question actually asks you to name and order. Your booklet lists all twelve with their dates, so here they are in one place. You are expected to know the period names in order within each era — the dates are for reference, and for the scale work on the practical sheet.
| Period | Million years ago | Lasted | What the rock record shows |
|---|---|---|---|
| PALAEOZOIC ERA · 541–251 mya · era of ancient life · the age of the trilobites | |||
| Cambrian | 541–488 | 53 my | The Cambrian explosion — marine animals develop hard shells, so the fossil record effectively begins |
| Ordovician | 488–443 | 45 my | Marine invertebrates diversify enormously; the first jawless fish |
| Silurian | 443–416 | 27 my | Reef-building spreads; the first vascular plants appear on land |
| Devonian | 416–359 | 57 my | The "age of fishes"; the first amphibians crawl out of the water |
| Carboniferous | 359–299 | 60 my | Vast swamp forests — the origin of most coal; giant insects |
| Permian | 299–251 | 48 my | The first reptiles; Pangaea complete. Ends in the worst extinction in Earth's history |
| MESOZOIC ERA · 251–65 mya · era of intermediate life · the age of the reptiles | |||
| Triassic | 251–199 | 52 my | Life recovers slowly; the first dinosaurs and the first mammals |
| Jurassic | 199–145 | 54 my | The first birds; giant marine reptiles — ichthyosaurs, plesiosaurs |
| Cretaceous | 145–65 | 80 my | The first flowering plants; Pangaea breaks up and the Atlantic opens. Ends at Chicxulub |
| CENOZOIC ERA · 65 mya–today · era of recent life · the age of the mammals | |||
| Palaeogene | 65–23 | 42 my | Mammals fill the empty niches — and one group walks back into the sea and becomes the whales |
| Neogene | 23–2 | 21 my | Modern whale, seal and fish groups appear; grasslands spread; the climate cools |
| Quaternary | ~2–today | ~2 my | Ice ages, and sea level swinging by more than 100 m. Two epochs: Pleistocene and Holocene. Recognisable humans |
📌 Palaeogene + Neogene together are the old name you will still see everywhere: the Tertiary. That is why the extinction at 65 mya is called the Cretaceous–Tertiary (K–T) event even though "Tertiary" is no longer an official period.
Local connection — Chapman's Peak Drive & the Karoo
South Africa has some of the world's best readable rock. Drive out of Cape Town along Chapman's Peak and the cliffs above the road are Table Mountain sandstone, ~510 million years old — laid down during the Palaeozoic era when this part of Africa was joined to South America in Gondwana. Further inland, the Karoo Basin contains one of the planet's most complete records of the Permian–Triassic extinction (~251 mya), the worst mass extinction in Earth's history. Scientists from around the world fly to the Karoo just to read those layers. And even older — the Barberton Greenstone Belt in Mpumalanga preserves 3.5-billion-year-old fossil stromatolites from the early Archean eon, among the oldest direct evidence of life on Earth.
In the Lab — the CAPS Investigations
CAPS sets four investigations for this block, and every one of them is about the same thing: making 4.6 billion years small enough to hold in your head. There is no apparatus. There is a toilet roll, a passage, a calculator and a clock face.
What CAPS asks for, word for word
Investigations column, block 8: "Develop a timeline in table form" · "See videos illustrating 'Toilet Roll' — unrolled down a passage, with humans arriving only in the last square of the roll" · "Compare geological time on a timeline" · "Compare the earth's history to a 24-hour day, in which humans exist only as of 1 minute before midnight". Resources column: "Timeline printed as a wall chart for the back of the classroom."
All four are on the practical sheet, plus the wall chart — and one extra drill that CAPS does not set but every paper asks for: reading an age off a decay curve.
Five investigations · no apparatus you do not already own
- 🟢 desk The timeline in table form — build the whole geologic time scale yourself, eon by era by period, and hang it on a wall. CAPS asks for both the table and the wall chart.
- 🟢 kitchen The toilet roll walk — unroll a whole roll down a passage, mark the events onto it to scale, and find out how much of the roll is Precambrian. Supplied roll data if you would rather not unroll one.
- 🟢 desk Compare geological time on a timeline — draw it to scale, discover that most of what you want to label is invisible, and solve that problem the way real time charts do.
- 🟢 desk The 24-hour day — convert every event to a clock time and check CAPS's claim that humans turn up one minute before midnight.
- 🟢 desk Read an age off a decay curve — the Cobalt-60 example from your booklet, plotted, then used backwards. Not a CAPS investigation; it is what the exam does with this topic.
→ Open the Topic 12 practical sheet — printable, with supplied data for every task.
Why this block's practical is a scale exercise and not an experiment
Look again at the Depth column: "for learners to conceptually understand the age of the planet". Not measure it — understand it. The problem with 4.6 billion years is not that it is hard to define; it is that it means nothing. Nobody has an intuition for a billion of anything.
So the investigations convert the number into something a body can experience — a distance you walk down a passage, and a day you have already lived. That is the whole method, and it is why CAPS sets four versions of the same task instead of one.
Key Vocabulary
Tap each card to flip it. CAPS states the definition of half-life in the curriculum text itself, so that one you must be able to give word-perfect.
Your booklet then carries a list headed "these definitions are examinable", and nine of the twelve cards below are on it: geologic time scale · half-life · igneous rock · isotopes · radiometric dating · sedimentary rock · strata · stratigraphy · extrude. The other three — stratification, relative dating and Phanerozoic eon — are not on that list but are named in the booklet's core concepts or needed to answer any "read the cliff face" question.
⚠️ Watch stratification and stratigraphy. They look almost identical, they are both examinable, and they mean different things — one is the layering itself, the other is the study of it.
The time taken for half the atoms of a radioactive isotope in a sample to decay into the stable daughter isotope. Always the same for a given isotope — Cobalt-60 is 5.26 years, Uranium-238 is 4.5 billion years, Samarium-147 is 106 billion years. (Your Term 2 Paper 1 scaled ¹⁴⁷Sm down to 106 million so the graph would work — see the note in Section 3.)
Atoms of the same element (same number of protons) but with different numbers of neutrons — so they have the same chemical behaviour but different atomic masses. Carbon-12, Carbon-13 and Carbon-14 are isotopes of carbon. Some isotopes are stable, some are radioactive.
The natural arrangement of rocks, sediments or any other material into distinct horizontal layers (strata). In geology, stratification is what makes rock cliffs look like layered cakes — and is the basis for reading the order of events in Earth's history.
A technique that determines a rock's absolute age by measuring the ratio of a radioactive parent isotope to its stable daughter isotope. Because half-life is constant, the ratio tells you exactly how many half-lives have passed.
The layers of sediment deposited over time that, once compressed, become sedimentary rock. Each layer (singular: stratum) records a period of Earth's history. The oldest is at the bottom; the youngest at the top — the Law of Superposition.
Rock formed from cooled magma or lava. Includes basalt, granite, volcanic ash. Contains lots of radioactive uranium and potassium, so it can be dated very precisely — but its molten origin means it contains no fossils.
Rock formed when sediment is compressed under pressure over time. Includes sandstone, shale, limestone. The cool, gentle conditions preserve fossils — but the rock itself is hard to date directly for deep time, so scientists use the "bracketing" technique with igneous layers above and below.
The current eon, from 541 million years ago to today. Starts with the Cambrian explosion when animals first developed hard shells preserved in the fossil record. Divided into three eras: Palaeozoic, Mesozoic, and Cenozoic. Where almost every fossil you'll ever see comes from.
A system of chronological dating that relates geological strata (stratigraphy) to time, used by Earth scientists to describe the timing and relationships of events that have occurred during Earth's history. In one phrase: the calendar of Earth's history, divided into eons, eras, periods, epochs and ages.
The correlation and classification of rock strata — the branch of geology that works out the order of the layers and matches layers in one place to layers somewhere else. Not the same as stratification, which is the layering itself. Stratification is the thing; stratigraphy is the study of the thing.
Working out the order in which events happened, without putting a number of years on any of them — done by reading the strata (Law of Superposition). Its partner is absolute dating, which gives an actual age in years and is done radiometrically. Relative = what came first. Absolute = when.
To force or push out. In geology, lava that is extruded from the interior of the Earth reaches the surface and cools there — which is why extrusive igneous rock (like basalt) has tiny crystals, while magma that cools slowly underground grows big ones (like granite). One of your booklet's examinable definitions.
Quick Check
Question 1
Well done!
Next up: Life Processes and the Chemistry of Life — the building blocks of every marine organism.