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Earth and Space · Chapter 2: Structure of the Earth
Fossils and the Age of the Earth
How fossils form, famous fossils from Pakistan such as Pakicetus, the law of superposition, half-life and radiometric dating, and the age of the Earth, with a radioactive-decay virtual lab.
Key termRememberExampleWatch out
01Concept
A fossil is the preserved remains or traces of a living thing from long ago, usually found in sedimentary rock. Fossils tell us what lived on Earth in the past and help us work out the age of rocks.
How fossils form
Most living things rot away completely. A fossil forms only when the remains are buried quickly (by mud, sand or volcanic ash) so that air and decomposers cannot destroy them. Hard parts such as bones, shells and teeth are the most likely to become fossils.
| Type of fossil | What it is | Example |
|---|---|---|
| Petrified (body) fossil | Hard parts slowly replaced by minerals, turning to stone | Fossil bones, petrified wood |
| Mould and cast | A hollow print left in rock (mould), sometimes later filled with minerals (cast) | Shell shapes in limestone |
| Trace fossil | Signs of activity, not the body itself | Footprints, burrows, eggs |
| Preserved whole | Kept almost unchanged in ice, amber or tar | Insects in amber; mammoths in Siberian ice |
Famous fossils from Pakistan
- Pakicetus: one of the earliest known whales, about 50 million years old, found in the hills of northern Punjab. It walked on land and lived near water. Its fossils show how whales came from land animals.
- Paraceratherium (formerly called Baluchitherium): a giant hornless rhinoceros, one of the largest land mammals ever, found in the Bugti Hills of Balochistan.
- The Salt Range: its rock layers contain fossils from more than 500 million years ago, and geologists from around the world come to study them.
Relative dating: which is older?
The law of superposition says: in undisturbed layers of sedimentary rock, the lowest layers are the oldest and the top layers are the youngest, because each layer was laid on top of the one before.
This tells us the order of events, but not the actual age in years.
Absolute dating: how old in years? (Class 10 / O Level)
Some rocks contain radioactive atoms (the parent), which slowly change into other atoms (the daughter) at a fixed rate. The half-life is the time it takes for half of the parent atoms to decay. By measuring how much parent is left, scientists can calculate the age. This is radiometric dating.
| Parent → daughter | Half-life | Used to date |
|---|---|---|
| Carbon-14 → nitrogen-14 | 5,730 years | Things that were once alive, up to about 50,000 years old |
| Potassium-40 → argon-40 | 1.25 billion years | Volcanic rocks |
| Uranium-238 → lead-206 | 4.47 billion years | The oldest rocks and meteorites |
Worked example: a piece of ancient wood has 25% of its original carbon-14 left. 100% → 50% → 25% is 2 half-lives. Age = 2 × 5,730 = 11,460 years.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Key Formulas
Parent atoms left
- = number of parent atoms at the start
- = number left now
- = number of half-lives passed
Age of a sample
- = age (years)
- = half-life (years)
After 1 half-life 50% is left, after 2 → 25%, after 3 → 12.5%.
04Real World Examples
Mohenjo-daro
Archaeologists used carbon-14 dating of charcoal and other remains to show that the cities of the Indus Valley Civilisation are about 4,500 years old.
Fossil fuels
Coal, oil and gas are made from the remains of plants and tiny sea creatures that lived millions of years ago: that is why they are called fossil fuels.
Finding oil
Geologists use tiny fossils in drill samples to match rock layers and find where oil and gas may be trapped.
Museums
Pakistan's natural history museums, such as the Pakistan Museum of Natural History in Islamabad, display fossils found in the country.
05Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.What is a fossil? Why are fossils usually found in sedimentary rocks?Show answer
Answer
A fossil is the preserved remains or traces of a living thing from long ago. They are found in sedimentary rocks because living things get buried in sediment, which slowly turns into rock. The heat of igneous and metamorphic rock formation would destroy them.
Q2.State the law of superposition.Show answer
Answer
In undisturbed layers of sedimentary rock, the lowest layer is the oldest and the top layer is the youngest.
Q3.Define half-life.Show answer
Answer
Half-life is the time taken for half of the radioactive (parent) atoms in a sample to decay.
Q4.(Class 10 / O Level) A bone has 12.5% of its original carbon-14 left. How old is it? (Half-life of C-14 = 5,730 years)Show answer
Answer
- 100% → 50% → 25% → 12.5% = 3 half-lives
- Age = 3 × 5,730 = 17,190 years
Q5.Why is Pakicetus an important fossil?Show answer
Answer
Pakicetus, found in Pakistan, is about 50 million years old and is one of the earliest known whales. It lived partly on land, so it shows that whales came from land animals.
Long questions
Q6.Explain how fossils form, and how scientists find the age of rocks by relative and absolute dating.Show answer
Answer
Formation:
- An animal or plant dies.
- It is quickly buried by mud, sand or ash.
- Over millions of years, minerals replace the hard parts, turning them to stone.
- Rocks are lifted and eroded, exposing the fossil.
Relative dating uses the law of superposition: lower layers are older. It gives the order of events but not the age in years.
Absolute (radiometric) dating measures how much of a radioactive parent isotope is left. Using its half-life: age = number of half-lives × half-life. Carbon-14 (5,730 years) for recent remains; uranium-238 (4.47 billion years) for the oldest rocks.