Fossils, fossilisation and dating fossils
Fossils are remains or traces of ancient living things kept in rock, ice or amber. Scientists find their age by radioactive carbon dating and other radioactive clocks.
What are fossils?
Fossils are the remains, impressions or traces of organisms that lived long ago. Most are found in sedimentary rocks (rocks formed from layers of mud and sand). Some are kept in ice, amber (hardened tree resin) or tar. Bones, shells, teeth, wood, leaf prints and footprints can all become fossils. The study of fossils is palaeontology. Older rock layers hold simpler fossils, and younger layers hold more complex ones. This shows that life changed from simple to complex forms. Birbal Sahni is called the father of Indian palaeobotany (the study of plant fossils).
How a fossil forms (fossilisation)
- 1An animal or plant dies and falls into mud, sand or water.
- 2Soft parts rot away quickly. Hard parts such as bones, shells and wood remain.
- 3Layers of sediment (fine mud and sand) cover the remains and protect them from decay.
- 4Over millions of years the layers are pressed and harden into sedimentary rock.
- 5★Minerals in water slowly seep in and replace the remains, turning them into stone.
- 6Earth movements and erosion bring the rock to the surface, where the fossil is found.
★ key step
Kinds of fossilisation
| Kind | What happens | Example |
|---|---|---|
| Preservation (actual remains) | The whole body is kept unchanged in ice, amber or tar | Woolly mammoth in Siberian ice; insects in amber |
| Petrifaction | Minerals replace the tissues bit by bit, and the remains become stone | Fossil tree trunks at Tiruvakkarai (Villupuram district) |
| Moulds and casts | The body dissolves and leaves a hollow (mould); minerals fill it (cast) | Shells and ammonites |
| Compression and carbonisation | Pressure flattens leaves or soft bodies, leaving a thin film of carbon | Fern leaf prints in coal |
| Infiltration (replacement) | Minerals carried by water fill the pores of hard parts, or slowly replace them | Fossil bones and shells |
Dating fossils
Scientists find a fossil's age by radioactive dating. Living things take in carbon, including a little radioactive carbon-14 (¹⁴C). After death, no new ¹⁴C enters the body. The ¹⁴C already present slowly decays. Its half-life (the time for half of it to decay) is about 5730 years. By measuring the ¹⁴C left, scientists find how long ago the organism died. This radiocarbon dating was developed by W. F. Libby. It works for remains up to about 50,000 years old. Much older rocks and fossils are dated with longer-lived elements, such as uranium (uranium-lead method) and potassium (potassium-argon method).
Worked example: age from carbon-14
A piece of fossil wood has only one-fourth (1/4) of the carbon-14 found in living wood. The half-life of carbon-14 is 5730 years. Find its age.
- After 1 half-life, 1/2 of the ¹⁴C is left.
- After 2 half-lives, 1/2 × 1/2 = 1/4 is left.
- So 2 half-lives have passed.
- Age = 2 × 5730 years = 11,460 years.
The fossil wood is about 11,460 years old.
Tamil Nadu has a National Fossil Wood Park at Tiruvakkarai (Villupuram district), where ancient tree trunks have turned to stone. Ariyalur district is known for sea-shell fossils, which show the area was once under the sea.
1 marks: “Who developed radiocarbon dating?” W. F. Libby.
2 marks: “What are fossils? Name the branch of science that studies them.” Remains, impressions or traces of ancient organisms preserved in rock, ice or amber. Palaeontology.
4 marks: “How are fossils formed? How is their age determined?” Burial in sediment; soft parts decay, hard parts remain; sediment hardens into rock; minerals replace the remains (petrifaction). Age by radiocarbon dating (¹⁴C, half-life 5730 years, Libby); uranium-lead or potassium-argon for older rocks.
¹⁴C halves every 5730 years: less ¹⁴C left means an older fossil.