Fossils and Relative Dating of Rock Layers — Answers
Answer sheet with model answers and success criteria for the Year 8 earth, rocks & weather worksheet.
Success TutoringWorksheetsAnswer sheetYear 8 · Science · Earth, Rocks & WeatherFossils and Relative Dating of Rock Layers — Answers
Part A: How fossils form
A fossil is the preserved remains or trace of an organism that lived long ago. Most fossils form when an organism is buried quickly by sediment, such as mud or sand, before it decays. Over time more layers build up, the sediment is compacted and cemented into sedimentary rock, and minerals in groundwater slowly replace the hard parts.
Body fossils are the remains themselves (bones, shells, leaves). Trace fossils show activity (footprints, burrows). A mould is a hollow imprint; a cast is a mould later filled with minerals.
Answer: B, D, E, A, C
- AIgneous
- BSedimentary
- CMetamorphic
- DAll types equally
Answer: Sedimentary
Fossils form as sediment buries remains. The heat of magma (igneous) or heat and pressure (metamorphic) usually destroy fossils.
Answer: Soft parts rot or are eaten quickly after death, while hard parts last long enough to be buried and replaced by minerals.
- Abody fossil
- Btrace fossil
- Ccast
- Dindex fossil
Answer: trace fossil
Trace fossils record an organism's activity, such as footprints, burrows or droppings, rather than the body itself.
Answer: A mould is the hollow space left in rock after the remains dissolve. A cast forms when minerals later fill that space, making a copy of the original shape.
Part B: Putting rock layers in order
Law of superposition: in undisturbed layers of sedimentary rock, the oldest layer is at the bottom and the youngest is at the top. Cross-cutting relationships: anything that cuts through rock layers (such as a fault or an igneous dyke) is younger than the layers it cuts.
A cliff face shows these layers from bottom to top: A sandstone; B shale with trilobite fossils; C limestone with shell fossils; E sandstone. A band of igneous rock, D (a dyke), cuts up through layers A, B and C but stops at the bottom of layer E.
- ALayer A
- BLayer B
- CLayer C
- DLayer E
Answer: Layer A
By the law of superposition, the bottom layer was laid down first.
Answer: Younger. The dyke cuts through layer C, so layer C must already have existed when the magma forced its way through it.
Answer: Older. The dyke stops at the bottom of layer E, so layer E was deposited on top after the dyke formed.
Answer: A, B, C, D, E
Answer: False
Relative dating gives only the order of events (older or younger). Finding an age in years needs other methods, such as radiometric dating.
Part C: Index fossils and deposition rates
Answer: The two layers formed at about the same time. The ammonite is an index fossil, so it can be used to match (correlate) layers between the sites.
Answer: The organism lived for only a short period of geological time, and it was widespread (found over a large area). It should also be common and easy to recognise.
Answer: 3000 years
- 3 m = 3000 mm
- 3000 ÷ 1 = 3000 years
Answer: 4000 years
- 2 m = 2000 mm
- 2000 ÷ 0.5 = 4000 years
Answer: About 5000 years ago
- 60 ÷ 12 = 5 lots of 1000 years
- 5 × 1000 = about 5000 years
Part D: Explain
Model answer: The shells belonged to animals that lived in a sea long ago. When they died, they were buried by sediment on the sea floor. Over a long time, more layers built up and the sediment was compacted and cemented into sedimentary rock, and the shells became fossils. Later, movements of Earth's crust (such as plates colliding) slowly pushed these rock layers upwards to form mountains. Weathering and erosion then exposed the fossils at the surface.
Success criteria:
- The organisms lived in an ancient sea
- Burial by sediment and formation of sedimentary rock
- Uplift of rock layers by movements of Earth's crust
- Weathering or erosion exposed the fossils
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.
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