What is the difference between relative and absolute (radiometric) dating?

Relative dating places events in order (older or younger) without ages in years, using stratigraphic principles: superposition (oldest bed at the base), original horizontality, lateral continuity, cross-cutting relationships (a fault or dyke is younger than what it cuts) and included fragments (an inclusion is older than its host). Absolute dating gives ages in years from radioactive decay: the parent-to-daughter ratio gives the fraction of parent remaining, hence the number of half-lives, and age equals half-lives times the half-life. The time scale is built by ordering strata relatively, then calibrating with radiometric dates from interbedded igneous rocks.

How do you calculate an age from a parent-to-daughter ratio?

In a closed system every daughter atom came from a decayed parent, so the original parent equals parent plus daughter now. Convert the ratio to the fraction of parent remaining: one parent to one daughter is a half (one half-life), one to three is a quarter (two half-lives), one to seven is an eighth (three half-lives). Then age equals the number of half-lives times the half-life. The common trap is confusing the fraction remaining with the number of half-lives, so always work out the fraction first.

What makes a good index (zone) fossil?

A good index fossil is widespread but short-lived: a wide geographical distribution so it appears in many regions, and a short stratigraphic (time) range so it pins a narrow age. It should also be abundant, easily preserved and distinctive so it is reliably found and identified. That combination ties widely separated rocks to the same brief interval, which is what biostratigraphic correlation requires. Ammonites, graptolites and trilobites are classic examples because they evolved rapidly and spread widely.

What is the difference between gradualism and punctuated equilibrium?

They are two models of the tempo of evolution. Gradualism is slow, steady, continuous change, appearing in the record as a smooth series of transitional forms. Punctuated equilibrium is long periods of little change (stasis) interrupted by short bursts of rapid change, appearing as stasis then the abrupt appearance of new forms with few intermediates. The real record often shows stasis and sudden appearances, supporting punctuation, but some apparent suddenness reflects gaps in the incomplete record, so both processes probably operate.

What is the single most common mistake in this concept?

Confusing the fraction of parent remaining with the number of half-lives in a dating calculation, and trusting superposition in overturned beds. A one-to-three parent-to-daughter ratio means a quarter remains, which is two half-lives, not three. And if a fold has overturned a succession, the lowest bed may be the youngest, so you must check a way-up indicator (graded bedding, mud cracks) before applying superposition. Both slips lose easy, high-value marks in the dating and map questions.

EnglandGeology

Eduqas A-Level Geology Time, past life and past climates: dating, fossils, evolution and the Quaternary

A deep-dive Eduqas A-Level Geology guide to the Time, past life and past climates concept. Covers relative dating and stratigraphic principles, radiometric dating and half-life, fossils and index fossils, evolution and mass extinctions, palaeoenvironments and palaeoclimate proxies, and the Quaternary option, with the exam patterns Eduqas repeats.

Generated by Claude Opus 4.819 min readEduqas-A220-Time-Past-Life-ClimatesUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this concept actually demands
Relative dating and stratigraphic principles
Radiometric dating and half-life
Fossils and index fossils
Evolution and mass extinctions
Palaeoenvironments and palaeoclimate
The Quaternary option
How this concept is examined
Check your knowledge

What this concept actually demands

Time, past life and past climates is where Eduqas turns the rock and fossil record into a history. It runs from working out the order of events (relative dating), through pinning ages in years (radiometric dating), to reading environments and climates out of the rocks, and finishes with the Quaternary option. The examiners test two linked skills: the quantitative dating calculation, and the interpretive reconstruction of a sequence or environment from a section, which dominates Components 1 and 3.

This guide walks through the six clusters in a sensible build order, then sets out the exam patterns Eduqas repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Relative dating and stratigraphic principles

Relative dating orders events using five principles: superposition (oldest bed at the base), original horizontality, lateral continuity, cross-cutting relationships (a fault or dyke is younger than what it cuts) and included fragments (an inclusion is older than its host). Where beds are folded or overturned, way-up indicators (graded bedding fining upwards, downward-narrowing mud cracks, truncated cross-beds) reveal the original top and bottom. Reconstructing a history means ordering the beds, checking the way up, then placing faults, intrusions and unconformities.

Radiometric dating and half-life

Radioactive parents decay to stable daughters at a fixed rate set by the half-life (a half remains after one, a quarter after two, an eighth after three). In a closed system the parent-to-daughter ratio gives the fraction remaining and hence the number of half-lives, so age = half-lives times the half-life. Long-half-life methods date old rocks (uranium-lead, potassium-argon, rubidium-strontium); carbon-14 dates recent organic material up to about 60,000 years. Reliable ages need a closed system and a known initial daughter.

Fossils and index fossils

Fossils are preserved by remaining unaltered, by recrystallisation, replacement (silica, pyrite), as moulds and casts, or by carbonisation, favoured by rapid burial, hard parts and anoxia. Trace fossils record activity rather than the body. A good index (zone) fossil is widespread but short-lived, abundant and distinctive, so it correlates widely separated rocks to the same brief interval (ammonites, graptolites, trilobites).

Evolution and mass extinctions

The fossil record preserves a succession of changing life, evidence for evolution, with two models of tempo: gradualism (smooth transitions) and punctuated equilibrium (stasis then rapid bursts). Five mass extinctions punctuate the record; the end-Permian (largest, Siberian flood basalts) and the end-Cretaceous (dinosaurs and ammonites lost, iridium-rich impact layer plus Deccan volcanism) are the key ones, each followed by radiations of survivors.

Palaeoenvironments and palaeoclimate

Environments are reconstructed from fossils, sedimentary structures and lithology combined, organised by facies and Walther's law (conformable vertical successions record laterally migrating environments, so a deepening-up sequence is a transgression). Palaeoclimate proxies include coal (warm wet), evaporites (hot arid), tillites (cold glacial), reef limestones (warm shallow seas), oxygen isotopes and fossil leaf shape, which record both climate change and continental drift.

The Quaternary option

The Component 3 Quaternary option covers glacial deposits (poorly sorted, unstratified, striated till; moraines; streamlined drumlins), periglacial features (frost shattering, ice wedges, solifluction), fluvioglacial deposits (sorted, stratified outwash and eskers), the glacial-interglacial cycles recorded by oxygen isotopes and ice cores, sea-level change (eustatic and isostatic), and the dating methods (radiocarbon, isotope stratigraphy, varves).

How this concept is examined

A typical Eduqas profile for Time, past life and past climates:

Calculation questions (Components 1 and 2). Radiometric ages from parent-to-daughter ratios and half-lives, the predictable quantitative task here.
Sequence-reconstruction questions (Components 1 and 3). Ordering the events in a cross-section using superposition, cross-cutting, inclusions and unconformities.
Fossil and correlation questions (Component 2). Modes of preservation, the characteristics of a good index fossil, and using fossils to correlate.
Levels-of-response extended answers (Component 2). Comparing gradualism and punctuated equilibrium, describing a mass extinction and its causes, and reconstructing an environment or climate from evidence.

Eduqas Time, past life and past climates covers relative dating (superposition, original horizontality, lateral continuity, cross-cutting, inclusions, plus way-up indicators), radiometric dating (half-life, parent-to-daughter ratios, age equals half-lives times the half-life, with uranium-lead and potassium-argon for old rocks and carbon-14 for recent material), fossils and index fossils (modes of preservation, and the widespread-but-short-lived index fossil), evolution (gradualism versus punctuated equilibrium, the end-Permian and end-Cretaceous mass extinctions), palaeoenvironments (fossils, structures and lithology combined, facies, Walther's law, and palaeoclimate proxies such as coal, evaporites and tillites), and the Quaternary option (till versus outwash, drumlins, oxygen isotopes and ice cores, eustatic and isostatic sea-level change). Master the half-life calculation and the sequence-reconstruction method.

Check your knowledge

A mix of recall and application questions covering the whole concept. Attempt them under timed conditions, then check against the solutions.

State the principle that places a dyke younger than the beds it cuts. (1 mark)
Explain how graded bedding indicates the way up. (2 marks)
A mineral has a parent-to-daughter ratio of 1:7 and a half-life of 100 million years. Calculate its age. (3 marks)
State why carbon-14 cannot be used to date ancient rocks. (2 marks)
State the two essential characteristics of a good index fossil. (2 marks)
Compare gradualism and punctuated equilibrium in one sentence each. (2 marks)
State the climate indicated by coal, evaporites and tillites. (3 marks)
State Walther's law. (2 marks)
Explain how to distinguish glacial till from fluvioglacial outwash. (3 marks)

Solutions

Q1: Cross-cutting relationships: a feature that cuts across a rock is younger than the rock it cuts.
Q2: A graded bed fines upwards because coarse grains settle first at the base and fine grains last at the top, so the fine end is the original top; coarse above fine means the beds are overturned.
Q3: A 1:7 parent-to-daughter ratio gives a total of 8 parts, of which 1 is parent, so the fraction remaining is one eighth. One eighth is reached after 3 half-lives. Age = 3 times 100 = 300 million years.
Q4: Carbon-14 has a short half-life (about 5,730 years), so after roughly 50,000 to 60,000 years almost none remains to measure; ancient rocks need long-half-life methods such as uranium-lead.
Q5: A wide geographical distribution and a short stratigraphic (time) range, so it is widespread but short-lived.
Q6: Gradualism is slow, continuous change with transitional forms; punctuated equilibrium is long stasis interrupted by short, rapid bursts of change with few intermediates.
Q7: Coal indicates a warm, wet (swampy) climate; evaporites a hot, arid climate; tillites a cold, glacial climate.
Q8: Facies that are conformably stacked vertically were originally deposited side by side in laterally adjacent environments, so a vertical succession records the lateral migration of environments.
Q9: Till is deposited by ice, so it is poorly sorted, unstratified and angular with striated clasts; outwash is deposited by meltwater, so it is sorted, stratified and rounded. The sorting, bedding and clast shape distinguish them.