G13: Geological Time & Stratigraphy
The geological timescale, principles of stratigraphy, and methods of dating rocks including relative and absolute dating.
Geological Time & Stratigraphy
The geological timescale, principles of stratigraphy, and methods of dating rocks including relative and absolute dating.
Key Fact: The geological timescale divides Earth's 4.6 billion year history into eons, eras, periods and epochs
Key Fact: Principle of superposition: in undisturbed sequences, the oldest rocks are at the bottom and youngest at the top
Key Fact: Principle of original horizontality: sedimentary rocks are deposited horizontally; tilting indicates later deformation
Key Fact: Principle of cross-cutting relationships: a feature that cuts another is younger than what it cuts
Key Fact: Relative dating puts rocks in order of age; absolute (radiometric) dating gives a numerical age in millions of years
Key Fact: Index fossils are organisms that lived for a short time but were widely distributed, used to correlate rock layers across regions
📋 Key Vocabulary and Concepts
For Geological Time & Stratigraphy, you must know:
- Superposition: The principle that in an undisturbed sedimentary sequence, the oldest beds are at the bottom and the youngest at the top
- Cross-cutting relationships: The principle that a geological feature (e.g. dyke, fault) that cuts across another must be younger than the feature it cuts
- Index fossil: A fossil of an organism that lived for a short time but was geographically widespread, used to correlate and date rock layers
- Radiometric dating: A method of determining the absolute age of a rock by measuring the ratio of parent to daughter isotopes and the known half-life
- Half-life: The time taken for half of the radioactive parent isotopes in a sample to decay into daughter isotopes
❓ Practice Questions
Q: State the three main principles of stratigraphy.
Q: What makes a good index fossil?
Q: How does radiometric dating work?
Q: What is the difference between relative and absolute dating?
Q: Name the four main eras of the Phanerozoic Eon in order.
✅ Answers
- 1) Superposition: oldest beds at the bottom. 2) Original horizontality: beds are deposited horizontally. 3) Cross-cutting relationships: a feature that cuts another is younger.
- Short geological time range (rapid evolution), wide geographical distribution, easily recognisable, and abundant in the rock record.
- A radioactive parent isotope decays into a stable daughter isotope at a known rate (half-life). By measuring the ratio of parent to daughter isotopes in a mineral, the age since the mineral crystallised (its closure temperature) can be calculated.
- Relative dating places rocks in chronological order (older/younger) without giving numerical ages. Absolute (radiometric) dating measures the actual age in years (typically millions of years) using radioactive isotope decay.
- Precambrian (oldest), Palaeozoic, Mesozoic, Cenozoic (youngest).
🎯 Exam Tips
- Three stratigraphic principles: superposition, original horizontality, cross-cutting
- Index fossil criteria: short range, wide distribution, recognisable, abundant
- Radiometric dating: parent/daughter ratio + half-life = absolute age
- Relative dating = order only; Absolute dating = numerical age in years
📝 Exam Technique
GCSE Geology Exam Tips — Geological Time & Stratigraphy:
1. For Geological Time & Stratigraphy questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Geological Time & Stratigraphy to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
1. For Geological Time & Stratigraphy questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Geological Time & Stratigraphy to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
⚠️ Common Errors
✗ Superposition applies to all rock types ✓ Superposition applies to SEDIMENTARY and volcanic rocks deposited in layers; it does not apply to intrusive igneous rocks or metamorphic rocks that have been restructured
✗ Radiometric dating can be used on any rock ✓ Radiometric dating requires minerals with suitable radioactive isotopes; sedimentary rocks cannot be directly dated (they give the age of the source material, not deposition); igneous and metamorphic minerals are typically used
✗ The geological timescale is based on radiometric dating alone ✓ The timescale was originally built using relative dating (fossils and stratigraphy); radiometric dating later provided absolute ages to calibrate it
✍️ Model Answer
Full-Mark Response
Explain the principles of stratigraphy and describe how both relative and absolute dating methods are used to determine the age of rocks. [6 marks]
Stratigraphy uses key principles to determine relative ages. The principle of superposition states that in undisturbed sedimentary sequences, the oldest beds are at the bottom and youngest at the top. The principle of original horizontality states that sedimentary rocks are deposited horizontally; any tilting indicates post-depositional deformation. The principle of cross-cutting relationships states that a feature cutting across another (e.g. a dyke cutting a sedimentary bed) must be younger than what it cuts. Index fossils further refine relative dating: organisms with short time ranges and wide distributions (e.g. ammonites, graptolites) allow correlation of rock layers across different regions. Fossil succession shows that organisms have evolved in a definite order, so the same fossil assemblage indicates the same relative age worldwide. Absolute dating uses radioactive decay: unstable parent isotopes (e.g. uranium-238, potassium-40) decay to stable daughter isotopes at a fixed rate (half-life). By measuring the parent/daughter ratio in a mineral, its crystallisation age can be calculated. For example, U-Pb dating of zircon in igneous rocks gives the eruption age. Absolute dating calibrates the relative timescale, providing numerical ages for geological periods. Together, these methods build the geological timescale spanning 4.6 billion years.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of geological time & stratigraphy, including key geological concepts, observational data and theoretical models relevant to OCR J357.
AO2 (Application of Knowledge): Apply knowledge and understanding of geological time & stratigraphy to both familiar and unfamiliar geological contexts, using field evidence and theoretical principles to explain phenomena.
AO3 (Analysis & Evaluation): Analyse geological data related to geological time & stratigraphy, evaluate evidence from observations and investigations, and construct reasoned arguments using scientific methodology.
📝 Exam Questions by Topic
🎬 Video Resources
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