Use relative-age rules

Supplied materials

• All measurements and observations below are supplied classroom examples. They are not data from actual learners or a claim about a particular real site.
• Paper or a browser document for a labeled drawing, table or explanation.
• Worked problem inputs: An intrusion cuts layers X and Y; later a fault cuts all three. Order the events allowed by the record.
• Assessment case 1: In an undisturbed stack, layer P underlies Q and an intrusion cuts both. What order is supported? Which inference or prediction is best supported?
• Assessment case 2: Which scientific label fits the specifically described step or feature? Supplied case: Intrusion cuts existing layers.
• Assessment case 3: Which scientific label fits the specifically described step or feature? Supplied case: Erosion removed an interval of deposits.
• Assessment case 4: Which scientific label fits the specifically described step or feature? Supplied case: Fault displaces layers.
• Assessment case 5: Which scientific label fits the specifically described step or feature? Supplied case: Dike younger than rock it crosses.
• Assessment case 6: A surface separates older layers from younger deposits with a preserved gap. What is warranted? Which inference or prediction is best supported?

Learn the idea

In an undisturbed deposited stack, lower layers are older. A cross-cutting feature is younger than the feature it cuts. Later disturbance can complicate the original layer order.

Superposition means lower undisturbed layers are older. Cross-cutting means cutting feature formed after what it cuts. Unconformity means gap in the preserved geological record. Disturbance means folding or faulting alters layer arrangement.

For the supplied mechanism problem, An intrusion cuts layers X and Y; later a fault cuts all three. Order the events allowed by the record. The deposited layers existed before the intrusion, and the fault came after it cuts both layers and intrusion. The record does not supply an exact numerical age.

1. Read the supplied problem and mark the structures, processes or quantities named: An intrusion cuts layers X and Y; later a fault cuts all three. Order the events allowed by the record.
2. Draw the relevant boundary, pathway or comparison and label it using the scientific ideas above.
3. Write a causal explanation or calculation, showing how each observation supports the result.
4. Compare your reasoning with the worked solution; name a condition that would change or limit the inference.

Check your reasoning

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Take it further

New record: undisturbed layers P and Q are cut by a dike; an erosion surface cuts the dike. Expected: deposition preceded intrusion, which preceded that erosion surface; exact dates remain unavailable.

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