Earth Science · Geology · Earth Interior / Principles and Exploration
Relative ages of rock layers — principles and investigation
The relative age of a stratum is an order of what came first rather than a numerical age. Try judging the order of formation by changing the relationships between stacked strata, intrusions, faults, and unconformities. This is a geological analysis experiment that looks at what is cutting what, rather than just looking at the top and bottom positions.
How does it work?
The order is determined by the law of superposition, law of penetration, and law of cutting. The unconformity represents time of loss of deposition and erosion.
The cutting incident is younger than the cutting subject.
Try this investigation
- Run the section forming process and place the 6 event cards in old order. Let's look at the relationship between intrusion and fault cutting.
- Execute and record the standard cross section where the intrusion cuts the fault, and check the order of A·B·C and unconformity.
- Reverse the cutting relationship and run again. Explain why the relative order of intrusions and faults is reversed.
Interpreting your results
What if a fault cuts the intrusive rock that cuts both A below and B above?
According to the law of superposition, B is younger than A, and according to the law of penetration and cutting, the cutting event is younger than the cut object.
Assumptions and limitations
It covers only the two sedimentary layers that have not been overturned and designated faulting, intrusion, and erosion events. Absolute age and duration of unconformity are not calculated. Tilt and displacement are for explanatory purposes only and do not restore the actual geological structure.