Geological Structures and Earth History
Reading the record
Stratigraphy rests on superposition (oldest at the bottom), original horizontality, lateral continuity and cross-cutting relationships (a feature cutting another is younger). Uniformitarianism assumes the same processes operated in the past.
Structures you must recognise:
- Folds: anticline (upward arch, oldest rocks in the core), syncline (downward trough, youngest in the core).
- Faults: normal (extension, hanging wall down), reverse/thrust (compression, hanging wall up), strike-slip (lateral).
- Unconformities: angular (tilted then eroded then buried), disconformity (erosion surface between parallel beds), nonconformity (sediment on igneous or metamorphic rock).
Dating
Relative dating orders events; absolute dating uses radiometric decay. Half-life: after n half-lives, the parent fraction remaining is (1/2)ⁿ. Useful systems: carbon-14 (short, organic), potassium-argon and uranium-lead (long, igneous minerals).
Traps
- Calling every arch an anticline - check whether the oldest rocks are in the core.
- Reversing fault terminology: normal faults result from tension, reverse from compression.
- Using radiometric dating on sedimentary rock directly; date cross-cutting igneous bodies instead.
- Forgetting that an unconformity represents missing time.
Worked example
A sequence has horizontal limestone, then a tilted dyke cutting it, then a horizontal sandstone on top. Order: limestone → dyke → erosion → sandstone. The boundary is an angular unconformity. Rule: cross-cutting is younger; inclusions are older; unconformities mark gaps.
Deep time context
Earth is about 4.54 billion years old; life appeared by ~3.5 Ga; the Cambrian explosion ~540 Ma; mass extinctions punctuate the record.