Science Puzzle
Which Came First in the Rock?
Fig. 1 shows a cliff face. Geologists have labelled six features: four layers of sedimentary rock (A to D), a band of dark rock that was once molten magma (X), a wavy, worn surface (E) and a crack where the rock has slipped (F).
Nobody was there to watch, but the rocks themselves record the order of events. In what order did these six things happen, from oldest to youngest?
The Answer
A, B, C, then X, then E, then D, and finally F.
A, B and C first. Sediment such as sand and mud settles in layers, each new one on top of the last. Unless something has turned the rocks over, the lowest layer is the oldest. This is the law of superposition, first written down by Nicolas Steno in 1669.
Then X. Magma forced its way up through A, B and C, so those layers must already have been there; magma can't cut through rock that doesn't exist yet. Anything that cuts across a rock must be younger than the rock it cuts. Geologists call this the principle of cross-cutting relationships.
Then E. The band of magma rock stops dead at the wavy surface; the top of layer C is uneven too. The land was worn down by wind, water or ice after X had formed, slicing the top off both. Geologists call a surface like this an unconformity. It can stand for millions of years that didn't leave any record at all.
Then D. Layer D lies on top of the worn surface, so it was laid down afterwards, perhaps when the area sank beneath a sea or lake again.
Finally F. The crack breaks every layer, D included, with the rock on one side slipped down past the other, so it's the youngest event of all.
The principle: Relative dating. In undisturbed rock the oldest layers lie at the bottom (superposition), and any feature that cuts through rock, such as a fault or a band of cooled magma, must be younger than the rock it cuts (cross-cutting relationships).