Layers are not the only things in rock. Magma can squeeze up through cracks and harden into a sheet of igneous rock called a dike, and rock can break and slip along a fault. These features cut across the layers, and that tells their age.
Key idea
Anything that cuts across rock is younger than the rock it cuts. A dike or fault is younger than every layer it crosses and older than any layer that lies unbroken on top of it.
Why it works
You can't cut a cake before it is baked. To slice through a layer, the layer must already be there. So a dike that crosses layers A, B and C formed after all three. If layer D lies across the top of the dike, unbroken, then D settled after the dike formed.
Worked example: Where the dike stops
A dike cuts through layers 1, 2 and 3 but stops at the bottom of layer 4, which lies on top. How old is the dike compared with the layers?
- It cuts layers 1, 2 and 3, so it is younger than all three.
- Layer 4 lies unbroken on top of it, so layer 4 is younger than the dike.
Answer: Younger than layers 1 to 3, older than layer 4
Worked example: Putting events in order
Flat layers A (bottom) and B (top) are cut by a fault F, and a dike D cuts both layers but is itself broken by the fault. Order the events.
- Layers first, bottom up: A, then B.
- D cuts both layers: it came next.
- F breaks the dike too, so the fault is youngest: A, B, D, F.
Answer: A, B, D, F
Common mistake
Dating a dike by the layer where it ends, as if it were the same age as that layer. A dike must be younger than every layer it cuts across. Check: trace the dike up from the bottom; it is younger than the highest layer it cuts, and older than the layer it stops under.