Superposition assumes layers pile up without a break. Often they don't. For long stretches of time, rock was worn away, or none was laid down at all, and then deposition began again. The buried erosion surface between the older and younger rocks is an unconformity: a gap in the record.
- Angular unconformity: older layers were tilted or folded, eroded flat, then buried by new, flat layers. The angle between them is the giveaway.
- Nonconformity: sedimentary layers rest on eroded igneous or metamorphic rock, which formed deep underground and had to be uplifted and exposed first.
- Disconformity: layers above and below are parallel, so only an uneven erosion surface, sometimes with pebbles or old soil along it, reveals the gap.
Why it works
Each kind tells a story. An angular unconformity needs deposition, then deformation, then uplift and erosion, then sinking and more deposition, a long history squeezed into one surface. A nonconformity means kilometers of rock were stripped away to expose a pluton or schist. Their time gaps can be huge: in the Grand Canyon, the Great Unconformity separates rocks hundreds of millions to more than a billion years apart.
Key idea
An unconformity is a buried erosion surface that marks missing time, not a layer. It is angular (tilted rocks below), a nonconformity (igneous or metamorphic rock below) or a disconformity (parallel layers).
Worked examples
Worked example: What kind?
Horizontal limestone sits on an uneven surface of schist and gneiss. What kind of unconformity?
- Schist and gneiss are metamorphic rocks, formed deep underground.
- Sedimentary rock on eroded metamorphic rock is a nonconformity.
Answer: A nonconformity.
Worked example: How much time is missing?
The youngest layer below an unconformity is 410 million years old; the oldest layer above it is 320 million years old. How much time does the surface represent?
- $410 - 320 = 90$ million years.
- For 90 million years, rock was eroded here, or never deposited.
Answer: 90 million years.
Common mistake
Many students think an unconformity is a special kind of layer. It is a surface, a gap: the rock that would have recorded that time is missing. Check the ages: across a normal layer boundary the ages change smoothly; across an unconformity they jump.