Relative Ages of Rocks Answers
What is a nonconformity?
A nonconformity is a gap in the geologic record where younger sedimentary rock rests on older igneous or metamorphic rock, indicating erosion or nondeposition.
If horizontal layers of sedimentary rock have a vertical fault running through them, how might a geologist use relative dating to determine when the fault occurred?
The geologist would use cross-cutting relationships: the fault is younger than every layer it cuts, so it formed after those layers and before any overlying layer that it does not cut.
Figure 13-1
Explain the principle of superposition in your own words.
The principle of superposition states that in an undisturbed sequence of sedimentary rock layers, the oldest layers are at the bottom and the youngest layers are at the top.
Explain what is meant by correlating rock layers.
Correlating rock layers means matching layers in different locations that are the same age or represent the same geologic event, using features such as rock type and fossils.
Compare and contrast disconformities and nonconformities.
Both are gaps in the geologic record caused by erosion or nondeposition. A disconformity separates parallel sedimentary rock layers, while a nonconformity separates sedimentary rock from older igneous or metamorphic rock.
Figure 13-1
What is an unconformity?
An unconformity is a boundary between rock layers that represents missing geologic time caused by erosion or a period of nondeposition.
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