A paleontologist unearths a well-preserved fern fossil within a horizontal layer of shale situated directly between two distinct volcanic ash beds. Radiometric dating reveals that the underlying ash bed is 65 million years old, while the overlying ash bed is 60 million years old. Based on these stratigraphic relationships, which conclusion about the age of the fossil is most accurate?
- AThe fossil is older than 65 million years because organisms buried in shale always predate underlying rock layers.
- The fossil organism lived between 60 and 65 million years ago.Cevap
- CThe fossil is exactly 60 million years old because it crystallized directly from cooling magma within the upper volcanic bed.
- DThe fossil is younger than 60 million years because sedimentary rock can only form after all volcanic activity has ceased.
Cevap
The fossil organism lived between 60 and 65 million years ago.
According to the Law of Superposition in undisturbed strata, lower layers are older than upper layers. Because the fossil-bearing shale was deposited on top of the 65-million-year-old ash bed and was subsequently covered by the 60-million-year-old ash bed, the fern fossil is reliably bracketed to an age between 60 and 65 million years old.
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Radiometric age bracketing and the Law of Superposition
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