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Difficulty: MediumEarth's History and Paleontology

A geologist analyzes a cross-section of undisturbed sedimentary rock containing three horizontal layers: shale at the bottom, sandstone in the middle, and limestone at the top. A vertical basalt dike cuts through both the shale and sandstone layers, but terminates directly beneath the limestone layer. Based on relative dating principles, which of the following statements correctly describes the relative age of the basalt dike?

  1. A
    It is older than both the shale and sandstone layers because igneous magma originates from deep below the crust.
  2. It is younger than the sandstone layer but older than the limestone layer.Answer
  3. C
    It is younger than the limestone layer because igneous intrusions always postdate every sedimentary layer in a formation.
  4. D
    It is the exact same age as the limestone layer because igneous intrusions and overlying strata solidify simultaneously.

Answer

The basalt dike is younger than the sandstone layer but older than the limestone layer.
According to the principle of cross-cutting relationships, an igneous intrusion is younger than any rock layers it cuts through. Because the basalt dike cuts through the shale and sandstone, it is younger than both of those units. Furthermore, because the dike does not penetrate the limestone layer above it, the limestone must have been deposited on top of the eroded dike at a later time, making the dike older than the limestone.

Step-by-Step Solution

1
Apply the Principle of Cross-Cutting Relationships to the penetrated layers
The basalt dike is younger than both the shale and sandstone layers.
A geologic feature (such as an igneous intrusion or fault) that cuts across rock layers must be younger than the rocks it disrupts.
2
Apply the Law of Superposition to the unpenetrated overlying layer
The limestone layer is younger than the basalt dike.
Because the basalt dike stops beneath the limestone and does not alter or cut it, the limestone was deposited horizontally on top after the dike intruded and cooled.
3
Synthesize the chronological sequence
Chronological order from oldest to newest: Shale →\rightarrow Sandstone →\rightarrow Basalt Dike →\rightarrow Limestone.
Combining relative dating principles places the dike between the deposition of the sandstone and the limestone.

Key Concept

Principle of Cross-Cutting Relationships and the Law of Superposition in Relative Dating
Estimated Time:1m 0s
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Topics

Earth's History and Paleontology

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