Using Figure 3: What feature does the gabbro represent?

Question

BlackTom AI · Accepted Answer

Gabbro is a coarse-grained mafic intrusive igneous rock, so it represents an intrusion of magma into preexisting rock. This key property immediately points toward an intrusive feature that cuts across or fills fractures in host rock.

1) Relevant knowledge points and concepts
- Gabbro: coarse-grained (plutonic) mafic intrusive igneous rock (composition like basalt but crystallized slowly at depth).
- Dike: a tabular, discordant intrusive body that cuts across bedding/structures of the host rock.
- Granite: a felsic intrusive rock (different composition and typically lighter colored).
- Fault: a brittle fracture along which significant displacement/offset of rock layers has occurred.
- Cross-bedding: a sedimentary internal bedding feature produced by currents, not an igneous body.

2) Step-by-step reasoning
- Identify lithology: the feature is labeled gabbro, so it is igneous and intrusive rather than sedimentary.
- Determine geometry and relationship: an intrusive body that cuts across layering is best described as discordant and tabular.
- Match with terminology: a discordant tabular intrusion is a dike. A sill would be concordant (parallel to bedding), which is not indicated here.

3) Intermediate steps / checks
- Check whether the feature shows offset (fault): if there were offset of layers along a plane, that would indicate a fault; the gabbro intrusion does not produce offset, so fault is incorrect.
- Check whether the feature is an internal sedimentary structure (cross-bedding): gabbro is igneous, so cannot be cross-bedding.
- Check composition match for granite: gabbro is mafic (dark, high Mg/Fe), granite is felsic (light, high Si/Al); they are different rock types.

4) Why the correct answer is correct
Because gabbro is an intrusive igneous rock that forms in fractures or tabular bodies cutting existing rocks, the feature represents a dike — a discordant magmatic intrusion.

5) Why other options are incorrect
- Granite: wrong composition/type (felsic, not mafic gabbro).
- Fault: would show displacement of layers; here the feature is an intrusion, not a displaced plane.
- Cross-bedding: a sedimentary internal structure, incompatible with an igneous gabbro body.

6) Conclusion
The feature represents a dike. This matches the provided correct answer: Dike.

Using Figure 3: What feature does the gabbro represent?Multiple choice

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