hard · Enhanced ACT science
Two geologists disagree about the origin of a roughly circular lake basin.
Geologist A (Glacial Scour): The basin was carved by an advancing glacier during the last ice age. Glacial scour produces a U-shaped bedrock cross-section and leaves unsorted rock debris (till) resting directly on the bedrock floor.
Geologist B (Volcanic Collapse): The basin is a collapsed volcanic caldera. Caldera collapse produces a roughly circular basin floored by layered volcanic ash and pumice, with no till present.
A new core sample through the basin floor reveals a U-shaped bedrock cross-section and a layer of unsorted till resting directly on bedrock; no volcanic ash or pumice is found anywhere in the core.
This new core-sample finding is best described as:
- consistent with Geologist A only, matching the U-shape and till on bedrock
- consistent with Geologist B only, since ash and pumice explain the till
- consistent with both geologists equally, since each predicts a closed basin
- consistent with neither geologist, since real basins need both ash and till
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