Question

which correctly lists three different geologic features that help scientists determine the relative ages of rocks by their positions?
○ magma, erosion, and intrusions
○ erosion, magma, and cross - cutting relationships
○ intrusions, index fossils, and magma
○ index fossils, intrusions, and cross - cutting relationships

Explanation:

• Index fossils: These are fossils of organisms that were widespread but existed for a relatively short period. They help in correlating rock layers across different locations.
• Intrusions: An intrusion is a body of igneous rock that forms from magma that cools and solidifies within the Earth's crust. Intrusions are younger than the rocks they intrude into.
• Cross - cutting relationships: The principle of cross - cutting relationships states that a geologic feature (such as a fault or an intrusion) that cuts through another feature is younger than the feature it cuts.

Magma is just molten rock material. While it can form intrusions (which are relevant), just "magma" alone (as in the first three options) is not a geologic feature used in the way of index fossils, intrusions (as a formed feature), and cross - cutting relationships for relative age determination based on position. Erosion can remove rock layers but is not a feature used in the same sense as the other three correct ones for relative age determination by position in the context of the question.

Answer:

index fossils, intrusions, and cross - cutting relationships