Chapter 1: Problem 9

Describe three kinds of relationships between two bodies of rock that indicate which of the two bodies is the younger one.

Short Answer

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Three kinds of relationships between two rock bodies that can help determine which one is younger are: 1. Cross-cutting Relationships - When one rock body cuts through another, the one cutting through is younger (e.g., an igneous intrusion cutting through sedimentary layers). 2. Inclusions - The rock body containing fragments (inclusions) of another is younger (e.g., a sedimentary rock layer containing fragments of an igneous rock). 3. Unconformities - The rock body above an unconformity surface (a gap in the geologic record) is younger (e.g., two sedimentary rock layers separated by an erosion surface).

Step by step solution

Cross-cutting Relationships

A cross-cutting relationship is a principle in geology that states the geologic feature which cuts through another is the younger of the two features. In the context of rock bodies, if one rock body cuts through another, the one cutting through is younger. For example, if there is an igneous intrusion (like a dike or sill) cutting through a sedimentary rock layer, we can determine that the igneous intrusion is the younger body because it cuts across the pre-existing sedimentary layers.

Inclusions

Inclusions occur when fragments of one rock body are enclosed within another. The rock body that contains the fragments (inclusions) is younger because it has formed around the fragments of the older rock. For instance, if we have a sedimentary rock layer containing fragments of an igneous rock, then the sedimentary rock layer is younger than the igneous rock. The reason is that the sedimentary rock formed after the igneous rock had already existed and eroded into fragments.

Unconformities

An unconformity is a surface that represents a gap in the geologic record, usually formed due to erosion or lack of deposition. When two rock bodies are separated by an unconformity, the body of rock that is above the unconformity surface is the younger one. For example, imagine two sedimentary rock layers separated by an erosion surface (unconformity). The lower rock layer was deposited, then was eroded and/or had a period of non-deposition. Later, the upper rock layer was deposited over the erosion surface. This indicates that the upper rock layer is younger than the lower one.

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Describe three kinds of relationships between two bodies of rock that indicate which of the two bodies is the younger one.

Short Answer

Step by step solution

Cross-cutting Relationships

Inclusions

Unconformities

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