Question

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

Short answer

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.

Key concepts

Cross-cutting Relationships

In geology, the principle of cross-cutting relationships is an essential tool when determining the relative ages of rock formations. This principle states that any geologic feature that cuts across another feature must be younger than the feature it cuts.

Imagine you have a cake (a layer of sedimentary rock) and someone inserts a knife into it (an igneous intrusion like a dike). The cut made by the knife is obviously formed after the cake was baked. Similarly, in geology, if you find a rock layer that is intersected by another, the one doing the intersecting is more recent.

Common examples include igneous intrusions, such as dikes or sills, which penetrate existing rock layers or faults that break through different strata. By observing these cross-cutting features, geologists can deduce the chronological order of rock formation events. This principle is straightforward yet powerful in reconstructing the geological history of an area.

Inclusions

Inclusions offer another compelling method for discerning the relative age of rock bodies. The principle of inclusions suggests that fragments from one rock unit found within another are older than the rock containing them.

Picture it like this: if you find pieces of chocolate embedded in a cookie, those chocolate pieces were created before the cookie dough encased them. The same concept applies to geologic materials.

For example, if sedimentary rock contains pieces of an older igneous rock, we conclude that the sedimentary rock is younger. This is because the sedimentary material must have formed around those igneous fragments. Geologists often observe this principle in different types of rocks to help accurately sequence geological events over time. It reflects an intricate yet fascinating way to interpret earth’s dynamic processes.

Unconformities

Unconformities are perhaps the most intricate among the concepts but are critical in understanding gaps in the geological timeline. An unconformity represents a period where deposition stopped, erosion may have occurred, or where there is no record of deposition at all.

Picture the geological record as a book. An unconformity is like a missing chapter, highlighting an interruption in the story of rock formation. The rocks above this missing segment or erosion surface are younger than those below.

There are several types of unconformities, including angular, disconformity, and nonconformity. In each instance, the rock layer sitting atop the unconformity is considered newer. Understanding unconformities allows geologists to piece together significant changes in Earth's geological history, identifying epochs of erosion or non-deposition that tell tales of ancient environmental shifts.