Part VII. Dating, Interpreting Geologic Histories, and Geologic Time. (Chapter 8 in your textbook)


A. Relative Age Determination (relative timing):

1. Law of Superposition: In a series of sedimentary layers, the bottom layer is the oldest.

*Assumes the layers have not been overturned by plate tectonic activity.


2. Cross Cutting Relationships: younger events (faults, igneous intrusions, erosion, metamorphism) "cut" or are superimposed on something. That something had to be there to be "cross cut" or modified.


Diagram showing basic Cross-cutting relationships

The lowermost sedimentary layer is the oldest (Law of Superposition ), so the relative timing of events shown would be deposition of layers #1-3. Then these layers are cut by a fault (Cross-cutting relationship -- the layers had to already be there to be cut). The faulting event would be #4.

See section 10.3 in your textbook (p. 279) and figures 10.1 and 10.2.


B. Interpreting Geologic Histories:

1, Use relative timing relationships (use basic information in rocks, superposition, and cross-cutting relationships) to work out general events.

2. We also use examples of processes and relationships observed in the modern world to help interpret the past.

This principle is called: uniformitarianism (sometimes also referred to actualism). This principle is summarized by: Understanding the present is key to understanding the past.


photo of Redgranite quarry Photo of mafic dike
Diagram Showing cross-cuttinge relationships

This is a simplified cartoon of the quarry at Redgranite, Wisconsin (see photos above) with relative timing of event interpretations (numbers 1-7, with #1 being the oldest event).

The baked contact metamorphic "halo" at the boundary between the granite intrusion and sedimentary rock layers 1-3 indicate that layers 1-3 were there to be baked (cross-cutting relationship). The bottom-most layer is the oldest (Law of Superposition).

There is a vertical sheet-like mafic intrusion (see photo above) cuts the granite; because it cuts the granite, it post dates the granite and because the granite baked layers 1-3, the mafic intrusion (#5) post dates layers 1-3 and the granite.

Sedimentary rock layer #6 contains inclusions of the granite and was not baked by the granite. This indicates that layer #6 post-date the granite.

We can't tell for sure that layer #6 came after the mafic intrusion because there are no cross-cutting relationships to constrain their timing. I drew the diagram with the assumption that there were inclusions of the mafic intrusion in layer #6, but as shown, there is no definitive answer.

The quarry operation cross-cut both the granite and the mafic intrusion, so we can say for sure that the quarry came last (#7).


Next: C. The vastness of Geologic time.

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