Dating relative rock
The simplest and most intuitive way of dating geological features is to look at the relationships between them.
There are a few simple rules for doing this, some of which we’ve already looked at in Chapter 6.
CC BY-SA 1.0], via Wikimedia Commons' src=" alt="" width="474" height="251" srcset=" sizes="(max-width: 474px) 85vw, 474px"/An atom that has different number of neutrons but the same number of protons.
While most properties are based on the number of protons in an element, isotopes can have subtle changes between them, including temperature fractionation and radioactivity.
The principle of inclusions states that any rock fragments that are included in rock must be older than the rock in which they are included.
So we can infer that coal seam is younger than the faults (because it disrupts them), and of course the upper sandstone is youngest of all, because it lies on top of the coal seam.
Exercise 8.1 Cross-Cutting Relationships The outcrop shown here (at Horseshoe Bay, B. Buff/pink felsic intrusive igneous rock present as somewhat irregular masses trending from lower right to upper left 2. A 50 cm wide light-grey felsic intrusive igneous dyke extending from the lower left to the middle right – offset in several places Using the principle of cross-cutting relationships outlined above, determine the relative ages of these three rock types.
[SE] The principle of cross-cutting relationships states that any geological feature that cuts across, or disrupts another feature must be younger than the feature that is disrupted.
An example of this is given in Figure 8.7, which shows three different sedimentary layers.
Recognizing unconformities is important for understanding time relationships in sedimentary sequences.