Relative dating worksheet geology
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.Recognizing unconformities is important for understanding time relationships in sedimentary sequences.An example of an unconformity is shown in Figure 8.8.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.Geologists have studied the order in which fossils appeared and disappeared through time and rocks. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.
Figure 8.8 The great angular unconformity in the Grand Canyon, Arizona.Some fossils, called index fossils, are particularly useful in correlating rocks.For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.For example, the principle of superposition states that sedimentary layers are deposited in sequence, and, unless the entire sequence has been turned over by tectonic processes or disrupted by faulting, the layers at the bottom are older than those at the top.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.