Relative dating diagram
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.
For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it (Figure 8.6).
Figure 8.6a A xenolith of diorite incorporated into a basalt lava flow, Mauna Kea volcano, Hawaii.
The lower sandstone layer is disrupted by two faults, so we can infer that the faults are younger than that layer.
But the faults do not appear to continue into the coal seam, and they certainly do not continue into the upper sandstone.
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.
There are a few simple rules for doing this, some of which we’ve already looked at in Chapter 6.
In this session we will introduce you to the idea of relative and absolute dating, and explore the difference between these two ideas.
Diagram 1 shows three sites, each with its own sequence of assemblages.
which friends we knew at which stage of our lives).
But unless we have been revising a CV or resum recently we may be less clear about the precise years in which particular things happened (that is, absolute chronology).