Dating relative rock
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.Using this time scale, geologists can place all events of Earth history in order without ever knowing their numerical ages.The specific events within Earth history are discussed in Chapter 8.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.
There are a few simple rules for doing this, some of which we’ve already looked at in Chapter 6.
The tilted rocks at the bottom are part of the Proterozoic Grand Canyon Group (aged 825 to 1,250 Ma).
The flat-lying rocks at the top are Paleozoic (540 to 250 Ma).
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.