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. 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).
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. 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.
Potassium-argon dating is a form of isotopic dating commonly used in paleontology. Scientists use the known natural decay rates for isotopes of potassium and argon to find the date of the rocks. The radioactive isotope converts to a more stable isotope over time, in this case decaying from potassium to argon. This is the convenience online New Dating Site Far Rockaway dating has brought to the current generation, and you can join us today and start searching. Our registration process is easy and will only take a few minutes of your time. Online Dating - Free to join Dating New Dating Site Far Rockaway Site - Girls Date For Free. Someone You can Love is Nearby. Browse Profiles & Photos of Doctor Singles in Picture Rocks, PA! Join Match.com, the leader in online dating with more dates, more relationships and more marriages than any other dating site. Dan Bacon Dan Bacon is a dating and relationship expert and the author of The Flow, an eBook that teaches you the easiest way to get laid or get a girlfriend.Dan has Picture Rocks Dating Ads been helping new men succeed with women for more than 14 years.
Exercises
Exercise 8.1 Cross-Cutting Relationships
The outcrop shown here (at Horseshoe Bay, B.C.) has three main rock types:
1. Buff/pink felsic intrusive igneous rock present as somewhat irregular masses trending from lower right to upper left
2. Dark grey metamorphosed basalt
3. 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.
On Line Dating In Picture Rocks Pictures
(The near-vertical stripes are blasting drill holes. The image is about 7 m across.) [SE photo]
On Line Dating In Picture Rocks Look
An unconformity represents an interruption in the process of deposition of sedimentary rocks. Recognizing unconformities is important for understanding time relationships in sedimentary sequences. An example of an unconformity is shown in Figure 8.8. The Proterozoic rocks of the Grand Canyon Group have been tilted and then eroded to a flat surface prior to deposition of the younger Paleozoic rocks. The difference in time between the youngest of the Proterozoic rocks and the oldest of the Paleozoic rocks is close to 300 million years. Tilting and erosion of the older rocks took place during this time, and if there was any deposition going on in this area, the evidence of it is now gone.
There are four types of unconformities, as summarized in Table 8.1, and illustrated in Figure 8.9.
| Unconformity Type | Description |
|---|---|
| Nonconformity | A boundary between non-sedimentary rocks (below) and sedimentary rocks (above) |
| Angular unconformity | A boundary between two sequences of sedimentary rocks where the underlying ones have been tilted (or folded) and eroded prior to the deposition of the younger ones (as in Figure 8.8) |
| Disconformity | A boundary between two sequences of sedimentary rocks where the underlying ones have been eroded (but not tilted) prior to the deposition of the younger ones (as in Figure 8.7) |
| Paraconformity | A time gap in a sequence of sedimentary rocks that does not show up as an angular unconformity or a disconformity |