Relative dating is the science of determining the relative order of past events i. In geology, rock or superficial deposits , fossils and lithologies can be used to correlate one stratigraphic column with another. Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating , archaeologists and geologists used relative dating to determine ages of materials. Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique. Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate. The regular order of the occurrence of fossils in rock layers was discovered around by William Smith.
The layers on top could only be laid down on top of the bottom layer so must be younger. However the relative ages of rocks is more commonly determined by the presumed ages of the fossils found in the sedimentary layers.
The sedimentary layers with the simplest fossils are assumed to be older even if the sedimentary layer is found on top of a sedimentary layer that has fossils that are more complex and therefore assumed to be younger.
What is relative dating of rocks and fossils
Fossils that are in violation of the law of superposition where the older fossil occurs above a younger fossil are said to be stratigraphically disordered.
The conclusion of some scientists is that the Law of Superposition just doesn't work Shindewolf Comments on Some Stratigraphic Terms American Journal of Science June " Historical geology relies chiefly on paleontology the study of fossil organisms. The Law of Superposition makes logical sense but in practice it is the nature of the fossils found in the sedimentary layers that determine the relative ages of the rocks. The theory of descent with modification trumps the empirical evidence of superposition.Fossil Rock Anthem
Law of Superposition i. Things get a little more complicated especially when the layers of strata are folded in such a way as to be completely overturned!
Principle of Cross-cutting Relationships i. The Law of Strata Identified by Fossils is a little bit more complex. In the succession of strata, each layer represents the geographical conditions that occurred over that area at the time the layer was deposited. The diagram right shows two distinct faunal assemblages.
What can be deduced from our two distinct faunas in terms of their usefulness in the relative dating of rocks? The table below summarises key features:.
Gastrioceras listeri is a particularly good example of a ZONE fossil. As it is free swimming it could have travelled a considerable distance.
There are a number of different types of intrusions, including stocks, laccoliths , batholiths , sills and dikes. The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.
Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.
The principle of inclusions and components explains that, with sedimentary rocks, if inclusions or clasts are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found.
These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them. The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds.
Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal.
The law of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it.
This is because it is not possible for a younger layer to slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed. The principle of faunal succession is based on the appearance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or sometimes absence may be used to provide a relative age of the formations in which they are found.
Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin 's theory of evolution , the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat facies change in sedimentary strata , and that not all fossils may be found globally at the same time.
The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous.
How can fossils be used to determine the relative ages of rock layers?
As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous. Layers of sediment do not extend indefinitely; rather, the limits can be recognized and are controlled by the amount and type of sediment available and the size and shape of the sedimentary basin.
Sediment will continue to be transported to an area and it will eventually be deposited. However, the layer of that material will become thinner as the amount of material lessens away from the source. Often, coarser-grained material can no longer be transported to an area because the transporting medium has insufficient energy to carry it to that location.
In its place, the particles that settle from the transporting medium will be finer-grained, and there will be a lateral transition from coarser- to finer-grained material. The lateral variation in sediment within a stratum is known as sedimentary facies. If sufficient sedimentary material is available, it will be deposited up to the limits of the sedimentary basin. Often, the sedimentary basin is within rocks that are very different from the sediments that are being deposited, in which the lateral limits of the sedimentary layer will be marked by an abrupt change in rock type.
Melt inclusions are small parcels or "blobs" of molten rock that are trapped within crystals that grow in the magmas that form igneous rocks. In many respects they are analogous to fluid inclusions. Melt inclusions are generally small — most are less than micrometres across a micrometre is one thousandth of a millimeter, or about 0.