The Laws of Relative Dating — Mr. Mulroy's Earth Science
2) absolute dating (isotopic, tree rings relative ages then try to get absolute age dates. • Determining relative age relies on a . Cross-cutting Relationships. We know that we can relatively date layers of rock by knowing that the layers mawatari.info Using relative and radiometric dating methods, geologists are able to answer the than the layers they cut through (principle of cross-cutting relationships).
Formation names are designated by geologists to identify rock units that have recognizable characteristics that can identify them in a region.
Thus, formations are used as units for mapping purposes and communication.
In the lowest parts of the Grand Canyon are the oldest formations with igneous and metamorphic rocks at the bottom. The Vishnu Schist is the oldest and the cross-cutting intrusions of Zoroaster Granite are younger.
How does the law of crosscutting relationships help scientists determine the relative age of rocks?
As seen in the figure, the other layers on the walls of the Grand Canyon are numbered in reverse order with 15 being the oldest and 1 the youngest. The Colorado Plateau, on which the Grand Canyon region lies, is characterized by strata that are horizontal or nearly so. These rocks were originally deposited horizontally Principle of Original Horizontality and have not been disturbed very much since they were deposited except by a broad regional uplift there are local exceptions.
In the Grand Canyon, there is a gentle tilt of the strata to the south, thus the strata of the North Rim are about a thousand feet higher than those of the South Rim about 18 miles away. Applying the stratigraphic principles, one can interpret that the slight tilting of the strata occurred after their deposition and that the Grand Canyon was cut by the Colorado River after the regional tilting.
This is an application of Cross Cutting Relationships to establish relative time and Lateral Continuity to correlate them across the canyon. The red, layered rocks of the Grand Canyon Supergroup on the dark-colored rocks of the Vishnu Complex.
On top of these basement rocks, lie the strata of the Grand Canyon Supergroup there are several formations included in this supergroup unit.
7 Geologic Time
These formations were originally deposited flat on top of the basement rocks Original Horizontality and have since been broken into tilted blocks by normal faulting see Chapter 9 which cut through both them and the underlying basement. Because the formation of the basement rocks and the deposition of these overlying sediments is not continuous deposition but broken by events of metamorphism, intrusion, and erosion, the contact between the Grand Canyon Supergroup and the older basement is termed an unconformity.
An unconformity represents a period during which deposition did not occur or erosion removed rock that had been deposited, so there are no rocks that represent events of Earth history during that span of time at that place.
Unconformities are shown on cross sections and stratigraphic columns as wavy lines between formations. There are three types of unconformities which will be discussed below. The first occurs when sedimentary rock lies on top of crystalline rock, and is a type of unconformity called a nonconformity. A nonconformity occurs when sediments are deposited on top of non-layered crystalline igneous and metamorphic rocks as is the case with the contact between the Grand Canyon Supergroup and the Vishnu basement rocks.
All three of these formations have an erosional unconformities at the two contacts between them. The pinching Temple Butte is the easiest to see, but even between the Muav and Redwall, there is an unconformity. The Grand Canyon Supergroup is a sequence of strata representing alternating marine transgressions and terrestrial deposition in this case regressions where the sea retreated.
During formation of this sequence, sea-level rose or the land sank leaving marine deposits on the surface and then fell or the land rose leaving the land exposed to erosion and to deposition of terrestrial sediments. In other words, layers of rock that could have been present, are absent.
The time that could have been represented by such layers is instead represented by the disconformity. Disconformities are unconformities that occur between parallel layers of strata indicating that there was no deformation during the period of nondeposition or erosion.
In the lower part of the picture, note the dipping toward the right rocks. These intersect the non-dipping rocks above at an angle, making an angular unconformity. On top of the Grand Canyon Supergroup lie the horizontal layers of the canyon walls showing unconformable contacts with the tilted layers of the Grand Canyon Supergroup below i.
The lower strata were tilted by tectonic processes that disturbed their original horizontality which of course also affected the underlying basement rocks. Thus there were cross-cutting processes that affected those rocks before the younger strata were deposited horizontally on top of them. After the deposition of the Grand Canyon Supergroup and the tectonic events that tilted and faulted them, there was an erosion-produced landscape with hills and valleys over which the sea transgressed again and deposited layers of three horizontal formations of sedimentary rock called the Tonto Group.
The upturned and eroded edges of the tilted older rocks of the Grand Canyon Supergroup lay at angles with the overlying Tonto Group. This third type of unconformity is called an angular unconformity. Here is the sequence of these events in order. The oldest rock is a body of deformed rock composed of brown and gray layers.
7 Geologic Time – An Introduction to Geology
Its deformation includes pretty severe deformation shown as folding. From the symbols used in the drawing, this rock looks like it was probably metamorphosed. The oldest event, therefore, is the formation of the brown and grey rock, followed by its deformation and metamorphism which we might call basement rock here. The brown and gray basement rock was cut by the fault A which cuts across and displaces it.
Both the basement rock and fault A are crosscut by rock mass B. Its irregular outline suggests that it is an igneous intrusion emplaced as magma into the region.
Since it cuts across both the basement rocks and the fault, it is younger than both. Principles of relative dating[ edit ] Methods for relative dating were developed when geology first emerged as a natural science in the 18th century. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events.
Uniformitarianism[ edit ] The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.
In geology, when an igneous intrusion cuts across a formation of sedimentary rockit can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccolithsbatholithssills and dikes.
Cross-cutting relationships[ edit ] Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures.
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. 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.
Original horizontality[ edit ] 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. 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. 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 evolutionthe 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 strataand that not all fossils may be found globally at the same time.
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. Inclusions of igneous rocks[ edit ] Multiple melt inclusions in an olivine crystal.