Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. The method of reading the order is called stratigraphy ( layers of rock are called This is the principle of 'superposition'. Describe the five Principles of Stratigraphy and explain how each applies to interpreting Apply relative dating principles to a block diagram and interpret the . oldest at the bottom to youngest on top based on the principle of superposition . decides older or younger (no precise data); based on stratigraphy; principle of layers of rock; very general relative dating; group things; no specific dates;.
Of course, all strata eventually end, either by hitting a geographic barrier or by a depositional process being too far from its source, either a sediment source or a volcano. Strata that are subsequently by cut by a canyon remain continuous on either side of the canyon. Dark dike cutting across older rocks, the lighter of which is younger than the grey rock.
Principle of Cross-Cutting Relationships: When one rock formation contains pieces or inclusions of another rock, the included rock is older than the host rock. Fossil succession showing correlation among strata. Principle of Fossil Succession: Assemblages of fossils contained in strata are unique to the time they lived and can be used to correlate rocks of the same age across wide geographic distribution.
Evolution has produced a succession of life whose fossils are unique to the units of the Geologic time Scale.
The figure shows the South Rim separated from the North Rim by approximately 18 miles. The predominant white layer just below the canyon rim is the Coconino Sandstone. This layer is laterally continuous, even though the intervening canyon separates its outcrops on either side by about 18 miles. These layers of rock are continuous over a wide region of the Colorado Plateau surrounding the Grand Canyon even though several canyons cut through the strata.
This is an example of the principle of lateral continuity. 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. 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. 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.
7 Geologic Time
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.
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.
7 Geologic Time – An Introduction to Geology
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. Individual inclusions are oval or round in shape and consist of clear glass, together with a small round vapor bubble and in some cases a small square spinel crystal. The black arrow points to one good example, but there are several others. The occurrence of multiple inclusions within a single crystal is relatively common 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.
Nevertheless, they can provide an abundance of useful information. Using microscopic observations and a range of chemical microanalysis techniques geochemists and igneous petrologists can obtain a range of useful information from melt inclusions. Two of the most common uses of melt inclusions are to study the compositions of magmas present early in the history of specific magma systems.
This is because inclusions can act like "fossils" — trapping and preserving these early melts before they are modified by later igneous processes.
In addition, because they are trapped at high pressures many melt inclusions also provide important information about the contents of volatile elements such as H2O, CO2, S and Cl that drive explosive volcanic eruptions.
Sorby was the first to document microscopic melt inclusions in crystals.