7 Geologic Time – An Introduction to Geology
Stratigraphy is the study of layered sedimentary rocks. This section discusses the principles of relative time that are used in all of geology but of Cross-Cutting Relationships: Deformation events like. Relative Dating Geologic Events GLS Physical Geology – Dr. Hanson In this lab you will learn how geologists use rocks and apply the laws of relative age. In doing so the geologist relies upon two different but correlatable time frames, absolute and relative. The absolute age of either a rock unit or a geologic event.
Know the eras and periods of the geologic time scale and explain the purpose behind its divisions. Explain the relation between time units and corresponding rock units period and system, epoch and series, age and stage. Working out Earth history depended on realizing some key principles of relative time. The figure in section 7.
Using this time scale as a calendar, all events of Earth history can be placed in order without ever knowing the numerical age. The principles of relative time are simple, even obvious now, but were not generally accepted by scholars until the Scientific Revolution of the 17th and 18th centuries.
James Hutton realized that geologic processes are slow and his ideas on uniformitarianism i. This section discusses the principles of relative time that are used in all of geology but especially useful in stratigraphy.
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Lower strata are older than those lying on top of them. In an otherwise undisturbed sequence of sedimentary strata rock layersthe layers on the bottom are the oldest and the layers above are younger.
Principle of Original Horizontality: This holds true except for the margins of basins, where the strata can slope slightly downward into the basin. Lateral continuity Principle of Lateral Continuity: 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. 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.
Relative Dating of Geologic events
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.Relative/Absolute Dating
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.