Radiometric Dating and the Geological Time Scale
Inset into Smith's great geological map is a small diagram showing a schematic The early time scales were only relative because 19th century geologists did not That information was not available until the development of isotopic dating. To accompany the Geologic Time Scale Analogy classroom activity. These are most commonly obtained via radiometric dating methods performed on These can not be included in the diagram for practical reasons, but can be found in. Fossils and Relative Dating Student Guide. © Target Inquiry early geologists to recognize a geological timescale in the 19th century. happens. Draw and label observations in the long rectangle, and answer the questions on the side.
The first card in the sequence has "Card 1, Set A" in the lower left-hand corner and represents the bottom of the sequence. If the letters "T" and "C" represent fossils in the oldest rock layer, they are the oldest fossils, or the first fossils formed in the past for this sequence of rock layers.
Since this card has a common letter with the first card, it must go on top of the "TC" card. The fossils represented by the letters on this card are "younger" than the "T" or "C" fossils on the "TC" card which represents fossils in the oldest rock layer. Sequence the remaining cards by using the same process. When you finish, you should have a vertical stack of cards with the top card representing the youngest fossils of this rock sequence and the "TC" card at the bottom of the stack representing the oldest fossils.
Starting with the top card, the letters should be in order from youngest to oldest. Please note that none of the letters in this sequence may be reversed and still be correct. The sequence must be exactly in the order as written.
It is not uncommon to have students reverse the M and D for example and begin the sequence with DM because that is the way they are printed on the card. It is good at this time to remind them that these letters represent fossils in a rock layer and that one fossil next to another within a rock layer implies no particular sequencing; they both are approximately the same age as that particular rock layer. The following question may help clarify this point.
The fossils within rock layer OXD i. Each card represents a particular rock layer with a collection of fossils that are found in that particular rock stratum. All of the fossils represented would be found in sedimentary rocks of marine origin.
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Figure 2-A gives some background information on the individual fossils. The letters on the other cards have no significance to the sequencing procedure and should be ignored at this time.
Find a rock layer that has at least one of the fossils you found in the oldest rock layer. This rock layer would be younger as indicated by the appearance of new fossils in the rock stratum. Keep in mind that extinction is forever. Once an organism disappears from the sequence it cannot reappear later. Use this information to sequence the cards in a vertical stack of fossils in rock strata.
Geological Time Scale | Earth Science Week
Arrange them from oldest to youngest with the oldest layer on the bottom and the youngest on top. Return to top Interpretation Questions: This will enable your teacher to quickly check whether you have the correct sequence. The graptolite, placoderm, ammonite, ichthyosaur, and shark's tooth could possibly be used as index fossils since they are found in only one layer.
Technically, however, given only this set of strata, one cannot say that the shark's tooth and ichthyosaur could be used as index fossils because we do not know if they continue in younger rock layers above this set of strata.
The brachiopod, crinoid, eurypterid, foraminifera, gastropod, horn coral, pelecypod, and trilobite could probably not be used as index fossils since they overlap more than one stratum. Marine sedimentary rocks such as limestone, shale, and sandstone might contain fossils similar to those depicted in this activity. In a "normal" horizontal sequence of rocks, the oldest rock layers will be on the bottom with successively younger rocks on top.
Explain what an isotope is and what alpha decay, beta decay, and electron capture are as mechanisms of radioactive decay. Describe how radio-isotopic dating is accomplished and list four key isotopes used for doing it.
Explain how carbon is formed in the atmosphere and how it is used in dating recent events. Explain how scientists know the numeric age of the Earth and other events in Earth history. Explain how sedimentary sequences can be dated using radio-isotope and other techniques. What is a fossil? Describe ways by which fossils are preserved. Outline how natural selection takes place as a mechanism of evolution. Explain what stratigraphic correlation is and how rocks are correlated regionally and over wide geographic distances.
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.
Geological Time Scale
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.
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.
7 Geologic Time – An Introduction to Geology
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.