Note that the geologic column was established and fairly well known before geologists had a means of determining numeric ages. Thus, in the geologic column shown below, the numeric ages in the far right-hand column were not known until recently. The Eons are divided into Eras only Phanerozoic Eras are shown in the chart. These include, from oldest to youngest:. The Eras are divided into Periods.
The Periods are often named after specific localities. Further subdivisions of Periods are called Epochs. Only Epochs of the Cenozoic Era are shown in the Chart.
Note that for this course, you need to know the Eons, Eras, and Periods in age order. You will not be asked about the Epochs at least for now. Also, you will not be asked to give the numeric ages for the above at least for now. Although geologists can easily establish relative ages of rocks based on the principles of stratigraphy, knowing how much time a geologic Eon, Era, Period, or Epoch represents is a more difficult problem without having knowledge of numeric ages of rocks.
In the early years of geology, many attempts were made to establish some measure of numeric time. In radioactivity was discovered, and it was soon learned that radioactive decay occurs at a constant rate throughout time. With this discovery, Radiometric dating techniques became possible, and gave us a means of measuring numeric age. Radiometric dating relies on the fact that there are different types of isotopes.
After the passage of two half-lives only 0. Some examples of isotope systems used to date geologic materials. Note that with the exception of 14C, all techniques can only be used to date igneous rocks. Some elements occur in such small concentration or have such long half lives, that they cannot be used to date young rocks, so any given isotope system can only be used if the material available is suitable for that method.
Example - Radiocarbon 14 C Dating Radiocarbon dating is different than the other methods of dating because it cannot be used to directly date rocks, but can only be used to date organic material produced by once living organisms. Thus the ratio of 14 C to 14 N in the Earth's atmosphere is constant. Living organisms continually exchange Carbon and Nitrogen with the atmosphere by breathing, feeding, and photosynthesis.
Thus, so long as the organism is alive, it will have the same ratio of 14 C to 14 N as the atmosphere. When an organism dies, the 14 C decays back to 14 N, with a half-life of 5, years. Measuring the amount of 14 C in this dead material thus enables the determination of the time elapsed since the organism died. Radiocarbon dates are obtained from such things as bones, teeth, charcoal, fossilized wood, and shells. Because of the short half-life of 14 C, it is only used to date materials younger than about 70, years.
Other Numeric Age Methods. There are other means by which we can determine numeric age, although most of these methods are not capable of dating very old materials. Among the methods are:.
Absolute Dating and the Geologic Column Using the methods of absolute dating, and cross-cutting relationships of igneous rocks, geologists have been able to establish the numeric ages for the geologic column. For example, imagine some cross section such as that shown below.
From the cross-cutting relationships and stratigraphy we can determine that:. By examining relationships like these all over the world, numeric age has been very precisely correlated with the Geologic Column. But, because the geologic column was established before radiometric dating techniques were available, note that the lengths of the different Periods and Epochs are variable. Theoretically we should be able to determine the age of the Earth by finding and dating the oldest rock that occurs.
So far, the oldest rock found and dated has an age of 3. Individual zircon grains in sandstones have been dated to 4. But, is this the age of the Earth?
Probably not, because rocks exposed at the Earth's surface are continually being eroded, and thus, it is unlikely that the oldest rock will ever be found. But, we do have clues about the age of the Earth from other sources:. We have now presented most of the tools necessary to interpret Earth history. These tools include knowledge of different kinds of rocks and the conditions under which they form and the laws of stratigraphy.
To make sure you have acquired the knowledge necessary to use these tools, make sure you understand how the interpretations were made in the production of the artwork on pages in your textbook and figure Questions on this material that might be asked on an exam. Stephen A.
Physical Geology. In order to do so we will have to understand the following: The difference between relative age and numeric age. The principles that allow us to determine relative age the principles of stratigraphy. How we can use fossils and rocks to understand Earth History. How rock units are named and correlated from one locality to another. How the Geologic Column was developed so that relative age could be systematically described.
How we can determine the numeric age of the Earth and events in Earth History. Relative age - Relative means that we can determine if something is younger than or older than something else. Relative age does not tell how old something is; all we know is the sequence of events. For example: The a volcano is younger than the rocks that occur underneath it.
Numeric age- Numeric age means that we can more precisely assign a number in years, minutes, seconds, or some other units of time to the amount of time that has passed. Thus we can say how old something is. For Example this metamorphic rock is 3. What can we say and learn from these excavations?
Relative age of trash layers - Because of the shape of the pits the oldest layers of trash occur below younger layers i. Thus the relative age of the trash layers is, in order from youngest to oldest. The civilizations that deposited the trash had a culture and industrial capabilities that evolved through time. The oldest inhabitants used primitive stone tools, later inhabitants used cups made of ceramics, even later inhabitants eventually used tin cans and then changed to Aluminum cans, and then they developed a technology that used computers.
This shows that society has evolved over the years. Similar cultures must have existed in both areas and lived at the same time. Thus we can make correlations between the layers found at the different sites by reasoning that layers containing similar distinctive discarded items artifacts were deposited during the same time period. Because the Ceramic Cups layer is found at the Tulane site, but not at the Zoo site, the civilization that produced the Ceramic cups probably did not live in the Zoo area.
Thus, we can recognize a break in the depositional sequence at the Zoo site. The surface marking the break in deposition would be called an unconformity in geologic terms, and represents time missing from the depositional record. The trash pits contain some clues to numeric age : The Tulane trash pit has an old license plate in the Tin Cans layer. This plate shows a date of , thus the Tin Cans layer is about 67 years old. The date on the doubloon is Thus the Al Cans layer is about 37 years old.
Principles of Stratigraphy Stratigraphy is the study of strata sedimentary layers in the Earth's crust. Principle of Uniformitarianism The principle of Uniformitarianism was postulated by James Hutton who examined rocks in Scotland and noted that features like mudcracks, ripple marks, graded bedding, etc.
Principle of Superposition Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.
Principle of Original Horizontality Sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface.
Principle of Original Continuity If layers are deposited horizontally over the sea floor, then they would be expected to be laterally continuous over some distance. These organisms include mollusks, echinoids, and corals. Limestone is found in beds, and most limestone beds form in marine environments in which big deposits of organisms and carbonate precipitation build up over the years, like an ocean or large lake. Sandstone is shaped from the breakdown of larger rocks due to weathering and erosion as well as from processes that occur inside the rock, usually biologic but now and again chemical in nature.
Sandstone is often categorized based totally on the composition and kind of grain it consists of in large quantity. Ferruginous sandstone, for example, denotes excessive iron content in its material composition.
More superior classifications, utilized by geologists, combine descriptions of texture and composition. Limestone can be informally described by the sort of carbonate it consists of—along with calcite, aragonite, and dolomite. Geologists use more complex classifications to describe limestone based totally on texture, formation, and composition.
Many sedimentary rocks, including sandstone, display a visible stratification into layers. This visual cue can help determine how a rock came to be primarily based at the size and intensity of each layer. Limestone does not have the stratification pattern that sandstone does. Some limestone is composed completely of organic matter that is impossible to see with just the naked eye.
They create some of the dramatic landscapes you can find across the U. As sedimentary rocks, they share certain similarities. However, their different origins and compositions make them unique. Limestone is defined as being primarily composed of calcium carbonate, which often comes from plant and animal material such as the shells of mollusks.
Sandstone is not defined by any one substance. It consists of sand sized particles, which range from 0. It often contains quartz,, though it does not have to. Other common components of sandstone include feldspar, mica, lithic fragments and biogenic particles.
Sandstone is formed from the breakdown of larger rocks due to weathering and erosion as well as from processes that occur within the rock, usually biologic but sometimes chemical in nature.
Limestone often forms from whole or pieces of a variety of organisms that contain calcium carbonate, such as mollusks, echinoids and corals. Most limestone beds form in marine environments where large deposits of organisms and carbonate precipitation build up over time.
Sandstone is often classified based on the type of grain it contains in large quantity. For example, ferriginous sandstone denotes a high iron content.
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