The geologic cross section below, illustrates a series of rock layers as they might appear in a roadcut or quarry wall. The rock layers and events are lettered. In order to determine the relative ages we use the four fundamental principles of relative age dating that were explained to you on the concepts page. Geologists think about cross sections of the earth like the side view of a layer cake. In general, the oldest units are on the bottom and the youngest units are on the top law of superposition. There can be lots of complexities, such as folding events, faulting, erosion events, intrusion by magma, etc. We put these events into the order in which they occurred, starting with the oldest, and working toward the youngest.
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The radiocarbon dating technique is thus useful for 57, years or so, about 10 half-lives back. Radiocarbon dating relies on daughter-to-parent ratios derived from a known quantity of parent 14 C. Early applications of carbon dating assumed the production and concentration of 14 C in the atmosphere remained fairly constant for the last 50, years.
However, it is now known that the amount of parent 14 C levels in the atmosphere. Comparisons of carbon ages with tree-ring data and other data for known events have allowed reliable calibration of the radiocarbon dating method.
Taking into account carbon baseline levels must be calibrated against other reliable dating methods, carbon dating has been shown to be a reliable method for dating archaeological specimens and very recent geologic events.
The work of Hutton and other scientists gained attention after the Renaissance see Chapter 1spurring exploration into the idea of an ancient Earth. In the late 19 th century William Thompson, a.
Lord Kelvin, applied his knowledge of physics to develop the assumption that the Earth started as a hot molten sphere. He estimated the Earth is 98 million years old, but because of uncertainties in his calculations stated the age as a range of between 20 and million years. This animation illustrates how Kelvin calculated this range and why his numbers were so far off, which has to do with unequal heat transfer within the Earth. Patterson analyzed meteorite samples for uranium and lead using a mass spectrometer.
The current estimate for the age of the Earth is 4. It is remarkable that Patterson, who was still a graduate student at the University of Chicago, came up with a result that has been little altered in over 60 years, even as technology has improved dating methods.
Radioactive isotopes of elements that are common in mineral crystals are useful for radioisotopic dating. Zircon is resistant to weathering which makes it useful for dating geological events in ancient rocks. During metamorphic events, zircon crystals may form multiple crystal layers, with each layer recording the isotopic age of an event, thus tracing the progress of the several metamorphic events.
Geologists have used zircon grains to do some amazing studies that illustrate how scientific conclusions can change with technological advancements. Zircon crystals from Western Australia that formed when the crust first differentiated from the mantle 4. The zircon grains were incorporated into metasedimentary host rocks, sedimentary rocks showing signs of having undergone partial metamorphism.
The host rocks were not very old but the embedded zircon grains were created 4. From other properties of the zircon crystals, researchers concluded that not only were continental rocks exposed above sea level, but also that conditions on the early Earth were cool enough for liquid water to exist on the surface. The presence of liquid water allowed the processes of weathering and erosion to take place.
Researchers at UCLA studied 4. Igneous rocks best suited for radioisotopic dating because their primary minerals provide dates of crystallization from magma. Detrital sedimentary rocks are less useful because they are made of minerals derived from multiple parent sources with potentially many dates. However, scientists can use igneous events to date sedimentary sequences. For example, if sedimentary strata are between a lava Liquid rock on the surface of the Earth.
Another example would be a 65 million year old volcanic dike A narrow igneous intrusion that cuts through existing rock, not along bedding planes. This provides an upper limit age on the sedimentary strataso this strata would be older than 65 million years.
Primary sedimentary minerals containing radioactive isotopes like 40 K, has provided dates for important geologic events. Thermoluminescence, a type of luminescence dating Luminescence aka Thermoluminescence : Radioisotopic dating is not the only way scientists determine numeric ages. Luminescence dating measures the time elapsed since some silicate mineralssuch as coarse- sediments of silicate mineralswere last exposed to light or heat at the surface of Earth.
All buried sediments are exposed to radiation from normal background radiation from the decay process described above. Some of these electrons get trapped in the crystal lattice of silicate minerals like quartz. When exposed at the surface, ultraviolet radiation and heat from the Sun releases these electrons, but when the minerals are buried just a few inches below the surface, the electrons get trapped again.
Samples of coarse sediments collected just a few feet below the surface are analyzed by stimulating them with light in a lab. This stimulation releases the trapped electrons as a photon of light which is called luminescence.
The amount luminescence released indicates how long the sediment has been buried. Luminescence dating is only useful for dating sediments young sediment that are less than 1 million years old.
In Utah, luminescence dating is used to determine when coarse-grained sediment layers were buried near a fault. This is one technique used to determine the recurrence interval of large earthquakes on faults like the Wasatch Fault that primarily cut coarse-grained material and lack buried organic soil A type of non-eroded sediment mixed with organic matter, used by plants.
Many essential elements for life, like nitrogen, are delivered to organisms via the soil. Apatite from Mexico. Fission Track: Fission track dating relies on damage to the crystal lattice produced when unstable U decays to the daughter product Th and releases an alpha particle.
These two decay products move in opposite directions from each other through the crystal lattice leaving a visible track of damage.
The image below shows a geologic cross section of the ground beneath our feet. 1) Identify the layers in order from oldest to youngest including the fault represented by "E". 2) Define how the Law of Superposition explains the relationship in the cross section. 3) Why is the absolute dating method more useful than the relative dating method? Homework # 4 Relative Dating Section: Name: Date: 1. The cross section below represents a portion of Earth's crust. Letters A through D are locations within the rock units. At which location is quartzite most likely found? A. A B. B C. C D. D 2. Base your answer(s) to the following question(s) on the geologic cross section below, whichFile Size: 1MB. Relative Dating and Cross Cutting Relationships. Do some searching on the Internet (or elsewhere) to find another simple geologic cross section of your choice. Include an image of this cross section in your write-up, and describe the order in which events occurred in the section.
This is common in uranium-bearing mineral grains such as apatite. The tracks are large and can be visually counted under an optical microscope.
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The number of tracks correspond to the age of the grains. Fission track dating has also been used as a second clock to confirm dates obtained by other methods. They may be actual remains of body parts rareimpressions of soft body parts, cast Material filling in a cavity left by a organism that has dissolved away.
The body parts of living organisms range from the hard bones and shells of animals, soft cellulose of plants, soft bodies of jellyfish, down to single cells of bacteria and algae. Which body parts can be preserved? The best environment for preservation is the ocean, yet marine processes can dissolve hard parts and scavenging can reduce or eliminate remains.
Thus, even under ideal conditions in the ocean, the likelihood of preservation is quite limited. For terrestrial life, the possibility of remains being buried and preserved is even more limited. In other words, the fossil record is incomplete and records only a small percentage of life that existed.
Although incomplete, fossil records are used for stratigraphic correlationusing the Principle of Faunal Successionand provide a method used for establishing the age of a formation on the Geologic Time Scale. Trilobites had a hard exoskeleton and are often preserved by permineralization. Remnants or impressions of hard parts, such as a marine clam shell or dinosaur bone, are the most common types of fossils. The original material has almost always been replaced with new minerals that preserve much of the shape of the original shell, bone, or cell.
The common types of fossil preservation are actual preservationpermineralizationmold Organic material making a preserved impression in a rock. Actual preservation is a rare form of fossilization where the original materials or hard parts of the organism are preserved.
Preservation of soft-tissue is very rare since these organic materials easily disappear because of bacterial decay. Examples of actual preservation are unaltered biological materials like insects in amber or original minerals like mother-of-pearl on the interior of a shell.
Another example is mammoth skin and hair preserved in post- glacial deposits in the Arctic regions. Rare mummification has left fragments of soft tissue, skin, and sometimes even blood vessels of dinosaurs, from which proteins have been isolated and evidence for DNA fragments have been discovered.
Mosquito preserved in amber. Permineralization in petrified wood. The shape of this cavity is an mold Organic material making a preserved impression in a rock. If the mold Organic material making a preserved impression in a rock.
Sometimes internal cavities of organisms, such internal cast Material filling in a cavity left by a organism that has dissolved away. If the chemistry is right, and burial is rapid, mineral nodules form around soft structures preserving the three-dimensional detail. This is called authigenic mineralization. Carbonized leaf. Trace fossils are indirect evidence left behind by an organism, such as burrows and footprints, as it lived its life.
Ichnology is specifically the study of prehistoric animal tracks. Dinosaur tracks testify to their presence and movement over an area, and even provide information about their size, gait, speed, and behavior.
Burrows dug by tunneling organisms tell of their presence and mode of life. Foot prints of the early crocodile Chirotherium. Evolution has created a variety of ancient fossils that are important to stratigraphic correlation.
The geologic cross section below, illustrates a series of rock layers as they might appear in a roadcut or quarry wall. The rock layers and events are lettered. In order to determine the relative ages we use the four fundamental principles of relative age dating that were explained to you on the concepts page. Exercise on Relative Dating and Geologic Cross Sections 1. Describe the geologic history of the cross section shown below. Note that the Precambrian basement (Unit A) has been dated by the U-Pb technique as having an age of + billion years.
The British naturalist Charles Darwin recognized that life forms evolve into progeny life forms. He proposed natural selection -which operated on organisms living under environmental conditions that posed challenges to survival-was the mechanism driving the process of evolution forward. The basic classification unit of life is the species : a population of organisms that exhibit shared characteristics and are capable of reproducing fertile offspring.
For a species to survive, each individual within a particular population is faced with challenges posed by the environment and must survive them long enough to reproduce. Within the natural variations present in the population, there may be individuals possessing characteristics that give them some advantage in facing the environmental challenges.
Relative dating geologic cross section
These individuals are more likely to reproduce and pass these favored characteristics on to successive generations. If sufficient individuals in a population fail to surmount the challenges of the environment and the population cannot produce enough viable offspring, the species becomes extinct.
The average lifespan of a species in the fossil record is around a million years.
That life still exists on Earth shows the role and importance of evolution as a natural process in meeting the continual challenges posed by our dynamic Earth. If the inheritance of certain distinctive characteristics is sufficiently favored over time, populations may become genetically isolated from one another, eventually resulting in the evolution of separate species. This genetic isolation may also be caused by a geographic barrier, such as an island surrounded by ocean.
Evolution is well beyond the hypothesis stage and is a well-established theory of modern science.
Historical Geology: Structure, Cross Section 1
Variation within populations occurs by the natural mixing of genes through sexual reproduction or from naturally occurring mutations. While some species in the fossil record show little morphological change over time, others show gradual or punctuated changes, within which intermediate forms can be seen. Image showing fossils that connect the continents of Gondwana the southern continents of Pangea.
relative dating practice 1. A geologic cross section is shown below. The most recently formed rock unit is at location A. A B. B C. C D. D 2. Base your answer(s) to the following question(s) on the geologic cross section below of the Grand Canyon. The numbers 1 through 10 represent Paleozoic sedimentary rock layers. Relative Dating. A Geologic Time Scale Relative dating is the process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages-i.e., Cross-section of the Permian El Capitan Reef at Guadalupe National Monument, Texas. The red line shows a chronostratigraphic time line that. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the other events represented in that sequence. A geologic cross section: Sedimentary rocks (A-C), igneous intrusion (D), fault (E). The principle of.
Wegener used correlation to help develop the idea of continental drift. Correlation is the process of establishing which sedimentary strata are of the same age but geographically separate.
Correlation can be determined by using magnetic polarity reversals Chapter 2rock types, unique rock sequences, or index fossils. There are four main types of correlation : stratigraphiclithostratigraphic, chronostratigraphic, and biostratigraphic. Stratigraphic correlation is the process of establishing which sedimentary strata are the same age at distant geographical areas by means of their stratigraphic relationship.
Geologists construct geologic histories of areas by mapping and making stratigraphic columns-a detailed description of the strata from bottom to top.
Based on the stratigraphic relationship, the Wingate and Moenave Formations correlate. These two formations have unique names because their composition and outcrop pattern is slightly different.
Other strata in the Colorado Plateau and their sequence can be recognized and correlated over thousands of square miles. Lithos is Greek for stone and -logy comes from the Greek word for doctrine or science. Lithostratigraphic correlation can be used to correlate whole formations long distances or can be used to correlate smaller strata within formations to trace their extent and regional depositional environments.
Stevens Arch in the Navajo Sandstone at Coyote Gulch some miles away from Zion National Park For example, the Navajo Sandstonewhich makes up the prominent walls of Zion National Park, is the same Navajo Sandstone in Canyonlands because the lithology of the two are identical even though they are hundreds of miles apart. Extensions of the same Navajo Sandstone formation are found miles away in other parts of southern Utah, including Capitol reef A topographic high found away from the beach in deeper water, but still on the continental shelf.
Typically, these are formed in tropical areas by organisms such as corals. Further, this same formation is the called the Aztec Sandstone in Nevada and Nugget Sandstone near Salt Lake City because they are lithologically distinct enough to warrant new names. Chronostratigraphic correlation matches rocks of the same age, even though they are made of different lithologies.
Different lithologies of sedimentary rocks can form at the same time at different geographic locations because depositional environments vary geographically. For example, at any one time in a marine setting there could be this sequence of depositional environments from beach to deep marine : beach, near shore area, shallow marine lagoonreef A topographic high found away from the beach in deeper water, but still on the continental shelf.
Each depositional environment will have a unique sedimentary rock formation. On the figure of the Permian Capitan reef A topographic high found away from the beach in deeper water, but still on the continental shelf.
All three of these unique lithologies were forming at the same time in Permian along this red timeline. Biostratigraphic correlation uses index fossils to determine strata ages.
Index fossils represent assemblages or groups of organisms that were uniquely present during specific intervals of geologic time. Assemblages is referring a group of fossils. Fossils allow geologists to assign a formation to an absolute date range, such as the Jurassic Period to million years agorather than a relative time scale.
In fact, most of the geologic time ranges are mapped to fossil assemblages. The most useful index fossils come from lifeforms that were geographically widespread and had a species lifespan that was limited to a narrow time interval. In other words, index fossils can be found in many places around the world, but only during a narrow time frame.
Some of the best fossils for biostratigraphic correlation are microfossilsmost of which came from single-celled organisms. As with microscopic organisms today, they were widely distributed across many environments throughout the world. Some of these microscopic organisms had hard parts, such as exoskeletons or outer shells, making them better candidates for preservation.
Foraminifera, single celled organisms with calcareous shells, are an example of an especially useful index fossil for the Cretaceous Period and Cenozoic era The second largest span of time recognized by geologists; smaller than a eon, larger than a period.
We are currently in the Cenozoic era. Rocks of a specific era are called eratherms. Conodonts are another example of microfossils useful for biostratigraphic correlation of the Cambrian through Triassic Periods. Conodonts are tooth-like phosphatic structures of an eel-like multi-celled organism that had no other preservable hard parts. The conodont-bearing creatures lived in shallow marine environments all over the world.
Upon death, the phosphatic hard parts were scattered into the rest of the marine sediments.
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These distinctive tooth-like structures are easily collected and separated from limestone in the laboratory. Artist reconstruction of the conodont animal right along side its teeth Because the conodont creatures were so widely abundant, rapidly evolving, and readily preserved in sedimentstheir fossils are especially useful for correlating strataeven though knowledge of the actual animal possessing them is sparse. Scientists in the s carried out a fundamental biostratigraphic correlation that tied Triassic conodont zonation into ammonoids, which are extinct ancient cousins of the pearly nautilus.
Up to that point ammonoids were the only standard for Triassic correlationso cross-referencing micro- and macro- index fossils enhanced the reliability of biostratigraphic correlation for either type.
If you find a fossil of a fish in a dry terrestrial environment did the fish flop around on land? Did the rock form in water and then move? Since fish do not flop around on land today, the explanation that adheres to the philosophy that natural laws do not change is that the rock moved.
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Ina young doctor named Nicholas Steno dissected the head of an enormous great white shark that had been caught by fisherman near Florence, Italy.
Most people at the time did not believe that fossils were once part of living creatures. Authors in that day thought that the fossils of marine animals found in tall mountains, miles from any ocean could be explained in one of two ways:. But for Steno, the close resemblance between fossils and modern organisms was impossible to ignore. Instead of invoking supernatural forces, Steno concluded that fossils were once parts of living creatures.
He then sought to explain how fossil seashells could be found in rocks and mountains far from any ocean. This led him to the ideas that are discussed below. Steno proposed that if a rock contained the fossils of marine animals, the rock formed from sediments that were deposited on the seafloor.
These rocks were then uplifted to become mountains. These laws are illustrated in Figure below. If a sedimentary rock is found tilted, the layer was tilted after it was formed.
When a valley cuts through sedimentary layers, it is assumed that the rocks on either side of the valley were originally continuous. The youngest layers are found at the top of the sequence, and the oldest layers are found at the bottom. Other scientists observed rock layers and formulated other principles. Geologist William Smith identified the principle of faunal succession, which recognizes that:. Scottish geologist, James Hutton recognized the principle of cross-cutting relationships.
This helps geologists to determine the older and younger of two rock units Figure below. If an igneous dike B cuts a series of metamorphic rocks Awhich is older and which is younger?
In this image, A must have existed first for B to cut across it.
The Grand Canyon provides an excellent illustration of the principles above. The many horizontal layers of sedimentary rock illustrate the principle of original horizontality Figure below. At the Grand Canyon, the Coconino Sandstone appears across canyons.
The Coconino is the distinctive white layer; it is a vast expanse of ancient sand dunes. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the other events represented in that sequence.
The relative age of a rock then is its age in comparison with other rocks. If you know the relative ages of two rock layers, 1 Do you know which is older and which is younger? In some cases, it is very tricky to determine the sequence of events that leads to a certain formation. The Canyon, incised by the Colorado River, is immense, averaging 4, feet deep for its entire miles. It is 6, feet deep at its deepest point and 15 miles at its widest.
The image below shows the layers of rock that are exposed in the canyon. This is a geologic cross section not drawn to scale that shows the stratigraphy of the Grand Canyon.
In other words, it represents what the rock layers would look like if you took a knife and sliced down through the Earth at a given location, like a piece of cake. Note that all three types of rock sedimentary, metamorphic, and igneous are present in this cross section. Note how the ages of the rock decrease in age from the bottom to the top.
Listen to scientists talk about the age of the Grand Canyon and the controversy over the dating.
Press the play button on the left of the page to listen. Transcripts available on the left menu as well.