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Copy of Studying the past

fossils and dating

April Kincaid

on 20 August 2013

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Transcript of Copy of Studying the past

Chapter 29
Fossils & Dating

29.1 Fossils
29.2 Relative Dating
Relative Dating
Relative dating is the process of placing events in the sequence in which they occurred.
Simply stated, this dates events relative to other events, ex. - We have first period, followed by second period, then break and so on.
In the Principle of superposition, the oldest layers of rock are at the bottom and the youngest at the top of a strata.
A strata is a sequence of layers of rock and sediments.
In the Principle of Cross Cutting Relationships, a magma intrusion cuts across the horizontal strata. The intrusion is always younger than the surrounding layers.
Embedded Fragments are rocks within rocks such as pebbles in a conglomerate. These pebbles or embedded fragments are always older than the conglomerate.
Formation of Fossils
Fossils include shells, bones petrified trees, impressions, and any preserved traces of prehistoric life. Paleontologists study these fossils. Paleontology is the study of life that existed in ancient times.
A fossil is any evidence of earlier life preserved in rock
Original remains are the unchanged remains of plants or animals. In rare cases, organisms may be preserved entirely. Examples of this include woolly mammoths preserved in permafrost, or insects preserved in amber.
Replaced remains are formed when remains are slowly replaced on a molecular level into rock forming minerals. Frequent examples are in shells, bones, teeth, and petrified wood.
Molds and Casts are another form of fossils.
A mold is a hollow depression in rock formed when a fossil dissolves out of the rock.
A cast is formed when minerals seep into a mold forming a copy of the original fossil.
Trace Fossils may include trails, bite marks, burrows, and any other indirect evidence of life. From trace fossils, scientists can better determine a dinosaurs (or that of other prehistoric life) habits and life.
29.3 Absolute Time
Carbonaceous Films are composed of a thin carbon film resulting in a detailed image resembling a silhouette. This occurrs when carbon compounds undergo chemical changes brought by heat and pressure.
Fossil Review
Relative dating places events in sequence, however absolute time places events in actual time. Absolute Time - is the method of recording events that identifies the actual date of an event.
Identify two ways remains may be fossilized.
Describe how palaeontologists use trace fossils to make inferences about the past.
Compare and contrast Original Remains, Replaced Remains, Molds, and Casts.
Gaps in Relative Time
Other ways of finding the rock record is by studying unconformities. An unconformity indicates where layers of rock are missing in the strata sequence. This can be caused by a layer of rock eroding away or shifted to another place. This can be related to missing pages in a book.
Angular unconformities are caused when the horizontal strata is tilted during uplift. Over time, the exposed layers on top erode and a new layer forms on top of it.
Disconformity is caused when top layers of a strata are eroded away and new layers form on top. This is much like an angular unconformity however, in a disconformity, no tilting occurs.
Nonconformities are the boundaries between different stratas such as between a tilted strata and a new layer in an unconformity.
''A" is a Folded rock strata cut by a thrust fault.
"B" Is a large intrusion
"C" is an angular unconformity on which a strata was deposited
"D" is a volcanic dike
"E" is the youngest strata
"F" is a normal fault
By examining layers of rocks, scientists can get an idea of geologic history and the relative time of prehistoric events.
Rock Layer Correlation
Correlation is the matching of rock layers from one area to another. For example, you can find the same strata in the Grand Canyon in Zion National Park.
Historical Methods
Scientists have many ways of examining these similar stratas. The most direct of which is simply walking the outcrop, which is simply walking over an area to observe the strata.
Matching rock characteristics is useful when examining a strata. Things such as color and composition enable scientists to recognize and correlate stratas.
Index Fossils are the remains of organisms that lived and died within a particular time segment of Earth's history. These fossils can be correlated much like matching rock characteristics in stratas. To be useful, index fossils must be unique in such a way that they are easily recognisable, they must also be abundant, widespread in occurrence, and found over a broad geographical area.
Fossils can also be used as environmental indicators. This allows scientists to see what environments existed in a particular area in a particular time. For instance, if you found a fossil of coral in the desert, you could assume that the area was once covered in warm shallow water.
A key bed is a single rock layer that, like an index fossil, is unique, easily recognizable, and widespread. Large volcanic eruptions make great key beds as well as impacts of large meteorites.
Stratigraphic matching is simply matching strata sequences. So, if a layer of sandstone sandwiched with volcanic rock is found in two separate places it would be called stratigraphic matching.
29.2 Section Review
What are the four characteristics of an index fossil?
How do unconformities represent gaps in geological time?
Would you expect to find dinosaur fossils above or below human fossils in an undisturbed strata sequence?
Before the 20th century, scientists had limited means of measuring absolute time. Some of these now primitive methods included estimating erosion rates of sediments (which were not constant.).
Another method is counting tree rings. This method is still useful today. Each ring is equal to one year.
A third method is counting varves. A varve is any sediment that is deposited on a yearly cycle. Much like counting tree rings, varves can be found in glacial samples and other bodies of water.
In the mid 1900s, scientists use radioactive isotopes to measure absolute time. Radioactive decay is the process of radioactive isotopes emit or capture tiny particles
Glacial Sample
Tree Rings
Each time a particle is emmited or captured, the isotope changes. Radiation is released until a stable isotope (without radiation) is formed. The original isotope is called the parent isotope while the product of decay is called the daughter isotope.
Half life
Radioactive elements decay at constant rates that are not affected by the isotopes enviornment. The moment igneous rock crystallizes its isotopes begin to decay. The ratio of radioactive element to stable element can be used to determine the absolute age of the rock.
The rate at which a radioactive element decays is called its half-life.
Half-life is the time it takes for half the radioactive atoms in a sample to decay to a stable product. After the first half life, half of the
radioactive element will have decayed and half would
remain. In the next half life, Half of the remaining
radioactive atoms will decay and so on. No matter
how small the sample, the half life remains the same
and it will decay into a stable product.
Radiometric Dating
Scientists can use radioactivity and half-lives to measure absolute time. In radiometric dating, scientists measure the amounts of a parent and a daughter isotope within a rock or mineral and use the ratio of the two to find the age of the rock.
Radio carbon dating is used to find the absolute time of the existence of prehistoric animals or tools. Living things take in radioactive carbon 14 and non-radioactive carbon 12. These remain constant until death when the carbon 14 begins to decay (Beta). Scientists can measure the ratios of carbon 14 and carbon 12 to tell when the plant or animal died. carbon 14 has a half life of 5730 years. This method can only be used on previously living things and is limited to dating things up to 70,000 years old.
There are three main types of decay: Alpha decay, beta decay, and electron capture.. In alpha decay, an alpha particle (two protons and two neutrons) are emitted. In beta decay, a neutron disintegrates into a proton and an electron (or beta particle) and the electron is emitted. In electron capture, a proton captures an electron and becomes a neutron.
Uranium-Lead dating uses the (Alpha, Beta) decay of radioactive uranium-238 to the stable Lead-206. Uranium-238 has a half-life of 4.5 billion years. This makes it possible to use this method to date some of the oldest rocks on the planet. Uranium-238 is usually found in igneous rocks. Because of its long half-life, Uranium-led dating is not accurate for dating rocks younger than 10 million years old.
29.3 review questions
Rubidium-strontium dating uses the (beta) decay of radioactive rubidium-87 to strontium-87. The half-life of rubidium-87 is 47 billion years (more than ten times the Earth's age)! This method is best used for dating extremely old rocks. Rubidium common in feldspars and micas and can be used to date almost all igneous rock. If both U-238 and rubidium-87 is found in a mineral, they can be used to check each other for higher accuracy.
Potassium-argon dating uses the (beta capture) decay of potassium-40 to argon-40. Potassium has a half life of 1.3 billion years and can be used to date rocks as young as 50,000 years. Potassium-40 is very common and is found in metamorphic and sedimentary rocks as well as in igneous rocks. This method is useful when dating young rocks that can not be dated by uranium or rubidium.
What are the similarities between tree rings and varves?
If an igneous rock is more a billion years old what is the best methods you could use to find the exact age of a rock.
If you were to date a piece of petrified wood, would you use carbon-14 or potassium-argon dating? Why?
Thank you for watching our slide. :)
What are the three types of radioactive decay?
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