Characteristics of Metamorphic Rocks

Characteristics of Metamorphic Rocks

The Radiometric Dating Game Radiometric dating methods estimate the age of rocks using calculations based on the decay rates of radioactive elements such as uranium, strontium, and potassium. On the surface, radiometric dating methods appear to give powerful support to the statement that life has existed on the earth for hundreds of millions, even billions, of years. We are told that these methods are accurate to a few percent, and that there are many different methods. We are told that of all the radiometric dates that are measured, only a few percent are anomalous. This gives us the impression that all but a small percentage of the dates computed by radiometric methods agree with the assumed ages of the rocks in which they are found, and that all of these various methods almost always give ages that agree with each other to within a few percentage points. Since there doesn’t seem to be any systematic error that could cause so many methods to agree with each other so often, it seems that there is no other rational conclusion than to accept these dates as accurate. However, this causes a problem for those who believe based on the Bible that life has only existed on the earth for a few thousand years, since fossils are found in rocks that are dated to be over million years old by radiometric methods, and some fossils are found in rocks that are dated to be billions of years old.

THE END of LONG AGE RADIOMETRIC DATING

Geologic Time The most obvious feature of sedimentary rock is its layering. This feature is produced by changes in deposition over time. With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?

Dating the particles which make up the rock wouldn’t give you the age of the rock itself. In addition, the redeposition process upsets the conditions necessary to achieve accurate results through radiometric dating. Scientists believe they can indirectly date sedimentary rocks using radiometric dating if they find igneous or metamorphic rock imbedded in or around a sedimentary rock layer.

Advanced Search Abstract Mapping in the Karas Mountains area of southern Namibia has shown the presence of a major lithotectonic boundary at the northern margin of the Namaqua-Natal Metamorphic Province. The area is subdivided into the Northern Volcanic Zone largely comprising mafic, intermediate and felsic volcanic and intrusive rocks metamorphosed to greenschist to amphibolite facies, and the Southern Sedimentary Zone, dominated by pelitic rocks, metamorphosed at amphibolite to granulite facies, and intruded by syntectonic granites.

The intervening Lord Hill Boundary Zone is characterized by anastomosing mylonite belts with dips and lineations plunging steeply to the south, within less-deformed metalavas and intrusions similar in character to those of the Northern Volcanic Zone. The intrusive and metavolcanic rocks of the Northern Volcanic and Lord Hill Boundary Zones have geological characteristics of present-day volcanic arcs.

The age and tectonic setting are very similar to those of the Jannelsepan and Copperton Formations in South Africa, and point to an extensive Mesoproterozoic volcanic arc at the southern edge of the Kalahari Craton.

Geologic Age Dating Explained

The initial ratio has particular importance for studying the chemical evolution of the Earth’s mantle and crust, as we discussed in the section on igneous rocks. K-Ar Dating 40K is the radioactive isotope of K, and makes up 0. Since K is one of the 10 most abundant elements in the Earth’s crust, the decay of 40K is important in dating rocks. But this scheme is not used because 40Ca can be present as both radiogenic and non-radiogenic Ca.

Radiometric age dating works best on igneous, volcanic, or metamorphic rocks. It is important to select “fresh” rock that has not been chemically or structurally altered by deformation, weathering, hydrothermal alteration, or metamorphism.

That camel skull was found associated with a particular layer of sedimentary rock — sandstone, mudstone, limestone, etc. Absent some colossal fluke, it is almost certain that the camel lived at the same time that the rock layer was being deposited; in other words, the rock and the fossil are the same age. So the question becomes, how old is the rock layer, and how do we know?

What follows is an oversimplified overview. Classification of Rocks Geologists classify rocks into three large families — igneous, sedimentary and metamorphic. Igneous rocks make up volcanoes and the backbones of many mountain ranges. They form by the freezing of a liquid material from deep in the earth. Basalt and granite are examples of igneous rock.

Absolute Age

Absolute ages are much different from relative ages. The way of determining them is different, too. Absolute ages are determined by radiometric methods, such as carbon dating. These methods depend on radioactive decay. Radioactive Decay Radioactive decay is the breakdown of unstable elements into stable elements. To understand this process, recall that the atoms of all elements contain the particles protons, neutrons, and electrons.

Where thick sequences of sedimentary rock layers have been deposited in large basins, the deepest layers at the bottoms of the sequences may subsequently have become folded by earth movements when subjected to elevated temperatures and pressures that were sufficient to transform them into metamorphic rocks.

April Koch April teaches high school science and holds a master’s degree in education. Discover how geologists study the layers in sedimentary rock to establish relative age. Learn how inclusions and unconformities can tell us stories about the geologic past. We’ll even visit the Grand Canyon to solve the mystery of the Great Unconformity!

Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time. Now imagine that you come upon a formation like this: Example of a rock layer that is not smooth or parallel What do you think of it? How do you study it? How can you make any conclusions about rock layers that make such a crazy arrangement? Geologists establish the age of rocks in two ways: Numerical dating determines the actual ages of rocks through the study of radioactive decay.

Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another. Relative dating requires an extensive knowledge of stratigraphic succession, a fancy term for the way rock strata are built up and changed by geologic processes.

The radiometric dating of an igneous rock provides..?

The principle states that in a sequence of undeformed sedimentary rocks the oldest beds are at the bottom and the youngest ones are at the top. Underlying assumptions are 1 that layers were originally deposited horizontally, 2 and that beds are not overturned sedimentary structures can be used to dermine whether a sedimentary succession is overturned or not.

More on index fossils here.

Relative dating utilizes six fundamental principles to determine the relative age of a formation or event. The first principle is the Principle of Superposition which states that in an undisturbed succession of sedimentary rock, the oldest layers are on the bottom.

It is captivating and compelling…covers all the bases. Igneous and metamorphic rocks, which were once extremely hot and have cooled into solid rock, are dated using the following methods: However, these methods make the following assumptions: Isotopes are several different forms of the same element. An isotope of uranium can decay into an isotope of another element at a certain rate. For example, uranium parent isotope decays into thorium , which itself is unstable, and the process continues until the atom changes into lead daughter isotope , which is stable.

Advocates of these methods assume that melting or liquid magma at the beginning resets the age clock to zero that is, there are no daughter isotopes in the rock at the time of cooling or solidification. If the rate of decay from uranium to lead is constant with a half-life of 4, million years , and if we know the amount of daughter isotopes in the rock at the beginning, and if we can measure how much parent and daughter isotopes are present in a given rock sample, we can supposedly calculate the age of the rock.

Relative Vs. Absolute Dating: The Ultimate Face-off

What is an example of radiometric dating? Carbon 14 dating is the best known example of radiometric dating, but there are many others. Another example of radiometric dating is the dating of the age of geological formations on earth. The oldest known rocks on the earth that have been analyzed, have been dated back some 4.

May 01,  · Metamorphic Rocks: Accurate radiometric dating of metamorphic rocks is more difficult. During metamorphism, preexisting minerals may be altered and new minerals may be formed. During metamorphism, preexisting minerals may be altered and new minerals may be formed.

Read each question carefully before selecting the BEST answer. Provide specific and detailed answers to essay questions. A Half life is the number of atoms that decay per unit of time. Radiometric dating means placing events in their proper sequence. After four half-lives there is no longer any of the original radioactive material remaining. Inclusions are pieces of one rock contained within another.

The radioactive isotope, potassium , has Argon as a daughter product. The percentage of radioactive atoms that decay during one half-life will change.

Why is it almost impossible to date sedimentary rocks using radiometric dating techniques?

Venus Absolute Age The standard geologic time scale was devised according to relative time relationships observed in rocks across the world. Determining the actual ages of these time spans, and thus establishing the beginning and ending dates of geologic eons, eras, periods, and epochs, became possible with the discovery of radioactivity.

Radioactive elements decay at known rates of speed. This radioactive decay begins after the elements are locked into crystalline mineral structures. Some elements have variations called isotopes, which are atoms that contain different numbers of neutrons in their nuclei.

dating sedimentary rocks. Sedimentary rock, rock formed at or near the earth s surface by the accumulation and lithification of sediment detrital rock or by the precipitation from solution at normal surface temperatures chemical ntary dating sedimentary rocks rocks are the rocks exposed on the earth s surface but are only a minor constituent of the entire crust, which is dominated.

Dinosaurs disappeared about 65 million years ago. That corn cob found in an ancient Native American fire pit is 1, years old. How do scientists actually know these ages? Geologic age dating—assigning an age to materials—is an entire discipline of its own. In a way this field, called geochronology, is some of the purest detective work earth scientists do. There are two basic approaches: Here is an easy-to understand analogy for your students: Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old.

To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This is called the Rule of Superposition. This rule is common sense, but it serves as a powerful reference point. Geologists draw on it and other basic principles http: Relative age dating also means paying attention to crosscutting relationships.

relative dating 7th

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The commonly accepted billion year age of the earth is derived from radiometric dating of lunar rocks and meteorites in addition to dating methods based on the Gerling-Holmes-Houtermans model.

These are K-Ar data obtained on glauconite, a potassium-bearing clay mineral that forms in some marine sediment. Woodmorappe fails to mention, however, that these data were obtained as part of a controlled experiment to test, on samples of known age, the applicability of the K-Ar method to glauconite and to illite, another clay mineral. He also neglects to mention that most of the 89 K-Ar ages reported in their study agree very well with the expected ages.

Evernden and others 43 found that these clay minerals are extremely susceptible to argon loss when heated even slightly, such as occurs when sedimentary rocks are deeply buried. As a result, glauconite is used for dating only with extreme caution. The ages from the Coast Range batholith in Alaska Table 2 are referenced by Woodmorappe to a report by Lanphere and others Whereas Lanphere and his colleagues referred to these two K-Ar ages of and million years, the ages are actually from another report and were obtained from samples collected at two localities in Canada, not Alaska.

There is nothing wrong with these ages; they are consistent with the known geologic relations and represent the crystallization ages of the Canadian samples.

Geochronology

Thus, the layers are successively younger, going from bottom to top. The convention in geology is to number the layers beds within a sequence such that the oldest layer has the lowest number. In the illustration, layer 1 was deposited at time 1. At time 2, layer 2 was deposited on top of layer 1. At time 3, layer 3 was deposited on top of layer 3.

Age-dating of metamorphic rocks also is usually accomplished using radioactive materials. The processes of determining the age of metamorphic rocks often can be fraught with formidable difficulties. Fortunately, vanishingly few fossils occur in metamorphic rocks, so we can pass on quickly to – sedimentary rocks.

April Koch April teaches high school science and holds a master’s degree in education. Learn how scientists determine the ages of rocks and fossils. We’ll explore both relative and numerical dating on our quest to understand the process of geological dating. Along the way, we’ll learn how stratigraphic succession and radioactive decay contribute to the work of paleontologists. Dating Dinosaur Fossils Consider the following scenario: Paul the Paleontologist is a very famous scientist who has studied dinosaur bones all over the world.

Recently, he appeared on the evening news to talk about a new dinosaur he just discovered. The dinosaur is called superus awesomus. Paul says he can tell from the fossils that superus awesomus lived on Earth about million years ago. Paul is super awesome, so I’m going to take him at his word.

40) Metamorphic Chemistry 1


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