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.
Age Dating the Earth
At the time that Darwin’s On the Origin of Species was published, the earth was “scientifically” determined to be million years old. By , it was found to be 1. In , science firmly established that the earth was 3. Finally in , it was discovered that the earth is “really” 4. In these early studies the order of sedimentary rocks and structures were used to date geologic time periods and events in a relative way.
At first, the use of “key” diagnostic fossils was used to compare different areas of the geologic column.
The theory that all geologic phenomena may be explained as the result of existing forces having operated uniformly from the origin of the earth to the present time.
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. If these dates are correct, this calls the Biblical account of a recent creation of life into question. After study and discussion of this question, I now believe that the claimed accuracy of radiometric dating methods is a result of a great misunderstanding of the data, and that the various methods hardly ever agree with each other, and often do not agree with the assumed ages of the rocks in which they are found.
I believe that there is a great need for this information to be made known, so I am making this article available in the hopes that it will enlighten others who are considering these questions. Even the creationist accounts that I have read do not adequately treat these issues. At the start, let me clarify that my main concern is not the age of the earth, the moon, or the solar system, but rather the age of life, that is, how long has life existed on earth. Many dating methods seem to give about the same ages on meteorites.
Thus radiometric dating methods appear to give evidence that the earth and meteorites are old, if one accepts the fact that decay rates have been constant.
Age Dating, Educational Resources for K
Page with 4 varve sections x17 Download each PDF page. For one class set, print 4 copies of the 10mm rulers and 1 page each of the 4 pages of varve sections. Cut the rulers and varve sections apart. Each varve section has a number You could put each varve section and an enlarged ruler into a plastic ziplock bag or envelope – for easy handout to each pair – and for easy storage.
Students can work in pairs, lay the enlarged 10mm ruler across the layers on their enlarged varve section so they can count the number of varves in say a 5mm segment, and record that number as x out of 5.
AGE OF THE EARTH. So far scientists have not found a way to determine the exact age of the Earth directly from Earth rocks because Earth’s oldest rocks have been recycled and destroyed by the process of plate tectonics.
Fossils and Geologic Time Fossils Fossils are physical evidence of preexisting organisms, either plant or animal. The most common and obvious fossils are the preserved skeletal remains of animals. Other fossils, which are also evidence of past organisms, include leaf impressions, tracks and trails, burrows, droppings, and root casts. Microfossils are the microscopic skeletons of previously existing plants or animals, and their examination requires an optical or an electron microscope for close study.
A very small fraction of the organisms that have lived on the Earth is found in the fossil record: Many did not possess skeletons or other hard parts that could be preserved; many did not survive the process of fossilization, wherein skeletons and tissues are replaced by minerals; and many were subsequently destroyed either by chemical or physical processes such as recrystallization, metamorphism, or erosion.
Fossils of any kind are useful in “reading the rock record,” meaning they help us decipher the history of the earth. They can help us determine the geologic age and environment the paleoenvironment in which they were deposited. Finally, and if the fossil record is complete enough, their study can help us better understand the evolution or progression of life through geologic time. Our understanding of the meaning of fossils is based on the accumulated knowledge from previous generations of investigators who carefully recorded the identity and distribution of fossils from numerous geologic exposures or samples from wells and recorded their findings in the scientific literature.
Geology[ edit ] The regular order of the occurrence of fossils in rock layers was discovered around by William Smith. While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers. As he continued his job as a surveyor , he found the same patterns across England. He also found that certain animals were in only certain layers and that they were in the same layers all across England.
Due to that discovery, Smith was able to recognize the order that the rocks were formed.
Mar 03, · Overview of the mechanics of radioactive decay as associated with radiometric or absolute rock dating. Coverage of key definitions (half-life, parent atom, daughter atom, etc).
So, how do we know how old a fossil is? There are two main methods determining a fossils age, relative dating and absolute dating. Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages. Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.
Relative Dating The majority of the time fossils are dated using relative dating techniques. Using relative dating the fossil is compared to something for which an age is already known. For example if you have a fossil trilobite and it was found in the Wheeler Formation. The Wheeler Formation has been previously dated to approximately million year old, so we know the trilobite is also about million years old. Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.
Index fossils are fossils that are known to only occur within a very specific age range. Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils. If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil.
Reliability of Geologic Dating
Richard Cowen’s Chapter Eight: Leaving the Water – images – curent page , , to , , years ago Eurypterids, otherwise known as sea scorpions. Wikipedia Late Ordovician survivals and extinctions: There were no land animals and extinctions were confined to water life.
Radiometric dating (often called radioactive dating) is a way to find out how old something is. The method compares the amount of a naturally occurring radioactive isotope and its decay products, in samples.
Mineralogy As a discipline, mineralogy has had close historical ties with geology. Minerals as basic constituents of rocks and ore deposits are obviously an integral aspect of geology. The problems and techniques of mineralogy, however, are distinct in many respects from those of the rest of geology, with the result that mineralogy has grown to be a large, complex discipline in itself. Nepheline greasy light gray , sodalite blue , cancrinite yellow , feldspar white , and ferromagnesian minerals black in an alkalic syenite from Litchfield, Maine, U.
About 3, distinct mineral species are recognized, but relatively few are important in the kinds of rocks that are abundant in the outer part of the Earth. Thus a few minerals such as the feldspars, quartz, and mica are the essential ingredients in granite and its near relatives. Limestones , which are widely distributed on all continents, consist largely of only two minerals, calcite and dolomite. Many rocks have a more complex mineralogy, and in some the mineral particles are so minute that they can be identified only through specialized techniques.
It is possible to identify an individual mineral in a specimen by examining and testing its physical properties. Determining the hardness of a mineral is the most practical way of identifying it.
More Bad News for 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? Although there might be some mineral differences due to the difference in source rock, most sedimentary rock deposited year after year look very similar to one another.
This means that a quartz sandstone deposited million years ago will look very similar to a quartz sandstone deposited 50 years ago. Making this processes even more difficult is the fact that due to plate tectonics some rock layers have been uplifted into mountains and eroded while others have subsided to form basins and be buried by younger sediments.
Geologic age dating 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: relative geologic age dating, and absolute geologic age dating.
Are they always accurate? When a new fossil is discovered, geologists assign a date for when they think the plant or animal lived. They normally use radiometric dating methods to date the fossil, and many promote these methods as being accurate. Yet when you look into the technical papers on these discoveries, you find that these dates are often questionable and are sometimes clearly in error.