Earth Floor: Geologic Time
Relative dating tells us the sequence in which events occurred, not how long ago they occurred. Draw it out! Absolute Dating – • the process of establishing the age of an object by determining the number of years it has. My Time Scale: Introduction to Relative and Absolute Dating Activity With Steno's law of superposition, geologists could identify the relative age of various Students should draw a horizontal line across the eon, era and period columns . The first method of finding the absolute age of an object is by examining tree rings. If we looked at a cross-section ofa tree or log we would notice that all through.
On the one hand, the half life should be short enough so that a measurable amount of the daughter element has formed. On the other hand, if the half life is too short, the amount of parent element left may not be measurable. Thus, K-Ar dating would not be appropriate for a material that is 50, years old, as hardly any daughter element would have formed. Similarly, C dating is not be appropriate for materials older than about 70, years as the amount of the parent element left becomes too small to be measured accurately.
Radiometric dating depends on certain assumptions. The most fundamental assumption is that the half life of a parent-daughter pair does not change through time.
Experimentally and theoretically, that assumption seems justified. Also, successful cross-checking of ages using different dating techniques on the same sample supports the constancy of half lives.
For example, C dates may be checked against ages determined through varve counting. A second assumption is that the system is closed.
That is, no parent or daughter material has been added to or lost from the material being dated. Such addition or subtraction may occur if the material mineral or rock has been weathered or metamorphosed. Therefore, material to be dated must be carefully examined to determine whether such processes may have taken place. Because the dating method depends upon comparing the ratio of parent to daughter element, the assumption must be made that the amount of daughter element initially present be zero or else be determinable.
Igneous rocks and highly metamorphosed rocks are the best candidates for radiometric dating because for them, for reasons that won't be discussed here, it can relatively easily be determined whether the initial amount of daughter element present was zero or, if it wasn't zero, what was the initial amount.
Geologic Age Dating Explained
The 'age' of an igneous rock refers to the time when the magma or lava from which it formed cooled below a certain temperature. A useful material for dating that time is the mineral zircon, a minor but common constituent of igneous rocks.
As magma or lava solidifies, the elements zirconium Zrsilicon Si and oxygen O link together to form zircon crystals. If uranium U atoms are in the vicinity, they may be incorporated into the zircon in place of Zr atoms. This substitution is possible because the size and charge of the U is similar to that of Zr.
That is, the U can 'fit' in the sites normally occupied by Zr. Any lead Pb in the vicinity cannot be incorporated in the zircon because it can't 'fit' in any of the sites. Assuming the zircon has not been affected by weathering or metamorphism, any Pb subsequently found in the zircon must have come from decay of the U; it was not there to start with.
It is true that not all minerals that crystallize from a magma or lava form simultaneously, but except for extremely young igneous rocks, the time required for solidification is very short compared compared to the age of the rock. Accurate radiometric dating of metamorphic rocks is more difficult. During metamorphism, preexisting minerals may be altered and new minerals may be formed.
For preexisting minerals, there is the distinct possibility that during metamorphism, parent or daughter elements may have been added or lost. If this happens, attempts to determine an accurate original premetamorphic age of the material will be frustrated.
For example, loss of some of the daughter element will give a deceptively young age; addition of daughter element will give a deceptively old age. However, if the rock is highly metamorphosed, the situation is more propitious.Anthropology optional for UPSC - Dating methods like relative and absolute dating for finding age
For example, in the K-Ar system, all of the daughter element Ar may be lost from some preexisting minerals. Or else, completely new mineral grains may develop that contain the parent element K but totally lack the daughter element Ar.
In either case, these minerals constitute new 'closed' systems with zero initial daughter element and, if dated, give the age of the metamorphic event. The age of a sedimentary rock refers to the time when loose sediment is turned into rock becomes 'lithified'. Sedimentary rocks are varied and complex, but for many of them, the sedimentary particles out of which they are made consist of material eroded from prexisting rocks. After transportation and deposition, the particles are bound together in some fashion, perhaps by a 'cement'.
Those processes do not reset the clock: Thus, if the particles are dated, the ages obtained refer to the ages of the rock from which they were derived. In consequence, for many sedimentary rocks, the constitutent grains have widely varying ages.
To get the age of the sedimentary rock itself, the material dated has to have formed at the time of consolidation of the rock. For most sedimentary rocks, there is no such material that is datable contains suitable parent-daughter elements.
Sedimentary rocks must, therefore, be dated by 'bracketing'. The method involves determining the absolute ages of slightly younger and slightly older objects to set limits within which the unknown age must lie. You know that Agnes is a 'middle child', younger than her sister Mary, who has just turned 7, and older than her brother John, who is 4 and about to celebrate his 5th birthday.
With this knowledge, you have Agnes' age 'bracketed'. Agnes is more than 4 and less than 7 years old. Sedimentary rocks whose absolute ages can't be determined directly may be established by dating associated lava flows. In the example to the right, the numbered layers are sedimentary rocks.
After deposition of layers 12 and 13, lava flow 'X' was erupted. Then, layers were deposited, followed by eruption of lava flow 'Y'. Finally, layers 17 and 18 were deposited. Radiometric dating of the lava flows established their ages as 2 million years for 'Y' and 1 million years for 'X'. Using the Law of Superposition: Layers 12 and 13 must be older than 2 million years. Layers 14 to 16 must be younger than 2 million years but older than 1 million years. Layers 17 and 18 must be younger than one million years.
In the example to the right, after sedimentary layers 21 to 25 were deposited, lava flow 'Q' was erupted. This use of fossils to identify the order of rock layers in the overall stratigraphic column allowed Smith to create a complete geologic map of England. Moreover, the principle of faunal succession allowed geologists worldwide to organize rocks and fossils into their relative temporal order. Based on these observations, major differences in the types of fossilized organisms found led scientists to organize the time scale into broad categories called eons, then subcategories called eras, and so on through finer and finer divisions of time.
Student Prerequisites None Activity Procedures 1. Begin the day with a discussion of time. How do you think about historical events? Do you categorize events by decade? Do you categorize events by major turning points in history like the American Revolution and the Civil War? How about events in your own life? Do you categorize things by what school you attended?
Do you categorize things by where you lived? Do you categorize things relative to major family events like the birth of a sibling or a divorce? Today we will be organizing events like geologists do and become familiar with the format of the time scale used to organize information about Earth history.
Pass out the handouts and explain the general directions described on the first page: Create a list of at least 10 major events in your lifetime b. Place a number beside each event according to which happened longest ago 1 and which happened most recently 10 c.
- Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
- Pre/Post-Test Key
Rewrite the list in reverse chronological order, with the most recent at the top and the most long ago at the bottom d. To the new list, add information about exactly how many years ago the event occurred 4.
My Time Scale: Introduction to Relative and Absolute Dating Activity
The list so far is what geologists refer to as a relative or sequential timeline. Pretty obvious that the dike came after the rocks it cuts through, right? With absolute age dating, you get a real age in actual years.
Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.
The narrower a range of time that an animal lived, the better it is as an index of a specific time. No bones about it, fossils are important age markers. But the most accurate forms of absolute age dating are radiometric methods. This method works because some unstable radioactive isotopes of some elements decay at a known rate into daughter products.
Geologic Age Dating Explained - Kids Discover
This rate of decay is called a half-life. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: There are a couple catches, of course. Not all rocks have radioactive elements.
Sedimentary rocks in particular are notoriously radioactive-free zones. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.
You might have noticed that many of the oldest age dates come from a mineral called zircon.