How to do absolute age dating

The meltwaters carry away sediment that was trapped in the ice. Where the sediment-laden meltwaters flow into a lake, a layer of sediment is deposited on the lake floor every year. Coarse sediment is deposited quickly; fine sediment remains suspended in the water until it has a chance to gradually settle out during the cold months when the surface of the lake is frozen over and the water is quiet.

Each annual layer, therefore, has two parts: The coarse-fine couplet is known as a varve 'varv' is Swedish for 'layer'. If the process of annual varve deposition continues today or only ceased in historic time, and the date of cessation was recorded, then simple counting can reveal the absolute age of any layer within the lake basin, using the formula developed above: However, deposition of varves may cease because the glaciers have melted away completely or because the lake basin has been completely filled by sediment.

Types of Absolute Age Dating

If annual deposits ceased before human records dating the event were kept, then the length of time that has elapsed since the top layer was deposited must be determined in some other fashion. Correlation of time-equivalent layers may be accomplished by 'pattern matching'. It has been observed that in any given year , varve thicknesses vary from places to place.

This is not surprising, since different meltwater streams carry different sediment loads. Also, in any given place varve thicknesses vary from year to year, being thicker when temperatures are higher and meltwaters are more voluminous and flow faster, and thinner when temperatures are lower and stream volume and velocity are diminished. Such yearly changes in stream volume and velocity and thickness of depositional layers tend to be regional: Throughout the region, all layer thicknesses may be double what they were the year before. Through time, therefore, although layer thicknesses vary from place to place, the ratio of layer thicknesses to each other at one site are the same as the ratio of layer thicknesses to each other at a different site.

The diagram on the right shows varves that accumulated simultaneously during the same 8-year time interval at locations A, B, and C. Note that although the thicknesses vary from location to location, the ratios of thicknesses remain constant. At location A, from oldest to youngest, the thicknesses for the layers are 0. The ratio of the thicknesses is 5: That is, the pattern is the same at at the three locations.

It is important to note that the pattern is random. The position within the series of any layer, therefore, is unique. Layers formed at the same time, such as layer 'X', may be recognized. That is, equivalent layers may be correlated. Correlation using pattern matching makes it possible to determine, in a location where deposition has ceased, the absolute ages of the layers. This is accomplished by comparison with a location where annual deposition continues. Consider the varve sequences at locations A, B and C.

At A, yearly deposition continues, so the absolute ages of layers can be determined by counting down from the top. At locations B and C, deposition stopped at some time in the past. Indeed, some of the top layers may have been removed by erosion. However, by pattern matching, five layers within the series at A can be correlated with five layers at the top of B. Similarly, by pattern matching, five layers in the series at B can be correlated with five layers at the top of C.

Since the ages of the layers at A are known by counting down from the top, layers at B that correlate with them can also be assigned ages. Then, the ages of the rest of the layers at B may be determined by counting down. In similar fashion, layers at C that correlate with layers at B may be assigned ages, and the rest of the layers at C may be assigned by counting down.


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Using this method, ages of varves that formed tens of thousands of years ago may be determined. For example, varves close to forty thousand years old have been dated in Japan. Pattern matching is also used to date trees by examining growth rings dendrochronology. Ages up to 14, years have been determined in this fashion. An archeologist finds a dried out, abandoned flood plain at location 'A'.

Geologic Time Absolute Age Dating

He drills a hole and extracts a drill core that shows a series of layers of sediment one of which contains pottery fragment 'X'. The archeologist then contacts his colleague who is working in a nearby area location 'B' where there is a modern floodplain to which a layer of sediment is added every year. He asks his colleague to extract and send him a drill core from location 'B', making sure to include and label the most recent layer, deposited in She does so, and also includes another drill core from a third location 'C', where she has recently worked.

She tells him that location 'C', like location 'A', is also a dried out, abandoned floodplain. The first archeologist wants to know in what year the layer containing pottery fragment 'X' was deposited. In what year was the layer formed that contains pottery fragment 'X'?


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My answer to Question 1: The layer containing 'X' was deposited in: Indeed, dating of lake sediments using varves was undertaken as early as Their disadvantage is that they are restricted to sites where annual deposition has occurred and the absolute age of at least one layer can be determined with confidence by some other means for example, by counting or by pattern matching with places where annual deposition continues through to today.

Places satisfying these requirements are relatively few. Another disadvantage is that over geologic time, preservation of such layers is limited. Absolute age determination by varve counting is only suitable for materials less than several tens of thousands of years old. These limitations are overcome in radiometric dating. Radioactive elements, such as certain isotopes of uranium, thorium, rubidium, potassium, carbon and others, have the property that over set periods of time, known as their 'half lives' which are different for each radioactive element , half of their atoms decay to form atoms of different elements.

Absolute-Age Dating | bombardier.blackhammer.com

For example, over the course of million years, half the atoms of the 'parent' element uranium U decay to form atoms of the 'daughter' element lead Pb Over the next million years, half of the remaining U atoms change to Pb, and so on. By comparing the ratios of U to Pb that are found in the material today, the time when the process started may be ascertained see table below. Examples of radioactive parent-daughter pairs and their half lives include: U - Pb 4.

An error of that magnitude may be quite acceptable for such old rocks. After careful analysis, a geochronologist determines that an unweathered, unmetamorphosed mineral sample contains 7 trillion atoms of the radioactive element K and trillion atoms of its decay product A How many years ago was the sample formed? The number of years ago that the sample formed is: It is important to choose a radioactive parent-daughter pair whose half life is appropriate for the age of the material being dated. 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.

Absolute Age: Definition & Dating

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.

What is Absolute Age?

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.

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 Zr , silicon 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.

Absolute dating

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. How can you tell the age of a rock or to which geologic time period it belongs? One way is to look at any fossils the rock may contain. If any of the fossils are unique to one of the geologic time periods, then the rock was formed during that particular time period.

Another way is to use the "What's on top? When you find layers of rocks in a cliff or hillside, younger rocks are on top of older rocks. But these two methods only give the relative age of rocks--which are younger and which are older. How do we find out how old a rock is in years? Or how do we know how long ago a particular group of fossilized creatures lived? The age of a rock in years is called its absolute age.