From the Precambrian to the present, each geologic era is associated with characteristic fossils. By identifying the species of the fossils, you can calculate the relative age of any rock layer that contains fossils. This is called relative dating. However, it only gives a rough range of possible ages, since each geologic era spans many millions of years.
Radiometric dating - Wikipedia
Some rock layers are surrounded by volcanic debris, or tuff, in situ, meaning they weren't broken by igneous intrusions; rather, local volcanic activity simply blanketed an area with ash at various times. These areas are the easiest to date because volcanic debris can usually be radiometrically dated with a high degree of accuracy. Dating the ash layers above and below a sedimentary rock layer to determine its age is called bracketing. Radiometric dating uses the decay of unstable isotopes -- atoms with specific electrical charges -- to calculate something's age. Tuff radiometry usually uses potassium-argon dating.
Volcanic debris contains feldspar crystals, full of an isotope called potassium Potassium 40 decays into argon 40 at a predictable rate over enormous spans of time. If you know this rate and you know the proportion of potassium 40 to argon 40 in the surrounding ash, you can estimate the age of the surrounded rock layer. Angela Libal began writing professionally in She has published several books, specializing in zoology and animal husbandry. With sedimentary rocks, one would end up dating the individual grains of sediment comprising the rock, not the rock as a whole.
These grains could have radically different ages. So, geologists prefer to work with igneous rocks. Useful to archaeologists, maybe, but system is not typically used on rocks at all. Thus, sedimentary and metamorphic rocks can't be radiometrically dated. Although only igneous rocks can be radiometrically dated, ages of other rock types can be constrained by the ages of igneous rocks with which they are interbedded. Magnetostratigraphy The Earth generates a magnetic field that encompasses the entire planet.
In the last fifty years, a new dating method has emerged that exploits two aspects of rocks' interactions with the Earth's magnetic field. It is, in essence a form of relative dating. Some magnetic minerals, such as magnetite occur naturally in igneous rocks.
Molten Intruders
When their grains form, they align themselves with the Earth's magnetic field. The Earth's magnetic field changes quickly i. Nevertheless, because of the orientation of their magnetic minerals, their intrinsic magnetic field records the orientation of the Earth's field as it existed when they formed.
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Such ancient magnetic fields are called remnant or paleomagnetism. The Earth's magnetic field has a north and south pole. For unknown reasons, at intervals of very roughly , years, the north and south poles trade places. The result is that the paleomagnetic polarity of igneous rocks is either: Magnetic north coincides roughly with geographic north. Magnetic north coincides roughly with geographic south. If we drill a core form layers of rocks with paleomagnetism, and color-code ones with normal and reverse polarity, we get a pattern like a bar code.
Any interval of time we designate will display a unique pattern of paleomagnetic reversals. What kinds of rocks retain paleomagnetism: Igneous, for reasons noted. Some sedimentary rocks retain paleomagentism when they contain minerals derived form earlier igneous rocks.
Three requirements need to be met: Sediments consist of very small grains that settle slowly from water Sediments include magnetic minerals Sediments were deposited in very quiet body of water, like a lake. The fact that sediments can record paleomagnetism is very useful. Remember, we have no means of directly measuring the radiometric age of sediments that aren't preserved in association with igneous rocks.
We can , however, hang a numerical age on them if their paleomagnetic "fingerprint" can be matched with that of a sequence of igneous rocks that can be radiometrically dated. By studying paleomagnetic polarity of rocks of different ages, geologists have developed a paleomagnetic time scale that is correlated with the regular time scale. The scale consists of chrons a.
I also recall reading that geologists assume the initial Pb isotope ratios vary from place to place anyway. Later we will see that mixing of two kinds of magma, with different proportions of lead isotopes, could also lead to differences in concentrations. Mechanism of uranium crystallization and falling through the magma We now consider in more detail the process of fractionation that can cause uranium to be depleted at the top of magma chambers. Uranium and thorium have high melting points and as magma cools, these elements crystallize out of solution and fall to the magma chamber's depths and remelt.
This process is known as fractional crystallization. What this does is deplete the upper parts of the chamber of uranium and thorium, leaving the radiogenic lead. As this material leaves, that which is first out will be high in lead and low in parent isotopes. This will date oldest.
Magma escaping later will date younger because it is enriched in U and Th. There will be a concordance or agreement in dates obtained by these seemingly very different dating methods. This mechanism was suggested by Jon Covey. Tarbuck and Lutgens carefully explain the process of fractional crystallization in The Earth: An Introduction to Physical Geology. They show clear drawings of crystallized minerals falling through the magma and explain that the crystallized minerals do indeed fall through the magma chamber.
Further, most minerals of uranium and thorium are denser than other minerals, especially when those minerals are in the liquid phase. Crystalline solids tend to be denser than liquids from which they came. But the degree to which they are incorporated in other minerals with high melting points might have a greater influence, since the concentrations of uranium and thorium are so low. Now another issue is simply the atomic weight of uranium and thorium, which is high. Any compound containing them is also likely to be heavy and sink to the bottom relative to others, even in a liquid form.
If there is significant convection in the magma, this would be minimized, however. At any rate, there will be some effects of this nature that will produce some kinds of changes in concentration of uranium and thorium relative to lead from the top to the bottom of a magma chamber. Some of the patterns that are produced may appear to give valid radiometric dates.
Radiometric dating
The latter may be explained away due to various mechanisms. Let us consider processes that could cause uranium and thorium to be incorporated into minerals with a high melting point. I read that zircons absorb uranium, but not much lead. Thus they are used for U-Pb dating. But many minerals take in a lot of uranium. It is also known that uranium is highly reactive. To me this suggests that it is eager to give up its 2 outer electrons. This would tend to produce compounds with a high dipole moment, with a positive charge on uranium and a negative charge on the other elements.
This would in turn tend to produce a high melting point, since the atoms would attract one another electrostatically. I'm guessing a little bit here. There are a number of uranium compounds with different melting points, and in general it seems that the ones with the highest melting points are more stable. I would suppose that in magma, due to reactions, most of the uranium would end up in the most stable compounds with the highest melting points.
These would also tend to have high dipole moments. Now, this would also help the uranium to be incorporated into other minerals. The electric charge distribution would create an attraction between the uranium compound and a crystallizing mineral, enabling uranium to be incorporated.
But this would be less so for lead, which reacts less strongly, and probably is not incorporated so easily into minerals. So in the minerals crystallizing at the top of the magma, uranium would be taken in more than lead. These minerals would then fall to the bottom of the magma chamber and thus uranium at the top would be depleted. It doesn't matter if these minerals are relatively lighter than others.
The point is that they are heavier than the magma. Two kinds of magma and implications for radiometric dating It turns out that magma has two sources, ocean plates and material from the continents crustal rock. This fact has profound implications for radiometric dating. Mantle material is very low in uranium and thorium, having only 0. The source of magma for volcanic activity is subducted oceanic plates.
Subduction means that these plates are pushed under the continents by motions of the earth's crust. While oceanic plates are basaltic mafic originating from the mid-oceanic ridges due to partial melting of mantle rock, the material that is magma is a combination of oceanic plate material and continental sediments. Subducted oceanic plates begin to melt when they reach depths of about kilometers See Tarbuck, The Earth, p.
In other words, mantle is not the direct source of magma.
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