Dating Rocks

been divided into the Paleogene period, the Neogene period, and the present Quaternary period.

DATING ROCKS

All rocks look the same at first, but many in the same pile of random rocks you might find have come from vastly different time periods on the geologic scale. A lot has happened in these few billion years since the earth was formed. It would be nice to see how to tell when in time a rock was first develop.

Fossils can help with this in some cases. You would need to know what the fossil is first and then find some relative way of seeing when the fossilized organism first died and became trapped in substances that later become rocks themselves. If you know when the organism died, you can see what else was laid down at the same time and possibly their relationships back in time.

There are three major approaches to dating materials in geology. You can do relative dating once you know the exact date of at least some of the species in the rock. Think of sediment as it is laid down over a long period in time. What is the sediment? How long did the sediment take to get as thick as it is in the rock? What is the date of any known fossil in that sediment? What about the dates of other things above and below this level?

The second approach involves actually dating the materials in the rock to a known date in time. This isn't always exact and the degree of exactness depends on your dating method. Carbon dating, for example, is only reliable for dating things from within the last 50,000 years and is most accurate when combined with tree-ring dating.

The third approach involves magnetism. The magnetic field direction and location has changed, as you know. This helps determine many things with regard to the dating of rocks, just as it proved the theories on plate tectonics.

Relative dating works best for sedimentary rock but it can be used for volcanic rock from volcanoes erupting more than once. Each layer or stratum is generally laid down horizontally. Exposed sedimentary rock is seen along the Grand Canyon or possibly in

your own back yard. Figure 14 shows strata near a river that was once much wider. This rock face was once sediment at the bottom of a wide ancient river:

Figure 14.

The study of strata like these is called stratigraphy. You should recognize that the older layers are deposited first and that they are at the bottom of any exposed rock face. If you see major changes in a strata's appearance or horizontal tendency, something is off, you can assume that something happened after the strata were laid down.

The three main rules of stratigraphy are called:

1. The Principle of Horizontality —layers are laid down in a near horizontal way initially

2. The Principle of Superposition —the top layers are generally younger

3. The Principle of Cross-cutting relationships —the layer that crosses other layers must have formed after the older strata have solidified into rock.

A cross-cutting layer would be one that extends across others, it is the youngest layer by definition. It happens if there are layers that are solid but then get fractured enough for another layer of sediment to slip into the cracks. This crack will be diagonal to the other layers and will cut across the others. This only works if the other layers they cut through have become rocks first.

A series of strata that are no longer horizonal on a rock face usually means they have gotten thrust upward or downward in some way. This is another way of determining what happened to any given area of land where you can see the layers of rock exposed.

You can add a fourth principle to fossilized rocks. This is the principle of faunal succession. It means that the various fossil species will appear and disappear in the same order. Once you have extinction of a given species, it will not be seen within younger rocks. You can get a time zone when known species existed and overlap them with another species to find a range at which the second fossil must have been laid down and when the species lived. Some species lived for a long range in time, while others a shorter range. Knowing these ranges helps you narrow down the range of a species you are wondering about.

Index fossils are those most used in dating fossilized specimens. They are great for dating because they did not exist in living form for a long period of time. Ideally, they should be widespread and easy to identify in their fossilized form. Primate fossils are not good as index fossils because they are so rare, even if they existed over a short time span. Other mammals (like pigs or rats) are better as index fossils because they are common and evolved more quickly over time compared to primates.

Numeric aging of rocks attempts to get an accurate date out of the rock in question. Radiometric dating is commonly used for this. This involves using isotopes of the same atom. Atoms exist in their common form, usually with the same number of protons and neutrons in the atom's nucleus. Carbon-12 has 6 protons and six neutrons. Isotopes of carbon are still made of carbon atoms but will differ in atomic weight. This is because neutrons weigh the same as protons and add weight but little else to the atom. Carbon14 has 2 extra neutrons per atom.

Carbon-14 isn't as stable as carbon-12. It will decay in ways that are not random but happen at a steady pace to make carbon-14 to become carbon-12. This is called the decaying of carbon-14. This is similar to radioactivity but in very slow motion compared to uranium, for example. It also involves changes in the neutron number and not the numbers of negatively charged electrons. Potassium does the same thing by turning a heavier isotope into a lighter one.

Radioactive decay takes a parent isotope and turns it into a daughter isotope that is more stable. The time it takes for half of an amount of a given substance to do this is called the half-life. You use the known decay rate and the amount of each substance (parent versus daughter isotope, for example) to see the exact date.

There are other methods used in absolute dating of rocks, such as thermoluminescence and electron spin resonance. These look at how radioactivity in a rock has affected the crystal structure of crystals in the rock.

This is a summary of the techniques used in dating rocks:

• Radiocarbon —it has limited usefulness because you need relatively recent material that has carbon in it to a great degree, such as charcoal, seashells, bones, or wooden materials. Based on the idea that the carbon atoms made don't decay until the substances have left the biosphere.

• Potassium-argon dating —this can extend to billions of years in the past and relies on the decay of potassium found in certain rocks that have potassium in them.

• Uranium-lead dating —this is based on the decay of uranium into lead and can work for items from 10,000 to billions of years ago.

• Uranium-series dating —this is based on the decay of uranium into thorium and is used for things like calcium carbonate, seashells, teeth, coral, and minerals with uranium in them. It would work for things dating billions of years ago.

• Fission track dating —this is used for substances from 1000 to up to billions of years ago in the past. It relies on uranium decay and measures the tracks of damage in glass and minerals affected by uranium decay.

• Thermoluminescence —this is good for things up to a million years ago and is used to date stone tools, pottery items, quartz, or feldspar. It works for items less damaged by the sun's rays or heat because they were buried and protected at some point in the past. It is also based on the trapping of electrons in mineral crystals.

• Electron spin dating —this works for things exposed to uranium and looks at mineral lattices affected by this radiation. It dates things up to 3 million years ago.

• Cosmogenic nuclides dating —this helps dates olivine or quartz coming from volcanoes and is based on the decay of substances by cosmic rays. It works for items up to 5 million years ago.

• Magneto-stratigraphy — this works for potentially billions of years in the past and looks at the layers of rock and their relative magnetic spin. It would work for volcanic rock and sedimentary rocks.

• Tephrochronology —this is a method that works for items that are at least 100 years old but also as far in the past as billions of years. It dates things ejected from volcanoes to link things found in distant successions of strata.

Radiation is decay but generally involves the decay of the electrons in an atom, turning these electrons into tiny missiles that get trapped into imperfections within crystals. The techniques of optical stimulating luminescence, thermoluminescence, and electron spin resonance all measure how these imperfections trap runaway electrons. Traps eventually get saturated and aren't able to trap more; this means that rocks older than 100,000 years old are less likely to be dated easily using this method.

Magneto-stratigraphy is helpful now that we know times when the earth has reversed polarity. This is measuring rocks laid down over geologic time and other dating methods to create a polarity signature. This geomagnetic polarity time scale is not equal so you get a barcode of sorts showing different strata having their own magnetic pole strength and direction. As you can imagine, you would have needed to have other corroborating dating methods the first time this barcode or scale was generated.