|Do you believe radiometric dating is an accurate way to date the earth? Why or why not? Could you also please explain further what radiometric dating is and the process to use it? Mahalo.
|Question Date: 2012-04-03|
Yes!! Absolutely. It is an accurate way to date specific geologic events. This is an enormous branch of geochemistry called Geochronology. There are many radiometric clocks and when applied to appropriate materials, the dating can be very accurate. As one example, the first minerals to crystallize (condense) from the hot cloud of gasses that surrounded the Sun as it first became a star have been dated to 4568 plus or minus 2 million years....!!
That is pretty accurate!!!
Other events on earth can be dated equally well given the right minerals.For example, a problem I have worked on involving the eruption of a volcano at what is now Naples, Italy, occurred 38500 years ago with a plus or minus of 300 years.So, when the materials are appropriate and one carefully avoids contamination and re setting radiometric clocks can be VERY ACCURATE.
Yes, radiometric dating is a very accurate way to date the Earth.We know it is accurate because radiometric dating is based on the radioactive decay of unstable isotopes. For example, the element Uranium exists as one of several isotopes, some of which are unstable. When an unstable Uranium (U) isotope decays, it turns into an isotope of the element Lead (Pb). We call the original, unstable isotope (Uranium) the "parent", and the product of decay (Lead) the "daughter". From careful physics and chemistry experiments, we know that parents turn into daughters at a very consistent, predictable rate.
For an example of how geologists use radiometric dating, read on:
A geologist can pick up a rock from a mountainside somewhere, and bring it back to the lab, and separate out the individual minerals that compose the rock. They can then look at a single mineral, and using an instrument called a mass spectrometer, they can measure the amount of parent and the amount of daughter in that mineral. The ratio of the parent to daughter then can be used to back-calculate the age of that rock. Pretty cool!
The reason we know that radiometric dating works so well is because we can use several different isotope systems (for example, Uranium-Lead, Lutetium-Halfnium, Potassium-Argon) on the same rock, and they all come up with the same age. This gives geologists great confidence that the method correctly determines when that rock formed. Hope that helps, and please ask if you'd like more details!
Great question! I think that I will start by answering the second part of your question, just because I think that will make the answer to the first question clearer. Radiometric dating is the use of radioactive and radiogenic (those formed from the decay of radioactive parents) isotopes (isotopes are atoms of the same element that have different numbers of neutrons in their nuclei) to determine the age of something. It is commonly used in earth science to determine the age of rock formations or features or to figure out how fast geologic processes take place (for example, how fast marine terraces on Santa Cruz island are being uplifted).
Radiometric dating relies on the principle of radioactive decay. All radioactive isotopes have a characteristic half-life (the amount of time that it takes for one half of the original number of atoms of that isotope to decay). By measuring the parent isotope (radioactive) and the daughter isotope (radiogenic) in a system (for example, a rock), we can tell how long the system has been closed (in our example, when the rock formed).
The process of radiogenic dating is usually done using some sort of mass spectrometer. A mass spectrometer is an instrument that separates atoms based on their mass. Because geochronologists want to measure isotopes with different masses, a mass spectrometer works really well for dating things.
I do think that radiometric dating is an accurate way to date the earth, although I am a geochronologist so I have my biases. The reason that I trust the accuracy of the age that we have determined for the earth (~4.56 billion years) is that we have been able to obtain a very similar result using many different isotopic systems. Most estimates of the age of the earth come from dating meteorites that have fallen to Earth (because we think that they formed in our solar nebula very close to the time that the earth formed). We have dated meteorites using Rb-Sr, Sm-Nd, Pb-Pb, Re-Os, and Lu-Hf isotope systems and have obtained very similar ages. The fact that the age we calculate is reproducible for these different systems is significant. We have also obtained a very similar age by measuring Pb isotopes in materials from earth. I should mention that the decay constants (basically a value that indicates how fast a certain radioactive isotope will decay) for some of these isotope systems were calculated by assuming that the age of the earth is 4.56 billion years, meaning that we will also calculate an age of 4.56 billion years if we use that decay constant. The decay constants for most of these systems have been confirmed in other ways, adding strength to our argument for the age of the earth.
Radiometric dating depends on the chemistry and ratios of different elements. It works like this:
Take, for example, zircon, which is a mineral; its chemical formula is ZiSiO4, so there is one zirconium (Zi) for one silicon (Si) for four oxygen (O). One of the elements that can stand in chemically for zircon is uranium. Uranium eventually decays into lead, and lead does not normally occur in zircon, except as the radioactive decay product of uranium. Therefore, by measuring the ratio of lead to uranium in a crystal of zircon, you can tell how much uranium there originally was in the crystal, which, combined with knowing the radioactive half-life of uranium, tells you how old the crystal is.
Obviously, if the substance you are measuring is contaminated, then all you know is the age since contamination, or worse, you don't know anything, because the contamination might be in the opposite direction - suppose, for example, you're looking at radio carbon (carbon 14, which is produced in the atmosphere by cosmic rays, and which decays into nitrogen). Since you are exposed to the atmosphere and contain carbon, if you get oils from your skin onto an archeological artifact, then attempting to date it using radio carbon will fail because you are measuring the age of the oils on your skin, not the age of the artifact. This is why crystals are good for radiometric dating: the atoms in a crystal are extremely efficiently packed, and it's very difficult to get anything into a crystal such as a contaminant by any means short of destroying the crystal and re-growing it anew.
The oldest crystals on Earth that were formed on Earth are zircon crystals, and are approximately 4.1 billion years old. Asteroids in the solar system have been clocked at 4.5 billion years old, however, based on the ages of the zircon crystals within them. We assume that the Earth is probably as old as the asteroids, because we believe the solar system to have formed from a collapsing nebula, and that the Earth, being geologically active, has simply destroyed any older zircon crystals that would be its true age, but we can't really be certain. The building blocks that the Earth is made of, the asteroids are 4.5 billion years old, and we presume that the Earth formed fairly quickly thereafter.
Based on astronomical models of how stars work, we also believe the Sun to be about 4.6 billion years old, slightly older than the rocky bodies in the solar system, so the timing of the solar system using zircon crystals in asteroids also makes sense from this angle as well.
Radiometric dating is a widely accepted technique that measures the rate of decay of naturally occurring elements that have been incorporated into rocks and fossils. Every element is defined by the particular number of protons, neutrons, and electrons that make up it's atoms. Sometimes, the number of neutrons within the atom is off. These atoms, with an odd number of neutrons, are called isotopes. Because they do not have the ideal number of neutrons, the isotopes are unstable and over time they will convert into more stable atoms. Scientists can measure the ratio of the parent isotopes compared to the converted isotopes. Because the rate of conversion of isotopes is known (how long it takes for a particular isotope to convert/decay), we can use the ratio to determine how old the object is that contains them.
The rate of isotope decay is very consistent, and is not effected by environmental changes like heat, temperature, and pressure. This makes radiometric dating quite reliable. However, there are some factors that must be accounted for. For example, sometimes it is possible for a small amount of new "parent" isotopes to be incorporated into the object, skewing the ratio. This is understood and can be corrected for. Also, techniques such as taking samples from multiple sections and dating with multiple isotopes, will help crosscheck/confirm the accuracy of the date.
Carbon-14 is the most commonly used isotope for dating organic material (plants, animals). Plants and animals continually take in carbon-14 during their lifespan. When they die, they no longer acquire carbon-14 and so we can measure the decay of the isotope to determine when the plant or animal died. Because carbon-14 decays relatively rapidly compared to other isotopes, it can only be used to date things that are less than 60,000 years old. Anything older would have so little carbon-14 left that you couldn't accurately measure it. However, the rapid decay allows precise dating - accuracy within just a couple decades.
When dating older objects, namely rocks, it is necessary to use other isotopes that take a much longer time to decay. The most common isotopes used are uranium-235 and uranium-238 (there are multiple isotopes of uranium). The uranium isotopes eventually convert into lead isotopes. Measuring the ratio of uranium to lead can have a margin of error as small as 2-5%. In other words, we can predict the age of a rock within two million years out of two-and-a-half billion years. That's pretty good.
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