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- I have heard of dating a nerd. Even dating a geek. But, dating a rock? What’s that all about?
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-. It's about dating Nuvvuagittug rocks. It's about radioactive dating. These rocks are about 20 miles from the Inukjuak. And, that happens to be the northeast edge of the Hudson Bay in Canada.
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-. The rocks are exposed because ancient ice age glaciers have carved out the land. Some geologists have dated rocks to be 3.8 billion years old. Another team of geologists say they have found rocks 4.4 billion years old. If this is true this age is close to Earth's earliest formation. Studies here should help science learn about the earliest evolution of our planet.
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-. 4,468,000,000 years was when the first rain filled the oceans. It was when the continents heaved up out of the waters to create dry land. Comets and asteroids were impacting Earth's surface. The biggest impact was the size of the planet Mars. That impact splashed the Earth's crust into orbit. The orbiting debris coalesced into what we now have as our Moon.
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-. These are major events in Earth's early evolution. Dating these rocks precisely could tell science the timing of what occurred when. There are thousands of acres of minerals to be studied in the Hudson Bay. The minerals contain the chemistry of the earliest oceans. Even the chemistry of the earliest atmosphere. This coupled with the studies of Moon rocks are treasures in geology.
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-. Most of Earth’s surface has been covered with sedimentary rock, or, it has become molten magma cooling and crystallizing into igneous rock. Only this tiny portion of rock on the Hudson Bay appears to have remained intact throughout Earth's history. Almost all the ancient rock everywhere else has been destroyed in one way or another.
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-. The second oldest rocks were found on the tundra in Canada's Northwest Territories dated to be 3,920,000,000 years old.
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- Some of the clues of Earth's early history have come from Zircon crystals. These crystals form in cooling magma. Zircon crystals can trap radioactive atoms such as uranium. Measuring the decay rates of the radioactive isotopes allow the age of the Zircon's to be calculated.
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-. Some of the Zircon crystals found in the outback's of Australia have been dated to be 4,400,000,000 years old. But, these are the crystals themselves and not the surrounding rock.
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-. The radioactive isotopes in the rocks decay over time at a clocklike pace. Measuring the ratios of the levels of atoms and isotopes reveals the age of the rocks.
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------------------------------ Neodymium 142 breaks down to
------------------------------ Samarium 146 , which has a half-life of 68 million years.
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- Samarium 146 was injected into the Earth's chemistry by the supernova that exploded and produced the shockwaves that condensed the gases and dust to allow gravity to produce a Sun and the Solar System. But, with the short half-life Samarium 146 would have decayed away in 500,000,000 years.
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-. The Hudson Bay rocks have specific proportions of Neodymium 142 to other Neodymium isotopes, indicating the rocks formed when Samarium 146 was still around These proportions give a date of 4,280,000,000 years old. Other samples found since 2008 have been dated at 4,400,000,000 years old. Other geologists studying Zircon crystals date the Hudson Bay rocks at 3,750,000,000 years old.
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-. Who is right?
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-. Another study of Quartzite containing Zircon crystals was dated to be 3,800,000,000 years old. Next, they studied the decay of Lutetium into Hafnium and got an age of 3,800,000,000 years old
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-. Uranium and Lead combinations found in rock would give the most reliable dating. The hunt is on for this combination of elements. The history is important. Right now fossil life dates back to 3,500,000,000 years. That is where bacteria were found preserved in ancient rocks.
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-. `Bacteria feed on carbon. There is a balance of carbon isotopes in the atmosphere. These comparisons have dated the earliest life found in the rocks to be 3,800,000,000 years old.
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-. How did life form?
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-. Could these old rocks reveal the story?
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- Over millions of years radioactive isotopes slowly break down at a steady, clocklike rate, unique to each element
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------------------------ Samarium 147 decays into Neodymium 143.
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-The ratio of the number of atoms of Samarium gradually decreases. Number of atoms of Neodymium 143 gradually increases. The amount of decay as a function of time is a logarithmic function.
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-. Neodymium 144 is a stable atom and it does not break down at all , its levels of abundance stay constant. Plotting the ratios of these decaying atoms to stable atoms creates a linear slope with time. The ratio of a decay is constant, but, the original sample of parent atoms that are decaying is decreasing and reapplying the constant rate creates a logarithmic decay curve of the ratio with time.
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-. Other ratios for dating include Uranium 238 decaying to Lead 206
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-. Ratios of Uranium 235 decaying to Lead 207.
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-. Carbon-14 isotope is the most familiar carbon-dating mechanism but it has a more rapid decay rate that only goes back 60,000 years.
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-. Quantum mechanics makes predictions only about probabilities of different outcomes. An example occurs with this radioactive dating. The probabilities become statistics with very large numbers of atoms, making the calculation results very reliable.
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- Atomic nucleus decays to some lower energy by the emission of electrons, helium nuclei, or photons.
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--------------------------- No = number of radioactive nuclei to start with.
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---------------------------- t = after time t
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--------------------------- N = the number of radioactive nuclei left
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---------------------------- N / No = e ^ ( -t / R)
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---------------------------- R = the mean lifetime of the radioactive nuclei.
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-. To learn how this is used in radioactive dating consider a piece of wood found in a cave used in an agent campfire.
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-. When a tree dies its wood stops breathing carbon dioxide from the atmosphere.
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-. Carbon-14 that is in the atmosphere decays to Carbon-12 with a lifetime of 8,270 years.
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---------------------------- . When “R” equals “t” ------ e^-1 = N / No = , 0.37
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-. The ratio of “N / No” equals 37 %, which means 37 % of the C14 atoms have decayed to C12 atoms over the mean lifetime of the decay. Every 8,270 years the ratio decays by 37 %. 37 % of the C14 atoms , which contain 6 protons and 8 neutrons loose 2 neutrons that Beta Decay into a helium nucleus of 2 protons, an electron and an anti-neutrino that go flying out of the sample. C12 carbon atoms having 6 protons and 6 neutrons are left behind.
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-. Carbon dating would not work for Moon rocks because with a mean lifetime of only 8,270 years all the C14 would have disappeared over the lifetime of the Moon. However, Uranium decays into the Lead much more slowly. Uranium-238 decays into Lead-206 over a half-life of 4,500,000,000 years meaning half of the atoms have decayed over that time span.
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-. Rock samples from the Moon were found to have about equal amounts of U238 and Pb206, which would mean that Moon rocks are about 4,500,000,000 years old. (A more precise study put them at 4,400,000,000 years)
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- Since this answer for the age is intuitively correct. Let’s use the radioactive decay formula to see if we get the same answer?
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------------------------------ (N / No ) / 1/2 = e ^ ( -t / R)
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------------------------------ ln (N / No ) / ln (½) = -t / R
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------------------------------- (0.69 ) / ( 0.69 ) = t / 4.5
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-------------------------------- t = 4.5 billion years
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-. Most decay ratios are specified as half-life's. For example: Potassium 40 into Argon 40 has a half-life of 1,250,000,000 years. We can measure decay rates for isotopes very precisely in the laboratory. Knowing the rate of decay and the ratio of the decaying elements we can calculate the time span the decaying has occurred. For example:
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----------------------------- If we find rock to have a ratio of radioactive elements to be:
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----------------------------- Potassium 40 = 0.85 units
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----------------------------- Argon 40 = 9.15 units
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-------------------Original amount of Potassium 40 = No = 0.85 + 9.15 = 10 units
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-------------------------- time / half-life = ln (N / No) / ln ( ½)
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-------------------------- time / 1.25 = ln (0.85 / 10) / ln (0.5)
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-------------------------- time / 1.25 = - 2.47 / - 0.69
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-------------------------- time / 1.25 = 3.58
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---------------------------- time = 4.45 billion years
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- If this is the ratio of the isotopes found in the rocks, the rocks are 4,450,000,000 years old. Which is about the same time that the Moon was formed. Dating rocks can be very rewarding.
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RSVP, with comments, suggestions, corrections. Index of reviews available ---
--- Some reviews are at: -------------------- http://jdetrick.blogspot.com -----
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