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By: David Rice To: Tyler A. Wunder Re: Earth Age. How we KNOW, #1 of 4 --------------------------------------------------------- You said to Marty Leipzig: TAW> I have a friend here and we were speaking about dating TAW> techniques, age of the earth, etc. What good evidence is TAW> there that the earth is 4.5 billion years old (whatever TAW> the figure is that's most often thrown around) and not TAW> in fact OLDER? I mean, is the age simply based on the TAW> oldest rocks that we've got, or is there some good reason TAW> to suspect that the oldest rocks we use to come up with TAW> the 4.5 billion year figure are in fact used, or are even TAW> the oldest rocks there are? TAW> TAW> Could someone, for example, happen upon some new evidence TAW> (e.g. new rocks) that would lead us to suspect that the TAW> earth is...say...10 billion years old, or is there some TAW> reason to suspect that this just isn't realistic? Hi! I'm a ignorant heathen and not a physicist or geologists, so let -ME- take a crack at this! The oldest known "rock" on Earth is younger than the solar system. No, really. It's about 3.8 American billion years old (3,800,000,000). The following is from Chris Stassen. ---------------------------------------------------------- Author: Chris Stassen Subject: FAQ: Age of the Earth Updated: 09/24/92 The most direct means for calculating the earth's age is a Pb/Pb isochron age, derived from samples of the earth and meteorites. This involves measurement of three isotopes of lead (Pb-206, Pb-207, and either Pb-208 or Pb-204). A plot is constructed of Pb-206/Pb-204 versus Pb-208/Pb-204. If the solar system formed form a common pool of matter, which was uniformly distributed in terms of Pb isotope ratios, then the initial plots for all objects from that pool of matter would fall on a single point. However, amounts of Pb-206 and Pb-207 will change in some samples, as these isotopes are decay end-products of U (U-238 decays to Pb-206, and U-235 decays to Pb-207). If the source of the solar system was also uniformly distributed with respect to U isotope ratios, then this change will cause the data points to move away from each other, but they will always fall on a single line. And from the slope of the line we can derive the amount of time which has passed since the pool of matter became separated into individual objects. A creationist would object to all of the "assumptions" listed above. However, the test for these assumptions is the plot of the data itself. The actual underlying assumption is that, if those requirements have not been met, there is no reason for the data to plot on a line. The resulting plot for five meteorites that contained uranium, a single data point for all meteorites that do not, and one for modern ocean sediments. It looks like this: Y-axis: ratio of Pb[207]/Pb[204] X-axis: ratio of Pb[206]/Pb[204]. +---------------------------------------------------------------- | 7 | 30 + | | 6 | | 20 + | | 4 5 | 3 | 2 10 + 1 | | +------+------+------+------+------+------+------+------+------+- 10 20 30 40 50 Data points: (1) Iron Meteorites; (2) Beardsley; (3) Modern sediments and young galenas; (4) Saratov; (5) Elenovka; (6) Richardton; (7) Nuevo Laredo. I can't really do it justice in ASCII, I recommend interested parties to get the original. (Dalrymple, 1986, Figure 12) The slope of the line in the above chart gives an age of 4.55 +/- 0.07 billion years. Most of the other measurements for the age of the earth rest upon calculating an age for the solar system by dating objects which are less geologically active (such as meteorites). Below is a table of radiometric ages derived from groups of meteorites: ======================= ====== ====== =============== Number Type Dated Method Age (x10^9 yr) ======================= ====== ====== =============== Chondrites 13 Sm-Nd 4.21 +/- 0.76 Carbonaceous chondrites 4 Rb-Sr 4.37 +/- 0.34 Chondrites (undist. H) 38 Rb-Sr 4.50 +/- 0.02 Chondrites (all) 50 Rb-Sr 4.43 +/- 0.04 H Chondrites (undist.) 17 Rb-Sr 4.52 +/- 0.04 H Chondrites 15 Rb-Sr 4.59 +/- 0.06 L Chondrites (rel. und.) 6 Rb-Sr 4.44 +/- 0.12 L Chondrites 5 Rb-Sr 4.38 +/- 0.12 LL Chondrites (undist.) 13 Rb-Sr 4.49 +/- 0.02 LL Chondrites 10 Rb-Sr 4.46 +/- 0.06 E Chondrites (undist.) 8 Rb-Sr 4.51 +/- 0.04 E Chondrites 8 Rb-Sr 4.44 +/- 0.13 Eucrites (polymict) 23 Rb-Sr 4.53 +/- 0.19 Eucrites 11 Rb-Sr 4.44 +/- 0.30 Eucrites 13 Lu-Hf 4.57 +/- 0.19 Diogenites 5 Rb-Sr 4.45 +/- 0.18 Iron (+ St. Severin) 8 Re-Os 4.57 +/- 0.21 ======================= ====== ====== =============== (After Dalrymple, 1991, p. 291; duplicate studies on identical meteorite types omitted.) As shown in the table, there is excellent agreement on about 4.5 billion years, between hundreds of different meteorites and by several different dating methods. Further, studies on individual meteorites generally give concordant ages by multiple radiometric means. For example: ======================= ====== ====== =============== Meteorite Dated Method Age (x10^9 yr) ======================= ====== ====== =============== Guarena w-rock Ar-Ar 4.44 +/- 0.06 13 sam Rb-Sr 4.46 +/- 0.08 ----------------------- ------ ------ --------------- Olivenza 18 sam Rb-Sr 4.53 +/- 0.16 w-rock Ar-Ar 4.49 +/- 0.06 ----------------------- ------ ------ --------------- Saint Severin 4 sam Sm-Nd 4.55 +/- 0.33 10 sam Rb-Sr 4.51 +/- 0.15 w-rock Ar-Ar 4.43 +/- 0.04


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