The earth is commonly estimated to be about 4.6 billion years old, and the oldest rocks on earth have been dated to 4.28 billion years ago. (Zircons found from grains in Western Australia are considered slightly older – 4.36 billion years old.) Geologists depend on radiometric age dating to give dates to different strata and the rocks and bones and artifacts found in those layers. Most will say that the dating methods have been confirmed over and over again, but there are questions about whether the dating methods used are reliable and whether they give scientists true dates – or whether they give dates that fit those secular geologists’ preconceived ideas.
During its history, the earth has suffered constant change. Erosion, volcanic eruptions, earthquakes, catastrophes change the surface of the earth, melting rocks, grinding them up and spitting them back out in different forms than they had when they started. Because geologists have a difficult time finding truly ancient earth rocks, they have dated meteorites – assumed to have formed at the same time as the earth (and the rest of the solar system) in order to give a reliable age for the earth.
Those who trust in radiometric dating methods point to G. Brent Dalrymple’s 1991 book The Age of the Earth, in which he lists the ages of various chondrite samples age-dated through several methods. Dalrymple shows that the dates of chondrite samples derived through different methods all give similar ages. A list of samples dated by the rubidium-strontium (Rb-Sr) method gives dates in a range from 4.37 +/- 0.34 billion years to 4.59 +/- 0.06 billion years. Samarium-neodymium (Sm-Nd) dating of chondrites gives a date of 4.21 +/- 0.76 billion years, and both eucrites dated by lutetium-hafnium (Lu-Hf) and iron dated by rhenium-osmium (Re-Os) gives dates of 4.57 billion years. Dalrymple lists other sets of samples and shows that the samples consistently date between 4.29 billion and 4.57 billion years, regardless of the dating method used.
This looks like convincing evidence that the earth is truly about 4.6 billion years old and the dating methods work reliably, (even though the Rb-Sr dating has a margin of error of 340 million years and Sm-Nd of 760 million years). Dalrymple’s results appear to be fairly consistent in giving the earth an age of well over 4 billion years.
As we noted last week, however, the use of these dating methods depends on a number of assumptions. When these methods give dates hundreds of millions of years apart, even without taking the margins of error into consideration, we can reasonably hesitate to put our full faith in them. On tests of rocks considered much younger, those result differences really matter.
Despite claims to the contrary, dating methods do not always give consistent dates, and can give widely different results for samples of rocks from the same layers.
In 2005, geologist Andrew Snelling and creationist researchers had multiple samples of Grand Canyon basalts age-dated using at least three of the main radiometric dating methods, and found the tests gave discordant results. The Cardenas Basalt samples gave an age of 516 (+/-30) million years when dated by the K-Ar method, 1111 (+/-80) million years when dated by the Rb-Sr method, and 1588 (+/-170) million years when dated with the Sm-Nd method. The range in dates for just the Cardenas Basalt was therefore more than a billion years. That’s a huge range considering the low age-date for the basalt was “only” 516 million years.
The Grand Canyon Brahma amphibolites samples dated by the Rb-Sr, Ur-Pb, and Sm-Nd methods gave ages that ranged from 1240 to1883 million years ago. In a couple of cases, layers farther down in the Grand Canyon (and therefore relatively older) were dated younger than layers higher up. These results, and other similar tests by Steve Austin and other creationists (see links below), have demonstrated that these dating methods are not necessarily as reliable as old earth geologists often claim.
Assuming A Great Age
One of the problems with using the Rb-Sr or K-Ar or other certain age-dating methods is that they do not give accurate ages for items that are young. Andrew Snelling reports that in 1996, samples were taken from 20th century lava flows on New Zealand’s Mt Ngauruhoe – “two each from the 11 February 1949, 4 June 1954, and 14 July 1954 flows and from the 19 February 1975 avalanche deposits, and three from the 30 June 1954 flow…” The samples were sent to Geochron Laboratories in Cambridge, Mass for whole-rock potassium-argon (K–Ar) dating. Even though the rocks were from lava flows less than 50 years old, the samples gave greatly exaggerated dates for the rocks.
Snelling reports, “The ‘ages’ range from <0.27 to 3.5 (± 0.2) million years for rocks which were observed to have cooled from lavas 25–50 years ago. One sample from each flow yielded ‘ages’ of <0.27 or <0.29 million years while all the other samples gave ‘ages’ of millions of years. ”
Geologists complain that this sort of testing is ridiculous to do because everybody knows that K-Ar dating will not give accurate dates for extremely young rocks. Ah, and that’s just the problem. When geologists do tests of ancient rocks, they assume extremely old ages, and so they use age-dating methods that would be appropriate for extremely old rocks. If the rocks truly aren’t that old, how can geologists know? What would indicate to a lab that the correct date is the lower date rather than the 3.5 million year date? If they used dating methods appropriate for young rocks, would those give relatively consistent young ages for the rocks?
Speed of Light and Inflated Dates
While there are definitely some difficulties with the age-dating methods, Andrew Snelling believes they can still be useful for the most part in giving relative dates – that is, determining that this sample is older than that sample.
Some young-earth physicists like Lambert Dolphin, Barry Setterfield, and Trevor Norman argue that these dating methods are not wrong in theory – but they give wildly exaggerated dates because radioactive decay rates have slowed down over time. The speed of light has a direct effect on the atom and atomic decay rates. These physicists argue that the speed of light has been slowing down, affecting the rate of radioactive decay. If decay rates were much higher in the past, then dating-methods based on today’s rates of change would be inflated.
Unless the physicists get on the ball and build a good time machine, not one of us can go back in time and watch the formation of the earth and its familiar geologic wonders. Yet, we believe we already have a good account of what happened. The first five books of the Bible were written by a man who glowed after his encounters with the I AM (Exodus 34:29-30) and who spoke with God “face to face” (Exodus 33:11). While secular scientists scoff at divine revelation, we have confidence that it is truly the best way to know what happened at the beginning of the world.