Science Web Assignment for Unit 42
Geologists play a difficult logic game, one with lots of partially known information, from which they must extrapolate and test different scenarios. In some cases, it must seem like they are using circular reasoning, that a rock is such-and-such an age because it contains a particular fossil, and the fossilized remains must have lived in a certain time period because the rock it is found in is such-and-such an age. In fact, geologists use a number of methods to determine the probable age of a given specimen or geological formation, then compare it with many similar objects or formations to determine whether their estimation is reasonable. From many such determinations, they have created a geological "yardstick" of ages and epochs that they can use to quickly classify new specimens, always subject to further checking for exceptional situations.
As we shall see in several more units, the atom isn't the solid unit of matter originally envisioned by Democritus or popularized by Dalton. Atoms have components: electrons in orbit around a nucleus of protons and neutrons. Most of the time, the nucleus is stable but under certain conditions, it can fly apart in a process called radioactive decay, releasing detectable bursts of energy as it does so; the resulting decay "product" element may be stable or may be another radioactive isotope that can decay further. By monitoring samples containing radioactive forms of atoms, we can determine how much radioactive material is present. We can compare this amount to the amounts of both the radioactive element and its decay products found in recently formed rocks. If we assume that methods of forming rocks are limited and haven't changed over the earth's duration, then rocks formed now should have the same composition and ratio of radioactive isotopes to decay elements as rocks formed a billion years ago. If the older rock has less radioactive material in it than we expect, then we can use that difference to estimate the age of the rock.
To do so, we need to find radioactive materials that don't decay too quickly. The popular carbon-14 radioactive molecule is useful only for dating organic objects in historical times; anything more than 30 000 years old has converted so much of its carbon-14 to nitrogen-14 that we can't accurately measure the remainder.
Geologists instead use other elements for different periods, each one matched to a given range where amounts can be measured with relative accuracy. Uranium 238 has a half-life of about 4.5 billion years. This means that in any given sample, about half the atoms will convert and decay within 4.5 billion years. The remaining half will decay to a quarter of the original amount in another 4.5 billion years, and so on. Potassium-argon is another commonly used radiometric pair, since K-40 has a half life of 1.3 billion years, and its decay product Ar-40 is stable. K-40 is found in mica, which occurs in all forms of rock. Because it is widely distributed, geologists can take many samples from different locations to compare their findings. The geological time scale that radiometry produces is subject to constant tweaking as we learn more about radiation of elements trapped in the earth's crust.
Geologists don't use radiometric methods alone. They also look at stratification and weathering to determine whether ages produced by radiometric dating are reasonable. For example, a rock dating back to 1.5 billion years found near the surface of a recent lava flow could not be considered indicative of the flow's age: it must have somehow come to rest on the flow, perhaps carried by snow avalanches or water (or a hiker) to the area.
Geologists compare the age of rocks in similar strata to see if their results are consistent. This process of correlation allows them to confirm or challenge dating results. Once they have confirmation that a given strata consistently dates to a particular age, then they may use fossils found only in that strata to date other strata as the same age. This kind of "bootstrapping" is similar to the classification of variable stars, nebula, and galaxies used to determine distances in the universe.
The result of this logical puzzle-solving is a geological time frame for the earth that maps fossil remains to their first appearance in a particular age.
MYA = million years ago
Microfossils in precambrian rocks dated to 3.5 billion years again. Major groups of bacteria, fungi, protists, some animals invertebrates) represented in deposits from the bottom of the Grand Canyon (Arizona), and from the Ediacaran Hills of South Australia which have been dated as 3.5 million years old.
248 MYA: MASS EXTINCTIONS: Cause unknown?
|Mesozoic "Age of Reptiles"
65 MYA: MASS EXTINCTIONS: due to radical climate change?
Current period: MASS EXTINCTIONS due to ice ages, environmental changes introduced by humans.
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