What property of radioactive isotopes allows them to be used to determine the age of rocks and fossil?

Answer 1

it's half-life should be similar to the age of the fossil. APEX

Answer 2

Radioactive isotopes decay at a known rate, and have a predictable half life; the time it will take for half of a given quantity of radioactive isotope to decay into a stable state. Using these known numbers, it is possible to estimate the relative age of an object. Different radioactive isotopes are useful for measuring different time scales, but not all are present in any given object (ie- different minerals or rocks). This method is not without issue, and estimates that are grossly inaccurate can occur, either by an error in execution of the methods, or because of incorrect assumptions of the original condition/quantity of component materials. It is also possible that rates of radioactive decay may not always be constant; the scale of time in which scientists have been able to consistently measure these rates has not been sufficient to confirm or deny this prediction.

Answer 3

The half life of radioactive isotopes can be used to determine the age of minerals within rocks. If an igneous rock, this will mark the time when the magma or lava cooled enough for the mineral to crystallise. It cannot be used to date sedimentary rocks, because the minerals within them will actually be older than the sedimentary rock itself.

Answer 4

The radioactive isotopes decay into other elements at known rates. So if we can determine what the original proportion of radioactive to non-radioactive isotopes was, we can measure the current proportion and figure out how old the sample is.

For example, the element carbon is a basic building block of all living things (on Earth, at least). The normal isotope is carbon-12, but cosmic rays interact with nitrogen in the atmosphere to form carbon-14. So the "natural" proportion of carbon 12 to carbon 14 is pretty much known.

Living things eat and incorporate the carbon atoms into their structures. When they die, they STOP eating and incorporating carbon atoms, and the carbon-14 atoms decaying without being replaced causes the carbon-14 level to fall (very slightly!) while the carbon-12 remains constant. So by measuring the proportion now, we can calculate - at least roughly - how long it has been since this sample was alive.

Carbon-dating is only accurate out to about 60,000 years; beyond that, too little of the carbon-14 remains to be accurately measured. However, there are other elements that provide a longer time scale, but not as accurately.

One problem would be that we assume that cosmic rays are producing carbon-14 at a steady rate, and for a short time period, we know that wasn't true. During the late 1940s and through the early 1960's, both the US and the Soviet Union conducted tests of nuclear weapons in the open atmosphere, which produced large spikes in the amount of carbon-14 in the atmosphere. If there had been similar spikes within the last 100,000 years or so (such as the explosion of a supernova star within about 300 light years or so) we would be unable to accurately account for the extra carbon 14 created in that event.

Answer 5

Isotopic dating relies on the amount and rate of decay of the isotope present in rock. Fossils are generally not dated directly by isotopic dating, however. Igneous intrusions found cutting through, or above or below the strata containing the fossil is used to narrow down its age.

Answer 6

By determining the age of igneous intrusions that cut through or are on top of or below a sedimentary layer containing fossils, a substantiated age range can be determined for the age of the fossils.

Answer 7

The property of radioactive decay and the use of the essentially constant half life of isotopes enable scientists to use nuclear physics for dating techniques.

Answer 8

Their half life and the proportions of their decay products.

Answer 9

its half life should be similar to the age of the fossil

Answer 10

This is the specific half-life and decay products.