How do scientists determine that things are millions or billions of years old?

Answer 1

Radiometric dating.

This excerpt from http://pubs.usgs.gov/gip/geotime/radiometric.html
should help explain it:

Radioactive decay is a spontaneous process in which an isotope (the parent) loses particles from its nucleus to form an isotope of a new element (the daughter). The rate of decay is conveniently expressed in terms of an isotope's half-life, or the time it takes for one-half of a particular radioactive isotope in a sample to decay. Most radioactive isotopes have rapid rates of decay (that is, short half-lives) and lose their radioactivity within a few days or years. Some isotopes, however, decay slowly, and several of these are used as geologic clocks. The parent isotopes and corresponding daughter products most commonly used to determine the ages of ancient rocks are listed below:


Parent Isotope / Stable Daughter Product / Half Life
Uranium-238 Lead-206 4.5 billion years
Uranium-235 Lead-207 704 million years
Thorium-232 Lead-208 14.0 billion years
Rubidium-87 Strontium-87 48.8 billion years
Potassium-40 Argon-40 1.25 billion years
Samarium-147 Neodymium-143 106 billion years

Only certain types of rocks, chiefly the igneous variety, can be dated directly by radiometric methods; but these rocks do not ordinarily contain fossils. Igneous rocks are those such as granite and basalt which crystallize from molten material called "magma".

When igneous rocks crystallize, the newly formed minerals contain various amounts of chemical elements, some of which have radioactive isotopes. These isotopes decay within the rocks according to their half-life rates, and by selecting the appropriate minerals (those that contain potassium, for instance) and measuring the relative amounts of parent and daughter isotopes in them, the date at which the rock crystallized can be determined. Most of the large igneous rock masses of the world have been dated in this manner.

Most sedimentary rocks such as sandstone, limestone, and shale are related to the radiometric time scale by bracketing them within time zones that are determined by dating appropriately selected igneous rocks.

Hope that helped.

Oh, Carbon -14 is only useful for dating things younger than about 50,000 years since it's half-life is only 5,730 years.

Also, as mountaingym mentioned, many fossils have been fairly accurately dated (there is always a margin of error in dating) by paleontologists. This is done by relating them to igneous layers (like perhaps ash flows) occuring above and below the sediment layer containing the fossil. Fossils dated in this manner can then be used, by comparison, to help establish relative ages of fossil-bearing strata in other regions that may not contain significant igneous deposits.

Another method widely used to help establish relative geologic age in areas without significant igneous deposits is the magnetic polarity timescale. Reversals of the Earth's magnetic field are recorded in rocks containing iron-bearing minerals. These minerals became magnetized in the direction of the current field at the time the rock formed. When a magnetometer is used to measure the remanent magnetism of a large section of rock the results are recorded as either positive or negative values and a characteristic pattern of stripes results. Any igneous rocks in the section can be dated using radiometric dating to get absolute ages to pin the section. Once enough of these sections were measured all over the world and dated as accurately as possible they were correlated to each other using the characteristic striped pattern to build a magnetic polarity timescale that covers about the last 155 million years.

Answer 2

Radiometric Dating, despite what another long-winded answer stated. In addition to C-14 dating (useful for tens of thousands of years), the is Potassium Dating, Uranium Dating, Argon-argon Dating, and others that I can't remember right of the top of my head. Also, Paleontology has Fossil Dating worked out very well, and a good Paleontologist (but not me) can give a reasonable date based on fossils.

Answer 3

Like Carbon 14 radioactive dating, I believe it is Uranium 232 dating that can do this for the items that are older than can be dated with Carbon 14.

Answer 4

By using elements that decay over time. By measuring how fast or slow a rock breaks down, one can determine the approximate age.

Answer 5

radiocarbon dating

also, the detection of certain isotopes of carbon, oxygen, and other elements helps in the deciding factor of age because these isotopes generally ahve a known half-life (the amount of years needed to reduce the mass of a substance by 1/2)

Answer 6

Usually using the halflives of certain radioactive elements contained in the object or sample studied.

Answer 7

Radiometric dating: Looking at the decay rate of isotopes of elements found in fossils.

http://en.wikipedia.org/wiki/Radiometric_dating

Answer 8

Carbon dating

Answer 9

they can tell by looking at the minerals in it and using carbonox they can tell around the era the object was in and like with stars they are able to tell by the mass how long it has lived and how long it has to live

Answer 10

I think they check the minerals inside and see what kind of sediments are in the stone/rock/bone etc. good question