[inti-net] How do scientists determine the age of dinosaur bones?

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http://science.howstuffworks.com/dinosaur-bone-age.htm/printable

How do scientists determine the age of dinosaur bones?
by Tracy V. Wilson

Browse the article How do scientists determine the age of dinosaur bones?
Introduction to How do scientists determine the age of dinosaur bones?

When paleontologist Mary Schweitzer found soft tissue in a Tyrannosaurus rex
fossil, her discovery raised an obvious question -- how the tissue could have
survived so long? The bone was 68 million years old, and conventional wisdom
about fossilization is that all soft tissue, from blood to brains, decomposes.
Only hard parts, like bones and teeth, can become fossils. But for some people,
the discovery raised a different question. How do scientists know the bones are
really 68 million years old?

Mark Wilson/Newsmakers/Getty Images
The 67-million-year-old Tyrannosaurus rex skeleton known as Sue stands on
display at Union Station on June 7, 2000, in Washington, D.C.

Today's knowledge of fossil ages comes primarily from radiometric dating, also
known as radioactive dating. Radiometric dating relies on the properties of
isotopes. These are chemical elements, like carbon or uranium, that are
identical except for one key feature -- the number of neutrons in their nucleus.

Usually, atoms have an equal number of protons and neutrons. If there are too
many or too few neutrons, the atom is unstable, and it sheds particles until
its nucleus reaches a stable state. Think of the nucleus as a pyramid of
building blocks. If you try to add extra blocks to the sides pyramid, they may
stay put for a while, but they'll eventually fall away. The same is true if you
take a block away from one of the pyramid's sides, making the rest unstable.
Eventually, some of the blocks can fall away, leaving a smaller, more stable
structure.

The result is like a radioactive clock that ticks away as unstable isotopes
decay into stable ones. You can't predict when a specific unstable atom, or
parent, will decay into a stable atom, or daughter. But you can predict how
long it will take a large group of atoms to decay. The element's half-life is
the amount of time it takes for half the parent atoms in a sample to become
daughters.

To read the time on this radioactive clock, scientists use a device called a
mass spectrometer to measure the number of parent and daughter atoms. The ratio
of parents to daughters can tell the researcher how old the specimen is. The
more parent isotopes there are -- and the fewer daughter isotopes -- the
younger the sample. The half-life of the isotope being measured determines how
useful it is at dating very old samples. Once all the parents have become
daughters, there's no more basis for comparison between the two isotopes.
Scientists can't tell whether the clock ran down a few days or millions of
years ago. This means that isotopes with a short half-life won't work to date
dinosaur bones.

The short half-life is only part of the problem when dating dinosaur bones --
researchers also have to find enough of the parent and daughter atoms to
measure. Read on to see what it takes to date a fossil and what volcanic ash
has to do with it.

Dating Sedimentary Rock

The most widely known form of radiometric dating is carbon-14 dating. This is
what archaeologists use to determine the age of human-made artifacts. But
carbon-14 dating won't work on dinosaur bones. The half-life of carbon-14 is
only 5,370 years, so carbon-14 dating is only effective on samples that are
less than 50,000 years old. Dinosaur bones, on the other hand, are millions of
years old -- some fossils are billions of years old. To determine the ages of
these specimens, scientists need an isotope with a very long half-life. Some of
the isotopes used for this purpose are uranium-238, uranium-235 and
potassium-40, each of which has a half-life of more than a million years.

GABRIEL BOUYS/AFP/Getty Images
An eagle flies over the Grand Canyon in Arizona, April 5, 2007. You can
see the layers of sedimentary rock
in the canyon's walls.

Unfortunately, these elements don't exist in dinosaur fossils themselves. Each
of them typically exists in igneous rock, or rock made from cooled magma.
Fossils, however, form in sedimentary rock -- sediment quickly covers a
dinosaur's body, and the sediment and the bones gradually turn into rock. But
this sediment doesn't typically include the necessary isotopes in measurable
amounts. Fossils can't form in the igneous rock that usually does contain the
isotopes. The extreme temperatures of the magma would just destroy the bones.

So to determine the age of sedimentary rock layers, researchers first have to
find neighboring layers of Earth that include igneous rock, such as volcanic
ash. These layers are like bookends -- they give a beginning and an end to the
period of time when the sedimentary rock formed. By using radiometric dating to
determine the age of igneous brackets, researchers can accurately determine the
age of the sedimentary layers between them.

Using the basic ideas of bracketing and radiometric dating, researchers have
determined the age of rock layers all over the world. This information has also
helped determine the age of the Earth itself. While the oldest known rocks on
Earth are about 3.5 billion years old, researchers have found zircon crystals
that are 4.3 billion years old [source: USGS]. Based on the analysis of these
samples, scientists estimate that the Earth itself is about 4.5 billion years
old. In addition, the oldest known moon rocks are 4.5 billion years old. Since
the moon and the Earth probably formed at the same time, this supports the
current idea of the Earth's age.

You can learn more about fossils, dinosaurs, radiometric dating and related
topics by reading through the links on the next page.

Other Dating Methods
Radiometric dating isn't the only method of determining the age of rocks.
Other techniques include analyzing amino acids and measuring changes in an
object's magnetic field. Scientists have also made improvements to the standard
radiometric measurements. For example, by using a laser, researchers can
measure parent and daughter atoms in extremely small amounts of matter, making
it possible to determine the age of very small samples [source: New Scientist].

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