Dating Methods
by David M.
Fitzpatrick
Last updated
Monday, 05 September 2005
In earth science, radiometric dating methods
are used to determine the ages of rocks and minerals. Geologists use
information gained from dating rocks to study the 4.6-billion-year history
of the earth. These methods are constantly challenged by "Creation
scientists" who claim they're unreliable. So, are they?
In three words: No, they're not. Here's
why.
Pre-Radiometric Dating
In the 19th century geologists could only construct a relative time
scale. They couldn't determine the earth's age, and they didn't know the
duration of the various divisions on the time scale as we do today. It was
radiometric dating that enabled these things, following the discovery of
radioactivity in 1896. Using radiometric dating methods, geologists could
measure how long each division of the relative geologic time scale lasted.
This enabled the development of an absolute time scale.
How Do They Work?
Unstable radioactive elements such as thorium and uranium decay
naturally and at constant rates. At the end of the decaying process, the
unstable elements become other elements or isotopes (isotopes are atoms that
differ in mass from other atoms of that element). This process is referred
to as "decaying into _____" where _____ is the new element or isotope.
The radioactivity of an element is
described in terms of "half-life." At the end of a half-life, half the
original quantity of the element has decayed. At the end of another
half-life, half of that has decayed, leaving a total of 1/4 the
original amount... and so on.
Carbon-14. The half-life of every
radioactive element is different. For example, carbon-14 has a half-life of
5,730 years. So, after 5,730 years, half the original quantity of the
element has decayed. After another 5,730 years, half of the remaining half
will have decayed (so 1/4 would be left). After 5,730 more years, we'd be
down to 1/8... and so on.
Uranium-238. Uranium-238, however,
has a significantly longer half-life of 4.5 billion years.
When Does Decay Begin?
Decay begins once a radioactive element becomes part of a growing
mineral crystal. The decay rate is steady and constant, a never-changing
rate. Creation scientists often argue that since we can't actually be around
for 5,730 or 4.5 billion years, we really can't know for certain. This is
flawed logic; we can indeed! We know that we don't need to wait until an
unstable radioactive element has decayed into something else to prove it,
because we can see the decay rate is steady through scientific measurements.
Radiometric Dating Methods
There are several types of dating methods.
Carbon-14 (radiocarbon): These
techniques are generally limited to a dating period of about 50,000 years
because carbon-14's rapid decay rate (half-life of 5,730 years).
Potassium-argon: In this method,
potassium decays into argon. It is widely used in dating rocks. How it
works is that argon may diffuse (leak) out of rocks if the rock has been
heated above 125° C (257° F). Of course, this dating method reflects the
last episode of heating, not the time of original rock formation.
Rubidium-strontium: This method,
in which rubidium-87 decays into strontium-87 is used to date very old
terrestrial and lunar rocks. Unlike the diffusion of argon under mild
heating, strontium is not so diffused.
Thorium-230: Commonly used to to
date oceanic sediments beyond the range of radiocarbon techniques.
Lead: Many dating methods use lead
to differentiate between Precambrian rocks (those that are more than 570
million years old) from those that are not Precambrian.
Fission-track: Involves the paths
(tracks) made by nuclear particles in glass or a mineral by the
spontaneous fission of uranium-238 impurities. The method has been used to
date the period from about 40,000 to 1 million years ago. This is an
interval not covered by carbon-14 (relatively young dates) or
potassium-argon (much older).
Rhenium and osmium: Dating methods
that determine the age of rock deposits by the concentrates of these
elements in them.
Other Dating Methods
There are other dating methods that are non-radiometric.
Dendrochronology: This is based on
the number, width, and density of annual growth rings of long-lived trees.
This can accurately date events and climatic conditions of the past 3,000
to 4,000 years.
Varve analysis: A varve is a sedimentary bed, or sequence of sedimentary beds, that are deposited in a
body of still water in a year. Analyzing varves can date Pleistocene
glacial deposits that occurred more than 10,000 years ago.
Obsidian hydration: Dates obsidian
artifacts by measuring the thickness of rims produced when water vapor
diffuses into chipped surfaces. It's useful in a range of 200 to 200,000
years.
Thermoluminescence (TL): This
measures the light (thermoluminescence) certain minerals give off when
heated. It can be used on objects several hundred thousand years old.
Earth's Magnetic Field
The Earth's magnetic field can also be used as a dating method. First of
all, the magnetic poles are not equivalent to the geographic North and South
Poles. Second, we know the magnetic poles move around over long periods of
time.
We can tell this by studying ancient
volcanic rocks. When volcanic rocks cool with their minerals oriented in the
magnetic field existing at that time, we can ascertain where the
magnetic poles were at that time. Coupled with radiometric dating methods to
determine the ages of the rocks, we can determine where the magnetic poles
were at those ages. Constant tracking enables us to chart a historical path
of the Earth's magnetic field.
As well, using these methods, studies have
shown that the magnetic field of Earth has reversed at least 170 times in
the past 100 million years. Dating these reversals from radioactive isotopes
in the rocks has had great influence on such theories as continental drift
and the spreading of ocean floors. |
