Earth Science
Chapter 9
Section 3
A. Changing From One Element to Another:
All matter is made of atoms. When all the atoms in a
particular type of matter are the same, the matter is called an
element.
Most elements are stable. They do not change under normal
condition.
Some elements exist in forms that are unstable. Over time,
these elements break down, or decay, by releasing particles and
energy in a process called radioactive decay.
During radioactive decay, the atoms of one element break
down to form atoms of another element.
Radioactive elements occur naturally in igneous rocks.
Scientists use the rate at which these elements decay to
calculate the rock’s age.
B. The Rate of Radioactive Decay:
The rate of decay of each radioactive element is constant - it
never changes.
This rate of decay is the element’s half-life.
The half-life of a radioactive element is the time it takes for
half of the radioactive atoms to decay to atoms of a stable
element. Examples- Potassium-40 decays to Argon-40,
Carbon 14 decays to Nitrogen-14.
C. Absolute Ages From Radioactive Dating:
Geologists use radioactive dating to determine the absolute age
of rocks.
How?
1. Scientists first determine the amount of a radioactive
element in a rock.
2. Scientists then compare that amount with the amount of the
stable element into which the radioactive element decays.
The two most commonly used radioactive elements in
radioactive dating are Potassium-40 and carbon-14.
Scientists use Potassium-40 to date the most ancient rocks,
because of its long half-life of 1.3 billion years. Potassium-40
decays to the stable element Argon-40.
Radioactive element Carbon-14 is useful in dating materials
from plants and animals that lived up to about 50,000 years
ago.
Here’s how.
1. As plants and animals grow, carbon-14 atoms are added to
their tissue.
2. After the organism dies, no more carbon is added.
3. The carbon-14 in the organism remains and begins to
decay to stable Nitrogen-14.
4. Scientists measure the amount of Carbon-14 in a tissue
sample with the amount of Nitrogen-14 to determine the
absolute age of an organism.
Carbon-14 has a half-life of 5,730 years, which makes
it useful for dating materials from plants and animals
that lived up to 50,000 years ago.
Older sample would not have enough Carbon-14 left to
get an accurate measurement.
D. Radioactive Dating of Rock Layers:
Igneous rock contains radioactive elements that make it easier
of the dating process.
Sedimentary rocks are made up of sediment of from a variety
of rocks; therefore an accurate age of the rock layer would not
be possible. An age of individual sediments would be
determined and not the age of the actual layer of sedimentary
rock.
Scientists age sedimentary rock layers by determining the age
of intrusions or extrusions in the sedimentary rock layers.
E. How Old is Earth?
The oldest rocks found on Earth have been dated to 4.0 billion
years old.
Moon rocks are dated to be approximately 4.5 billion years old.
Scientists infer that the Earth is roughly 4.6 billion years old.