Transcript Absolute Dating - Jefferson School District

Absolute Dating
Chapter 7
Lesson 2
Absolute Dating
Main Idea
Unstable parent isotopes change to
stable daughter isotopes at a constant
rate. Geologists measure the amounts of
these isotopes in minerals to determine
how long ago the minerals formed which
is called Absolute Dating.
Isotopes
Atoms of the same element that have the
same number of protons but have a different
number of neutrons are called isotopes.
Most isotopes are stable , meaning they stay in
their original form.
But some isotopes are unstable-we call these
radioactive.
Radioactive Decay
The breakdown of a radioactive, unstable
isotope (parent) of one element into a stable
isotope (daughter) of another element is
called radioactive decay.
Scientists compare the amount of parent
isotope with the amount of daughter isotope
to determine the absolute age of a rock.
The more daughter isotope there is the older
the rock is.
Half Life &
Radiometric Dating
Radiometric dating is finding the absolute
date of a rock using radioactive decay
and the half life of an isotope.
A half life is the time it takes for onehalf of a radioactive, unstable parent
isotope to decay by ½ into daughter
isotopes.
2 Forms of
Radiometric Dating
Potassium-40
Potassium 40 has a half life of 1.3 billion years.
It decays to 2 other stable elements called
argon and calcium. This is used to date rocks
older than 100,000 years old.
Uranium-238
Uranium-238 is a radioactive isotope that
decays to lead-206. Uranium-238 has a half
life of 4.5 billion years. This is used to date
rocks older than 10 million years
Half Dome
Using Uranium-238 to measure the ignous
rock of half dome determined that Half
Dome in Yosemite formed about 85
million years ago.
Age of Earth
Even though there are no rocks on earth
that we can measure the date that are
from when the Earth was formed, rocks
from our solar system, the moon, and
from meteorites have been measured.
The Earth and our Solar system are about
4.6 billion years old