Absolute Age
• Scientists can use several methods to
determine absolute age:
– Radioactive decay
– Radiocarbon dating
– Tree ring dating
– Ice cores
– Varves
Radioactive decay
• The emission of radioactive particles and the
resulting change into other isotopes over time is
called radioactive decay.
• This decay happens at a constant rate, so it can be
used to determine the age of the minerals in a rock.
• For sedimentary rocks, scientists date layers of
igneous rock or volcanic ash above and below the
sedimentary layers
Radiocarbon dating
• This is used to determine the age of carbon
material, or things that were alive.
• The amount of carbon decreases over time
which allows scientists to determine how long
ago the object died.
Dendrochronology – Tree Ring dating
• Every year, a tree produces
two rings (a dark-colored
and a light-colored ring).
• Counting the rings allows
scientists to know how old
the tree was when it died.
Used in combination with
carbon-dating, this can help
determine the age of a tree.
Ice Cores
• Ice cores contain a record of
past environmental
conditions in annual layers of
snow.
• Geologists use ice-cores to
study glacial cycles.
Varves
• Bands of alternating lightand dark-colored
sediments of sand, clay,
and silt are called varves.
• Using varved cores,
scientists can date cycles
of glacial sedimentation
over periods as long as
120,000 years.
Geologic Time Scale p.591
• Activity p. 167
• Instead of drawing the scale, write the name
of the period next to the questions for
numbers 4 – 15 at the top.