Chapter 1 Geologic Time and Earth Dynamics47
layer of the varve is light-coloured, composed of sand,
pollen, and coarse material that was deposited during
the spring and summer when water from melting ice or
rain flowed into a lake. The second layer is dark, and is
composed of very fine clay particles and organic matter
that settled to the bottom of the lake when the top was
covered with ice and no water flowed. Each pair of layers,
therefore, represents one year. The age of a varve formation can be calculated by counting the number of pairs,
just as the age of a tree can be determined by counting
its rings. An example of varves can be seen where the
Yukon and Teslin rivers cut through varve layers that
were deposited during the last ice age when the area was
a glacial lake.
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Activity 6
field activity
lab activity
library activity
4 classroom activity
chapter activity
4 research team activity
Investigating Relative
and Absolute Dating
Purpose
To examine how relative dating and absolute dating can be
used to determine the age of snow/ice layers.
Background Information
The same principles of dating that apply to rock layers also
apply to snow/ice deposits.
Tephra is a term used to describe all the fragmental
material produced from a volcano during an eruption, including
dust, pumice, glass shards, blocks, and shattered rocks. When
volcanic tephra falls in a place where snow/ice is not melting
but accumulating each year—such as on a glacier or in
Antarctica—knowing the date when the tephra was deposited
can be used to determine the age of ice layers above and
below it.
Materials and Equipment
•notebook
Procedure
1. Work in your research teams. Look at diagram (a). It
represents the top layers of compacted snow/ice taken
from a glacier this year. What is the age (in years) of the
top layer? How old is the bottom layer? How do you know?
Which principles of dating are illustrated here?
a)
2. Now look at diagram (b). Between the second layer from
the bottom and the layer above it, volcanic tephra has
been deposited. Carbon-14 dating shows that the tephra
was deposited in 1985. Is this a relative or absolute dating
technique? If you know that the tephra was deposited in
1985, how old today is the ice layer above the tephra and
how old is the layer beneath it? How can you determine
the age of these layers?
b)
Conclusions
1. If you know when a layer of tephra was deposited in an
ice field, can you determine the exact age of the snow/ice
layers immediately above and below the tephra layer?
Explain your reasoning.
2. If the tephra was deposited thousands of years ago, what
does the principle of relative dating tell you about the ages
of the layers of ice/snow above it? Can you determine the
exact age of all the layers above the tephra layer? Explain
your reasoning.