Page 245 - Laboratory Manual in Physical Geology

A CTIVIT Y 8.1 Geologic Inquiry for Relative Age Dating

Name: ______________________________________ Course/Section: ______________________ Date: ____________

A. Analyze this block of layer cake. Each side of the block of cake is a vertical cross section of the layers. Also notice the surfaces
between the layers, where two different layers touch each other. Geologists refer to surfaces between layers or other bodies of
rock as contacts .

1. Think about the process used to construct the layer cake, from making and depositing (laying down) the first layer to
making and depositing the last layer. On the left edge of the cake, number the layers to show the sequence of steps in
which they were deposited to make the layer cake from 1 (first step) to n (the number of the last step).

2. Using a pen, draw lines on the layer cake to mark all of the contacts between layers. Then place arrows along the right edge of
the cake that point to each contact. Label each arrow (contact) to show its relative age from 1 (the time when the first contact
was created; the oldest contact) to “n” (the number corresponding to the last time a contact was created; the youngest contact).

B. The picture below is an outcrop about 5 meters thick near Sedona, Arizona. The red rock is an ancient body of soil. The

brown layer in which grass is rooted is modern soil. The blocky brown-gray rock with wide fractures (cracks) is an ancient
lava flow (basalt, a volcanic rock). This outcrop is a natural geologic cross section of rock layers, analogous to the cake.
1. Which layer is the oldest? How do you know?

2. Using a pen, draw a line on the picture that marks the exact position of:
a. the contact between the red ancient soil and the lava flow.

b. the exact contact between the top of the lava flow and the base of the
darker brown modern soil in which grass is growing.
3. Notice the fractures (cracks) that cut across the lava flow layer. Are they
older or younger than the lava flow? How do you know?

4. Notice that clasts (broken pieces) of the lava flow are included in the
brown soil. Are they older or younger than the brown soil? How do
you know? 1 meter

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