Name _______________________________
Science Class Period 1 2 3 6 7
Date ________________________________
A model of the three laws of relative dating.
Relative dating is simply figuring out the order in which a series of rock layers formed.
Geologists use three laws to do this:
1. The law of superposition: This law states that the oldest rock layer is the layer on the
bottom and the youngest layer is the layer on top. Unless rocks have been overturned by
extreme mountain building, the layers are in the same order that they formed. (Picture a
camper putting together a s’more’s bar at a cook-out. First he (she) places the graham
cracker on the plate. Then he sets half a chocolate bar directly on the graham cracker.
(The chocolate has not been on the plate as long as the graham cracker. It is younger.)
Finally, the camper tops the s’more with a toasted marshmallow. The marshmallow is on
top of the other two layers, has been there for the least amount of time, and is the
youngest layer. Try to think of some other examples to make this law clear in your mind.
How about the mattress, sheets, blankets and bedspread on a bed, for example? It is
helpful to remember an obvious analogy for this law because students who think they
understand this law tend to get confused on tests!)
2. The law of included fragments: This law states that included fragments in a rock are
always older than the rock itself. Just as the chocolate chip in a chocolate chip cookie
must be older than the cookie (The chocolate chip was sitting in a package at the grocery
store long before the cookie dough was mixed and baked to make the cookie), a pebble
that is part of a conglomerate rock is older than the rock. The pebble had to exist first in
order to become part of the conglomerate when it formed.
3. The law of cross-cutting relationships: Magma under pressure squeezes into nearby
rock whenever fractures form in the rock. Then the magma slowly cools and becomes
rock itself. This newly formed igneous rock is, of course, younger than the rock it
intruded. In the same way, fractures and faults are younger than the rock that they cross.
Faults cannot occur in rock that does not already exist. The rock must be present first,
which means it will be older than the fault, when the fault occurs.
http://www.wesleyan.edu/ctgeology/LISproject/Relative%20ages.htm
http://www.sciencelearn.org.nz/Contexts/Dating-the-Past/Sci-Media/Animations-and-Interactives/Relative-rock-layers
http://www.tulane.edu/~sanelson/geol111/geotime.htm
Part 1. Relative Rock Layers Directions: Write the letter that describes the rock in the box.
1. Relative Rock Layers
A. This is the thinnest rock layer.
B. This is the youngest rock above all
the others.
C. This was the last layer to be
formed before the rocks tilted.
D. The oldest rock is below all the
others.
E. This layer formed on top of earlier
rocks after they were tilted and
eroded away.
F. This rock is just above the oldest.
Part 2. Directions: Interpret the diagram below and answer the questions
2. What is the possible age range of the Oligocene Sedimentary Rocks?
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Explain your answer________________________________________
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3. What is the possible age range of the Eocene Sedimentary Rocks?
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Explain your answer ________________________________________
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4. The Paleocene Sedimentary Rocks must be older than ______ m.y. Explain your answer _______________________
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