Determining the Relative Age of Rocks
Skill Builder
The Law of Superposition states that in undisturbed sedimentary rock layers,
younger rocks are on the top while older rocks are on the bottom. However, forces such
as folding, faulting and uplifting can rearrange the rock layers so the youngest is not
always found on top. Over millions of years, the forces of plate movement can change a
flat plain into landforms such as anticlines and synclines, folded mountains, fault-block
mountains, and plateaus. A fold in rock that bends upward into an arch is an anticline. A
fold in rock that bends downward to form a valley is a syncline. Anticlines and synclines
are found on many parts of the Earth’s surface where compression forces have folded the
crust. Plate movements and stress also create reverse, normal, strike slip and thrust
faults. The constantly shifting plates make it difficult to date the age of the Earth’s
rocks.
Four Rules for the Relative Dating of Rock
To find the relative age of rock layers you need to remember some simple rules.
A. Principle of Superposition – In undisturbed in sedimentary rock layers, younger
rocks are on the top while older rocks are on the bottom.
B. Principle of Cross Cutting Relations – Any geologic feature is younger than
anything else it cuts across. This means igneous intrusions, extrusions, faults and
folds are younger than the rock they are in.
C. Deposition and Erosion - Generally an area is under water where sedimentary rock
is being deposited in layers or it is above water where erosion is occurring.
D. Unconformities – an unconformity exists when flat newer layers of sedimentary
rock cover older tilted, eroded or bent layers.
Study the following diagrams. Use the four rules from above to determine the age of
each rock layer from oldest to youngest.
Relative Age of the Rock Layers
____________ Oldest
____________
____________
____________
____________
____________ Youngest
Explain (History)
____________________________
____________________________
____________________________
____________________________
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________________________________________________________________________
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Relative Age of the Rock Layers
____________ Oldest
____________
____________
____________
____________
____________
____________
____________
____________ Youngest
Explain (History)
____________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Relative Age of the Rock Layers
____________ Oldest
____________
____________
____________
____________
____________
____________ Youngest
Explain (History)
____________________________
____________________________
____________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Questions
1. What is the law of superposition?
2. How can faulting rearrange rock layers so that the youngest is not always on the
top?
3. How can folding rearrange rock layers so that the youngest is not always on the
top?
4. Explain why a fault, extrusion or intrusion has to be younger than the rock it cuts
across.