1.
Relative Age Using Relative Dating
The relative age of a rock is its age compared to the ages of other rocks. This
means a layer of rock can be older than another layer, while being younger than
another. It all depends on its position. It is not the age of rock in the number of
years since the rock formed. That is absolute age.
Rock layers form in different ways based on what kind of rock it is. The
sediment that forms sedimentary rocks is deposited in flat layers. Over years, the
Law of Superposition
sediment hardens and changes into sedimentary rock. These rock layers
provide a record of Earth’s geologic history.
It can be difficult to determine the absolute age of a rock because rocks don’t come with a birth
certificate. So, geologists use the law of superposition to determine the relative ages of sedimentary
rock layers. According to the law of superposition, in horizontal sedimentary rock layers, the oldest
layer is at the bottom. Each higher layer is younger than
the layer below it. This is only if there has not been any
geologic disturbances to
uplift or fold the layers,
such as an earthquake or
colliding boundaries, or
there wasn’t a lot of
Unconformities
erosion of top layers.
When rock layers don’t
match up or it’s hard to determine which layer came first it is called an unconformity. Unconformities,
disconformities, and nonconformities all show a gap in the geologic record.
There are other clues to the relative ages of
rocks. To determine relative age, geologists also
study extrusions and intrusions of igneous rock,
faults, and gaps in the geologic record. Igneous
rock forms when magma or lava hardens. Lava that
hardens on the surface is called an extrusion. The
rock layers below an extrusion are always older
than the extrusion. Beneath the surface, magma
may push into bodies of rock. The magma cools and
hardens into a mass of igneous rock called an intrusion. These
Igneous intrusions (magma)
masses can be a dike, sill, or stock, etc. An intrusion is always younger than the rock layers around and
beneath it.
More clues come from the study of faults. A fault is a break Earth’s crust. A fault is always
younger than the rock it cuts through. Movements along faults can make it harder for
geologists to determine the relative age of rock layers because the layers no longer line up as
they would be expected to.
To date rock layers, geologists first give a relative age
to a layer of rock at one location and then give the same
age to matching layers at other locations. This is how
scientists have constructed the geologic column. In some
of the rock layers are fossils. Certain fossils, called index
fossils, help geologists match rock layers. To be useful as
an index fossil, a fossil must be widely distributed,
abundant, and represent a type of organism that existed
only briefly. Index fossils are useful because they tell the
relative ages of the rock layers in
which they occur. Geologists use
Geologic columns with index fossils
particular types of organisms, such as ammonites, as index fossils.
Ammonites were a group of hard-shelled animals that evolved in shallow
seas more than 500 million years ago. They later became extinct.
Ammonite fossils have been found in many different places.
Ammonite fossil
Name____________________________
The Relative Age of Rocks
1. What is relative age? ______________________________________________________________________
2. What is absolute age? ______________________________________________________________________
3. What does the law of superposition state?
_____________________________________________________________________________________
4. How would you explain it if you found layers of rocks that were younger under older rock.
____________________________________________________________________________________
5. An unconformity is a _____ in the _______________________ record.
6. True or False? The deeper one travels into the Grand Canyon, the younger the rocks become.
7. Complete the table below about the clues that geologists use to find the relative ages of rock.
Clues to the Relative Ages of Rocks
Clues
How It Forms
What does it tell geologists about the other layers
around it?
Extrusion
A.
B.
Intrusion
C.
D.
Fault
E.
F.
8. Give three examples of an intrusion. __________________________________________________________
9. Geologists use _____________fossils to match rock layers in different locations.
10. A fault cuts through an extrusion. Which layer is the older? __________________________________
11. How have scientist constructed the geologic column? ________________________________________________
_____________________________________________________________________________________________
12. How do geologists use index fossils? _______________________________________________________
_______________________________________________________________________________________.
13. Not all fossils are index fossils. How does a fossil get to be an index fossil?
1. ____________________________________________________
2. ____________________________________________________
3.____________________________________________________
14. Give an example of an index fossil. __________________________________