Ms. Bowie - Geology 12
Relative Age Dating
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Relative Age Dating Activity (& Absolute Age Dating Methods)
Name: ____________________________________________
Date: ____________________________
Purpose:
In this activity you will learn to determine the sequence of geological events from cross-sections of strata
(rocks) in a given area. It's a bit of a brain teaser. Prepare for some fun and challenge.
Background:
Geologists often need to know the age of material that they find. They use absolute dating methods,
sometimes called numerical dating, to give rocks an actual date, or date range, in number of years. This is
different to relative dating, which only puts geological events in time order.
Absolute Age Dating
Radiometric dating (To Determine Absolute Age)
Most absolute dates for rocks are obtained with radiometric methods. These use radioactive minerals in rocks
as geological clocks.
The atoms of some chemical elements have different forms, called isotopes. These break down over time in a
process scientists call radioactive decay. Each original isotope, called the parent, gradually decays to form a
new isotope, called the daughter. Each isotope is identified with what is called a ‘mass number’. When
‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.
Isotopes are important to geologists because each radioactive element decays at a constant rate, which is
unique to that element. These rates of decay are known, so if you can measure the proportion of parent and
daughter isotopes in rocks now, you can calculate when the rocks were formed.
Because of their unique decay rates, different elements are used for dating different age ranges. For example,
the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of
uranium-238 to lead-206 is used for rocks older than 1 million years.
Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the
decay of carbon-14 to nitrogen-14. Because of the fairly fast decay rate of carbon-14, it can only be used on
material up to about 60,000 years old. Geologists use radiocarbon to date such materials as wood and pollen
trapped in sediment, which indicates the date of the sediment itself.
Measuring isotopes is particularly useful for dating igneous and some metamorphic rock, but not
sedimentary rock. Sedimentary rock is made of particles derived from other rocks, so measuring isotopes
would date the original rock material, not the sediments they have ended up in. However, there are
radiometric dating methods that can be used on sedimentary rock, including luminescence dating.
Luminescence dating measures the energy from radioactive decay that is trapped inside nearby crystals.
Ms. Bowie - Geology 12
Relative Age Dating
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Relative Age Dating
Relative age dating refers to a method of determining the approximate age based on the strata (geological
cross section or profile). Relative age dating assumes that the lower layers in any particular cross section
are older than the upper layers in that cross section (this is known as the "Law of Superposition") and that
an object cannot be older than the materials of which it is made. Igneous rocks are dated according to
whether they caused metamorphism in the surrounding rock (proof that they intruded into the pre-existing
rock), whether they cross cut pre-existing rocks or whether sediments were deposited on them after they
were formed. The profile from one location is then compared with profiles from surrounding sites to
determine the geological history of a larger area. If fossils are present in the rocks, they may also be used
to correlate rock layers across large distances, this is especially useful now that absolute time has been
established to determine the age of the rocks.
Principles you must understand to carry out relative dating:
The easiest way to do relative age dating is to work from oldest to youngest. Try to find the oldest rock
(usually located near the bottom) in the diagram below and work your way up.
Figure A shows the first
example. Review of the
Principle of Original
Horizontality and the Principle
of Superposition allows you to
clearly see that Layer A is the
oldest and Layer D is the
youngest because Layer A
appear below Layer D.
Ms. Bowie - Geology 12
Relative Age Dating
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Tricks, Tips and Hints for Determining the Relative Age:
Sedimentary Rocks:



If rocks are folded, the folding is younger that
the youngest rock affected.
If they are folded into a syncline (a U-shaped
fold) the youngest rocks are in the core of the
fold. The opposite is true for a anticline (domeshaped fold).
Sedimentary rocks that contain fragments of
another rock are younger than the rocks that
the fragments came from.
In this diagram, Layer A is oldest, followed by Layer B, then
Layer C, Layer D. Layers E, then F were deposited later.
Igneous Rocks:





If an igneous body crosscuts another rock, the igneous rock is younger than that rock.
If a body of granite contains unmelted inclusions of another rock, the granite is younger that the
included rock.
Granites can intrude into other rocks, even though they may be on the bottom of a geological
profile. Look carefully for the granitic pattern and for irregular contacts between the granite and the
country (pre-existing) rock). The granite may also metamorphose the country rocks.
Intrusive rocks produce contact metamorphism (shown as a starred pattern in the country rock
pattern) along their contacts with the older rocks into which they intrude.
Lava flows may cause contact-metamorphism with the older rocks they lie upon.
Metamorphic Rocks:
These are pre-existing rocks that have been metamorphosed (changed into a different rock) by large
amounts of heat and pressure in a region. These rocks have usually been deformed by large, mountain
forming events, and therefore if they are in contact with layered of unmetamorphosed rocks, they are
usually the oldest rocks in the sequence. Always look for the metamorphic pattern to determine if there is
a metamorphic rock in your sequence.

Metamorphic rocks are older than sedimentary rocks deposited above them or with igneous rocks
that may intrude them.
Ms. Bowie - Geology 12
Relative Age Dating
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Let's See how you are doing so far. Answer the following based on the previous pages:
1)
Which of the principles apply to sedimentary rocks?
Principles of superposition, original horizontality, faunal succession, & principle of inclusions.
2)
Are only sedimentary rocks used for relative age determinations? Explain.
No, igneous rocks can be used (principle of cross-cutting relations) and metamorphic can also be used
(Metamorphic contact zones.).
3)
Explain the relative age relationship of faults to the rocks they cut.
Faults are younger than the rocks they cut.
4)
Look at the diagrams in the next session.
A)
In this figure, could it be possible to determine an
absolute age of these rocks? If yes, explain in detail how you
may be able to do this:
Yes, if the faults contain fossils, particularly index fossils.
B)
In the figure seen here, could it be possible to
determine an absolute age of these rocks? If
yes, explain in detail how you may be able to do this:
Yes, this figure contains igneous rocks and it may be possible to find an absolute age by using radiometric dating.
5)
How do you determine the relative ages of igneous rocks (list the ways)?
Cross-cutting relationships, contact metamorphic zones and inclusions.
Ms. Bowie - Geology 12
Relative Age Dating
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Ms. Bowie - Geology 12
Relative Age Dating
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Learning to Read the Clues
Practice 1:
For each of the following sections, determine the relative age sequence of the rocks. Place the answers
in the spaces on the right. Remember, always start by looking for the oldest rocks first and working your way
from the oldest to the youngest. Don't forget to consider all intrusions and faults. The diagrams will
get progressively more challenging as you move along.
B
A
D
E
C
A
D
C
B
E
Ms. Bowie - Geology 12
Relative Age Dating
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B
A
C
E
I
D
H
B
F
A
C
G
Ms. Bowie - Geology 12
Relative Age Dating
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E
B
I
F
G
C
A
H
D
K
C
I
E
N
G
D
M
F
A
J
L
B
H
O