Earth Systems 3209 - Unit 2 Lesson 2 - Relative dating - Part 1
Many geologic events that Earth Scientists study occurred millions of years ago. The ages of these
events can be determined in two different ways in order to determine Geologic Time.
1) Relative Dating
2) Absolute Dating
Both of these dating methods are needed to accurately record geologic time chronologically and to
organize the geologic rock record.
Reference: (Tarbuck Text)
Relative Dating: pages 264 – 267; Absolute Dating: pages 273 – 277
1) Relative Dating
Relative dating attempts to place events in a sequence of formation, but does not identify their actual
date of occurrence. Comparing events to each other often does this.
This method of dating can’t tell us how long ago something happened; only that it followed one event
and preceded another.
6 relative dating techniques include;
1) Principle (Law) of Superposition
2) Principle of Original Horizontality
3) Principle of Cross-Cutting Relationships
4) Principle of Inclusions
5) Unconformities
6) Fossils
1) Law of Superposition
This states that in any undisturbed sequence of sedimentary rocks, a sedimentary layer is older than
the layers above it and younger than the layers below it. The youngest is always at the top.
If we use the previous diagram as an example, you can assume that the youngest layer is always at
the top.
Question 1: Label the oldest and youngest layers in the diagram below:
2) Principal of Original Horizontality
This states that most layers of sediment are deposited in a horizontal position. If rock layers are
folded or inclined, then the layers must have been moved into that position by crustal disturbances.
A folded rock sequence is a secondary feature caused by compressional (squeezing together) forces,
and can be seen in the diagram below.
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Types of compressional Forces:
a) Faulting
b) Folding
Faulting and Folding
When pressure (compressional or tensional forces) has been added to rocks, depending on the
type and environment, they may break or bend. This is a concept we will get into much greater
detail with in future lessons.
For now, the basics are:
Faulting – Movement of rock units along a crack in the rock. The fault below is caused by
compressional forces.
Folding – Bending of rock units caused by compressional forces, as seen below.
Question 2: What is one reason why the layers in the following diagram are not horizontally aligned?
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3) Law of Crosscutting Relationships
This states that an igneous rock or geologic feature is younger than the rocks it has intruded, or cuts
across.
2 types of crosscutting relations
a) Faults
b) Dykes
Dykes: A sheet of rock that formed in a fracture in a pre-existing rock body.
Question 3: Label the two types of Cross Cutting in the following diagram.
Question 4: Order the layers from oldest to youngest
Contact Metamorphism: When molten rock comes into contact with older rock
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The heat causes a kind of baking that changes the original rock.
Often represented as x’s on diagrams
Group Work:
1. Give three examples of Relative Dating and how they are used to determine the age of
something.
2. Using the diagram, determine the relationship between superposition and relative time.
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3. Describe: Superposition, Horizontality and Uniformitarianism. How are they different? How are
they related?
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4. In the diagram below write which of the labelled events occurred from oldest to youngest.
Earth Systems 3209 - Unit 2 Lesson 2 - Relative dating - Part 2
Continuing the 6 relative dating techniques;
1) Principle (Law) of Superposition
2) Principle of Original Horizontality
3) Principle of Cross-Cutting Relationships
4) Principle of Inclusions
5) Unconformities
6) Fossils
4) Law of Included Fragments (inclusions)
This states that pieces of one rock found in another rock must be older than the rock in which they
are found.
Question 1: Identify the inclusions in the diagram. Which is older E or D?
Question 2: In each example which layer is older?
5) Unconformities
For Unconformities, the process of formation involves land above sea level that is eroded. Then
it sinks when more sediment is deposited over the erosional surface.
Places in the rock record where layers or units of rock are missing, represent gaps in geologic time.
In general, it represents a buried erosional surface.
Three types of unconformity:
a) Angular Unconformity
Compressional forces cause sedimentary layers to fold as the layers uplifted.
The folded layers are eroded and sink where sediment is deposited over the erosional surface.
Rock layers below the erosional surface are inclined at an angle to the erosional surface and
layers above. This is illustrated in the diagrams below.
b) Disconformity
Rock layers above and below the erosional surface are parallel, as seen in the diagrams below.
c) Nonconformity
In this unconformity, younger sedimentary layers bury the eroded surface of metamorphic or
plutonic igneous rocks. The steps can be seen in the diagram below.
In nature, it wouldn’t be uncommon to see all three of these features in the one rock unit.
Identification of these will be one of the tasks that you are expected to perform for this course,
and an example that illustrates all three unconformities at once is seen below.
Question 3: Name each of the unconformities below:
6) Fossils – correlation
Fossils: The remains, traces, impressions, or any other evidence of plants and animals preserved in
rock.
When plants and animals die they get buried in sediment. The soft parts usually decay with the
hard parts being fossilized when the sediment turns to solid rock.
Conditions necessary for fossilization include:
a) Hard body parts.
b) Rapid burial.
c) Low-oxygen environment
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Fossils are used to match up rock layers between widely separated areas or between
continents
any time period can be recognized by its fossil content.
if there are missing rock layers, you can use fossils embedded in the rocks to tell when each
layer was deposited
Interpreting and Drawing well-labelled, detailed cross-sectional diagrams (Lab)
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In order to do these, rock types need to be correlated based on color, texture, rock types and
fossils.
Examples – a key is needed
Group Work:
1. Describe the three types of unconformity, and where you would find them. Draw Diagrams for
each. Also, Identify if any of the unconformities relate to other principles.
2. Arrange the letters in the order they occur, beginning with the oldest event and ending with the
youngest event.
Oldest
Youngest
3. What evidence is provided to indicate that H is NOT a buried lava flow?
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4. What is represented by the letter B? How do you know?
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5. Which of the features “B” or “I” formed first?
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