Relative Dating Geologic Events
GLS 100 Physical Geology – Dr. Hanson
In this lab you will learn how geologists use rocks and apply the laws of relative age
dating to determine sequences of geologic events. Once you are comfortable with
the concepts presented here we will go to Forest River Park and try to interpret some
local bedrock exposures.
Part I. General Concepts
A Geologic Time
A historian who studies the evolution of civilizations may think that the Stone age
represents a very ancient time in history. When viewing time, the reference
frame for most humans is very short -- many regard time in terms of years,
decades, or generations. However a geologist, who is concerned with the history
of the earth (spanning over 4.6 billion years) views time in terms of millions and
billions of years. Geological time encompasses a much broader perspective than
human time. To a geologist, an event that occurred 10 million years ago is
considered to be relatively young.
B. The Rock Record
Earth history is recorded in rocks. Each rock layer and structure that crosses it
records an event—such as deposition, faulting, igneous activity, or uplift and
erosion. Rocks record climate change, advances and regressions of the sea,
tectonic and volcanic events, and prevailing depositional environments.
However, the rock record is somewhat limited. Rocks, like ancient writings over
time, are destroyed or become so badly damage as to be almost indecipherable.
Rocks are repeatedly subject to change through weathering, melting, and
metamorphism. The continuous cycle of change experienced by rocks is called
the rock cycle.
C. Unconformities
Unconformities are buried erosional surfaces representing breaks in the rock
record. Like pages torn from a book they tell us that part of the story is missing.
Nevertheless, their location in the rock column does offer some information-often recording periods of uplift, orogenic activity, or lowering sea level. The
amount of rock removed reflects the magnitude and duration of the event.
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Types of Unconformities
The three types of unconformities that geologist encounter are illustrated
below. Note that each cross-section tells a story. More details could be obtains
if we could see the bedforms, fossils, mineralogy, and sediment characteristics of
each layer.
Figure 1. The three types of unconformities.
Part 2. Relative Age Dating
A. Relative time
Relative time refers to the timing of events with respect to others without regard
for their absolute age in years. When the Geologic Periods were first defined,
geologists had no concept of their actual age. All they knew, from applying
simple common-sense laws, was the sequence in which they occurred.
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B. Relative time indicators
Listed below are the five basic principles, or laws, used to determine the relative
age of rock bodies and the events affecting them.
1. Law of original horizontality
Assumes that sedimentary rocks are deposited in horizontal layers. Therefore if
the rocks are tilted or folded we can infer that they have been subjected to a
deformational event.
2. Principle (or law) of superposition
Assuming that most sedimentary rocks were originally deposited in horizontal
layers (principle of original horizontality), the oldest rocks will be on the bottom
of the sequence and the youngest on the top.
3. Principle of cross cutting relationships
Igneous intrusions and geologic structures such as faults, folds, and
unconformities must be younger than the rocks they affect or cut across.
4. Principle of inclusions
Any part of a pre-existing rock that has been incorporated into another rock body
is older than the host rock. For example, rock fragments (clasts) in a
conglomerate must have been derived from rocks that are older than the
conglomerate itself.
5. Principle of faunal succession
Lifeforms evolved a definite and non-repeating order. Fossils can therefore be
used to date and correlate sedimentary rocks. Fossils used for age dating and
correlation of strata from one region to another are called index fossils.
Typically these organisms had a wide geographical distributions, short geologic
life spans (as a species) and preservable hard parts. Soft-bodied organisms do not
make good index fossils. Some examples are shown in the figure 2.
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Figure 2. The Geologic Time Scale and associated index fossils. (Source
http://pubs.usgs.gov/gip/geotime/fossils.html)
PART III. EXERCISES IN RELATIVE AGE DATING
The object of the exercise is to interpret a sequence of geologic events by
studying the relationships between rocks and structures in the geologic crosssection in figure 4. Read the following explanations and answer the questions
before filling out the table. The key to the rock layers is presented in figure 3.
When you fill out the table for figure 4 the oldest events/features is always on
the bottom.
1. Unit F is a felsite dike feeding a lava flow. An absolute age of 249 million
years was obtained from zircons in the flow.
2. Unit G is a basalt dike feeding a lava flow.
3. Structure E is a fault.
4. Structure I is an unconformity.
5. Unit C is a deposit of gravel in the floor of a stream. It contains clasts of
basalt weathered from Unit G.
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Questions:
1. Letter I represents an unconformity between the shale and limestone
formations. During which geologic periods were each of these rocks deposited?
Shale: ___________________________
Limestone: _______________________
2. Name the type of unconformity represented by I. _________________________
3. Which law of relative age dating would you use to date F, G and E?
_________________________________
4. Which laws would you used to sequence the sedimentary strata?
___________________________ and __________________________
5. Which laws would you use to date the river channel deposits (Unit C).
_____________________________, __________________________
___________________________
Figure 3. Key to rock units in figure 4.
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Figure 3. Relative age dating exercise.
Relative Age
Letter?
12.
Feature
Event
11.
10
9.
8.
7.
6.
5.
4.
3.
2.
1.
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