Geology 2 – Physical Geology Lab
Lab #8 – Geologic Time - Relative Dating
Student Outcomes:
 Become familiar with the principles used to determine the relative ages of geologic
formations and features, and be able to decipher the chronological sequence of geologic
events.
 Be able to develop a reasonable sequence of geologic events by examining the features of an
outcrop.
 Understand the meaning and differences between relative dating and absolute dating.
Materials:
 Part I - Six stations with color photographs of outcrops.
 Part II – Key of standard lithologic symbols
 Part III – Relative dating cartoons
 Part IV – Reconstruction of a geologic history
Logistics: The lab is in three parts.
Part I – Circulate to six stations with color photographs of outcrops. Answer the
questions on the lab worksheet. Use the worksheet questions as a guide to aid in
developing a geologic interpretation of the outcrop.
Part II – Complete the key of standard lithologic symbols.
Part III – Examine the geologic profile cartoons to determine the relative dating of the
geologic units and features.
Principles of Relative Dating
Rocks are a record of events in Earth’s history. The rock bodies in the Earth’s crust are not all
the same age and were not formed in the same way. They were formed over a vast period of
time by a variety of geologic processes and each is a record of significant events in geologic
history. Igneous rocks are records of thermal events, resulting in igneous intrusions or volcanic
eruptions. Sedimentary rocks record changing events and environments on Earth’s surface, such
as the rise and fall of sea level, climatic changes, and changes in life forms. Metamorphic rocks
are records of other types of events, such as the collision of tectonic plates and the formation and
erosion of mountain belts. Each rock body is much more than just solid material beneath the
soil. It is a document of the past, a record of our planet’s history.
Geology deals with two different but complementary methods of dating: relative time and
absolute time. Relative dating is simply determining the chronological order of a sequence of
events. No quantitative or absolute number of years is deduced, only that one event occurred
earlier or later that another. Absolute dating designates a specific age in units of hours, days, or
years. This lab deals only with relative dating.
Relative dating is important in geology because many physical events – volcanism, canyon
cutting, the deposition of sediments, and the deformation of the Earth’s crust – can be recognized
in the field. Their relative age can be established by applying several principles of remarkable
simplicity and universality. These principles are:
1) Principle of Superposition, which states that in a sequence of undeformed sedimentary
rocks, the oldest beds are on the bottom and the youngest are on the top.
MPC Geology 2
Lab 8 – Relative Dating
2) Principle of Crosscutting Relations, which states that igneous intrusions and geologic
fractures, such as faults, are younger than the rocks they cut.
3) Principle of Inclusion, which states that a fragment of a rock incorporated or included in
another is older than the host rock.
4) Principle of Faunal Succession, which states that groups of fossil plants and animals
occur in the geologic record in a definite and determinable order and that a period of
geologic time can be recognized by the fossils contained in the rocks.
5) Succession in Landscape Development. Many landforms evolve through a definite series
of stages, so that a landscape’s relative age can be determined from the degree of erosion.
This characteristic is especially obvious in volcanic features such as cinder cones and
lava flows.
(From Exercises in Physical Geology, W.K. Hamblin and J.D. Howard, 1999)
The interpretation of events in Earth’s history is based on the principle of Uniformitarianism –
the concept that the processes that have shaped Earth in the geologic past are essentially the same
as those operating today. Because the laws of nature do not change with time, we can apply the
principles of relative dating to reconstruct the geologic history of our planet. These are the skills
you will practice in lab today.
Part I: Geologic Interpretations of Outcrop Photographs
At each station examine the color photographs of outcrops and answer the questions. Recall
what you have learned about the origin of the three rock types and refer to the principles of
relative dating described above. You may work with your peers. Remember that each of you
must understand the geologic interpretation before you move on to the next question. The lab
stations have color photographs that may be easier to interpret.
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Lab 8 – Relative Dating
Station 1: Carmel Valley Gravels
We stopped at this outcrop on our Saturday fieldtrip a few weeks ago.
a. What is the name of the fine-grained unit in the lower right of this photograph?
b. In what depositional environment was this sedimentary rock deposited?
c. What is the depositional environment of the gravels above the fine-grained unit in the lower
right?
d. What kind of contact separates the unit in the lower right from the gravels?
e. Is it a conformable contact? Why or why not?
f. What is the geologic history of the rocks and sediments shown here?
g. The fellow in the lower right with the red shirt on is Rich Keziran, a well-known history
teacher at MPC. What do you think he is thinking as he gazes up at these rocks?
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Lab 8 – Relative Dating
Station 2: The Grand Canyon
Notice the two series of sedimentary rocks: the Grand Canyon Series occurs in the lower right of
the photograph and is dipping towards the right. The Paleozoic series is flat-lying and lies above
the Grand Canyon series.
a. Draw a sketch or cartoon that shows the difference in dip between the two groups of
sedimentary rocks.
b. What is the type of contact between the two groups of sedimentary rocks?
c. What must have happened during the time period represented by this contact? In other
words, what happened to create this contact?
d. What is the geologic story or sequence of events that can be inferred by looking at the rocks
that comprise the Grand Canyon, as seen in this image?
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3. Plutons in Chile
The larger diagram shows a mountain-sized view of the contact, whereas the inset shows a closeup view of the contact.
a) Which of the rocks, the light rock or the dark rock is younger? What evidence is seen on the
close-up (the inset) to support your answer?
b) Where within or on the Earth did these plutonic rocks crystallize?
c) What must have happened after crystallization to expose these rocks at the Earth’s surface?
Station 4: Inclusion
a. Which is older, the dark rock or the light rock?
b. How can you tell?
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Station 5: Garrapata Beach
a) What is the origin of the sedimentary rocks in the lower half of the outcrop?
b) Which way are these rocks dipping (the rocks in the lower half of the image)?
c) What is the origin of the pinkish sedimentary rocks in the upper right of the outcrop?
d) Suggest a scenario for the origin of the large boulders between the two sedimentary units.
e) Describe the sequence of geologic events that created the outcrop we see at this place today.
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Lab 8 – Relative Dating
Station 6: Contacts.
a) What kind of sedimentary
rock occurs above the
granite?
b) Assume the granite clast
(lower image) is the same
granite that occurs below
the sedimentary rock.
Which is older, the granite
or the sedimentary rock?
c) Note that the slightly
weathered granite on both
side of the fresh granite.
Which is older, the
slightly weathered granite
or the fresh granite?
Why? (Hint: the
difference in weathering is
probably because of a
difference in composition
rather than a difference in
age.)
d) What kind of contact separates the slightly weathered granite from the fresh granite?
e) Did the granite form above or below the surface of the Earth? How do you know?
f) How did the granite get to the surface where it is today?
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Lab 8 – Relative Dating
Part II Key of Standard Lithologic Symbols.
Geologic maps and illustration use standard symbols to represent specific rock types.
Understanding these lithologic or rock symbols is essential to interpreting geologic maps and
graphics. It will also help you complete the next part of lab. Copy the symbols from the front of
class onto your lab worksheet.
Rock type
Sedimentary rock
Symbol
Rock type
Limestone
Metamorphic rock
Erosional contact
Igneous rock
Fault
Sandstone
Alluvium
Symbol
Conglomerate
Part III Relative Dating Cartoons
In the following diagrams, number the formation of the units or features in their correct
chronological order. Place the letter of the oldest unit on the bottom of the list and letter of the
youngest unit on the top and fill in the rest appropriately. You may choose to do either cartoon B
or C: you do not need to do both
Cartoon A.
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Cartoon B. (You may choose to do B or C: You do not have to do both)
Cartoon C.
What kind of unconformity separates unit B from unit P?
What kind of unconformity separates unit E from the underlying units?
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Cartoon D.
____
Hints: N refers to the time the house was built. Fault T cuts Formation A.
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Part IV Reconstruction of Geologic History
Looking at the structures and rock relationships in the following diagrams, reconstruct a
plausible geologic history or sequence of events. Describe the geologic events that must have
occurred in the correct chronological order to create the picture in the diagram. Use a list format.
Geologic History Diagram A
Sequence of events for diagram A: (List oldest event first.)
1) Metamorphism of rock Formation A
2)
3)
4)
5)
6)
7)
Geologic History Diagram B
Sequence of events for diagram B: (List oldest event first.)
1) Deposition of rock Formation A
6)
2)
7)
3)
8)
4)
9)
5)
Geology 2/Labs/Time/Lab #8 time.doc/LTS 10/06/06
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