Name __________________________________________
ESC 101
Laboratory Exercise: Geologic Time
Part A: Understanding Geologic Time.
As you learn about past events in Earth history you may be surprised when you hear that something occurred
“only” a million years ago. In the context of a human lifetime this is an enormous amount of time. Yet, geologists may
consider this to be a short time ago when compared to Earth’s age. The enormity of geologic time is difficult to
understand. One way to help visualize geologic time is to create a time-line.
To illustrate the enormity of geologic time the length of football field will be used as a time-line for geologic
events (Figure 1). Because distances in American football are measured in yards, this analogy will not use metric units of
measure.
The present will be placed on the goal line near the “Geologists” end zone. As we move away from the goal line
towards the “Astronomers” end zone we are moving further into the past. As you move back towards the “Geologists”
goal line you are moving closer to the present. On this scale, one yard is equal to 46 million years. Using this scale the
estimated time of origin of Earth and solar system (4.6 billion years) is 100 yards away, at the “Astronomers” goal line.
You can see that on this scale the extinction of the dinosaurs occurred only 1.4 yards from the present (the “Geologists”
goal line). It must be noted that many scientists believe that not all dinosaurs became extinct. There is evidence that
birds are the last surviving branch of the dinosaurs! Earliest humans lived 2.4 million years ago. This would be only
0.05 yards (1.8 inches) from the present. This illustrates that even a million years isn’t a long time in terms of Earth’s
age. On this scale the length of a football field from goal line to goal line is equal to the entire time Earth has existed.
One million years is equal to only 0.7 inches!
Figure 1: Geologic Time-Line © Brian Vorwald (2009)
ESC 101: Professor Vorwald
Geologic Time
Page 1 of 7
Part A: Understanding the Geologic Time Scale
In Laboratory One in your lab manual read the section, Part 1A on pages 2 and 3, up to the section titled, “Processes and
Cycles of Change.” Also study the Geologic Time Scale, Figure 1.3 on page 4. All Earth history has been organized into
what is called the geologic time scale, which like a calendar. In this calendar we can refer to many events and rock units.
As in a calendar, the geologic time scale has subdivisions with longer time periods being broken into smaller time
periods. The basis for these subdivisions is the fossil record and extinctions.
1.
The Precambrian the an informal name for geologic time from Earth’s origin up to 542 million years ago.
a.
List the names of the eons that comprise the Precambrian, starting with the oldest eon.
____________________
___________________
____________________
Age decreases (closer to the present)
b.
The graph below (Figure 2) represents the percentage of time for each of the major eons.
Figure 2 - The Eons of Geologic Time
Find the percent of total Earth History represented by the geologic time units using the directions listed
below. Record your answers in Table 1.
# Using a protractor measure the total degrees of each eon’s portion of the circle.
# Find the percentage of the circle for each eon using the following equation:
Percentage =
Measured Angle in degrees X 100
360 degrees
Table 1
Time Unit
Measured Angle
Calculations
Percent
Cenozoic Era
Mesozoic Era
Paleozoic Era
Precambrian Eon
ESC 101: Professor Vorwald
Geologic Time
Page 2 of 7
c.
Explain why Geologic Time Scale shown in Figure 1.3 is not accurately constructed.
2.
Which is the largest subdivision of time? (Eon, era, period, epoch)
3.
What is the name for the oldest eon?
4.
List the periods of the Mesozoic Era starting with the oldest period.
5.
Which period ended 416 Ma?
6.
Which period started 200 Ma?
7.
What is the significance of the terms enothem, erathem, system, and series?
8.
Determine the total duration of time for the Paleozoic Era.
9.
Based on Table 1 and Figure 1.3, what percentage of Earth history is well documented by fossils?
_____________________________
____________________
________________ %
7.
Answer questions 1a and 1b found on page 14 of your Laboratory 8 in your laboratory manual.
1a: ____________________________________________________________________________________
1b: ____________________________________________________________________________________
Part B:
Historical Time Grid Model for Geologic Time
Figure 3 is a grid representing the last millennium. In this model, you will determine how many pieces of paper,
each representing 1,000 years it would take to represent all of Earth history. Then if you were to place each piece of
paper representing 1,000 years of Earth history one on top of the other, you will find how high the stack of paper would
be.
1.
On the “Historical Time Grid”, each box represents one year and the entire grid represents 1,000 years.
#
Plot each of the historical events listed in Table 2 on the gridsheet by placing an X in the appropriate box.
#
Note that the millennium starts with the year 1001 and ends with 2000. When determining the date for
each box count from right to left.
Table 2
Column 1
Date
Column 2
Event
Date
Event
Your birth date
1776
Declaration of Independence signed
1976
Viking I land on Mars
1620
Pilgrims land in New England
1969
First Men landed on the Moon
1453
Hundred Years War ended
1963
President John F. Kennedy Assassinated
1337
Hundred Years War began
1945
End of World War II
1215
Magna Carta signed
1918
End of World War I
1095
Pope Urban II called for the Crusades
1865
End of the Civil War
1066
W illiam of Normandy conquered England
ESC 101: Professor Vorwald
Geologic Time
Page 3 of 7
2.
The height of gridsheets stacked one on another for selected events
Table 3 shows several important geologic events. Because they will not fit on one gridsheet, it will be necessary
to calculate the number of gridsheets that will represent the number of years before the present for each event
using the following steps. Record all data in Table 3.
a.
Determine the number of gridsheets for each event. Since one gridsheet represents 1,000 years divide the
number of years by 1,000.
b.
Paper Height: Because the thickness of each piece of paper is so small, we will measure the paper height
using reams (packages) of paper. Each ream has 500 gridsheets and is 5.0 cm thick.
#
Determine how many reams of paper would be needed for each event by dividing the number of
gridsheets by 500.
#
Find the height of the reams by multiplying the number of reams by 5.
#
Convert the ream height to meters. Since one meter is equal to 100 cm, divide the height in
centimeters by 100.
Table 3
Events
Time in Years
Before the
Present
Farthest advance of last glacial
ice
22,000
1st Pleistocene glacial advance
1,000,000
First ancestral human, Homo
Habilis (First member of the
genus Homo)
2,400,000
Extinction of “non-avian”
dinosaurs
65,000,000
Earliest Dinosaurs
232,000,000
Beginning of Cambrian Period
544,000,000
Oldest rocks found on Earth
3,900,000,000
Age of Earth
4,600,000,000
Number of
Gridsheets
Reams
of Paper
Ream Height
centimeters
meters
Questions:
1.
For about what percent of Earth’s total history have humans (represented by the genus Homo) _________ %
been in existence. Show your calculations below.
2.
Compare Earth’s age to the age of the oldest Earth rocks found.
3.
How many reams of paper would be needed for the total time non-avian dinosaurs
existed on Earth? Show your calculations below.
4.
How does the existence of humans compare with Earth’s age?
ESC 101: Professor Vorwald
Geologic Time
_______reams
Page 4 of 7
Historical Time Grid
From Explorations in Earth Science by Osmun R, Vorwald B, and W egner S., UPCO: ©2007.
Preprinted with permission
ESC 101: Professor Vorwald
Geologic Time
Page 5 of 7
Part C: Radiometric Dating
Complete questions 8 through 11 in Part 8C of Laboratory Eight on page 162 of your laboratory manual.
8a.
__________________
8b.
__________________ yr
Calculations:
8c.
____________________________________________________________________________________________
8d.
____________________________________________________________________________________________
9.
________________________
Explanation: _________________________________________________________________________________
_________________________________________________________________________________________________
10a. ____________________________________________________________________________________________
_________________________________________________________________________________________________
10b. ____________________________________________________________________________________________
_________________________________________________________________________________________________
11a. ________
Explanation:__________________________________________________________________________________
_________________________________________________________________________________________________
11b. ____________________________________________________________________________________________
_________________________________________________________________________________________________
Go on to the next page L
ESC 101: Professor Vorwald
Geologic Time
Page 6 of 7
Part D: Additional questions on radimetric (isotopic) dating
1.
A small sample of granite is taken from a boulder. Both the boulder and the small sample are found to
have the same age, based on the decay of Uranium-238. Why isn’t the size of a sample a factor in
determining it’s age?
2.
A rock is dated using the Potassium-Argon method. What would be the effect on the calculated age of
the rock if it was discovered that some of the Argon daughter isotope had been lost from the mineral
crystals? Explain why.
3.
An igneous rock that melted during a volcanic eruption was mixture of many minerals. These
included mica that contained Potassium-40 and Argon-40 in the ratio of 1:3. The magma eventually
cooled and solidified into an igneous rock 1.3 x 109 years ago.
Which of the circles shown below shows the correct shading of the percentage 40K compared to
40
Ar found in the mica crystals of the igneous rock at the time it solidified 1.3 x109 years ago. The
darkened portion represents the daughter isotope, 40Ar.
Circle Letter _____
4.
If the amount of the parent isotope in the original sample of a rock had been 48 grams, about how much
would have been left after three half-lives?
ESC 101: Professor Vorwald
Geologic Time
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