LAB F: EARTH HISTORY PRE-LAB Back to main page
This is an interactive lab with lots of graphics and animations. Do not
try to print it!
Print the Lab F Notes (without graphics)
Earth History Pre - Lab Instructions: The " [Show Me] " link used throughout this lab links to instructional animations that go with the lab. Click the small images with blue borders to see a larger version.
You will turn this pre-lab worksheet and personal time scale in at the beginning of your next lab. Be sure to staple the sheets together! It is worth 5 points.
Pre-lab F Activity/ Worksheet Personal Time Scale Sheet
Recognizing the enormous amount of geologic time and establishing methods to place geologic events into a sequence are some of the greatest intellectual accomplishments of science. Recent estimates place the age of Earth at 4,600,000,000 years old (4.6 billion years), forming at the same time as the rest of the planets in our solar system. It is very difficult for us, with an average lifespan of 75 years, to comprehend time periods of millions and billions of years. It is easier to place our understanding of major events in Earth’s history by using a simple clock metaphor. Try the Clock Metaphor activity [Show Me]
Geologists use two basic methods of determining the age of
rocks, landforms, and geologic events, such as mountain building.
Relative dating simply places geologic events in their proper
chronological order (1st, 2nd, 3rd, etc.)
without determining how many years old they are.
Numerical dating attempts to place a date on a rock or geologic
event in years (for example, dinosaurs became extinct 66 million years ago).
This pre-lab exercise is designed to give you some experience using the
principles of relative dating and an appreciation for the complexity of geologic
Geologists have several basic tools they use to place
geologic events in their proper chronological order. Following is a brief description of several of these
principles with links to photographs and diagrams to help you understand their
In a sequence of undisturbed sedimentary rocks or lava
flows, the oldest layer will be at the bottom of the sequence with the youngest
layer at the top. Therefore, each
rock layer is older than the layer above it and younger than the layer below it. [show me]
make sense that sand, mud, or lava would spread out over a surface that is
already there; therefore the pre-existing land surface is older.
Features which cut across other rocks are younger than the
rocks they cut across. These
features can be faults, igneous intrusions, or landforms.
Example 1: A fault is a fracture in the rocks along which the rock layers have shifted. Logically speaking, rocks can only be broken after they form.
Example 2: An igneous intrusion occurs where magma is forced between or across pre-existing rock layers (hence the word “intrusion”). Magma often flows upward to the surface through whatever cracks or fractures it can find. These cracks often run across the pre-existing rocks. Thus, the igneous intrusion is younger than the pre-existing rocks. [show me]
Another example would be a canyon. A canyon is carved into the rock the water is flowing over. Therefore, the rocks that make up the canyon’s walls existed before erosion exposed them. [show me]
A rock fragment can only occur if an existing rock is
broken. Any rock fragment included
within another rock therefore must be older than the rock it is included within.
rock layer incorporated that fragment within itself.
This principle has application to both large-scale and small-scale
a lave flow or other igneous rock will have fragments of pre-existing rocks that
the magma moved through before it crystallized. These
inclusions within igneous rocks are often called “xenoliths”.
already learned that detrital sedimentary rocks, such as conglomerates, are made
from the weathering products of pre-existing rocks.
Therefore, the pebbles included within the conglomerate layer are older
than the conglomerate layer. It
took time for the pebbles to be transported, deposited, and lithified into the
Long before the discovery of radioactive decay allowed us
to assign specific dates to rock layers, geologists used the principles of
relative dating to produce a hierarchical scale into which the recognized
enormous amount of geologic time could be divided into smaller units
me]. The process of
connecting rocks across distances is called "correlation".
By using the Law of Superposition and the
Principle of Fossil
Succession together, rock units from one section of the country could be
correlated to rock units in other areas so that eventually, a composite section
of Earth’s geologic history could be pieced together.
Once the use of radiometric dating became available to geologists and
applied to the geologic time scale (largely done with relative dating
techniques), the ages of the rocks were in the correct order.
This is a testimony to the remarkable accuracy and fieldwork accomplished
by the 19th century geologists who first compiled the geologic time
scale. Two of these geologists are
During the lab session, you will be asked to mark certain events on the
geologic time scale. To give you
some practice with this activity, and to learn how unevenly distributed the
subdivisions of the geologic time scale are, you will make your own personal
Just as fossils and rock layers record specific times in
Earth’s history, you probably have photographs, books, trophies, and other
physical objects that are evidence of your life story.
You should be able to combine this physical evidence with your memories
in order to divide your life story into a sequence of named chapters or phases
arranged in relative time and bracketed by numerical dates in years.
An example is provided for you to see.
[show me] Notice that
it is set up like the geologic time scale, with a column for the names of
relative ages and a column for numerical ages.
Now, try it for yourself.
Step 1: Open and print the "Personal Time Scale" sheet from the link at the top of this pre-lab.
Step 2: The youngest numerical age is this year (Zero years before present = 0 B.P.). Fill
in your age in years B.P. at the bottom of the scale and the year (B.P.) that you
Step 3: Fill in the “Names of Relative Ages” column with a sequence of names that you
think represent intervals of time in your life arranged in their correct sequence.
Horizontal lines should separate the names, as shown in the example, . Notice that the
“Preschoolian” age has already been added. It is bracketed by horizontal lines
with numerical ages for how many years ago you were born and when you began school.
Step 4: Complete your time scale by labeling the remaining horizontal lines with
numerical ages (in years B.P.). Ignore the ********* line on the chart for now.
Step 5: After completing your personal time scale, answer the questions on the first page of the worksheet to turn into
your lab instructor.
Notice that room exists for more information on your time scale. You could keep adding more and more information until a rather complete record of your history has been charted. For example, let’s assume an elderly aunt has died and you are going through her box of photographs. A photo of you at age two with that aunt shows the two of you at a beach in California. You probably don’t remember that incident and may not have it on your time line. With this new information, you can add an event to your personal time scale to fill in the “gap” between your birth and the time you entered school. Geoscientists are doing the same thing with the geologic time scale of Earth’s history with new discoveries and more detailed work at field locations.
The instructional Animations on this page use the Flash Player. Click on the logo to get the FREE Player.