LAB F: EARTH HISTORY PRE-LAB (for printing)
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 the 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.
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.
Absolute 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
Efforts to unravel the geologic events that have shaped the Earth began over 330
years ago in 1669. Nicolaus
Steno, a Danish anatomist, geologist, and priest (1636-1686) was the first
person to apply simple observations of nature to the more complex work of
deciphering Earth’s past. Working
in Italy, Steno studied deposits left by river floodwaters, the layers of rocks
at quarries, mines, and caves, and recognized fossils in rocks as having an
organic origin. Based on his
extensive field studies, he published a book in which he was the first to
clearly state some of the basic rules of relative dating used today.
Another important figure in the history of geology as a science and geologic
time is James Hutton (1726-1797). James
Hutton, a Scottish physician and gentleman farmer, explored the land around his
homeland and observed the steady work of erosion tearing rocks down into pebbles
and depositing them into streams. He
acknowledged that if erosion is constantly wearing down the land, other forces
must build it back up or ultimately all the land would disappear.
All of this work must be simultaneous and continuous.
From these field observations came his Doctrine of Uniformitarianism, an
underlying approach to geologic studies today.
Doctrine of Uniformitarianism
Simply stated, uniformitarianism says that the physical, chemical, and
biological processes at work shaping Earth today are the same processes that
have been working throughout Earth’s history.
Thus, by examining the present processes and their results, we can
identify the same processes from clues in the rocks.
The processes are the same, but the rates at which they are active have
changed through time. For
example, evidence suggests that volcanic activity in the early part of
Earth’s formation was much more frequent than present day rates.
A simple phrase that encapsulates uniformitarianism is “the present
is the key to the past.”
Finally, an historic figure closer to home is Major John
Wesley Powell. Powell was the
first person to navigate the rough passage of the Colorado River through Grand
Canyon, Arizona, in 1869. As a
scientist, Powell was mapping the unique geology and biology of the relatively
unexplored Southwestern U.S. Powell
likened the rock layers of Grand Canyon to the pages from Earth’s history
book. Powell captured the
nation’s attention with his poetic and descriptive writings on the rocks,
plants, animals, and native people of the Southwest.
Relative Dating Techniques
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
Law of Superposition
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. It should
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.
Principle of Original Horizontality
Sedimentary rocks and lava flows are laid down parallel to Earth’s surface.
Therefore, these layers begin essentially horizontal.
If rocks layers are observed tilted at an angle or folded, the events
that caused this deformation occurred after the rocks were already deposited.
In other words, the rocks were deposited first and the folding came
Principle of Cross-Cutting Relations
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.
Example 3: A landform is shaped
from the land. Therefore, whatever
rocks make up Earth’s surface must be older than the landform carved from it.
For example, mountains usually form by rocks being pushed up from
pressure in Earth’s interior. These
rocks are often twisted or tilted, another clue that they existed before the
mountains (see “Principle of Original Horizontality”).
Another example would be a canyon A canyon is often carved into the rock
over which water is flowing. Therefore,
the rocks that make up the canyon’s walls existed before erosion exposed them.
Principle of Inclusions
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. The
younger rock layer incorporated that fragment within itself.
This principle has application to both large-scale and small-scale
Large-scale: Often 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
THE GEOLOGIC TIME SCALE
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.
It wasn’t developed by any one person but compiled over a long time by
the efforts of many people, mostly during the 19th century.
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.
There is no one place where the entire rock section of Earth’s history
can be seen, but the integration of these rocks is indisputably accurate.
Once the use of radiometric dating became available to geologists and
applied to the developed 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. Adam Sedgwick, Roderick
During the lab session, you will be asked to make a scale model of Earth’s 4.6
billion-year-old history, marking certain events on the geologic time scale.
To give you some practice with this activity, and to learn some basic
Personal Time Scale
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 absolute dates in years.
An example is provided for you to see Notice
that it is set up like the geologic time scale, with a column for the names of
relative ages and a column for absolute ages.
Now, try it for yourself.
Step 1: Return to the [Earth
Science Pre-Lab Home Page] and click on the worksheet
to print for Lab E. In
that worksheet is a personal time scale chart to fill out.
Step 2: The
youngest absolute age is this year (zero years before present – 0 B.P.).
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, as in the example, should separate
the names. Notice that the
“Preschoolian” age has already been added.
It is bracketed by horizontal lines
with absolute ages for how old you are and when you began
Step 4: Complete
your time scale by labeling the remaining horizontal lines with
absolute ages (in years B.P.).
Ignore the ********* line on the chart for now.
Step 5: Answer
the questions following the time scale on the worksheet.
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. New
discoveries and more detailed work at field locations are being …….