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 time.

The Players

  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 usefulness. 

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 second.  

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 situations.

  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”.

  Small-scale:  You’ve 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 conglomerate. 

  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 Murchison

  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.).  Fill

 in your age in years B.P. at the bottom of the scale and the year (B.P.) that you

 started school.

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 school.

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 …….