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.
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] 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.
Sedimentary rocks and lava flows are laid down parallel to Earths 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. [show me]
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]
Example 3: A landform is shaped from the land. Therefore, whatever rocks make up Earths surface must be older than the landform carved from it. For example, mountains usually form by rocks being pushed up from pressure in Earths 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 carved into the rock the water is flowing over. Therefore, the rocks that make up the canyons 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. 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. [show me]
Small-scale: Youve 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. [show me]
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 [show me]. T he 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 Earths 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 19 th century geologists who first compiled the geologic time scale. Two of these geologists are Roderick Murchison and Adam Sedgwick.
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 time scale.
Just as fossils and rock layers record specific times in Earths 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 5: After completing your personal time scale, a nswer the questions on the first page of the worksheet to turn into
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, lets 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 dont 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 Earths history with n ew discoveries and more detailed work at field locations.
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