Scientific measurements such as radiometric dating use the natural radioactivity of certain elements found in rocks to help determine their age.
Scientists also use direct evidence from observations of the rock layers themselves to help determine the relative age of rock layers. Specific rock formations are indicative of a particular type of environment existing when the rock was being formed. For example, most limestones represent marine environments, whereas, sandstones with ripple marks might indicate a shoreline habitat or a riverbed.
The study and comparison of exposed rock layers or strata in various parts of the earth led scientists in the early 19th century to propose that the rock layers could be correlated from place to place. Locally, physical characteristics of rocks can be compared and correlated.
On a larger scale, even between continents, fossil evidence can help in correlating rock layers. The Law of Superposition, which states that in an undisturbed horizontal sequence of rocks, the oldest rock layers will be on the bottom, with successively younger rocks on top of these, helps geologists correlate rock layers around the world. This also means that fossils found in the lowest levels in a sequence of layered rocks represent the oldest record of life there.
By matching partial sequences, the truly oldest layers with fossils can be worked out. By correlating fossils from various parts of the world, scientists are able to give relative ages to particular strata. This is called relative dating. Relative dating tells scientists if a rock layer is "older" or "younger" than another. This would also mean that fossils found in the deepest layer of rocks in an area would represent the oldest forms of life in that particular rock formation.
In reading earth history, these layers would be "read" from bottom to top or oldest to most recent. If certain fossils are typically found only in a particular rock unit and are found in many places worldwide, they may be useful as index or guide fossils in determining the age of undated strata. By using this information from rock formations in various parts of the world and correlating the studies, scientists have been able to establish the geologic time scale.
This relative time scale divides the vast amount of earth history into various sections based on geological events sea encroachments, mountain-building, and depositional events , and notable biological events appearance, relative abundance, or extinction of certain life forms. When you complete this activity, you will be able to: Explore this link for additional information on the topics covered in this lesson: The nonsense syllables or letters sometimes overlap other cards and are being used to introduce the students to the concept of sequencing.
The cards should be duplicated, laminated, and cut into sets and randomly mixed when given to the students. It is recommended that students complete Procedure Set A and answer the associated Interpretation Questions correctly before proceeding to Set B. The cards in Set B represent rock layers containing various fossils. For Set B , you may want to color code each organism type i. Sequencing the rock layers will show the students how paleontologists use fossils to give relative dates to rock strata.
Return to top To enhance this activity, have students match the fossil sketches to real fossils. The following is a list of fossils in the John Hanley Fossil Teaching Set that may be useful in this activity.
It may be useful to share with students after they have completed Set B and answered the Interpretation Questions. The first card in the sequence has "Card 1, Set A" in the lower left-hand corner and represents the bottom of the sequence.
If the letters "T" and "C" represent fossils in the oldest rock layer, they are the oldest fossils, or the first fossils formed in the past for this sequence of rock layers.
Since this card has a common letter with the first card, it must go on top of the "TC" card. The fossils represented by the letters on this card are "younger" than the "T" or "C" fossils on the "TC" card which represents fossils in the oldest rock layer. Sequence the remaining cards by using the same process.
When you finish, you should have a vertical stack of cards with the top card representing the youngest fossils of this rock sequence and the "TC" card at the bottom of the stack representing the oldest fossils. Starting with the top card, the letters should be in order from youngest to oldest.
Please note that none of the letters in this sequence may be reversed and still be correct. The sequence must be exactly in the order as written. It is not uncommon to have students reverse the M and D for example and begin the sequence with DM because that is the way they are printed on the card. It is good at this time to remind them that these letters represent fossils in a rock layer and that one fossil next to another within a rock layer implies no particular sequencing; they both are approximately the same age as that particular rock layer.
The following question may help clarify this point. The fossils within rock layer OXD i. Each card represents a particular rock layer with a collection of fossils that are found in that particular rock stratum. All of the fossils represented would be found in sedimentary rocks of marine origin.
Figure 2-A gives some background information on the individual fossils. The letters on the other cards have no significance to the sequencing procedure and should be ignored at this time.
Find a rock layer that has at least one of the fossils you found in the oldest rock layer. This rock layer would be younger as indicated by the appearance of new fossils in the rock stratum. Keep in mind that extinction is forever. Once an organism disappears from the sequence it cannot reappear later. Use this information to sequence the cards in a vertical stack of fossils in rock strata.
Arrange them from oldest to youngest with the oldest layer on the bottom and the youngest on top. Return to top Interpretation Questions: This will enable your teacher to quickly check whether you have the correct sequence. The graptolite, placoderm, ammonite, ichthyosaur, and shark's tooth could possibly be used as index fossils since they are found in only one layer. Technically, however, given only this set of strata, one cannot say that the shark's tooth and ichthyosaur could be used as index fossils because we do not know if they continue in younger rock layers above this set of strata.
The brachiopod, crinoid, eurypterid, foraminifera, gastropod, horn coral, pelecypod, and trilobite could probably not be used as index fossils since they overlap more than one stratum. Marine sedimentary rocks such as limestone, shale, and sandstone might contain fossils similar to those depicted in this activity. In a "normal" horizontal sequence of rocks, the oldest rock layers will be on the bottom with successively younger rocks on top.
This activity illustrates this law because when the cards are placed in the correct order, the vertical stack shows the oldest fossils in a rock layer in the bottom of the stack and the youngest fossils in rock stratum on the top. Stratigraphic Section for Set B.