Lesson : Finding Relative Age of Rock Layers


I can determine the relative age of rock layers based on index fossils, superposition, and cross cutting relationships.





Finding the age of rock layers is kind of like solving a mystery.  Take a look at the picture below and see if you can determine the thief in this whodunit!

whodunnit, relative age,<br />rock layers, mystery

 Did you get it right?!  Check here.


The activity from the introduction uses some of the same rules that scientists use when determining the relative age, or age compared to rocks surrounding, of rock layers.  Basically this looks at the order of how geological event occurred.  Here are some principles that scientists follow:


  • Uniformitarianism- the way rock layers form and settle now is the same as they did in the past.
  • Superposition- rocks on bottom layer are older than those on top.
  • Horizontality- Sedimentary rock was formed in a horizontal pattern.  If no longer horizontal, this is due to movements of Earth's crust.
  • Cross Cutting Relationship-  A layer or fault that "cuts through" other layers is younger than those layers.


A few terms you may need to know:

  • Intrusion- emplacement of molten rock into existing rock (does not reach surface).
  • Extrusion- emplacement of molten rock into existing rock (does reach surface).
  • Index Fossils- Fossils of organisms that were found in many parts of the world, but for a short period of time.  If this fossil is found in a particular layer, it will help determine the layer's age and that of surrounding layers.

cross cutting

Watch this video which will help you understand the laws better.

Here's a clip to help you understand index fossils.


Examples of index fossils include:

Ammonites were common during the Mesozoic Era (245 to 65 mya), They were not found after the Cretaceous period, as they went extinct during the K-T extinction (65 mya).

Brachiopods (mollusk-like marine animals) appeared during the Cambrian (540 to 500 mya); some examples still survive.

Graptolites (widespread colonial marine hemichordates) that lived from the Cambrian period (roughly 540 to 505 million years ago) to the early to mid-Carboniferous (360 to 320 million years ago).

Nanofossils are microscopic fossils (the remains of calcareous nannoplankton, coccolithophores) from various eras. Nanofossils are very abundant, widely distributed geographically, and time-specific, because of their high evolutionary rates. There are enormous numbers of useful nanofossils, including radiolarians and foraminifera. Nanofossils are the primary method of dating marine sediments.

Trilobites were common during the Paleozoic Era (540 to 245 mya); about half of the Paleozoic fossils are trilobites. They evolved at the beginning of the Paleozoic Era and went extinct during the late Permian period (248 million years ago). 


Quick check:  Go to http://www.phschool.com/webcodes10/index.cfm?wcprefix=cfp&wcsuffix=2042&fuseaction=home.gotoWebCode&x=0&y=0 and try the questions.  How did you do??


Try It!  Try a few examples, determining the order from oldest to newest.  Make sure to follow the principles that were listed above.


relative age, practice

Click here for the answer.


Let's try one more.


relative age practice


Click here for the answer.

Challenge!!  Were the two above too easy?!  Give this a try!


relative age, practiceClick here for the answer.


Time to show what you know! Show your answers and work on your own paper to hand into your teacher, or in your science notebook.

First, write the correct answer to the sketch given.  Include reasons why you chose the order, based on the principles you read about earlier. (A geological history)

relative age 4

Second, draw your own layers.  Include at least 5 different layers, show at least one example of a cross-cutting relationship, and at least one index fossil. Make sure you have an answer key for your sketch and an explanation for the order of the layers using the principles. (Another geologic history)


1 Worksheet to use while working through this module
2 relative age principles
3 relative age quizlet
4 stratigraphy practice
5 relative age practice