19.3. Measuring Geological Time – Review Questions

Relative dating

  • Describe each of the principles of stratigraphy.








  • Draw, describe, and label examples of each principle of stratigraphy.
    • principle of superposition
    • principle of cross-cutting relationships
    • principle of original horizontality
    • principle of lateral continuity
    • principle of inclusions
    • principle of unconformities
    • principle of fossil succession
    • principle of baked contacts
    • principle of chilled margins















  • Draw examples of the four kinds of unconformities. Label the key features of each type of unconformity.
    • disconformity
    • nonconformity
    • paraconformity
    • angular unconformity

  • Search for “relative dating cross-section” on Google Images.
    • Test yourself on determining relative stratigraphic ages using the images that come up from this search.
    • Name the types of unconformities that you see in the cross-sections.
    • Come up with a hypothesis about the geological history of each cross-section you describe (e.g., sedimentary deposit, followed by uplift and folding, erosion, deposition of additional strata, tectonic compression, etc.).
  • Draw a cross-section through a section of Earth’s crust in the box below, similar to the ones you found on Google Images in the previous exercise. This time, create your own unique cross-section! Include different kinds of unconformities, layers of sedimentary rock, faults that offset the strata, and cross cutting igneous intrusions such as dikes and sills. Consider how fossils in the sedimentary rocks could help you date the rocks.
  • Determine the relative ages of the strata and features in your cross-section, and provide a description of the geological history of the cross-section area.  If you want to, you can show your cross-section to your professor or a TA and see if it makes sense geologically. Then you could show your cross-section to a friend/colleague in the class, and they can use the principles of superposition to determine the relative ages of the strata.






  • Scale is important when assessing relative ages of strata; if we look at strata at a single outcrop or part of an outcrop, sometimes we will get a different answer about relative ages than we would if we looked at the same strata at numerous outcrops. Draw a few examples of scenarios where looking at a rock outcrop might give you a different answer for the relative ages of strata than if you looked at the same strata at a series of outcrops at a regional scale.

  • You are mapping the geology of a rock outcrop, and you think you may have identified a paraconformity. What kinds of information or data would you need to get to assess whether a paraconformity is present or not?


  • Describe how you can use fossils to determine the relative ages of rock strata.


  • What kinds of geologic features can cross-cut strata?


  • What are the basic types of divisions of the geologic time scale?


  • What important event happened at the boundary between the Paleozoic and Mesozoic Eras?


  • What do scientists believe happened to cause this event?


Extra review questions that may be covered in lecture (depending on your professor) that are not completely covered in the the textbook readings:

  • There are five geologic events that can give us information that we can use to determine relative ages of rocks: intrusion, erosion, deposition, faulting, and rock deformation. Draw pictures of each type of geologic event.

  • Describe how human understanding of the age of the Earth has changed as we have learned how to “read the rocks” over the past few hundred years.


  • What is the Burgess shale known for? Where is it located?


Absolute dating

  • What is the difference between relative and absolute dating? What tools do geologists use to determine ages of rocks for each approach?


  • What is a half-life?


  • What is uranium-lead dating useful for? What mineral is commonly used for this kind of dating?


  • Refer to table 19.2 in the textbook. What radioisotopes could you use to date a rock that you expect is on the order of 1 Ga old? 500 Ma?


  • Is carbon-nitrogen dating useful for dating rocks? Why or why not?


Extra review questions that may be covered in lecture (depending on your professor) that are not completely covered in the the textbook readings:

  • What is an isotope? What is the difference between different isotopes of an atom?


  • Describe how radioactive decay works. What are three types of radioactive decay?


  • Where are the oldest rocks on Earth (that we have identified so far) located? How old are they?


  • What radioisotopes were used to date the oldest rocks on Earth? Which minerals do we analyse to examine these radioisotopes?


  • Name the four Eons of the geologic time scale, and describe event(s) that occurred in each one. What did the Earth’s surface look like in each of the Eons?


  • Using your arm as a representation of the age of the Earth, practice pointing and naming the positions of the Eons along your arm. Point to a few positions where major life forms evolved: the earliest life, the first multicellular life, the first land plants, vertebrates, and humans. Where is 4.6 Ga on your arm? Where is 2.6 Ga? Where is 542 Ma? 2 Ma?


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